HR: 15:10h
AN: G13A-06 INVITED     [Abstracts]
TI: Enhancing Tsunami Warning With Real Time GPS Magnitudes for the Largest Earthquakes
AU: * Freymueller, J T
EM: jeff@giseis.alaska.edu
AF: University of Alaska Fairbanks, Geophysical Institute, Fairbanks, AK 99775 United States
AB: Rapid determination of earthquake magnitude from seismic data alone is straightforward for most earthquakes, but remains problematic for the largest events. In large part this is due to the increase in low-frequency energy for the largest earthquakes, which means longer recording times are needed to reliably estimate the moment (proportional to the static slip) from dynamic seismic waveforms. Although new techniques are being developed now to rapidly estimate rupture duration and rupture length (a reasonable proxy for magnitude for the largest events), the near-real-time magnitude estimate for the disastrous December 2004 Sumatra-Andaman earthquake, the initial magnitude estimate was dramatically underestimated. Although the most critical failure in that event was the lack of a regional and local tsunami warning system, this event shows that we need to improve rapid magnitude estimation for the largest events. Real time GPS data from near-regional distances can reliably 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 increases. This makes real time GPS recordings highly complementary to seismic methods. Displacement records from large earthquakes show that a reliable estimate of the static displacement can be made as soon as the large dynamic displacements at a site die down. Even an extremely simple point source inversion, requiring a hypocenter location but no knowledge of fault mechanism, can provide an estimate of moment to within about 20-30%, and can provide starting values for a finite fault inversion that can determine moment to better than 10%. Simple inversions work because no possible earthquake with a small moment, regardless of mechanism, can cause large displacements a few hundred km from the fault. The critical requirement is to have real-time GPS data from multiple sites that are close enough to the rupture to have several cm to a few decimeters of static displacement. This can be accomplished by having a network of real-time GPS sites at a spacing of 100-200 km across potentially tsunamigenic subduction zones. For rapid magnitude determination for the largest events, sites farther from the trench than the volcanic arc simplify the inversion, but sites closer to the trench are valuable for resolving details of the slip and may constrain uplift related to tsunamigenesis.
DE: 1240 Satellite geodesy: results (6929, 7215, 7230, 7240)
DE: 1241 Satellite geodesy: technical issues (6994, 7969)
DE: 1242 Seismic cycle related deformations (6924, 7209, 7223, 7230)
SC: Geodesy [G]
MN: Fall Meeting 2005