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Tsunamis & Earthquakes

USGS PCMSC Tsunami and Earthquake Studies

 

Preliminary Analysis of the April 2007 Solomon Islands Tsunami, Southwest Pacific Ocean

The tsunami that was triggered by a magnitude 8.1 earthquake on April 1, 2007, in the Solomon Islands caused significant damage and loss of life. In the hopes that disasters such as this can be avoided in the future, we attempt to understand the mechanism and impact of this tsunami. The information presented here is focused on geologic aspects of the disaster.

Links for More Information on Earthquake from the National Earthquake Information Center:

Event page for April 2007 M=8.1 Solomon Islands Earthquake

Earthquake Summary Poster

Links for More Information on Tsunami from NOAA Center for Tsunami Research:

Event page for April 2007 Solomon Islands Tsunami

Tectonic Background

The M=8.1 earthquake that occurred in the Solomon Islands on April 1, 2007 (UTC), was located along the Solomon Islands subduction zone, part of the Pacific "Ring of Fire". A subduction zone is a type of plate tectonic boundary where one plate is pulled (subducted) beneath another plate. For most subduction zones that make up the western half of the Ring of Fire, the Pacific plate is being subducted beneath local plates. In this case, however, the Pacific plate is the overriding or upper plate. There are three plates being subducted along the Solomon Islands subduction zone: the Solomon Sea plate, the Woodlark plate, and the Australian plate (see figure below). A spreading center separates the Woodlark and Australian plates. More detailed information on the plate tectonics of this region can be found in Tregoning and others (1998) and Bird (2003).

Tectonic Map of Solomon Islands Region
Plate tectonics of the Solomon Islands region.
Water depth shown in color (cool colors-deeper water) (metadata).
Approximate area of fault that ruptured during April 2007 earthquake shown in hachure pattern.
(See a larger version of this image, 404 kb.)

In 1982 and 1984, the USGS, as part of the CCOP/SOPAC Australia-New Zealand-United States Tripartite Agreement, conducted marine geologic investigations of the Solomon Islands region. This resulted in the acquistion of multichannel seismic reflection data aboard the R/V S.P. Lee across the Solomon Islands subduction zone. A trackline map of one of the seismic reflection lines (line 401, cruise L-6-84-SP) is shown below. Both cruises L-7-82-SP and L-6-84-SP acquired seismic reflection and other data along the Solomon Islands subduction zone.

Trackline for line 401 and epicenter

Track line for seismic reflection line 401 (metadata).
Yellow circle is epicenter for April 2007 M 8.1 earthquake.
(Download Google Earth kmz file of cruise L-6-84-SP seismic reflection lines).

Line 401 in particular images the shallow part of the interplate thrust that is the boundary between the downgoing Woodlark plate and the overriding Pacific plate. The fault at this latitude most likely ruptured during the April 2007 M=8.1 earthquake. An interpretation of the data based on Bruns and others (1989) is shown below.

Seismic reflection profile 401

Multichannel seismic reflection line 401, cruise L-6-84-SP,
crossing the Solomon Islands subduction zone.
(See a larger version of this image, 2.4 Mb, as originally interpreted by Bruns and others, 1989.)

Several splay faults (secondary faults) that come off the shallow part of the interplate thrust, or subduction décollement, are evident in this profile. One or more of these faults may have been activated during the April 2007 earthquake.

Seismological Background

The Solomon Islands subduction zone is noted for producing an unusual pattern of earthquakes called "doublets". These are two earthquakes of similar magnitude that occur close to each other in space and time. Most of the historic doublets in the Solomon Islands have occurred north of the 2007 earthquake in the vicinity of Bougainville Island and along the New Britain subduction zone. The largest of these doublets are a pair of M=8.0 and 8.1 earthquakes that occurred 12 days apart in 1971 (Schwartz and others, 1989). The portion of the fault that ruptured in the first earthquake of the 1971 doublet reruptured in a different manner during a M=7.7 earthquake in 1995 (Schwartz, 1999). Other doublets have occurred in 1919 and 1920, 1945 and 1946, and 1975 (both occurred in the same year), all in the M=7-8 range. In the southeastern part of the Solomon Islands subduction zone, there were doublets in 1931, 1939, and a triplet in 1977 (Lay and Kanamori, 1980). It is unclear what mechanism causes earthquake doublets to occur, although stress triggering from the first earthquake of the doublet is likely a significant a factor. Timing between earthquakes that compose doublets is discussed in general by Kagan and Jackson (1999).

From the inversion of waveforms from the April 2007 earthquake recorded on seismograph stations around the world, rupture started at a point on the interplate thrust fault known as the hypocenter and broke approximately 250 km of the fault to the northwest. Bathymetric ridges entering the subduction zones (the Woodlark Rise and Woodlark Ridge in figures above) appear to influence the distribution of slip during the earthquake. It is intersting to note that the fault ruptured directly across where an active spreading center is being subducted. In contrast, subduction of the Woodlark rise to the northwest may have been a factor in arresting rupture.

Over geologic time, ridge subduction contributes to the uplift of the overriding plate and the creation of islands such as Simbo, Gizo, and Ranunga very near the Solomon trench (see Geist and others, 1993, for ridge subduction models). As ridges are subducted, scars in the overriding plate called "re-entrants" are left that can often be identified in the bathymetry and seismic reflection profiles (see above figures). Many of these processes can affect tsunami generation and will be investigated in the future.

Preliminary Simulation of Tsunami

To create a preliminary simulation of the April 2007 tsunami, we start with the fault mechanism determined by the Global CMT Project. The length of the fault that ruptured can be determined from the distribution of aftershocks or from the seismic inversion. In this case, however, we used the results from ShakeMap soon after the event to obtain an estimate of rupture length. Shown below is the preliminary simulation of the tsunami as viewed from different directions. The source and propagation model is based on an earlier study (Geist and Parsons, 2005) [Download PDF (6.5 MB)] that investigated tsunamis from the November 2000 New Ireland earthquake sequence (tsunami also observed at Gizo for the New Ireland event).

Initial tsunami wavefield - regional view to northwest
Tsunami wavefield at 2.4 minutes.
Regional view across the Solomon Sea.
See the animation: Large (6.2 MB) or Small (1.5 MB).

Initial tsunami wavefield - local view to northwest
Initial tsunami wavefield looking to the NW.
Close-up view near the earthquake source.
See the animation: Large (6.7 MB) or Small (1.8 MB).

Initial tsunami wavefield - local view to southeast
Initial tsunami wavefield looking to the SE.
Close-up view near the earthquake source.
See the animation: Large (3.5 MB) or Small (1.0 MB).

The islands near the trench are located in a region that is uplifted during the earthquake. The onset of the tsunami, therefore, will be very rapid and there might not be a withdrawal of the ocean prior to the initial wave peak. For islands such as Choiseul and Bougainville (Papua New Guinea), which are located at a typical distance from the trench for a subduction island arc, the first tsunami peak will often be preceeded by withdrawal of the ocean.

References

Bird, P., 2003, An updated digital model of plate boundaries: Geochemistry, Geophysics, Geosystems, v. 4, doi:10.1029/2001GC000252, 52 p.

Bruns, T.R., Vedder, J.G., and Cooper, A.K., 1989, Geology of the Shortland Basin region, central Solomons Trough, Solomon Islands--review and new findings, in Vedder, J.G., and Bruns, T.R., eds., Geology and offshore resources of Pacific island arcs--Solomon Islands and Bougainville, Papua New Guinea Regions: Houston, Texas, Circum-Pacific Council for Energy and Mineral Resources, Earth Science Series, v. 12, p. 125-144. [Download PDF (2592 K)]

Geist, E.L., and Parsons, T., 2005, Triggering of tsunamigenic aftershocks from large strike-slip earthquakes: Analysis of the November 2000 New Ireland earthquake sequence: Geochemistry, Geophysics, Geosystems, v. 6, doi:10.1029/2005GC000935, 18 p. [Download PDF (6.5 MB)]

Geist, E.L., Fisher, M.A., and Scholl, D.W., 1993, Large-scale deformation associated with ridge subduction: Geophysical Journal International, v. 115, p. 344-366. [Download PDF (2095 K)]

Kagan, Y.Y., and Jackson, D.D., 1999, Worldwide doublets of large shallow earthquakes: Bulletin of the Seismological Society of America, v. 89, p. 1147-1155.

Lay, T., and Kanamori, H., 1980, Earthquake doublets in the Solomon Islands: Physics of the Earth and Planetary Interiors, v. 21, p. 283-304.

Schwartz, S.Y., 1999, Noncharacteristic behavior and complex recurrence of large subduction zone earthquakes: Journal of Geophysical Research, v. 104, p. 23,111-123,125.

Schwartz, S.Y., Lay, T., and Ruff, L.J., 1989, Source process of the great 1971 Solomon Islands doublet: Physics of the Earth and Planetary Interiors, v. 56, p. 294-310.

Tregoning, P., Lambeck, K., Stolz, A., Morgan, P., McClusky, S.C., van der Beek, P., McQueen, H., Jackson, R.J., Little, R.P., Laing, A., and Murphy, B., 1998, Estimation of current plate motions in Papua New Guinea from Global Positioning System observations: Journal of Geophysical Research, v. 103, p. 12,181-112,203.

 

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