Study on Rupture Initiation by Broadband Records for Local Earthquakes.

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Body wave data recorded at a network of broadband seismograph stations were analysed to investigate local events with focal depths deeper than about 50 km. For these events the initial portion of P wave displacement represents well the source time function, but only if the amplitude is normalized to the maximum moment rate. The magnitudes of the analysed earthquakes range from Mw=3.1 to 6.5. It is shown than the seismic moment Mo and the pulse width T are well correlated as Mo/T^3=const., indicating that the stress drop is largely constant. This dynamic similarity seems to be valid for a vast range of earthquake sizes : Mw=1-8. Closer inspection of Mo/T^3 indicates that the stress drop slightly depends on the source depth while it is almost independent of whether the source is an inter-plate event or an intra-plate event. It is also shown that source complexity such as a multiple shock nature is not a characteristic of large earthquakes but is often observed even for small earthquakes.