Beginning of Earthquakes Determined from Seismic Initiation Phase

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First, we developed an inversion method for determining the source parameters of initial rupture process. The initial rupture process is simulated with the Sato and Kanamori's model (1999) in which a rupture starts from the ends of a pre-existing crack. The source parameters in this model are the radius of pre-existing crack, dynamic stress drop, and the trigger factor that controls the ensuing rupture propagation. In the real attenuating medium, the quality factor Qp stands as an additional unknown parameter. Numerical experiments showed that the radius of pre-existing crack, the trigger factor and Qp are not well retrieved because of the trade-off among the three parameters. Therefore we aimed at estimating those parameters in a robust manner as follows. The trigger factor was assumed to take only two values ; 10^<-3> corresponding to the Spontaneous model and unity corresponding to the Trigger model. For each of Qp values varied from 40 to 280 in an increment of 40, we determined th … More e radius of pre-existing crack, stress drop and the radius of the eventual crack by minimizing the sum of squared residuals for the P-wave velocity waveform from onset to the first peak. Finally, we chose a set of parameters that gives the minimum of sum of squared residuals for the waveform from onset to the end of the first cycle. This algorithm was tested with several synthetic data and found able to recover the expected parameters. Then we applied it to initial P-waves of seven microearthquakes (M0.1〜2.6) which occurred in the source region of the 1984 Western Nagano earthquake. As a result, five events were found better fit to the Spontaneous model and two others fit to the Trigger model. For the events categorized as the Spontaneous model, the estimated radii of pre-existent cracks range in 3-9 m. At the same time, we found that the theoretical waveform cannot be well fitted to the observed one for the time after the first peak. This probably results from too simple an assumption on the stopping of rupture in our source model, i.e., the rupture stops instantaneously at the final stage. Less