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| 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.
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| 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.
