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EARTHQUAKE
AND TSUNAMI OF 1 APRIL 2007 IN THE SOLOMON ISLANDS
George Pararas-Carayannis
INTRODUCTION
A great earthquake
on April 1, 2007 struck the New Georgia Group of the Northwest
Solomon Islands and generated a destructive tsunami. There were
numerous fatalities in the Solomon Islands and in Southeast Papua
New Guinea. A second quake of 6.7 magnitude a few minutes later,
occurred 75 miles west-southwest of Chirovanga, Choiseul, Solomon
Islands, and 1,410 miles north of Brisbane, Australia.
THE EARTHQUAKE OF
APRIL 2 2007 IN THE SOLOMON ISLANDS
Epicenter - The earthquake's
epicenter was at 8.6° S., 157.2° E., or about 45km (25
miles) south-southeast of Gizo, a small fishing town on Gizo
Island in the New Georgia Islands and 345km (215 miles) north-west
of Honiara, the capital of the Solomon Islands.
Time of Origin - April 1, 2007, 20:40 UTC, 7:40
a.m. local time (April 2, local date).
Magnitude - 8.1
Focal Depth - 10 km (6.1 miles)
Fault Plane - The following values are based
on the U.S.G.S. Centroid solution:
Fault plane: strike=331 dip=38 slip=120
Fault plane: strike=115 dip=58 slip=69
Aftershocks - There were several strong aftershocks
following the main earthquake. The largest of these aftershocks,
with a magnitude 6.4 struck at_11:45 hrs on _3 April 2007 near
Gatokae and the surrounding areas of Marovo Lagoon in this Western
province of the Solomon Islands.
 Felt
Reports - According
to residents at Gizo, the strong ground motions of the earthquake
lasted for almost two minutes. The quake's motions were felt
as far as Honiara, the capital of the Solomon Islands.
Death Toll and Damages
- According to reports
at least 50 people were killed (mainly by the tsunami) in the
Solomons and the neighboring Papua New Guinea. However, the death
toll may be greater since communications with remote islands
were affected. According to the initial government damage assessment,
around 916 houses were damaged or destroyed and about 50,000
people were affected. There was substantial damage to the Western
Provincial capital of Gizo and along the Entire Township and
villages on the Island. There was extensive damage to Gizo's
main airport, hospital, and to coastal roads. There was power
failure and damage to telephone lines throughout the island.
Graphic
showing Bathymetry, Spreading Center, the Woodlark Ridge, and
the Woodlark Spreading Basin in relation to the San Cristobal
Trench in the vicinity of the New Georgia islands Group of the
Solomon islands (Modified after Goodliffe et al. 1999)
THE
TSUNAMI OF APRIL 2, 2007 IN THE SOLOMON ISLANDS
Destructive tsunami
waves of up to five meters in height struck parts of Choiseul,
Vella La Vella, Kolombangara, New Georgia, Gizo, Simbo and Ranoggah,
in the western province of the Solomon Islands. The tsunami was
particularly destructive on Gizo, Noro and Taro islands. The
first tsunami wave reached the Gizo Township about five minutes
after the earthquake. Fortunately the earthquake occurred during
the day and most people seeing the sea receding moved to higher
ground - thus many lives were saved.
Gizo - Gizo, the main town of about
1,000 people in the Western Solomon Islands, was hit by waves
several meters high that destroyed buildings and washed people
out to sea. Witnesses described the waves as inundating 50-70
meters (164-230 feet) inland. The waves were particularly destructive
to building and homes. Large boats washed ashore and were deposited
in the middle of the town. Communication links were wiped out.
There was damage a Nusa Tupe island where the airport is located.
The Gizo hospital was inundated and damaged extensively - making
it inoperative.
Choiseul Island - In South Choiseul Island, waves
of up to 10 meters in height swept through the village of Sasamunga.
The waves penetrated up to 500 meters inland and destroyed at
least 300 houses. Sasamunga lost its hospital and health centers.
Also, the villages of Nukiki, Zepa and Luta sustained considerable
damage.
Mono Island - Four people were reported missing.
Simbo Island - Waves penetrated 200 meters
inland.
Ranunga Island - Stuck by the tsunami waves.
Tsunami
Warning Issued
The Pacific Tsunami
Warning Center (PTWC) in Honolulu issued a regional tsunami warning
for the immediate area of the Solomon Islands. The tsunami warning
was expanded to include Papua New Guinea, Vanuatu, Nauru, New
Caledonia, Northeastern Australia, Fiji, Chuuk, Pohnpei, Kosrae,
Indonesia, Tuvalu, Kiribati, Kermadec is., Marshall Islands and
New Zealand. An advisory was issued for Hawaii but not a watch
or warning. The warnings and advisory were cancelled nine hours
later.
Small tsunami
Recorded by Distant Tide Gauge Stations
PORT KEMBLA (Australia) 0.2FT T/14MIN
VANUATU (Vanuatu) 0.15M = 0.5FT T/22-28MIN
CAPE FERGUSON (Australia) 0.11M = 0.4FT T/12MIN
MANUS (PNG) 0.09M = 0.3FT T/40MIN
HONIARA (solomons) 0.20M = 0.6FT T/62MIN
Past Great
Earthquakes and Tsunamis in the Solomon Islands Region
Most of the earthquake
activity on the northeast side of the Solomon Island Arc occurs
along a 350 km segment of the North Solomon Trench close to Guadalcanal
and San Christobal Islands. The region has produced about 10
major earthquakes in the last fifty years including the Ms 7.7
on June 15, 1966 and the Ms 7.7 on February 7, 1984 events (Richter
magnitudes). The two great earthquakes of 14 and 26 July 1971
(about 12 days apart) had moment magnitudes of Mw=9 and Mw=8.1.
The earthquake of July 14, 1971 had its epicenter at 5.50 South
153.90 East in the New Ireland, Bismark Sea Region. The July
26, 1971 earthquake had its epicenter at 4.94 South, 153.17 East
in the same region. Earthquakes in this region appear to occur
as doublets, often within a few days of each other.
 Also, a great deal of earthquake
activity occurs along the southeast side of the Solomon Island
Arc.
Illustration of
Oceanic/Oceanic Crust Convergence along certain regions of the
Solomon Island Arc
As the April 2, 2007
event demonstrated, the region is capable of triggering great
earthquakes and destructive tsunamis. On July 21, 1975, a magnitude
7.9 earthquake (epicenter at 6.60 South, 154.90 East, further
north along the San Cristobal Trench, generated a large tsunami
which hit Bougainville Island and killed an estimated 200 people.
It is believed that the same tsunami was destructive also in
the same western province of the Solomon Islands, but no details
are available.
Major and great earthquakes
can occur on either side of the Solomon Island Volcanic Arc.
On the northeast side of the Solomon Island Arc larger rupture
of adjacent slabs are possible which could involve the New Ireland
segment or the North Solomon Trench. The most significant earthquake
that would occur in the area that could approach an M9 magnitude
earthquake would be expected near the Solomon Sea-Bismarck Sea
triple junction, or one closer to the 1971 events. Any large
earthquake in the region could generate a destructive local tsunami.
Seismotectonic
Setting of the Solomon Islands Region - Understanding Regional
Mechanisms of Tsunami Generation Along Active Boundaries of Young,
Marginal Sea Basins and Spreading Ridges
The Solomon Islands - an archipelago of 492 islands - is a volcanic
arc along an extensive tectonic zone situated at an active margin
boundary of two converging plates, where earthquakes occur frequently.
The seismotectonic dynamics, geometry and direction of subduction
in this volcanic arc region are complicated.
The spatial distribution
of earthquakes on both sides of the Solomon Island Arc supports
the existence of several subduction zones. Along the entire plate
margin, there is not one simple plate boundary but a cluster
of small plate boundaries which accommodate the mechanisms of
the total interaction in the region (Brooks, 1965; Denham, 1969).
The geometries of subduction differ for segments along the entire
Papua-New Guinea and Solomon Islands region. One has to look
at the geologic history of the entire Southwest Pacific to understand
the complex evolutional dynamics that control present seismicity
(and tsunami generation mechanisms) on both sides of the Solomon
Island Arc.
The Solomon Island Arc is migratory arc system which developed
from the early Eocene to Late Miocene as part of a continuous
Outer Melanesian Arc (Rodd,). The Arc extended from Papua New
Guinea through the Solomon Islands, Vanuatu, Fiji and Tonga/Lau,
to New Zealand. In the Late Miocene, the oceanic Ontong-Java
Plateau begun to collide with the Solomon Islands section of
the Outer Melanesian Arc, but several major events that followed
resulted in the break-up and segmentation of the Arc. At the
outset, the direction of subduction beneath the Solomon Islands
and Vanuatu arcs was reversed. The subduction of the Pacific
Plate stopped, and eastward subduction of the back-arc basins
beneath the Solomon and Vanuatu Arc segments, begun.
 In recent years, several research investigations
have been undertaken in the region to help understand present
processes of subduction, accretion and fragmentation of oceanic
plateaus at subduction zones and their deformational effects
on the overriding Solomon island arc (Mann, 1997; Mann et al.
1997, 2004; Goodliffe et al. 1997, 1999; Phinney et al. 1999,
2004; Martinez et al. 1999; Miura et al. 2005; Taira et al. 2004;
Taylor et al. 1995, 1999; Cowely et al. 2004). The researchers
are looking at different models of tectonic interaction. For
example, one of the models postulates wedging of the Solomon
Island Arc beneath the Ontong Java plateau - the largest oceanic
plateau in the world - Northeast of the Solomon Islands. The
other model postulates an oceanic accretionary wedge geometry
with northeastward component towards the Ontong Java plateau.
Map of the University
of Texas (Institute of Geophysics from studies conducted in the
Solomons) showing the Ontong Java Oceanic Plateau, and the North
Solomon Trench on the Northeast of the Solomon Island Arc and
the San Cristobal Trench in relation to the Woodlark Ridge and
Spreading Woodlark Basin on the Southwest.
On the Southwest side
of the Solomon Island Arc near the New Georgia Islands where
the April 1, 2007 earthquake occurred, the tectonics are also
very complicated. The earthquake occurred close to the San Cristobal
Trench, on the east end of the Woodlark Basin and near the triple
junction formed by the subduction of the Woodlark Spreading Ridge.
The quake's estimated rupture, with an orientation of about 331
degrees, parallels the orientation of the direction of the axis
of the San Cristobal trench. The Woodlark Basin is a young marginal
basin which is both propagating westward (from a spreading center)
into the Papuan Peninsula while - at the same time - spreading
and being subducted eastward beneath the Solomon Islands (Taylor
et al. 1995, 1999, quoting Weissel et al., 1982) - in this case
beneath the New Georgia Island Group.
It is difficult to
comment further on the dynamics of the region and its potential
for future tsunami generation as there is no known historical
tsunami data. As pointed out (Shinohara et al. 2003), without
detailed and accurate seismicity studies, it is difficult to
accurately describe the plate subduction processes of this complex
zone. The region does not represent a typical subduction zone
as other tsunamigenic areas of the world - since there is spatial
progression from continental rifting to seafloor spreading and
to shallow subduction at the eastern margin of the Woodlark Basin.
Such mechanism of shallow subduction beneath a volcanic island
arc can account for large earthquakes and destructive tsunami
generation. However - and although earthquakes in this region
that can occur may be large in magnitude - the rupture lengths
may be limited, the tsunami generating areas may be relatively
small and the tsunami impact may be confined by the physical
barriers and the local bathymetry of the Woodlark basin and of
surrounding island groups trapping tsunami energy. Thus, no tsunami
with far reaching impact can be expected from this region.
REFERENCES
and ADDITIONAL BIBLIOGRAPHY
Brooks, J. A., 1965.
Earthquake activity and seismic risk in Papua and New Guinea.
Australian Bureau of Mineral Resources, Geology & Geophysics,
Report No. 74.
Cowley, S., Mann, P., Coffin, M.F., and Shipley, 2004, Oligocene
to Recent tectonic history of the Central Solomon intra-arc basin
as determined from marine seismic reflection data and compilation
of onland geology, Tectonophysics, vol. 389(3-4), 267-307.
Denham, D., 1969.
Distribution of earthquakes in the New Guinea-Solomon Islands
region. J. Geophys. Res., 74: 4290-4299.
Goodliffe, A., B. Taylor, F. Martinez, R. Hey, K. Maeda, and
K. Ohno, 1997, Synchronous reorientation of the Woodlark Basin
spreading center, Earth Planet. Sci. Letts., 146, 233-242.
Goodliffe, A., B. Taylor, and F. Martinez, 1999, Data Report:Marine
geophysical surveys of the Woodlark Basin region, in Taylor,
B., P. Huchon, A. Klaus et al., Proc. ODP, Init. Repts., 180,
Ocean Drilling Program, College Station, TX, 1-20 [CD-ROM]
Paul Mann, Thomas
H. Shipley, and Millard F. Coffin US-Japan Marine Geophysical
Study_of the Solomon Island Arc_Ontong Java Convergent Zone,
Southwest Pacific Ocean
Mann, P., 1997. Model
for the formation of large, transtensional basins in zones of
tectonic escape: Geology, v. 25, p. 211-214.
Mann, P. and Taira, A., 2004, Global tectonic significance of
the Solomon Islands and Ontong Java Plateau convergent zone,
Tectonophysics, vol. 389, 137-190.
Martinez, F., B. Taylor, and A. M. Goodliffe, 1999, Contrasting
styles of seafloor spreading in the Woodlark Basin: Indications
of rift-induced secondary mantle convection, J. Geophys. Res.,
104, 12,909-12,926.
Miura, S., Suyehiro, K., Shinohara, M., Takahashi, N., Araki,
E., and Taira, A.. 2005, Seismological structure and implications
of collision between the Ontong Java Plateau and Solomon island
arc from ocean bottom seismometer-airgun data; Tectonophysics,
v. 389(3-4), 191-220.
Phinney, E., Mann, P., Coffin, M., and Shipley, T., 1999. Sequence
stratigraphy, structure, and tectonics of the southwestern Ontong
Java Plateau adjacent to the North Solomon trench and Solomon
island arc: Journal of Geophysical Research, v. 104, p. 20449-20466.
Phinney, E.J., Mann, P., Coffin, M.F., and Shipley, 2004, Sequence
stratigraphy, structural style, and age of deformation of the
Malaita accretionary prism (Solomon arc-Ontong Java Plateau convergent
zone), Tectonophysics, vol. 389(3-4), 221-246.
Rodd Jonathan A. The
Petroleum Potential of Fiji. Section 1: Summary, History &
Geology.
Shinohara Masanao,
Suyehiro Kiyoshi, and Takayuki Murayama (2003). _Microearthquake
seismicity in relation to double convergence around the Solomon
Islands arc by ocean-bottom seismometer observation _Geophysical
Journal International 153 (3), 691-698.
Taira, A., P. Mann
and R. Rahardiawan, 2004, Incipient subduction of the Ontong
Java Plateau along the North Solomon trench, Tectonophysics,
vol. 389, issue 3-4, 247-266.
Taylor, B., A. Goodliffe
and F. Martinez, 1999, How continents break up: Insights from
Papua New Guinea, J. Geophys. Res., 104, 7,497-7,512.
Taylor, B., A. Goodliffe, F. Martinez, and R. Hey, 1995, Continental
rifting and initial seafloor spreading in the Woodlark Basin,
Nature, 374, 534-537.
F. W. Taylor, Paul
Mann, M. G. Bevis, R. L. Edwards, Hai Cheng, Kirsten B. Cutler,
S. C. Gray, G. S. Burr, J. W. Beck, David A. Phillips. (2005)
Rapid forearc uplift and subsidence caused by impinging bathymetric
features: Examples from the New Hebrides and Solomon arcs. Tectonics
24:6, TC6005
Tectonophysics (Special
issue): "Tectonics, Seismicity, and Crustal Structure of
the Ontong Java Plateau-Solomon Island Arc Convergent Zone, Southwest
Pacific Ocean" (P. Mann and A. Taira, guest editors)
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