An earthquake source model with a ripple generating core.

Abstract

The rupture processes of large shallow earthquakes are investigated based on the broad-band seismograms recorded at relatively short focal distances as well as on various field surveys near the source regions. Predominant shortperiod waves are recorded only during a short period at the initial parts of P and S waves. These waves give a prominent spectral peak at high frequencies. The results from coseismic and postseismic field surveys in some source regions clearly show that there must be a particular region characterized by a large dislocation, large acceleration and extremely low aftershock activity. This specific region seems to have a relatively small dimension compared with the length of the main fault.A source model is proposed so as to satisfy various aspects of the observed results. In this model, the rupture processes in a source region may be divided into two stages. The first stage is the smoothing dislocation propagating over the main fault and the second one is the complex ruptures of many small faults distributed in a confined small region. The long-period waves are produced by the former stage. The latter stage radiates the predominant short-period waves, yielding a spectral peak at high frequencies. It is assumed that a large number of small faults with the same fault geometry and the same fault motion are arranged en echelon or parallel in this specific region, and that these small elements are dislocated simultaneously and successively in a short time after the onset of main rupture. We call this specific region in the earthquake source area "a ripple generating core."The stress drop by each small fault may be of the same order as that of the main fault, if static and dynamic fault parameters are estimated on the basis of the earthquake scaling law. Extremely low activity of aftershocks within the specific region may be caused by the distribution of many small ruptures. A large amount of dislocation around this specific region appear to arise from the accumulation of these abundant dislocations.