HR: 09:30h
AN: U51A-05    [Abstracts]
TI: Real Time GPS Magnitudes for the Largest Earthquakes, and Application to Tsunami Warning
AU: * Freymueller, J T
EM: jeff@giseis.alaska.edu
AF: University of Alaska Fairbanks, Geophysical Institute, Fairbanks, AK 99775 United States
AB: Seismology has two well-known limitations in the rapid determination of magnitudes and source models for the largest earthquakes. The first is that all magnitude scales used in seismology saturate, with the exception of moment magnitude determined from very long period data. This causes initial magnitudes to be underestimated. For example, in the case of the December 2004 Sumatra-Andaman earthquake, the initial magnitude estimate was 8.0. About 2 hours after the earthquake the magnitude was revised upward to 8.5 based on additional longer-period data, and the Harvard CMT solution with Mw 9.0 was released about 5 hours after the earthquake. The second limitation is that for ruptures that last longer than several minutes, finite source modeling becomes difficult because secondary phases from the initial part of the rupture arrive at the same time as direct phases from the later part of the rupture. Both of these limitations can be overcome given time, effort, and later-arriving data, but for near real time applications, magnitude determination of the largest earthquakes remains problematic. Real time GPS data from near-regional distances can be used to determine magnitude for the largest events within minutes of the earthquake, with no upper limit on magnitude. In fact, the larger the earthquake, the better GPS will perform because the signal to noise ratio for displacements increases. This makes real time GPS recordings highly complementary to seismic methods. Displacement records from large earthquakes show that the static displacements are largely complete after the passage of the direct shear waves. After this point, a reliable estimate of the static displacement can be made as long as the dynamic displacements from the surface waves are avoided. In the Sumatra-Andaman earthquake, a continuous GPS site in Medan, Sumatra, about 400 km from the epicenter, was displaced by ~14 cm horizontally. An epoch-by-epoch GPS solution shows that this displacement could be measured with a precision of ~2 cm using data no more than 15-20 minutes after the start of the rupture. Production of a GPS displacement record in near real time is feasible, and given data from a few sites within near-regional distances (within a few to several hundred km of the rupture), seismic moment can be estimated reliably using a simple and reliable inversion scheme. I use real data from the 2002 Denali earthquake (Mw 7.9) and synthetic data from the 2004 Sumatra-Andaman earthquake to illustrate the feasibility of the system, and evaluate how many GPS sites would be required to provide accurate magnitude and finite source model determination.
DE: 1208 Crustal movements--intraplate (8110)
DE: 1242 Seismic deformations (7205)
DE: 7215 Earthquake parameters
SC: Union [U]
MN: 2005 Joint Assembly