LARGE EARTHQUAKES AND TSUNAMIS AT THE SAMOA CORNER IN THE CONTEXT OF THE 2009 SAMOA EVENT

We examine the seismic properties of the 2009 Samoa earthquake in the context of its tsunami, the first one in 45 years to cause significant damage on U.S. soil. The event has a normal faulting geometry near the bend ending the 3000-km long Tonga-Kermadec subduction zone. Other large normal faulting tsunamigenic earthquakes include the 1933 Sanriku, 1977 Sumba and 2007 Kuril events. The 2009 Samoa earthquake shares with such intraplate earthquakes a slightly above average E/M0 (THETA = -4.82), but has a more complex geometry, a relatively long duration, and large CLVD (11%). Same-day seismicity appears detached to the SW of the fault plane, and 7 out of the 8 CMT regional solutions following the main shock are rotated at least 69 deg. away from its own mechanism. This points out to a mechanism of stress transfer rather than genuine aftershocks, in a pattern reminiscent of the 1933 Sanriku earthquake. Most of the seismic moment release around the Samoa corner involves normal faulting. To the South (16.5-18 deg. S; 1975, 1978, 1987, 2006), solutions consistently feature a typical intraplate lithospheric break. To the NW (15.5 deg. S), the 1981 event features a tear in the plate along Govers and Wortel's [2005] STEP model. The 2009 event is more complex, apparently involving rupture along a quasi-NS plane. An event presumably similar to 2009 took place on 26 June 1917, for which there is a report of a 12-m tsunami at Pago Pago. That event relocates 200 km to the NW, but its error ellipse includes the 2009 epicenter. The 1917 moment, tentatively 1.3 10**28 dyn*cm, is comparable to 2009. As suggested by Solov'ev and Go [1984], the report of a 12-m wave in Samoa during the 01 May 1917 Kermadec earthquake is most probably erroneous. We will present studies of the other large earthquakes of the past century in the area, notably the confirmed tsunamigenic events of 01 Sep. 1981 (damage on Savaii), 26 Dec 1975 (24 cm at PPG), 02 Apr 1977 (12 cm at PPG), 06 Oct 1987 and 07 Apr 1995 (only centimetric at PPG).