PLANETARY TREMORS: Strong 5.8 Magnitude Earthquake Hits Tonga - USGS! [MAPS + TECTONIC SUMMARY]

USGS earthquake location.

April 17, 2016 - TONGA - A 5.8-magnitude has hit the coast of the Pacific island nation of Tonga, USGS reported. The tremor with a depth of 66km, struck 287km from the county's capital of Nuku'alofa.

The part of the South Pacific Ocean where Tonga lies is considered to be one of the most seismically active areas in the world due to convergence between the Australia and Pacific plates, the USGS said.

USGS shakemap intensity.

In March, a 6.2-magnitude earthquake struck off Tonga and the Samoa islands, Geoscience Australia and the US Geological Survey (USGS) reported. The quake occurred just 96km from the town of Hihifo in Tonga.

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

- RT | USGS.

PLANETARY TREMORS: Strong 6.0 Magnitude Earthquake Strike Northwest Of Pangai, Tonga - No Tsunami Warning! [MAPS + TECTONIC SUMMARY]

USGS earthquake location.

May 19, 2015 - TONGA - According to the U.S. Geological Survey (USGS), the Tonga archipelago was rocked by a large earthquake today. The 6.0 magnitude tremor, struck 128 kilometres west of Pangai late Tuesday.

The under-sea quake struck at 19:30:51 UTC, with coordinates at 19.303°S 175.461°W.

The earthquake had a depth of 180.3 km (112.0 miles).

USGS earthquake shakemap intensity.

According to the Pacific Tsunami Warning Center, a destructive Pacific-wide tsunami is not expected and there is no threat to any land mass.

The Hawaii-based Pacific Tsunami Warning Centre said there was no threat of a tsunami.

Tonga, almost 2,000 kilometres (1,240 miles) northeast of New Zealand, lies on the so-called Pacific “Ring of Fire,” where continental plates collide causing frequent seismic activity. 

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: New Pacific Volcanic Island Off Tonga Is Already "Disappearing" - Due To Rain Erosion!

A photograph of the new Island off the coast of Nuku'alofa Photo: Mosese Fakatou

May 9, 2015 - TONGA - A new island that formed in the Pacific after an underwater volcano eruption off Tonga has begun to erode and is "getting smaller".

The cone-shaped island - about half a mile wide and just under a mile long - formed in January about 40 miles north-west of Nuku'alofa, the capital, after an eruption at the Hunga Tonga volcano.

The newly formed island is on the left of the picture (Mosese Fakatou)

Scientists predicted that the land mass would be fragile and was likely to gradually disappear - a prediction that has now been borne out by an expedition by locals to examine the island.

A Tongan shipping firm, Friendly Island Shipping Agency, sent a boat to the island last week and carried local geologists who briefly went ashore.

Mosese Fakatou, a manager from the firm who travelled to the island, said the island was stable but its surface had begun to erode.

"The geologists think it is getting smaller," he told The Telegraph.

"It looks like streaks of erosion probably from rain in the past few weeks. We could see the marks on the island."

Mr Fakatou said the island "looks all right" and the volcano was no longer active.

View of eastern side of new island (Mosese Fakatou)

"We sent some people ashore - some geologists - and they took some measurements of the island," he said.

New islands have been known to form due to volcanic activity but tend to disappear, often within weeks.

The latest island, which has yet to be officially named, is believed to have formed after a volcano exploded underwater and then expanded.

The new island was first photographed earlier this year

"This is not the first time - it has happened before," Pesi Fonua, editor of the Matangi Tonga news website, told The Telegraph.

"It is still very much there but it is not what it was after the eruption was formed. It is slowly eroding into the sea. It is very slow."

Photographs of the island first emerged in March after a group of locals went ashore. They reported that the surface remained still hot and a green lake in the crater smelt strongly of sulphur.

The newly formed island is pictured on the right of the picture (Mosese Fakatou)

The island formed following a volcano explosion that began in December and lasted weeks.

Gianpiero Orbassano took this picture of crater on the island (file photo)

Its birth was recorded by Tonga's lands and natural resources ministry, which stated: "During our observations the volcano was erupting about every five minutes to a height of about [1,300 feet], accompanied by some large rocks." - Telegraph.

MONUMENTAL EARTH CHANGES: Global Volcanism And Geological Upheavals - Tongan Underwater Volcano Creates SUBSTANTIAL NEW ISLAND, First Photographs Emerge Of The New Pacific Island!

New volcanic island at Hunga area of Tonga, Pacific Islands Photo: GP Orbassano

March 11, 2015 - TONGA, PACIFIC OCEAN - The first photographs have emerged of a newly formed volcanic island in the Pacific Ocean after three men climbed to the peak of the land mass off the coast of Tonga.

The pre-eruption satellite view before the island on the left became fused with the volcanic crater created by Hunga Tonga (Pleiades © CNES 2015)

The post-eruption satellite view after the island on the left became joined to the crater which created a larger land mass (Pleiades © CNES 2015)

The new volcanic island (centre) forms a trio with the older Hunga Tonga and Hunga Ha'apai volcanic islets, Tonga (All pictures taken by GP Orbassano)

The three locals from Tonga visited the island on Saturday, landing on a black beach and climbing to the rim of the crater.

They said the surface was still hot and the green lake in the crater smelt strongly of sulphur.

"It was a perfect day, with fantastic views – bright blue sky and the sea was the same colour as the sky," GP Orbassano, one of the locals, told Tonga's Matangi Online.

The one-mile long cone-shaped island began forming last month, about forty miles from the nation's capital, and is now safe to walk on.

Experts believe a volcano exploded underwater and then expanded until an island formed. The island is expected to erode back into the ocean in a matter of months.

Mr Orbassano said he believed the island was high enough for it to remain for some time – and potentially attract tourists.

"There are thousands of seabirds – all kinds, laying eggs on the island," he said.

Tonga's lands and natural resources ministry said last month the island was half a mile wide and just under a mile long. It is believed to be about 820 feet high.

Mr Orbassano, 63, an Italian who moved to Tonga more than 20 years ago, previously had a career in photography and took a series of photos of the new island. - Telegraph.

MONUMENTAL EARTH CHANGES: Global Volcanism And Geological Upheavals - Tongan Underwater Volcano Creates SUBSTANTIAL NEW ISLAND!

Smoke rises from a volcano, some 65 kilometres southwest of the South Pacific nation Tonga's capital Nuku'alofa, is shown in this image from
New Zealand's Ministry of Foreign Affairs and Trade on January 15, 2015 (AFP Photo/MFAT)

January 16, 2015 - TONGA - A Tongan volcano has created a substantial new island since it began erupting last month, spewing out huge volumes of rock and dense ash that has killed nearby vegetation, officials said Friday.

The volcano, about 65 kilometres (40 miles) southwest of the South Pacific nation's capital Nuku'alofa, rumbled to life on December 20 for the first time in five years, the Lands and Natural Resources Ministry said.

It said the volcano was erupting from two vents, one on the uninhabited island of Hunga Ha'apai and the other underwater about 100 metres offshore.

The ministry said experts took a boat trip to view the eruption on Thursday and confirmed it had transformed the local landscape.

"The new island is more than one kilometre wide, two kilometres long and about 100 metres high," it said in a statement.

Smoke rises from a volcano, some 65 kilometres southwest of the South Pacific nation Tonga's capital Nuku'alofa, is shown in this image from
New Zealand's Ministry of Foreign Affairs and Trade on January 15, 2015 (AFP Photo/MFAT)

"During our observations the volcano was erupting about every five minutes to a height of about 400 metres, accompanied by some large rocks... as the ash is very wet, most is being deposited close to the vent, building up the new island."

It said ash and acidic rain was deluging an area 10 kilometres around the volcano, adding: "Leaves on trees on Hunga Tonga and Hunga Ha'apai have died, probably caused by volcanic ash and gases."


WATCH: Underwater volcano continues to spew ash high into the air off Tonga.

A number of international flights were cancelled earlier this week amid concerns about the volcano's ash plume but they resumed on Wednesday, with authorities saying debris from the eruption was not being thrown high into the atmosphere.

Tonga, which is almost 2,000 kilometres northeast of New Zealand, lies on the so-called Pacific "Ring of Fire", where continental plates collide causing frequent volcanic and seismic activity. - Yahoo.

GLOBAL VOLCANISM: Tonga's Underwater Volcano Hunga Tonga-Hunga Ha'apai Erupts - Sends Volcanic Ash Up To 30,000 Feet Into The Air; Causing Ocean To Turn Blood Red; Cancelling Airline Flights!

The Hunga Tonga-Hunga Ha'apai underwater volcano, located about 65 km (40 miles) north of the capital Nukualofa,
was sending volcanic ash up to 4,500 metres (14,765 feet) into the air.

January 13, 2015 - TONGA - An underwater volcano off Tonga was spewing ash high into the air on Tuesday, causing several carriers to suspend air travel to the South Pacific island nation and turning the surrounding ocean blood red, residents and officials said.

The Hunga Tonga-Hunga Ha'apai underwater volcano, located about 65 km (40 miles) north of the capital Nuku‛alofa, was sending volcanic ash up to 4,500 meters (14,765 feet) into the air, the Wellington Volcanic Ash Advisory Centre (VAAC) said.

The volcano, which first erupted in 2009, had been rumbling in recent weeks before exploding violently in the past few days, The New Zealand Herald newspaper reported.

An Air New Zealand flight between Auckland and Nuku‛alofa on Monday was diverted to Samoa and later returned to New Zealand because of the volcano, the airline said in a statement.

A return service had been scheduled to fly on Tuesday, the airline said, but had been canceled because of volcanic activity. The volcano was not disrupting flights to other regions, it said.

Regional carrier Fiji Airlines also turned around a flight to Tonga on Monday, citing safety concerns, and domestic flights were grounded in Tonga.

Residents in Tonga described a spectacular sight, with the enormous plume from the volcano shooting high into the sky and a muddy discharge underwater turning areas of the sea off the island blood red.

"I feel it is healthily letting off steam but it is growing ... the base has doubled in size since December 24 when we first saw it again. And it's getting higher," resident Chris Egan told the Stuff.nz website.

"This one will not give up."

There was no damage reported and New Zealand's VAAC said the ash cloud was expected to dissipate by the end of Tuesday.  - Reuters.

Tongan volcano eruption puts flights on alert

The ash cloud as seen from the main island. Photo / Andrew Allison

A large ash cloud from a volcano in Tonga is starting to affect flights in the region and aviation authorities have issued an alert.

Air New Zealand said the volcanic activity had caused the cancellation of some services between New Zealand and Tonga.

Flight NZ970 operating between Auckland and Nuku'alofa yesterday was diverted to Samoa and later returned to Auckland. The return leg - flight NZ973 - from Nuku'alofa to Auckland was also cancelled, the airline said in a statement.

An additional return service which was due to operate today has also been cancelled.

Today's scheduled return service NZ970 Auckland - Tonga and NZ973 Tonga - Auckland had also been cancelled, the statement said.

The airline had scheduled additional services later in the week.

"The airline apologises for the disruption and will continue to monitor the situation closely," it said.

"The volcanic conditions in Tonga are not currently affecting Air New Zealand services to other destinations."

Andrew Allison, who is in Tonga on holiday, said his flight had been cancelled twice due to the ash cloud.

The Wellington Volcanic Ash Advisory Centre shows the reach of the plume. Photo / VAAC Wellington

The Hunga Tonga-Hunga Ha'apai underwater volcano - about 30km southeast of Falcon Island volcano - has been having small eruptions over the past few weeks, but a larger eruption in the past few days has resulted in a huge plume of ash covering the area.

An advisory has been put out by the Volcanic Ash Advisory Centre in Wellington.

New Zealand's Civil Aviation Authority is keeping a close eye on the volcanic activity.

Meteorological manager Peter Lechner said: "Looking at the advisory, they've still got an ash cloud extending from the southeast.

"From the ocean surface, the plume is up to 30,000 feet (9km) high."

The volcano is about 63km north of the island kingdom's main island and capital city: Nuku'alofa.

The advisory says it is expected the ash cloud will dissipate by about midnight (NZT) tonight. - NZ Herald.

PLANETARY TREMORS: Powerful 7.1 Magnitude Earthquake Strikes Off Fiji - No Tsunami Warning!

USGS earthquake location map

November 2, 2014 - FIJI - A major earthquake has hit in the south Pacific, about 140km northeast of Fiji's Ndoi Island.

The US Geological Survey says the 7.1-magnitude quake, which hit at 6:57 am local time (05:57 AEDT on Sunday), registered at a depth of 434km.

No tsunami warning was issued, according to the Hawaii-based Pacific Tsunami Warning Centre.

"Based on all available data, there is no tsunami threat from this earthquake," said the Hawaii-based Pacific Tsunami Warning Center.

USGS earthquake shakemap intensity

Fiji is an archipelago about 3,100 miles southwest of Hawaii and is located in the so-called Ring of Fire area of the Pacific Ocean basin, where a large number of earthquakes and volcanic eruptions occur.

Last month, a 7.2-magnitude quake struck some 570km south of Easter Island. - SKY News.



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 (>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 earthquake 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". - USGS.

EXTREME WEATHER: Category Five Tropical Cyclone Ian Devastates Tonga - One Dead; State Of Emergency In The Ha'apai Region; Villages Destroyed; Thousands Of Homes Damaged; Trees Flattened; Power Lines Down; The Most Powerful Cyclone To Ever Hit The Country!

January 12, 2014 - TONGA - The most powerful cyclone ever to slam into the South Pacific tourist destination of Tonga has destroyed villages, flattened trees and left at least one person dead, officials said Sunday.

GOES-Floater AVN color IR satellite image of TC Ian at 01:52 UTC on January 11, 2014
(Credit: NOAA/GOES)

Up to 70 percent of houses and buildings in the central Ha'apai islands group, which is home to about 8,000 people and bore the brunt of Cyclone Ian, were damaged or destroyed.

The Tongan government declared a state of emergency in the Ha'apai region after it was pounded by winds in excess of 105 knots (200 kilometres per hour) which whipped up mountainous seas around coastal villages.

Although initial reports when the cyclone hit on Saturday said there had only been minor damage, the full extent of the destruction began to emerge when communications were partially restored a day later.

"Seventy percent of houses (on Ha'apai) are damaged or blown away, and the rest of the 30 percent are affected by water," Tongan military commander Satisi Vunipola told reporters.

Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite acquired this image at
01:55 UTC on January 9, 2014. The storm had winds of 85 knots (157 kph or 98 mph), making it the equivalent of
a category 2 hurricane. (NASA image courtesy Jeff Schmaltz, LANCE/EOSDIS MODIS Rapid Response Team
at NASA GSFC. Caption by Holli Riebeek.)

Residents on Ha'apai's main island of Lifuka were reported to have huddled in churches for shelter as houses were destroyed in the furious cyclone.

Ian Wilson, a New Zealand emergency management official, said Lifuka was in the direct path of the cyclone.

"Whatever was on the island has been damaged, whether it's buildings, crops, roading or infrastructure, it's all been damaged," said Wilson, warning that it remained difficult to get a detailed picture of the destruction.

"There is no communication. We did have a satellite phone but that also died. It is serious. The eye of the storm went right across the top of the island."

Ian was downgraded to a category four cyclone on Saturday morning, but increased in intensity later in the day to be restored to the most severe rating of category five as it hit Ha'apai, knocking out contact with outlying islands.

MTSAT-2 visible satellite image taken at 01:32 UTC on January 11, 2014.
(Credit: MTSAT/FNMOC)

Ian is the first category five cyclone to belt into Tonga and Ha'apai governor Tu'i Ha'angana said he could see from one side of the island to the other -- "that's how devastated it is".

The head of the Tonga Red Cross, Sione Taumoefolau, said he had been informed of one death in Ha'apai but did not have further details as communication remained sketchy.

He said staff in the region told him by satellite phone that Lifuka was devastated.

The Red Cross established a policy last year of maintaining containers of relief supplies on most islands, and Taumoefolau said they were able to provide immediate assistance.

Tupou Ahomee Faupula, from Tonga's cell phone provider Digicel, said his field officer in Ha'apai reported widespread devastation.

"He told us that this was the worst ever damage from a cyclone. Most houses are flattened, roofs are off, trees and power lines are down."

The Tonga navy has sent two patrol boats to Ha'apai, and the Matangi Tonga news website reported the government was considering a request for overseas aid.

This NASA/NOAA GOES Project infrared image obtained January 11, 2014 shows
Tropical Cyclone Ian lashing the Tonga Islands

New Zealand offered immediate assistance of NZ$50,000 ($41,500) and an Air Force Orion was sent Sunday to begin an immediate aerial surveillance of the devastated areas.

The flight was not expected to return to the capital Nuku'alofa until late at night.

"Our thoughts are with the people of Tonga as they begin to come to terms with the damage caused by this cyclone," said Foreign Minister Murray McCully.

"Further support will be considered as the full extent of the damage becomes clear and the government of Tonga determines its priority response areas."

The Fua'amotu Tropical Cyclone Warning Centre downgraded Ian again to category four Sunday, with wind gusts of up to 140 knots (161 mph, 259 kph).

The storm was expected to continue weakening as it moved south over open waters, away from the island nation, according to meteorologists. - Yahoo.

PLANETARY TREMORS: Powerful 6.5 Magnitude Earthquake Strike Southeast Of Neiafu, Tonga!

November 02, 2013 - TONGA - According to the U.S. Geological Survey (USGS), the Tonga archipelago was rocked by a large earthquake today. The 6.5 magnitude tremor, struck 187 kilometres (116 miles) southwest of Neiafu late Saturday.

USGS earthquake location.

The under-sea quake struck at 518:53:49 UTC, with coordinates at 19.200°S 172.300°W.

The earthquake had a depth of 10 kilometres (6.2 miles).

According to the Pacific Tsunami Warning Center, a destructive Pacific-wide tsunami is not expected and there is no threat to any land mass.

USGS earthquake shakemap intensity.

Tonga, almost 2,000 kilometres (1,240 miles) northeast of New Zealand, lies on the so-called Pacific “Ring of Fire,” where continental plates collide causing frequent seismic activity.

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 (>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". - USGS.

PLANETARY TREMORS: Strong 6.0 Magnitude Earthquake Strikes Near The Kermadec Islands - Tremor Confused Seismic Monitors, Setting Off A Civil Defence Alert!

August 12, 2013 - NEW ZEALAND - A magnitude 6 earthquake near the Kermadec Islands confused monitoring systems this afternoon and set off a civil defence alert.

USGS earthquake location.

The quake was located 815 kilometres northeast of Whakatane at a depth of 470km, deep beneath the Kermadec Islands at 4.16pm.

It was initially recorded by Geonet as being a magnitude 4.5 shake 75km north of Te Araroa and a second 6.1 earthquake 10km southwest of Tokomaru Bay.

GNS duty seismologist Bill Fry said the Kermadec event had confused their systems and prompted two false readings.

"We measure earthquakes using the two different types of waves it produces. Sometimes the system can get confused when it thinks the later arriving wave is the first one, producing an incorrect reading."

Duty officers quickly assess quakes when the data comes in, and they work to make the readings more accurate, Fry said.

"We get a notification and the system comes up with its best location and we work to make sure it is accurate."

An earthquake so far away and so deep would not usually be felt anywhere in New Zealand, but the waves it produced could have resulted in smaller, more localised tremors.

"There's an ephemeral increase or decrease in stress on neighbouring areas whenever there is an earthquake. It's like if you are on a boat and the paddle going into the water sends small waves across the water's surface."

USGS earthquake shakemap location.

Seismologist Anna Kaiser said their systems had not noticed any other quakes elsewhere in the North Island after the Kermadec event, but it was not impossible that people could have felt something.

"Sometimes when you get an earthquake event like that you get a smaller local event embedded in the earthquake waves.

"You could get passing, low frequency waves felt lightly."

The Ministry of Civil Defence and Emergency Management sent out a national advisory just after 4.30pm stating the National Crisis Management Centre was activated and advising people to check their homes and workplaces for damage.

It was not clear if the alert had been lifted once the earthquake was redefined. - 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.

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 (>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". - USGS.

PLANETARY TREMORS: Strong Magnitude 6.0 Earthquake Strikes South Of The Kermadec Islands!

June 17, 2013 - KERMADEC ISLANDS - A strong 6.0 magnitude earthquake struck south of Kermadec Islands on June 15, 2013 at 11:20:34 UTC.

USGS earthquake location.

The epicentre was located at 313 km (194 miles) southwest of L'Esperance Rock, New Zealand at coordinates of 33.895°S 179.455°E.

The U.S. Geological Survey (USGS) registered a depth of 172.4 km (107.1 miles).

USGS earthquake shakemap intensity.

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 (Magnitude 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.

USGS plate tectonics for the region.

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". - USGS.

PLANETARY TREMORS: Powerful Magnitude 7.4 And 6.6 Earthquakes Strike The Tonga Region - USGS Indicates That It Ruptured A Fault Within The Subducting Pacific Lithosphere!

May 23, 2013 - TONGA -  According to the US Geological Survey (USGS), within the space of a couple hours, the Tonga archipelago was rocked by three large earthquakes. The first was a powerful 7.4-magnitude earthquake, which struck 255 kilometres (158 miles) southwest of Tonga early Friday.

USGS earthquake location.

USGS earthquake shakemap intensity.

The under-sea quake struck at 5:19 a.m. (1719 GMT Thursday) and sparked a tsunami alert, although no damage was expected due to its depth of 171 kilometres, according to the Pacific Tsunami Warning Center.

The earthquake, centred 282 kilometers southwest of Tonga’s capital Nuku’alofa, was followed four hours later by a second powerful 6.6 tremor 84 kilometres northwest of the town.

The second quake, at a depth of 103 kilometers, also generated a tsunami warning.

USGS earthquake damage estimates.

USGS earthquake population exposure.

“A destructive tsunami was not generated based on earthquake and historical tsunami data,” the center said of each quake.

Several minutes later, a third quake measuring 5.0 magnitude hit 80 kilometres northwest of Nuku`alofa, with a depth of 153.5 kilometres.

The three quakes come 12 days after a similar 6.5-magnitude earthquake struck the area, at a depth of 205 kilometers.

Tonga, almost 2,000 kilometers (1,240 miles) northeast of New Zealand, lies on the so-called Pacific “Ring of Fire,” where continental plates collide causing frequent seismic activity. - Inquirer News.

Tectonic Summary.

The May 23, 2013 Mw 7.4 earthquake southwest of Vaini, Tonga, occurred as a result of normal faulting at a depth of approximately 170 km. At the location of this earthquake, the Pacific and Australia plates are converging at a rate of approximately 73 mm/yr in an east-west direction, resulting in the westward subduction of the Pacific plate beneath Tonga at the Tonga-Kermadec trench. The depth and faulting mechanism of the May 23rd earthquake indicate it ruptured a fault within the subducting Pacific lithosphere rather than on the shallower thrust interface between the two plates.

The Tonga-Kermadec arc has frequent moderate-to-large earthquakes, and has hosted over a dozen M6.5+ earthquakes within 500 km of the May 23rd earthquake over the past 40 years. Most of these also occurred at intermediate depths; the largest was an Mw 7.7 earthquake in October of 1997, approximately 110 km to the north-northeast of the May 23 2013 event. None are known to have caused significant damage. Intermediate-depth (70-300 km) and deep-focus (depth > 300 km) earthquakes are distinguished from shallow earthquakes (0-70 km) by the nature of their tectonic setting, and are in general less hazardous than their shallow counterparts, though they may be felt at great distances from their epicenters. The Tonga-Kermadec slab in the region of the May 23 2013 earthquake is seismically active to depths of over 650 km.

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 (>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". - USGS.

PLANETARY TREMORS: Powerful 6.5 Magnitude Earthquake Strikes Off Tonga - No Tsunami Warning Issued!

May 11, 2013 - TONGA - A strong 6.5-magnitude earthquake struck off the Pacific nation of Tonga on Sunday, US seismologists said, but no tsunami warning was issued.

The under-sea quake hit at 08:46 am (2046 GMT Saturday), 139 kilometres (86 miles) from the town of Neiafu, on the south coast of the island Vava'u, at a depth of 205 kilometres, the US Geological Survey (USGS) said.

USGS earthquake map and location.

The Pacific Tsunami Warning Centre said based on the depth of the quake, no tsunami was expected.

The epicentre of the tremor was 353 kilometres north of the capital Nuku'alofa.

USGS earthquake map and location.

Tonga, almost 2,000 kilometres (1,240 miles) northeast of New Zealand, lies on the so-called Pacific "Ring of Fire", where continental plates collide causing frequent seismic activity. - Times of India.

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 (>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 earthquake shakemap intensity.

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.

USGS earthquake population exposure.

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". - USGS.