PLANETARY TREMORS: "It Was Pretty Big,... I Was Shaken By It,..." - Strong 5.7 Magnitude Earthquake Felt Throughout New Zealand! [MAPS + TECTONIC SUMMARY]

People reported feeling the quake from Dunedin to Auckland.
© GNS

February 9, 2016 - NEW ZEALAND - A strong 5.7 magnitude earthquake has rocked central New Zealand.

The 1.38pm quake was 35km south-east of St Arnaud, Nelson, at a depth of 65km.

Geonet duty seismologist Anna Kaiser said the shake was felt throughout the upper South Island and lower North Island.

"It was widely felt from Taranaki to Canterbury," she said.

There were also five reports from Auckland and one from Otago.

"It was a decent shake," she said.

"With that level of shaking, it would rattle things around but we wouldn't expect major structural damage."

USGS earthquake location.

People have reported feeling the slow-rolling quake in the lower North Island and upper South Island. Several people in Wellington have posted about the earthquake online.

"It was pretty big. I was shaken by it," one government department worker said.

Adris Cochrane wrote: "Whoa for a sec I thought it was #auckland property prices falling, but no, actually, an earthquake in Wellington."

Emergency services say they've had no calls about the earthquake in St Arnaud.

Latest on the 5.7 earthquake in St. Arnaud: https://t.co/5qFWzhZrqB pic.twitter.com/v3JJs5Wena

— GeoNet (@geonet) February 9, 2016

Meanwhile, Red Cross worker Hanna Butler said everyone at the Red Cross headquarters dived under their desks.

Some users say they felt the earthquake quite strongly in Christchurch. Margery Chilton, who owns St Arnaud House Bed & Breakfast in the Nelson Lakes, said her two-storey wooden house swayed for about a minute.

"It wasn't as bad as the earthquake a few months ago but it did sway for a little bit," she said.

"Nothing fell down and there wasn't any damage.

"We've got guests that are out for a walk in the park today, though. I'll be interested to hear how they got on when they return tonight."

USGS shakemap intensity.

"Strong jolt, room shook," said another Nelson resident.

Steve Smith, who is in South Taranaki, said he felt a "long rolling sensation" for about 30 seconds.

The earthquake was felt in the Parliamentary precinct, where Prime Minister John Key remarked on it to reporters on his way into the debating chamber.

So many little shakes so close to the 5yr anniversary of the #Christchurch #earthquake ... Okay, I'm starting to freak out here. #nature

— Judy L Mohr (@JudyLMohr) February 9, 2016

One Twitter user made a link between the quake and the first day back at Parliament.

One reader said "felt in Palmerston North, I watched my sideboard doors open and door swinging. I think I have a shaky house though." - New Zealand Herald.

Tectonic Summary - Seismotectonics of the Eastern Margin of the Australia Plate

The eastern margin of the Australia plate is one of the most sesimically active areas of the world due to high rates of convergence between the Australia and Pacific plates. In the region of New Zealand, the 3000 km long Australia-Pacific plate boundary extends from south of Macquarie Island to the southern Kermadec Island chain. It includes an oceanic transform (the Macquarie Ridge), two oppositely verging subduction zones (Puysegur and Hikurangi), and a transpressive continental transform, the Alpine Fault through South Island, New Zealand.

Since 1900 there have been 15 M7.5+ earthquakes recorded near New Zealand. Nine of these, and the four largest, occurred along or near the Macquarie Ridge, including the 1989 M8.2 event on the ridge itself, and the 2004 M8.1 event 200 km to the west of the plate boundary, reflecting intraplate deformation. The largest recorded earthquake in New Zealand itself was the 1931 M7.8 Hawke's Bay earthquake, which killed 256 people. The last M7.5+ earthquake along the Alpine Fault was 170 years ago; studies of the faults' strain accumulation suggest that similar events are likely to occur again.

North of New Zealand, the Australia-Pacific boundary stretches east of Tonga and Fiji to 250 km south of Samoa. For 2,200 km the trench is approximately linear, and includes two segments where old (Greater than 120 Myr) Pacific oceanic lithosphere rapidly subducts westward (Kermadec and Tonga). At the northern end of the Tonga trench, the boundary curves sharply westward and changes along a 700 km-long segment from trench-normal subduction, to oblique subduction, to a left lateral transform-like structure.

USGS plate tectonics for the region.

Australia-Pacific convergence rates increase northward from 60 mm/yr at the southern Kermadec trench to 90 mm/yr at the northern Tonga trench; however, significant back arc extension (or equivalently, slab rollback) causes the consumption rate of subducting Pacific lithosphere to be much faster. The spreading rate in the Havre trough, west of the Kermadec trench, increases northward from 8 to 20 mm/yr. The southern tip of this spreading center is propagating into the North Island of New Zealand, rifting it apart. In the southern Lau Basin, west of the Tonga trench, the spreading rate increases northward from 60 to 90 mm/yr, and in the northern Lau Basin, multiple spreading centers result in an extension rate as high as 160 mm/yr. The overall subduction velocity of the Pacific plate is the vector sum of Australia-Pacific velocity and back arc spreading velocity: thus it increases northward along the Kermadec trench from 70 to 100 mm/yr, and along the Tonga trench from 150 to 240 mm/yr.

The Kermadec-Tonga subduction zone generates many large earthquakes on the interface between the descending Pacific and overriding Australia plates, within the two plates themselves and, less frequently, near the outer rise of the Pacific plate east of the trench. Since 1900, 40 M7.5+ earthquakes have been recorded, mostly north of 30°S. However, it is unclear whether any of the few historic M8+ events that have occurred close to the plate boundary were underthrusting events on the plate interface, or were intraplate earthquakes. On September 29, 2009, one of the largest normal fault (outer rise) earthquakes ever recorded (M8.1) occurred south of Samoa, 40 km east of the Tonga trench, generating a tsunami that killed at least 180 people.

Across the North Fiji Basin and to the west of the Vanuatu Islands, the Australia plate again subducts eastwards beneath the Pacific, at the North New Hebrides trench. At the southern end of this trench, east of the Loyalty Islands, the plate boundary curves east into an oceanic transform-like structure analogous to the one north of Tonga.

Australia-Pacific convergence rates increase northward from 80 to 90 mm/yr along the North New Hebrides trench, but the Australia plate consumption rate is increased by extension in the back arc and in the North Fiji Basin. Back arc spreading occurs at a rate of 50 mm/yr along most of the subduction zone, except near ~15°S, where the D'Entrecasteaux ridge intersects the trench and causes localized compression of 50 mm/yr in the back arc. Therefore, the Australia plate subduction velocity ranges from 120 mm/yr at the southern end of the North New Hebrides trench, to 40 mm/yr at the D'Entrecasteaux ridge-trench intersection, to 170 mm/yr at the northern end of the trench.

Large earthquakes are common along the North New Hebrides trench and have mechanisms associated with subduction tectonics, though occasional strike slip earthquakes occur near the subduction of the D'Entrecasteaux ridge. Within the subduction zone 34 M7.5+ earthquakes have been recorded since 1900. On October 7, 2009, a large interplate thrust fault earthquake (M7.6) in the northern North New Hebrides subduction zone was followed 15 minutes later by an even larger interplate event (M7.8) 60 km to the north. It is likely that the first event triggered the second of the so-called earthquake "doublet".

More information on regional seismicity and tectonics

- USGS.

PLANETARY TREMORS: Strong Magnitude 6.6 Temblor Shake Near The Kermadec Islands Triggering "GHOST QUAKES" In North Island, New Zeland - But Why?!

Tuesday's earthquake was centred near the Kermadec's Raoul Island, about 1000km northeast of New Zealand.© Massey University

February 2, 2016 - NEW ZEALAND - The magnitude 6.6 quake that struck off the coast of the North Island on Tuesday was widely felt and triggered false reports of shakes in New Zealand.

It is not uncommon for earthquakes to confuse seismic readings as energy from tremors travels large distances.

These "ghost quakes" register as local earthquakes when the GNS Science system starts to receive data.

Seismic graphs show the earthquake waves at 8am on Tuesday detected by the northernmost seismographs.© John Ristau

Let's dive right in to the world of ghost earthquakes.

What happened on Tuesday?

A large magnitude 6.6 quake centres about 850 kilometres north of Whakatane at a depth of 360km.

The epicentre was near the Kermadec Islands and the Kermadec Trench.

The location of the quake. Deep quakes off the coast can be initially recorded as multiple quakes.© USGS

The United States Geological Survey located the earthquake 120km north-northwest of a barren outcrop known as L'Esperance Rock.

Below the North Island, the Pacific plate is subducting beneath the Australian plate, which causes deeper quakes.© GeoNet

ell me more about this part of the Pacific.

The trench is the fifth deepest point of the world's oceans.

It marks the point where the Australian plate meets the Pacific plate.

A national seismic trace shows the earthquake detected progressively, from Raoul Island southwards.© GNS Science

Geographically, the entire region is one of the most seismically active areas in the world.

So, about all that shaking people "felt" in New Zealand.

A still from the Raoul Island webcam on Tuesday at 9.20am.© GNS Science

New Zealanders felt the quake's energy, but it wasn't in New Zealand.

Seismographs interpreted the waves as locally sourced and triggered alerts for the North Island.

"Ghost" quakes sometimes appear on the seismic monitoring system, the GeoNet array of around 200 seismograms dotted around New Zealand, after a large regional earthquake.

Magnitude 6.6 is a big shake and has the potential to cause a disaster.

Sensitive equipment picks up the seismic waves created by earthquakes. Equipment gets confused and pushes out an earthquake alert interpreted as locally sourced to the public, when the shake could be hundreds of kilometres away.

As in this case?

Yes, the earthquake was around 1000km away and deep.

This is all a bit confusing.

It's simple.

As the seismic waves travelled south from the source they are picked up progressively by detectors.

First Raoul Island, which has a webcam by the way, picked up the earthquake then the network detected the waves as they quickly moved southwards.

The "ghost" or "false" quakes, reported on Tuesday as three severe quakes in the Bay of Plenty region, were removed from the GeoNet alert system after initial reports.

This kind of thing has happened before.

In 2013, a quake near the location of Tuesday's shake triggered ghost earthquake readings in New Zealand.

Why don't scientists wait to confirm an earthquake's location?

In short, it's important to get information and data out to the public quickly.

As soon as readings start coming in from the seismic network that information is automatically publicly notified.

Later, once GNS Science review data, they can revise the reported magnitude and pinpoint the precise location.

Let's get technical shall we?

OK, it's all about different types of seismic waves, known as P-waves and S-waves, and the types of detection equipment.

P stands for primary waves, S for secondary.

Broadly, the equipment confuses the secondary waves for primary shaking, hence the three severe shakes initially reported by GeoNet and felt by New Zealanders.

GNS seismologist Dr John Ristau says:

"People actually felt the quake. It was quite deep.

"Our automatic system, generally, for 99 per cent of the time it actually works quite well.

"We know there's a problem when we have large earthquakes north of the North Island, particularly when they are deep.

"Our automatic system gets fooled.

"Primary waves are the fastest. That triggers the system. Then you have the secondary waves. The S-waves come in well defined...so the automatic system gets fooled into thinking it's another earthquake."

Can scientists do anything about the confusion?

The current system is the best available.

Ristau says GNS Science would rather have the system detect earthquakes as false than re-calibrate it and risk failing to detect a locally-sourced shake.

Plus, it's important to get information out to the public as quickly as possible in New Zealand and the Pacific.

Seismograms are extremely sensitive aren't they?

They can pick up wind and traffic noise, so it's no surprise when they detect tremors from earthquakes hundreds, or even thousands, of kilometres away.

They picked up the Foo Fighters blasting Auckland in 2011, remember?

That area of the ocean sounds interesting.

The Kermadec Islands and the surrounding area lie within a marine sanctuary created in 2015.

Once fully enacted as a reserve by the Government in 2016, the sanctuary will be one of the largest and most protected marine regions in the world.

It's even possible to visit, although you'll need a permit. - Stuff.

PLANETARY TREMORS: Very Strong 6.6 Magnitude Earthquake Strikes North Of New Zealand - Geonet! [MAPS + TECTONIC SUMMARY]

USGS earthquake location.

February 2, 2016 - NEW ZEALAND - A 6.6 magnitude earthquake 1100km off the North Island was responsible for the tremors that shook New Zealanders this morning.

Geonet seismologist John Ristau said the tremors felt in Bay of Plenty, Tararua and Canterbury were not official earthquakes.

There had been reports that a 5.3 magnitude quake was recorded 35 km south of Murupara and that 5.0 magnitude quake struck 20 km north west of Pongaroa.

A third quake was said to have hit 15km south-west of Amberley.

USGS shakemap intensity.

The shakes weren't official earthquakes but ripple effects of a 6.6. earthquake near Raoul Island.

Geonet received more than 500 reports of people who had felt shakes, Mr Ristau said.

There would be few aftershocks because the earthquake was so deep, he said.

Old Shaky Isles living up to its reputation! A few #eqnz in the past 15 mins pic.twitter.com/BdE1p26aCS

— Paola Brett (@paolabrett) February 1, 2016

Several people have reported feeling the quakes on social media, in areas including Bay of Plenty and Wellington.

Fire Service and police northern communications have received no reports of damage.

A spokesman for the Murupara police said he was outside when the earthquake was recorded as happening and he didn't feel a thing. He said there had been no calls from members of the public.

A woman spoken to at Murupara Area School was also surprised to hear reports of a quake - saying she didn't feel anything and nobody at the school had felt the quake. - New Zealand Herald.

Seismotectonics of the Eastern Margin of the Australia Plate

The eastern margin of the Australia plate is one of the most sesimically active areas of the world due to high rates of convergence between the Australia and Pacific plates. In the region of New Zealand, the 3000 km long Australia-Pacific plate boundary extends from south of Macquarie Island to the southern Kermadec Island chain. It includes an oceanic transform (the Macquarie Ridge), two oppositely verging subduction zones (Puysegur and Hikurangi), and a transpressive continental transform, the Alpine Fault through South Island, New Zealand.

Since 1900 there have been 15 M7.5+ earthquakes recorded near New Zealand. Nine of these, and the four largest, occurred along or near the Macquarie Ridge, including the 1989 M8.2 event on the ridge itself, and the 2004 M8.1 event 200 km to the west of the plate boundary, reflecting intraplate deformation. The largest recorded earthquake in New Zealand itself was the 1931 M7.8 Hawke's Bay earthquake, which killed 256 people. The last M7.5+ earthquake along the Alpine Fault was 170 years ago; studies of the faults' strain accumulation suggest that similar events are likely to occur again.

USGS plate tectonics for the region.

North of New Zealand, the Australia-Pacific boundary stretches east of Tonga and Fiji to 250 km south of Samoa. For 2,200 km the trench is approximately linear, and includes two segments where old (greater than 120 Myr) Pacific oceanic lithosphere rapidly subducts westward (Kermadec and Tonga). At the northern end of the Tonga trench, the boundary curves sharply westward and changes along a 700 km-long segment from trench-normal subduction, to oblique subduction, to a left lateral transform-like structure.

Australia-Pacific convergence rates increase northward from 60 mm/yr at the southern Kermadec trench to 90 mm/yr at the northern Tonga trench; however, significant back arc extension (or equivalently, slab rollback) causes the consumption rate of subducting Pacific lithosphere to be much faster. The spreading rate in the Havre trough, west of the Kermadec trench, increases northward from 8 to 20 mm/yr. The southern tip of this spreading center is propagating into the North Island of New Zealand, rifting it apart. In the southern Lau Basin, west of the Tonga trench, the spreading rate increases northward from 60 to 90 mm/yr, and in the northern Lau Basin, multiple spreading centers result in an extension rate as high as 160 mm/yr. The overall subduction velocity of the Pacific plate is the vector sum of Australia-Pacific velocity and back arc spreading velocity: thus it increases northward along the Kermadec trench from 70 to 100 mm/yr, and along the Tonga trench from 150 to 240 mm/yr.

The Kermadec-Tonga subduction zone generates many large earthquakes on the interface between the descending Pacific and overriding Australia plates, within the two plates themselves and, less frequently, near the outer rise of the Pacific plate east of the trench. Since 1900, 40 M7.5+ earthquakes have been recorded, mostly north of 30°S. However, it is unclear whether any of the few historic M8+ events that have occurred close to the plate boundary were underthrusting events on the plate interface, or were intraplate earthquakes. On September 29, 2009, one of the largest normal fault (outer rise) earthquakes ever recorded (M8.1) occurred south of Samoa, 40 km east of the Tonga trench, generating a tsunami that killed at least 180 people.

Across the North Fiji Basin and to the west of the Vanuatu Islands, the Australia plate again subducts eastwards beneath the Pacific, at the North New Hebrides trench. At the southern end of this trench, east of the Loyalty Islands, the plate boundary curves east into an oceanic transform-like structure analogous to the one north of Tonga.

Australia-Pacific convergence rates increase northward from 80 to 90 mm/yr along the North New Hebrides trench, but the Australia plate consumption rate is increased by extension in the back arc and in the North Fiji Basin. Back arc spreading occurs at a rate of 50 mm/yr along most of the subduction zone, except near ~15°S, where the D'Entrecasteaux ridge intersects the trench and causes localized compression of 50 mm/yr in the back arc. Therefore, the Australia plate subduction velocity ranges from 120 mm/yr at the southern end of the North New Hebrides trench, to 40 mm/yr at the D'Entrecasteaux ridge-trench intersection, to 170 mm/yr at the northern end of the trench.

Large earthquakes are common along the North New Hebrides trench and have mechanisms associated with subduction tectonics, though occasional strike slip earthquakes occur near the subduction of the D'Entrecasteaux ridge. Within the subduction zone 34 M7.5+ earthquakes have been recorded since 1900. On October 7, 2009, a large interplate thrust fault earthquake (M7.6) in the northern North New Hebrides subduction zone was followed 15 minutes later by an even larger interplate event (M7.8) 60 km to the north. It is likely that the first event triggered the second of the so-called earthquake "doublet".

More information on regional seismicity and tectonics

- USGS.

PLANETARY TREMORS: Strong And Shallow Magnitude 6.0 Earthquake Hits Balleny Islands Region - USGS! [MAPS + TECTONIC SUMMARY]

USGS earthquake location.

January 31, 2016 - BALLENY ISLANDS  REGION - An earthquake measuring 6.0 on the Richter scale jolted Balleny Islands region at on Sunday, the U.S. Geological Survey said.

The epicenter, with a depth of 10.0 km, was initially determined to be at 63.2868 degrees south latitude and 169.1522 degrees east longitude.

The Balleny Islands (66°55′S 163°45′E) are a series of uninhabited islands in the Southern Ocean extending from 66°15' to 67°35'S and 162°30' to 165°00'E.

The group extends for about 160 km (99 mi) in a northwest-southeast direction. The islands are heavily glaciated and are of volcanic origin.

The islands were formed by the so-called Balleny hotspot. It is also the first land on Earth to have sunrise each day.The group includes three main islands: Young, Buckle and Sturge, which lie in a line from northwest to southeast, and several smaller islets and rocks.

USGS shakemap intensity.

The Buckle, Sturge and Young Islands are examples of stratovolcanoes. Strong earthquakes very close to the islands are rare, but tremors of moderate strength do occur over the Pacific–Antarctic Ridge, Macquarie Triple Junction and Pacific Rim between the Balleny Islands and Macquarie Island.

Other earthquakes occur near the Southeast Indian Ridge and Balleny Fracture Zone, including a magnitude 8.1 earthquake in 1998 that struck just over 700 km (430 mi) west-northwest of the Islands.

Seismotectonics of the Eastern Margin of the Australia Plate

The eastern margin of the Australia plate is one of the most sesimically active areas of the world due to high rates of convergence between the Australia and Pacific plates. In the region of New Zealand, the 3000 km long Australia-Pacific plate boundary extends from south of Macquarie Island to the southern Kermadec Island chain. It includes an oceanic transform (the Macquarie Ridge), two oppositely verging subduction zones (Puysegur and Hikurangi), and a transpressive continental transform, the Alpine Fault through South Island, New Zealand.

Since 1900 there have been 15 M7.5+ earthquakes recorded near New Zealand. Nine of these, and the four largest, occurred along or near the Macquarie Ridge, including the 1989 M8.2 event on the ridge itself, and the 2004 M8.1 event 200 km to the west of the plate boundary, reflecting intraplate deformation. The largest recorded earthquake in New Zealand itself was the 1931 M7.8 Hawke's Bay earthquake, which killed 256 people. The last M7.5+ earthquake along the Alpine Fault was 170 years ago; studies of the faults' strain accumulation suggest that similar events are likely to occur again.

USGS plate tectonics for the region.

North of New Zealand, the Australia-Pacific boundary stretches east of Tonga and Fiji to 250 km south of Samoa. For 2,200 km the trench is approximately linear, and includes two segments where old (greater than 120 Myr) Pacific oceanic lithosphere rapidly subducts westward (Kermadec and Tonga). At the northern end of the Tonga trench, the boundary curves sharply westward and changes along a 700 km-long segment from trench-normal subduction, to oblique subduction, to a left lateral transform-like structure.

Australia-Pacific convergence rates increase northward from 60 mm/yr at the southern Kermadec trench to 90 mm/yr at the northern Tonga trench; however, significant back arc extension (or equivalently, slab rollback) causes the consumption rate of subducting Pacific lithosphere to be much faster. The spreading rate in the Havre trough, west of the Kermadec trench, increases northward from 8 to 20 mm/yr. The southern tip of this spreading center is propagating into the North Island of New Zealand, rifting it apart. In the southern Lau Basin, west of the Tonga trench, the spreading rate increases northward from 60 to 90 mm/yr, and in the northern Lau Basin, multiple spreading centers result in an extension rate as high as 160 mm/yr. The overall subduction velocity of the Pacific plate is the vector sum of Australia-Pacific velocity and back arc spreading velocity: thus it increases northward along the Kermadec trench from 70 to 100 mm/yr, and along the Tonga trench from 150 to 240 mm/yr.

The Kermadec-Tonga subduction zone generates many large earthquakes on the interface between the descending Pacific and overriding Australia plates, within the two plates themselves and, less frequently, near the outer rise of the Pacific plate east of the trench. Since 1900, 40 M7.5+ earthquakes have been recorded, mostly north of 30°S. However, it is unclear whether any of the few historic M8+ events that have occurred close to the plate boundary were underthrusting events on the plate interface, or were intraplate earthquakes. On September 29, 2009, one of the largest normal fault (outer rise) earthquakes ever recorded (M8.1) occurred south of Samoa, 40 km east of the Tonga trench, generating a tsunami that killed at least 180 people.

Across the North Fiji Basin and to the west of the Vanuatu Islands, the Australia plate again subducts eastwards beneath the Pacific, at the North New Hebrides trench. At the southern end of this trench, east of the Loyalty Islands, the plate boundary curves east into an oceanic transform-like structure analogous to the one north of Tonga.

Australia-Pacific convergence rates increase northward from 80 to 90 mm/yr along the North New Hebrides trench, but the Australia plate consumption rate is increased by extension in the back arc and in the North Fiji Basin. Back arc spreading occurs at a rate of 50 mm/yr along most of the subduction zone, except near ~15°S, where the D'Entrecasteaux ridge intersects the trench and causes localized compression of 50 mm/yr in the back arc. Therefore, the Australia plate subduction velocity ranges from 120 mm/yr at the southern end of the North New Hebrides trench, to 40 mm/yr at the D'Entrecasteaux ridge-trench intersection, to 170 mm/yr at the northern end of the trench.

Large earthquakes are common along the North New Hebrides trench and have mechanisms associated with subduction tectonics, though occasional strike slip earthquakes occur near the subduction of the D'Entrecasteaux ridge. Within the subduction zone 34 M7.5+ earthquakes have been recorded since 1900. On October 7, 2009, a large interplate thrust fault earthquake (M7.6) in the northern North New Hebrides subduction zone was followed 15 minutes later by an even larger interplate event (M7.8) 60 km to the north. It is likely that the first event triggered the second of the so-called earthquake "doublet".

More information on regional seismicity and tectonics

- USGS.

PLANETARY TREMORS: Strong 6.0 Magnitude Earthquake Strikes East Of Fiji - USGS! [MAPS + TECTONIC SUMMARY]

USGS earthquake location.

January 18, 2016 - FIJI - A very shallow earthquake registered by Geoscience Australia as M6.1 hit south of Fiji at 18:24 UTC on January 18, 2016.

The agency is reporting a depth of 0 km. USGS is reporting M6.0 at a depth of 14.8 km (9.2 miles).

According to the USGS, the epicenter was located 171 km (106 miles) E of Ceva-i-Ra, 425 km (264 miles) SSW of Nadi and 436 km (271 miles) SSW of Suva, Fiji.

USGS shakemap intensity.

There are no people living within 100 km (62 miles) radius.

USGS issued a green alert for shaking-related fatalities and economic losses. There is a low likelihood of casualties and damage

Overall, the population in this region resides in structures that are vulnerable to earthquake shaking, though some resistant structures exist. - The Watchers.

Seismotectonics of the Eastern Margin of the Australia Plate

The eastern margin of the Australia plate is one of the most sesimically active areas of the world due to high rates of convergence between the Australia and Pacific plates. In the region of New Zealand, the 3000 km long Australia-Pacific plate boundary extends from south of Macquarie Island to the southern Kermadec Island chain. It includes an oceanic transform (the Macquarie Ridge), two oppositely verging subduction zones (Puysegur and Hikurangi), and a transpressive continental transform, the Alpine Fault through South Island, New Zealand.

Since 1900 there have been 15 M7.5+ earthquakes recorded near New Zealand. Nine of these, and the four largest, occurred along or near the Macquarie Ridge, including the 1989 M8.2 event on the ridge itself, and the 2004 M8.1 event 200 km to the west of the plate boundary, reflecting intraplate deformation. The largest recorded earthquake in New Zealand itself was the 1931 M7.8 Hawke's Bay earthquake, which killed 256 people. The last M7.5+ earthquake along the Alpine Fault was 170 years ago; studies of the faults' strain accumulation suggest that similar events are likely to occur again.

North of New Zealand, the Australia-Pacific boundary stretches east of Tonga and Fiji to 250 km south of Samoa. For 2,200 km the trench is approximately linear, and includes two segments where old (greater than 120 Myr) Pacific oceanic lithosphere rapidly subducts westward (Kermadec and Tonga). At the northern end of the Tonga trench, the boundary curves sharply westward and changes along a 700 km-long segment from trench-normal subduction, to oblique subduction, to a left lateral transform-like structure.

USGS plate tectonics for the region.

Australia-Pacific convergence rates increase northward from 60 mm/yr at the southern Kermadec trench to 90 mm/yr at the northern Tonga trench; however, significant back arc extension (or equivalently, slab rollback) causes the consumption rate of subducting Pacific lithosphere to be much faster. The spreading rate in the Havre trough, west of the Kermadec trench, increases northward from 8 to 20 mm/yr. The southern tip of this spreading center is propagating into the North Island of New Zealand, rifting it apart. In the southern Lau Basin, west of the Tonga trench, the spreading rate increases northward from 60 to 90 mm/yr, and in the northern Lau Basin, multiple spreading centers result in an extension rate as high as 160 mm/yr. The overall subduction velocity of the Pacific plate is the vector sum of Australia-Pacific velocity and back arc spreading velocity: thus it increases northward along the Kermadec trench from 70 to 100 mm/yr, and along the Tonga trench from 150 to 240 mm/yr.

The Kermadec-Tonga subduction zone generates many large earthquakes on the interface between the descending Pacific and overriding Australia plates, within the two plates themselves and, less frequently, near the outer rise of the Pacific plate east of the trench. Since 1900, 40 M7.5+ earthquakes have been recorded, mostly north of 30°S. However, it is unclear whether any of the few historic M8+ events that have occurred close to the plate boundary were underthrusting events on the plate interface, or were intraplate earthquakes. On September 29, 2009, one of the largest normal fault (outer rise) earthquakes ever recorded (M8.1) occurred south of Samoa, 40 km east of the Tonga trench, generating a tsunami that killed at least 180 people.

Across the North Fiji Basin and to the west of the Vanuatu Islands, the Australia plate again subducts eastwards beneath the Pacific, at the North New Hebrides trench. At the southern end of this trench, east of the Loyalty Islands, the plate boundary curves east into an oceanic transform-like structure analogous to the one north of Tonga.

Australia-Pacific convergence rates increase northward from 80 to 90 mm/yr along the North New Hebrides trench, but the Australia plate consumption rate is increased by extension in the back arc and in the North Fiji Basin. Back arc spreading occurs at a rate of 50 mm/yr along most of the subduction zone, except near ~15°S, where the D'Entrecasteaux ridge intersects the trench and causes localized compression of 50 mm/yr in the back arc. Therefore, the Australia plate subduction velocity ranges from 120 mm/yr at the southern end of the North New Hebrides trench, to 40 mm/yr at the D'Entrecasteaux ridge-trench intersection, to 170 mm/yr at the northern end of the trench.

Large earthquakes are common along the North New Hebrides trench and have mechanisms associated with subduction tectonics, though occasional strike slip earthquakes occur near the subduction of the D'Entrecasteaux ridge. Within the subduction zone 34 M7.5+ earthquakes have been recorded since 1900. On October 7, 2009, a large interplate thrust fault earthquake (M7.6) in the northern North New Hebrides subduction zone was followed 15 minutes later by an even larger interplate event (M7.8) 60 km to the north. It is likely that the first event triggered the second of the so-called earthquake "doublet".

More information on regional seismicity and tectonics

- USGS.

PLANETARY TREMORS: Strong 4.0 Magnitude Earthquake Felt In Batangas, Philippines - PHIVOLCS!

Epicenter map. © Phivolcs

December 21, 2015 - THE PHILIPPINES - A moderately strong earthquake measuring 4.0-magnitude on the Richter scale has jolted parts of Batangas and Laguna afternoon of December 21, 2015, the Philippine Institute of Volcanology and Seismology (Phivolcs) reported.

The 4.0-magnitude earthquake was recorded at 12:42 p.m. and originated at 10 kilometers northeast of Tanauan City, Batangas.

It was felt at Intensity 4 or moderately strong in Tanauan City and Sto. Tomas in Batangas, and Los Baños in Laguna.

Phivolcs said an earthquake with this intensity is comparable to the "vibration of a passing heavy truck that is felt by people indoors and some people outdoors and may awaken light sleepers."

Intensity 3 was felt in Malvar, Batangas and San Pablo City in Laguna, which is relatively weak, "but some people may experience dizziness and nausea."

It was "slightly felt" at Intensity 2 in Lipa City and Talisay in Batangas; Calamba City in Laguna, and Muntinlupa City.

It was felt at Intensity 1 or scarcely perceptible in Batangas City and Tagaytay City.

The earthquake was tectonic in origin meaning the ground shaking was primarily due to the sudden movement in the earth's plates.

Phivolcs said there was no reported damage to property or loss of lives due to the earthquake. Likewise, aftershocks are not expected to occur. - Manila Bulletin.

PLANETARY TREMORS: Very Strong Magnitude 6.8 Earthquake Strikes Southwest Of Dadali, Solomon Islands - No Tsunami Expected! [MAPS + TECTONIC SUMMARY]

USGS earthquake location.

November 18, 2015 - SOLOMON ISLANDS - A strong earthquake with a preliminary magnitude of 6.8 has struck the Solomon Islands, seismologists say.The earthquake, which struck at 5:31 a.m. local time on Thursday, was centered about 119 kilometers (74 miles) southwest of Dadali, or 176 kilometers (109 miles) west-northwest of the capital Honiara. It struck about 10.8 kilometers (6.7 miles) deep, making it a shallow earthquake, according to the U.S. Geological Survey (USGS).

The earthquake was centered in the ocean, but close to several islands where tremors could be widely felt. It was not immediately known whether the early morning earthquake had caused any damage or casualties.

USGS shakemap intensity.

The Pacific Tsunami Warning Center, which initially measured the earthquake at 7.0, said there was no threat of a Pacific-wide tsunami. "Based on earthquake information and historic tsunami records the earthquake was not sufficient to generate a tsunami," the center said.

Computer models from the USGS estimated that some 615,000 residents on nearby islands may have felt Thursday's earthquake, including some 19,000 people who may have felt moderate to strong shaking. - BNO News.

Seismotectonics of the New Guinea Region and Vicinity

The Australia-Pacific plate boundary is over 4000 km long on the northern margin, from the Sunda (Java) trench in the west to the Solomon Islands in the east. The eastern section is over 2300 km long, extending west from northeast of the Australian continent and the Coral Sea until it intersects the east coast of Papua New Guinea. The boundary is dominated by the general northward subduction of the Australia plate.

Along the South Solomon trench, the Australia plate converges with the Pacific plate at a rate of approximately 95 mm/yr towards the east-northeast. Seismicity along the trench is dominantly related to subduction tectonics and large earthquakes are common: there have been 13 M7.5+ earthquakes recorded since 1900. On April 1, 2007, a M8.1 interplate megathrust earthquake occurred at the western end of the trench, generating a tsunami and killing at least 40 people. This was the third M8.1 megathrust event associated with this subduction zone in the past century; the other two occurred in 1939 and 1977.

Further east at the New Britain trench, the relative motions of several microplates surrounding the Australia-Pacific boundary, including north-south oriented seafloor spreading in the Woodlark Basin south of the Solomon Islands, maintain the general northward subduction of Australia-affiliated lithosphere beneath Pacific-affiliated lithosphere. Most of the large and great earthquakes east of New Guinea are related to this subduction; such earthquakes are particularly concentrated at the cusp of the trench south of New Ireland. 33 M7.5+ earthquakes have been recorded since 1900, including three shallow thrust fault M8.1 events in 1906, 1919, and 2007.

USGS plate tectonics for the region.

The western end of the Australia-Pacific plate boundary is perhaps the most complex portion of this boundary, extending 2000 km from Indonesia and the Banda Sea to eastern New Guinea. The boundary is dominantly convergent along an arc-continent collision segment spanning the width of New Guinea, but the regions near the edges of the impinging Australia continental margin also include relatively short segments of extensional, strike-slip and convergent deformation. The dominant convergence is accommodated by shortening and uplift across a 250-350 km-wide band of northern New Guinea, as well as by slow southward-verging subduction of the Pacific plate north of New Guinea at the New Guinea trench. Here, the Australia-Pacific plate relative velocity is approximately 110 mm/yr towards the northeast, leading to the 2-8 mm/yr uplift of the New Guinea Highlands.

Whereas the northern band of deformation is relatively diffuse east of the Indonesia-Papua New Guinea border, in western New Guinea there are at least two small (less than 100,000 km²) blocks of relatively undeformed lithosphere. The westernmost of these is the Birds Head Peninsula microplate in Indonesia's West Papua province, bounded on the south by the Seram trench. The Seram trench was originally interpreted as an extreme bend in the Sunda subduction zone, but is now thought to represent a southward-verging subduction zone between Birds Head and the Banda Sea.

There have been 22 M7.5+ earthquakes recorded in the New Guinea region since 1900. The dominant earthquake mechanisms are thrust and strike slip, associated with the arc-continent collision and the relative motions between numerous local microplates. The largest earthquake in the region was a M8.2 shallow thrust fault event in the northern Papua province of Indonesia that killed 166 people in 1996.

The western portion of the northern Australia plate boundary extends approximately 4800 km from New Guinea to Sumatra and primarily separates Australia from the Eurasia plate, including the Sunda block. This portion is dominantly convergent and includes subduction at the Sunda (Java) trench, and a young arc-continent collision.

In the east, this boundary extends from the Kai Islands to Sumba along the Timor trough, offset from the Sunda trench by 250 km south of Sumba. Contrary to earlier tectonic models in which this trough was interpreted as a subduction feature continuous with the Sunda subduction zone, it is now thought to represent a subsiding deformational feature related to the collision of the Australia plate continental margin and the volcanic arc of the Eurasia plate, initiating in the last 5-8 Myr. Before collision began, the Sunda subduction zone extended eastward to at least the Kai Islands, evidenced by the presence of a northward-dipping zone of seismicity beneath Timor Leste. A more detailed examination of the seismic zone along it's eastern segment reveals a gap in intermediate depth seismicity under Timor and seismic mechanisms that indicate an eastward propagating tear in the descending slab as the negatively buoyant oceanic lithosphere detaches from positively buoyant continental lithosphere. On the surface, GPS measurements indicate that the region around Timor is currently no longer connected to the Eurasia plate, but instead is moving at nearly the same velocity as the Australia plate, another consequence of collision.

Large earthquakes in eastern Indonesia occur frequently but interplate megathrust events related to subduction are rare; this is likely due to the disconnection of the descending oceanic slab from the continental margin. There have been 9 M7.5+ earthquakes recorded from the Kai Islands to Sumba since 1900. The largest was the great Banda Sea earthquake of 1938 (M8.5) an intermediate depth thrust faulting event that did not cause significant loss of life.

More information on regional seismicity and tectonics

- USGS.

 

PLANETARY TREMORS: Three Earthquakes Rock Southland, New Zealand!

A 'light' earthquake struck north-west of Te Anau on Saturday night.  © GeoNet

November 15, 2015 - NEW ZEALAND - Three earthquakes shook Southland overnight, the strongest measuring 4.6.

GeoNet reported a 3.8 magnitude rumble centred 50km north-west of Te Anau at 10.57pm on Saturday. It was 39km deep and described as being of "light" intensity.

Just over two hours later at 2.05am, a 4.6 magnitude shake was felt 120km west of Tuatapere. The earthquake was described as "strong" and was 12km deep.

The third earthquake to strike came at 3.31am. GeoNet reported the third shake, measuring 4.1, was centred 75km west of Tuatapere at a depth of 15km. It was described as being of "moderate" intensity. - Stuff.

Tectonic Summary - Seismotectonics of the Eastern Margin of the Australia Plate

The eastern margin of the Australia plate is one of the most sesimically active areas of the world due to high rates of convergence between the Australia and Pacific plates. In the region of New Zealand, the 3000 km long Australia-Pacific plate boundary extends from south of Macquarie Island to the southern Kermadec Island chain. It includes an oceanic transform (the Macquarie Ridge), two oppositely verging subduction zones (Puysegur and Hikurangi), and a transpressive continental transform, the Alpine Fault through South Island, New Zealand.

Since 1900 there have been 15 M7.5+ earthquakes recorded near New Zealand. Nine of these, and the four largest, occurred along or near the Macquarie Ridge, including the 1989 M8.2 event on the ridge itself, and the 2004 M8.1 event 200 km to the west of the plate boundary, reflecting intraplate deformation. The largest recorded earthquake in New Zealand itself was the 1931 M7.8 Hawke's Bay earthquake, which killed 256 people. The last M7.5+ earthquake along the Alpine Fault was 170 years ago; studies of the faults' strain accumulation suggest that similar events are likely to occur again.

North of New Zealand, the Australia-Pacific boundary stretches east of Tonga and Fiji to 250 km south of Samoa. For 2,200 km the trench is approximately linear, and includes two segments where old (greater than 120 Myr) Pacific oceanic lithosphere rapidly subducts westward (Kermadec and Tonga). At the northern end of the Tonga trench, the boundary curves sharply westward and changes along a 700 km-long segment from trench-normal subduction, to oblique subduction, to a left lateral transform-like structure.

USGS plate tectonics for the region.

Australia-Pacific convergence rates increase northward from 60 mm/yr at the southern Kermadec trench to 90 mm/yr at the northern Tonga trench; however, significant back arc extension (or equivalently, slab rollback) causes the consumption rate of subducting Pacific lithosphere to be much faster. The spreading rate in the Havre trough, west of the Kermadec trench, increases northward from 8 to 20 mm/yr. The southern tip of this spreading center is propagating into the North Island of New Zealand, rifting it apart. In the southern Lau Basin, west of the Tonga trench, the spreading rate increases northward from 60 to 90 mm/yr, and in the northern Lau Basin, multiple spreading centers result in an extension rate as high as 160 mm/yr. The overall subduction velocity of the Pacific plate is the vector sum of Australia-Pacific velocity and back arc spreading velocity: thus it increases northward along the Kermadec trench from 70 to 100 mm/yr, and along the Tonga trench from 150 to 240 mm/yr.

The Kermadec-Tonga subduction zone generates many large earthquakes on the interface between the descending Pacific and overriding Australia plates, within the two plates themselves and, less frequently, near the outer rise of the Pacific plate east of the trench. Since 1900, 40 M7.5+ earthquakes have been recorded, mostly north of 30°S. However, it is unclear whether any of the few historic M8+ events that have occurred close to the plate boundary were underthrusting events on the plate interface, or were intraplate earthquakes. On September 29, 2009, one of the largest normal fault (outer rise) earthquakes ever recorded (M8.1) occurred south of Samoa, 40 km east of the Tonga trench, generating a tsunami that killed at least 180 people.

Across the North Fiji Basin and to the west of the Vanuatu Islands, the Australia plate again subducts eastwards beneath the Pacific, at the North New Hebrides trench. At the southern end of this trench, east of the Loyalty Islands, the plate boundary curves east into an oceanic transform-like structure analogous to the one north of Tonga.

Australia-Pacific convergence rates increase northward from 80 to 90 mm/yr along the North New Hebrides trench, but the Australia plate consumption rate is increased by extension in the back arc and in the North Fiji Basin. Back arc spreading occurs at a rate of 50 mm/yr along most of the subduction zone, except near ~15°S, where the D'Entrecasteaux ridge intersects the trench and causes localized compression of 50 mm/yr in the back arc. Therefore, the Australia plate subduction velocity ranges from 120 mm/yr at the southern end of the North New Hebrides trench, to 40 mm/yr at the D'Entrecasteaux ridge-trench intersection, to 170 mm/yr at the northern end of the trench.

Large earthquakes are common along the North New Hebrides trench and have mechanisms associated with subduction tectonics, though occasional strike slip earthquakes occur near the subduction of the D'Entrecasteaux ridge. Within the subduction zone 34 M7.5+ earthquakes have been recorded since 1900. On October 7, 2009, a large interplate thrust fault earthquake (M7.6) in the northern North New Hebrides subduction zone was followed 15 minutes later by an even larger interplate event (M7.8) 60 km to the north. It is likely that the first event triggered the second of the so-called earthquake "doublet".

More information on regional seismicity and tectonics

- USGS.

 

GEOLOGICAL UPHEAVALS: Nepal Landslides Spark Fear Of Flash Floods!

As survivors of the Nepal earthquakes struggle with the aftermath, a landslide has caused fears of flash flooding. 
© Philippe Lopez/AFP/Getty Images

May 23, 2015 - NEPAL - Villagers evacuated after landslips block fast-flowing river, with emergency workers hoping to drain water masses before they destroy scores of homes

Thousands of people have been told to evacuate their homes in Nepal after massive landslides blocked a river in the west of the country, sparking fears of flash flooding.

Two powerful earthquakes devastated Nepal on 25 April and 12 May, killing nearly 8,700 people and injuring 16,800 others.

Authorities fear tremors could unleash a flash flood that could destroy dozens of villages in the remote Myagdi district, 80 miles (130km) north-west of Kathmandu, the capital.

Emergency workers are hoping to drain the lake created behind the debris now blocking the normally fast-flowing Kali Gandaki river.

Kamal Singh Bam, a spokesman for Nepal police, said at least 123 people had already been evacuated from areas flooded by the newly formed lake. "Luckily they are safe, but the water level has risen covering all the houses up to around a kilometre upstream of the landslide," Bam said.

Nepal's quake-shattered villages: 'there's nothing to stay for now'

Sesh Narayan Poudel, a senior bureaucrat, said at least 10 villages had been evacuated overnight. "The water level of Kali Gandaki flowing below the dam is dropping, and that's not a good sign," Poudel said.

Although police and army teams have reached the site, the threat of new tremors has prevented work to drain the lake and ease the pressure on the natural dam. Continuing aftershocks and the effect of the first major earthquake have left many areas in Nepal unstable.

"Once the landslides have stopped, our technical team in the area will assess the situation, and once we get their report we can try and get the water flowing again," Rishiram Sharma, head of the department of hydrology and meterology, said.

There have also been warnings that natural dams holding back massive glacial lakes high in the Himalayas could have been damaged in the earthquake last month. If one such dam gave way, flood water could kill thousands and destroy huge numbers of homes.

Two lakes in particular - one close to Everest and the other about 18 miles (30km) from the mountain - have caused concern in the past. But specialists say that unless there is a further tremor with an epicentre close to the lakes, they are safe for now.

"We have assessed all the glacial lakes potentially under threat, but there is no any difference in the assessment before and after the earthquake," Sharma said.

There are 1,466 glacial lakes in Nepal, with 21 big enough to cause serious concern. Many are swollen with glacial meltwater as a result of climate change, scientists say.

Myagdi district, where the villagers were evacuated on Saturday night, is in the west of the south Asian country and some distance from the areas worst affected by last month's tremor.

Geologists say the case should be a "a wakeup call for the Nepal", especially with summer rains forecast. "With the coming monsoon it's sure that Nepal will see several such landslides," Ranjan Kumar Dahal, a geologist said. - The Guardian.

 

PLANETARY TREMORS: Major Global Seismic Uptick As The Black Celestial Event Nears - Two Powerful Earthquakes, 6.8 Magnitude And 6.9 Magnitude, Strikes Off Solomon Islands; The Third Day In A Row Islands Rocked By 6.0 Or Higher Magnitude Tremors! [MAPS + TECTONIC SUMMARY]

USGS earthquake locations.

May 23, 2015 - SOLOMON ISLANDS - It is the third day in a row that the Solomons have been rocked by a quake of 6.0-magnitude or higher.

Two strong 6.8-magnitude earthquakes struck off the Solomon Islands early Saturday, May 23 US geologists said, but there were no initial reports of damage and no tsunami warnings were issued.

The first quake hit at a depth of 10 kilometers (6 miles), 205 kilometers from Kirakira and 448 kilometers from the capital Honiara and the second shallow quake struck just over two hours later about 159 kilometers from Kirakira.

The Solomons have been rocked by several quakes of 6.0-magnitude or higher in recent days, with the most recent a 6.0 magnitude quake which hit the islands early Friday, May 22 and a 6.9 magnitude tremor the day before. No major damage was caused by the earlier tremors.

The Hawaii-based Pacific Tsunami Warning Center said there was no threat of a Pacific-wide tsunami from the Saturday tremors. Geoscience Australia initially estimated the two latest quakes at the magnitudes of 7.1 and 6.9 but said they were unlikely to cause local tsunamis in the quake-prone region, in part due to their location.

Seismologist Mark Leonerd said it was slightly unusual to have two big quakes “right next to each other” but that seismic activity was common in the region. The quakes have been followed by fairly typical aftershocks, he said, adding there had also been recent activity in nearby Papua New Guinea.“That sort of area is putting on a little bit of activity at the moment,” he said.

The Solomons are part of the Pacific “Ring of Fire,” a zone of tectonic activity known for its frequent earthquakes and volcanic eruptions. In 2013, the Solomons were hit by a tsunami after an 8.0-magnitude quake, leaving at least 10 people dead and thousands homeless after buildings were destroyed. - Rappler.

Tectonic Summary - Seismotectonics of the Eastern Margin of the Australia Plate

The eastern margin of the Australia plate is one of the most sesimically active areas of the world due to high rates of convergence between the Australia and Pacific plates. In the region of New Zealand, the 3000 km long Australia-Pacific plate boundary extends from south of Macquarie Island to the southern Kermadec Island chain. It includes an oceanic transform (the Macquarie Ridge), two oppositely verging subduction zones (Puysegur and Hikurangi), and a transpressive continental transform, the Alpine Fault through South Island, New Zealand.

Since 1900 there have been 15 M7.5+ earthquakes recorded near New Zealand. Nine of these, and the four largest, occurred along or near the Macquarie Ridge, including the 1989 M8.2 event on the ridge itself, and the 2004 M8.1 event 200 km to the west of the plate boundary, reflecting intraplate deformation. The largest recorded earthquake in New Zealand itself was the 1931 M7.8 Hawke's Bay earthquake, which killed 256 people. The last M7.5+ earthquake along the Alpine Fault was 170 years ago; studies of the faults' strain accumulation suggest that similar events are likely to occur again.

North of New Zealand, the Australia-Pacific boundary stretches east of Tonga and Fiji to 250 km south of Samoa. For 2,200 km the trench is approximately linear, and includes two segments where old (greater than 120 Myr) Pacific oceanic lithosphere rapidly subducts westward (Kermadec and Tonga). At the northern end of the Tonga trench, the boundary curves sharply westward and changes along a 700 km-long segment from trench-normal subduction, to oblique subduction, to a left lateral transform-like structure.

USGS plate tectonics for the region.

Australia-Pacific convergence rates increase northward from 60 mm/yr at the southern Kermadec trench to 90 mm/yr at the northern Tonga trench; however, significant back arc extension (or equivalently, slab rollback) causes the consumption rate of subducting Pacific lithosphere to be much faster. The spreading rate in the Havre trough, west of the Kermadec trench, increases northward from 8 to 20 mm/yr. The southern tip of this spreading center is propagating into the North Island of New Zealand, rifting it apart. In the southern Lau Basin, west of the Tonga trench, the spreading rate increases northward from 60 to 90 mm/yr, and in the northern Lau Basin, multiple spreading centers result in an extension rate as high as 160 mm/yr. The overall subduction velocity of the Pacific plate is the vector sum of Australia-Pacific velocity and back arc spreading velocity: thus it increases northward along the Kermadec trench from 70 to 100 mm/yr, and along the Tonga trench from 150 to 240 mm/yr.

The Kermadec-Tonga subduction zone generates many large earthquakes on the interface between the descending Pacific and overriding Australia plates, within the two plates themselves and, less frequently, near the outer rise of the Pacific plate east of the trench. Since 1900, 40 M7.5+ earthquakes have been recorded, mostly north of 30°S. However, it is unclear whether any of the few historic M8+ events that have occurred close to the plate boundary were underthrusting events on the plate interface, or were intraplate earthquakes. On September 29, 2009, one of the largest normal fault (outer rise) earthquakes ever recorded (M8.1) occurred south of Samoa, 40 km east of the Tonga trench, generating a tsunami that killed at least 180 people.

Across the North Fiji Basin and to the west of the Vanuatu Islands, the Australia plate again subducts eastwards beneath the Pacific, at the North New Hebrides trench. At the southern end of this trench, east of the Loyalty Islands, the plate boundary curves east into an oceanic transform-like structure analogous to the one north of Tonga.

Australia-Pacific convergence rates increase northward from 80 to 90 mm/yr along the North New Hebrides trench, but the Australia plate consumption rate is increased by extension in the back arc and in the North Fiji Basin. Back arc spreading occurs at a rate of 50 mm/yr along most of the subduction zone, except near ~15°S, where the D'Entrecasteaux ridge intersects the trench and causes localized compression of 50 mm/yr in the back arc. Therefore, the Australia plate subduction velocity ranges from 120 mm/yr at the southern end of the North New Hebrides trench, to 40 mm/yr at the D'Entrecasteaux ridge-trench intersection, to 170 mm/yr at the northern end of the trench.

Large earthquakes are common along the North New Hebrides trench and have mechanisms associated with subduction tectonics, though occasional strike slip earthquakes occur near the subduction of the D'Entrecasteaux ridge. Within the subduction zone 34 M7.5+ earthquakes have been recorded since 1900. On October 7, 2009, a large interplate thrust fault earthquake (M7.6) in the northern North New Hebrides subduction zone was followed 15 minutes later by an even larger interplate event (M7.8) 60 km to the north. It is likely that the first event triggered the second of the so-called earthquake "doublet".

More information on regional seismicity and tectonics

- USGS.

 

PLANETARY TREMORS: 6.0 Magnitude Earthquake Strikes Near Honiara In The Solomon Islands - No Tsunami Warning! [MAPS + TECTONIC SUMMARY]

USGS earthquake location map.

May 21, 2015 - HONIARA, SOLOMON ISLANDS - A earthquake magnitude 6 (mg/mb) has struck on Friday, 56 kilometers (35 miles) from Honiara in Solomon Islands.

The temblor was reported at 06:32:57 / 6:32 am (local time epicenter) at a depth of 10 km (6 miles). Global time of event 21/05/15 / 2015-05-21 19:32:57 / May 21, 2015 @ 7:32 pm UTC/GMT.

A tsunami warning has not been issued, according to the U.S. Geological Survey.

Exact location of event, longitude 160.3296 East, latitude -9.7761 South, depth 10 km, unique identifier, us10002b8f. Ids that are associated to the event, ,us10002b8f,.

Close countries, Solomon Islands (c. 559 000 pop). The epicenter was 56 km (35 miles) from Honiara (c. 56 300 pop), 119 km (74 miles) from Auki (c. 6 800 pop), 78 km (48 miles) from Tulaghi (c. 1 800 pop), 114 km (71 miles) from Taro’a (c. 500 pop). Nearest city, towns to epicentrum/hypocenter was Auki, Taro’a, Tulaghi.

USGS earthquake location map.

No injuries were immediately reported.

The Solomon Islands are a sovereign country consisting of a large number of islands in Oceania, lying to the east of Papua New Guinea and covering a land area of 28,400 square kilometers.

They are located on the eastern margin of the Australia plate, which is one of the most seismically active areas of the world due to high rates of convergence between the Australia and Pacific tectonic plates.

Tectonic Summary - Seismotectonics of the Eastern Margin of the Australia Plate

The eastern margin of the Australia plate is one of the most sesimically active areas of the world due to high rates of convergence between the Australia and Pacific plates. In the region of New Zealand, the 3000 km long Australia-Pacific plate boundary extends from south of Macquarie Island to the southern Kermadec Island chain. It includes an oceanic transform (the Macquarie Ridge), two oppositely verging subduction zones (Puysegur and Hikurangi), and a transpressive continental transform, the Alpine Fault through South Island, New Zealand.

Since 1900 there have been 15 M7.5+ earthquakes recorded near New Zealand. Nine of these, and the four largest, occurred along or near the Macquarie Ridge, including the 1989 M8.2 event on the ridge itself, and the 2004 M8.1 event 200 km to the west of the plate boundary, reflecting intraplate deformation. The largest recorded earthquake in New Zealand itself was the 1931 M7.8 Hawke's Bay earthquake, which killed 256 people. The last M7.5+ earthquake along the Alpine Fault was 170 years ago; studies of the faults' strain accumulation suggest that similar events are likely to occur again.

North of New Zealand, the Australia-Pacific boundary stretches east of Tonga and Fiji to 250 km south of Samoa. For 2,200 km the trench is approximately linear, and includes two segments where old (greater than 120 Myr) Pacific oceanic lithosphere rapidly subducts westward (Kermadec and Tonga). At the northern end of the Tonga trench, the boundary curves sharply westward and changes along a 700 km-long segment from trench-normal subduction, to oblique subduction, to a left lateral transform-like structure.

USGS plate tectonics for the region.

Australia-Pacific convergence rates increase northward from 60 mm/yr at the southern Kermadec trench to 90 mm/yr at the northern Tonga trench; however, significant back arc extension (or equivalently, slab rollback) causes the consumption rate of subducting Pacific lithosphere to be much faster. The spreading rate in the Havre trough, west of the Kermadec trench, increases northward from 8 to 20 mm/yr. The southern tip of this spreading center is propagating into the North Island of New Zealand, rifting it apart. In the southern Lau Basin, west of the Tonga trench, the spreading rate increases northward from 60 to 90 mm/yr, and in the northern Lau Basin, multiple spreading centers result in an extension rate as high as 160 mm/yr. The overall subduction velocity of the Pacific plate is the vector sum of Australia-Pacific velocity and back arc spreading velocity: thus it increases northward along the Kermadec trench from 70 to 100 mm/yr, and along the Tonga trench from 150 to 240 mm/yr.

The Kermadec-Tonga subduction zone generates many large earthquakes on the interface between the descending Pacific and overriding Australia plates, within the two plates themselves and, less frequently, near the outer rise of the Pacific plate east of the trench. Since 1900, 40 M7.5+ earthquakes have been recorded, mostly north of 30°S. However, it is unclear whether any of the few historic M8+ events that have occurred close to the plate boundary were underthrusting events on the plate interface, or were intraplate earthquakes. On September 29, 2009, one of the largest normal fault (outer rise) earthquakes ever recorded (M8.1) occurred south of Samoa, 40 km east of the Tonga trench, generating a tsunami that killed at least 180 people.

Across the North Fiji Basin and to the west of the Vanuatu Islands, the Australia plate again subducts eastwards beneath the Pacific, at the North New Hebrides trench. At the southern end of this trench, east of the Loyalty Islands, the plate boundary curves east into an oceanic transform-like structure analogous to the one north of Tonga.

Australia-Pacific convergence rates increase northward from 80 to 90 mm/yr along the North New Hebrides trench, but the Australia plate consumption rate is increased by extension in the back arc and in the North Fiji Basin. Back arc spreading occurs at a rate of 50 mm/yr along most of the subduction zone, except near ~15°S, where the D'Entrecasteaux ridge intersects the trench and causes localized compression of 50 mm/yr in the back arc. Therefore, the Australia plate subduction velocity ranges from 120 mm/yr at the southern end of the North New Hebrides trench, to 40 mm/yr at the D'Entrecasteaux ridge-trench intersection, to 170 mm/yr at the northern end of the trench.

Large earthquakes are common along the North New Hebrides trench and have mechanisms associated with subduction tectonics, though occasional strike slip earthquakes occur near the subduction of the D'Entrecasteaux ridge. Within the subduction zone 34 M7.5+ earthquakes have been recorded since 1900. On October 7, 2009, a large interplate thrust fault earthquake (M7.6) in the northern North New Hebrides subduction zone was followed 15 minutes later by an even larger interplate event (M7.8) 60 km to the north. It is likely that the first event triggered the second of the so-called earthquake "doublet".

More information on regional seismicity and tectonics

- USGS.

PLANETARY TREMORS: Major Seismic Uptick - Powerful 6.9 Earthquake Strikes Santa Cruz Islands - No Tsunami Threat! [MAPS + TECTONIC SUMMARY]

Earthquake 3D map.

May 20, 2015 - LATA, SOLOMON ISLANDS - The Pacific Tsunami Warning Center says a Pacific-wide tsunami is not expected after a 6.9-magnitude earthquake struck at the Santa Cruz Islands, far southwest of Hawaii.

The earthquake struck at around 12:48 p.m. Hawaii time on Wednesday. 

The epicenter is 114 miles west of Lata, Solomon Islands.

USGS earthquake location map.

The initial data shows a depth of 12.3 miles. 

Light shaking has been felt in the Santa Cruz Islands as well as the Solomon Islands.

No injuries were immediately reported.

USGS shakemap intensity.

The Solomon Islands are a sovereign country consisting of a large number of islands in Oceania, lying to the east of Papua New Guinea and covering a land area of 28,400 square kilometers.

They are located on the eastern margin of the Australia plate, which is one of the most seismically active areas of the world due to high rates of convergence between the Australia and Pacific tectonic plates. - KITV [Edited].

Tectonic Summary - Seismotectonics of the Eastern Margin of the Australia Plate

The eastern margin of the Australia plate is one of the most sesimically active areas of the world due to high rates of convergence between the Australia and Pacific plates. In the region of New Zealand, the 3000 km long Australia-Pacific plate boundary extends from south of Macquarie Island to the southern Kermadec Island chain. It includes an oceanic transform (the Macquarie Ridge), two oppositely verging subduction zones (Puysegur and Hikurangi), and a transpressive continental transform, the Alpine Fault through South Island, New Zealand.

Since 1900 there have been 15 M7.5+ earthquakes recorded near New Zealand. Nine of these, and the four largest, occurred along or near the Macquarie Ridge, including the 1989 M8.2 event on the ridge itself, and the 2004 M8.1 event 200 km to the west of the plate boundary, reflecting intraplate deformation. The largest recorded earthquake in New Zealand itself was the 1931 M7.8 Hawke's Bay earthquake, which killed 256 people. The last M7.5+ earthquake along the Alpine Fault was 170 years ago; studies of the faults' strain accumulation suggest that similar events are likely to occur again.

North of New Zealand, the Australia-Pacific boundary stretches east of Tonga and Fiji to 250 km south of Samoa. For 2,200 km the trench is approximately linear, and includes two segments where old (greater than 120 Myr) Pacific oceanic lithosphere rapidly subducts westward (Kermadec and Tonga). At the northern end of the Tonga trench, the boundary curves sharply westward and changes along a 700 km-long segment from trench-normal subduction, to oblique subduction, to a left lateral transform-like structure.

USGS plate tectonics for the region.

Australia-Pacific convergence rates increase northward from 60 mm/yr at the southern Kermadec trench to 90 mm/yr at the northern Tonga trench; however, significant back arc extension (or equivalently, slab rollback) causes the consumption rate of subducting Pacific lithosphere to be much faster. The spreading rate in the Havre trough, west of the Kermadec trench, increases northward from 8 to 20 mm/yr. The southern tip of this spreading center is propagating into the North Island of New Zealand, rifting it apart. In the southern Lau Basin, west of the Tonga trench, the spreading rate increases northward from 60 to 90 mm/yr, and in the northern Lau Basin, multiple spreading centers result in an extension rate as high as 160 mm/yr. The overall subduction velocity of the Pacific plate is the vector sum of Australia-Pacific velocity and back arc spreading velocity: thus it increases northward along the Kermadec trench from 70 to 100 mm/yr, and along the Tonga trench from 150 to 240 mm/yr.

The Kermadec-Tonga subduction zone generates many large earthquakes on the interface between the descending Pacific and overriding Australia plates, within the two plates themselves and, less frequently, near the outer rise of the Pacific plate east of the trench. Since 1900, 40 M7.5+ earthquakes have been recorded, mostly north of 30°S. However, it is unclear whether any of the few historic M8+ events that have occurred close to the plate boundary were underthrusting events on the plate interface, or were intraplate earthquakes. On September 29, 2009, one of the largest normal fault (outer rise) earthquakes ever recorded (M8.1) occurred south of Samoa, 40 km east of the Tonga trench, generating a tsunami that killed at least 180 people.

Across the North Fiji Basin and to the west of the Vanuatu Islands, the Australia plate again subducts eastwards beneath the Pacific, at the North New Hebrides trench. At the southern end of this trench, east of the Loyalty Islands, the plate boundary curves east into an oceanic transform-like structure analogous to the one north of Tonga.

Australia-Pacific convergence rates increase northward from 80 to 90 mm/yr along the North New Hebrides trench, but the Australia plate consumption rate is increased by extension in the back arc and in the North Fiji Basin. Back arc spreading occurs at a rate of 50 mm/yr along most of the subduction zone, except near ~15°S, where the D'Entrecasteaux ridge intersects the trench and causes localized compression of 50 mm/yr in the back arc. Therefore, the Australia plate subduction velocity ranges from 120 mm/yr at the southern end of the North New Hebrides trench, to 40 mm/yr at the D'Entrecasteaux ridge-trench intersection, to 170 mm/yr at the northern end of the trench.

Large earthquakes are common along the North New Hebrides trench and have mechanisms associated with subduction tectonics, though occasional strike slip earthquakes occur near the subduction of the D'Entrecasteaux ridge. Within the subduction zone 34 M7.5+ earthquakes have been recorded since 1900. On October 7, 2009, a large interplate thrust fault earthquake (M7.6) in the northern North New Hebrides subduction zone was followed 15 minutes later by an even larger interplate event (M7.8) 60 km to the north. It is likely that the first event triggered the second of the so-called earthquake "doublet".

More information on regional seismicity and tectonics

- USGS.