PRELIMINARY ANALYSIS OF THE EARTHQUAKE (MW 8.1) AND TSUNAMI OF APRIL 1, 2007, IN THE SOLOMON ISLANDS, SOUTHWESTERN PACIFIC OCEAN

The Mw-8.1 earthquake in the Solomon Islands that occurred at 20:40 on April 1, 2007 (UTC), struck along a complicated plate boundary in the southwestern Pacific Ocean. The earthquake had a thrust-fault focal mechanism and occurred at shallow depth (between 15 km and 25 km) beneath the island arc. The combined effects of the earthquake and tsunami caused 52 fatalities and left tens of thousands homeless. Multichannel seismic-reflection data collected in 1984 show the geologic structure of the arc's frontal prism within the earthquake's rupture zone. Modeling tsunami-wave propagation indicates that some of the islands are so close to the earthquake epicenter that they were hard hit by tsunami waves as soon as 5 min. after shaking began, allowing people scant time to react.

Several aspects of geography and geology make this earthquake and tsunami unique. First, although young oceanic crust is being subducted eastward at the New Britain Trench, the subducting plate bends sharply downward and dips steeply (30˚ to 45˚) into the mantle, and the earthquake's epicenter is located near the trench axis. Second, in the past 30 years, numerous earthquake doublets have struck this island arc, and the rupture zone of the 2007 earthquake includes the epicenters of two doublets, having magnitudes of about 7. To date (7/1/2007), however, the 2007 event has produced aftershocks as large as Mb 6.6, but no second M-8 earthquake and tsunami have occurred. Third, complex bathymetric and tectonic elements, including an active spreading ridge and transform fault, are being subducted where the earthquake struck. The effect of ridge subduction on seismogenesis is evident from the fact that earthquake slip began southeast of where the spreading ridge enters the subduction zone; slip was reduced directly over this ridge; and northwest of the ridge, slip resumed with increased amplitude.