Recently there has been a push within the seismological community to produce maps of ground shaking intensity in near real time for emergency response purposes. In densely instrumented regions, such as Southern California, Japan, and Taiwan, these maps can be generated using ground-truth measurements. However, other approaches are needed for regions with sparse station coverage. One of them is to derive finite source parameters in near real time, then forward model the ground shaking in the regions of interest (e.g. Dreger and Kaverina, (2000)). To successfully predict ground motion we need to have good waveform fits at all azimuths from the hypocenter. An initial set of good source parameters will dramatically reduce the time required to derive a good slip model and enable us to generate synthetic ground shaking information. It is also beneficial to have correct finite source information soon after the earthquake. The information then can help to identify the causative fault plane, design temporary portable seismic networks to monitor the aftershocks surrounding the main event, and forward calculate the stress perturbation due to the earthquake.
We inverted strong motion data for the finite source parameters of 6 large aftershocks of the 1999 Chi-Chi, Taiwan earthquake. The locations and origin times are depicted in Figure 1. For each event, we derived a preferred model by testing different focal mechanisms, hypocenters, and other source parameters in more than 1000 inversions. We documented how the fits between the observed waveforms and the corresponding synthetics deteriorated as the hypocenter and focal mechanism deviate from those of the preferred model. These results will help to determine how accurate these parameters must be if we wish to derive slip models in near real-time for generating ShakeMaps.
We found that, if the deviation in hypocenters and focal mechanisms were less than 5 km and 20, respectively, we generally recovered more than 80% of the preferred model's synthetic waveform fit, measured by variance reduction. The length of 5 km (Figure 14.1) is similar, and maybe related, to the widths of the slip patches we modeled. For the thrust events, the input dip angle of the fault must be correct to within 20. For the strike-slip event, the input fault strike must also be within 20 of the true strike (Figure 14.2).
For each of the six Chi-Chi, Taiwan aftershock events, we performed more than 1000 sensitivity tests by varying the source parameters used in the inversions and we documented the influence these parameters have on the slip model and the waveform fits. Good waveform fits can mostly be achieved if the errors in hypocenters and focal mechanisms are within 5 km and 20, respectively. However, in some cases good waveform fits can also be achieved outside of the preferred ranges of the input source parameters. These results provide the criteria needed to evaluate the performance of the seismic network if we want to invert the finite fault parameters of magnitude 6 earthquakes in real time and use the source model to forward-model the ShakeMaps, which can be used by the seismic response authorities for seismic mitigation purposes.
We thank Dr. Willie Lee for providing the strong motion data from Central Weather Bureau (CWB) of Taiwan and Dr. Win-Gee Huang for the strong motion data from IES, Academia Sinica of Taiwan. We thank CWB for providing their aftershock seismicity data. This research is funded by NSF Grant EAR-0000893 and PEER Lifelines 1E06.
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