EARTHQUAKE AND TSUNAMI OF 1 APRIL 2007 IN THE SOLOMON ISLANDS
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 2 APRIL 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 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 coastal roads. There were power failure and damage to telephone lines throughout the island.
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.
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 canceled 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 Cristobal Islands. The region has produced about 10 major earthquakes in the last fifty years including Ms 7.7 on June 15, 1966, and 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 is 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.
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 a spatial progression from continental rifting to seafloor spreading and shallow subduction at the eastern margin of the Woodlark Basin. Such a 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.
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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)