Initial rupture process of microearthquakes recorded by high sampling borehole seismographs at the Nojima fault, central Japan

Details Initial rupture process of microearthquakes recorded by high sampling borehole seismographs at the Nojima fault, central Japan Initial rupture process of microearthquakes recorded by high sampling borehole seismographs at the Nojima fault, central Japan

Oct 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002pepi..132..269h&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 132, Issue 4, p. 269-279.

Physics

4

Scientific paper

We analyze high sampling waveforms of the initial part of P-wave recorded at the 1800-m-deep borehole seismographs at the Nojima fault from December 1999 to May 2000 to clarify the initial rupture process of microearthquakes. We select 12 events with high S/N, whose magnitudes range from -0.3 to 2.2 and hypocentral distances from 1 to 11km. We adopt the two different source models by and by . The former (model by Sato and Hirasawa (SH model)) generates only a ramp-like onset of velocity pulse. The later (model by Sato and Kanamori (SK model)) is able to generate a weak initial phase that is controlled by a trigger factor and the length of pre-existing crack. We perform the waveform inversion to estimate the optimum source parameters of each model. Waveforms of 5 of the 12 events are clearly reproduced by both SH model and SK model with a large trigger factor and a small length of pre-existing crack. The others are explained by not SH model but only SK model with a small trigger factor and a large length of the pre-existing crack, indicating that the weak initial phase is a nucleation phase and reflects the source process. These seven events satisfy roughly a relation that a large event has a large length of the pre-existing crack; the final crack length is proportional to the length of the pre-existing crack.

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