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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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