Finite Fault Modeling in Near-Real Time for Tsunami Warning Applications

The Pacific Tsunami Warning Center (PTWC) provides notification to its clients in the Pacific basin of large earthquakes with an assessment of their potential for tsunamigenesis. It is our goal to issue our message products as soon as possible after the detection of a Pacific basin earthquake. Typically, our first official messages are issued within 15 minutes of an earthquake. The Achilles heel of the warning system has been its inability to predict the size of the waves when they arrive on shore. Due to taking a conservative posture, this has resulted in false evacuations. The tsunami modeling community has developed tools that give the tsunami warning system a nascent ability to forecast wave heights. This study is a step towards using a more realistic source, rather than rule of thumb source prescriptions based on Mw, which may be inadequate for complex events resulting in modeling errors in the near and regional fields. We are experimenting with computing slip distributions in near real time in an effort to provide a more realistic source for the purpose of tsunami modeling. The PTWC currently receives broadband data from as many as 100 stations at any given time. Hence we are well situated to do the fault modeling in near real time. The procedure requires a fault plane solution. We use a variant of the method used by Mendoza (SRL, Vol. 67, 19-26, 1996) to perform the inversion. Tests on synthetic earthquakes illustrate the limitations and parameter trade-offs inherent to bodywave inverse solutions. We have so far applied this technique to the Mw 8.4 2001 Peru earthquake and have obtained results that are broadly consistent with those found by previous studies Giovanni et al., ( GRL, Vol. 29, 14-1 - 14-4, 2002), Bilek and Ruff (GRL Vol. 29, 21-1 - 21-4, 2002) and Kikuchi ( wwweic.eri.u-tokyo.ac.jp/EIC/EIC_News/105E.html ). The resulting source time function as found by other studies is primarily bimodal, with one peak occurring at 15 sec, and a much larger peak at 75 sec. The rupture propagates almost exclusively to the Southeast. The first peak in the source time function corresponds to a section of fault in the immediate vicinity of the epicenter, and the main moment release over a region 130-200km Southeast of the hypocenter. The slip in this region tends to be distributed over more of the vertical extent of the fault. The solutions display some sensitivity to the sub-event source time function, however the moment is usually ~2 \ast 10^{28} ergs.