A method for Real-time inversion of earthquake source ruptures

With the enhancement of regional seismic networks, real-time seismology becomes a practical way for effective damage mitigation. For example, using P-wave arrival times and amplitudes from only a few stations, a real-time earthquake analysis system developed by Horiuchi et al. (2004) can precisely locate the earthquake within a few seconds and issue an early-warning for the S-wave shaking. This automatic procedure, however, models an earthquake as a point source, which is insufficient to issue shaking intensity or evaluate strong motion distribution caused by a large earthquake, since large earthquakes are usually observed as several asperities. Several methods to rapidly and automatically determine earthquake fault plane or to reveal rupture process have been proposed (e.g., Dreger and Kaverina, 2000; Kuge, 2003; Hori, 2004). Unfortunately, all the methods are time consuming and unpractical for a real-time warning system. In this study, we proposed an automatic method toward real-time determination of finite-fault slip distribution. Instead of waiting for moment tensor solutions of a larger earthquake, we determine the focal mechanism using the amplitude (with polarity) of the P-wave first motions as soon as P-wave arrivals are detected. Of the two nodal planes of the focal mechanism, the fault plane is identified by comparison of moment indexes of the two nodal planes by using P-wave only. Given that magnitude of a larger earthquake is known (it has been practically issued by Horiuchi system), waveform inversion for a finite fault plane source model is then carried out Here we show three simulation results for the 1999 Taiwan Chi-Chi earthquake, the 2007 Noto hanto, Japan, earthquake and the 2007 Chuetsu-oki, Japan earthquake. The inversion results can be obtained in one minute given that the hypocenter location and the magnitude are known (note that all these parameters can be in fact reliably issued the Horiuchi system). Our simulation results show that a real-time inversion of the source process is able to reveal a slip distribution,reasonable and comparable with that of an off-line analysis. Due to time delay for inversion, the kinematic source models might not be helpful for shaking intensity predictions. However, it may be of great importance for tsunami warning since the tsunami wave arrives at a relatively slow speed.