A SIMPLE DISCRETE ELEMENT MODEL FOR LARGE MULTIPLET EARTHQUAKES

Adjacent segments of the plate boundary may fail separately as earthqu akes that occur very close in time. When the time between events is a small fraction of the recurrence interval, then the sequence is referr ed to as ''doublets'' or ''multiplets''. In the first part of this pap er, we determine the frequency of occurrence of large multiplet earthq uakes. We count the number of multiplet earthquakes present in the Abe catalogue with aftershocks removed (M(s) greater than or equal to 7.1 ) using a series of space, time and magnitude filters. We compare this number to the number of multiplet earthquakes present in the Abe cata logue where origin times of earthquakes are randomized and epicentral locations and magnitudes are kept constant. The difference between the observed and random number of multiplets gives us the percentage of m ultiplets that do not occur randomly. We find that this percentage ran ges from 2% to 7% of all large earthquakes based on the best choice of space, time and magnitude filters. In the second part of this paper, we model multiplet earthquakes using three simple frictional slider mo dels. The models consist of slider blocks resting on a frictional ''co nveyer belt'' and interconnected by springs. The seismogenic portion o f the plate interface is represented by blocks that exhibit stick-slip behavior (asperities). Aseismic creep within the seismogenic zone is represented by ''creepers''. The ''creepers'' are blocks that obey a s imple linear creep law. The three models differ in that the positions of the creeping blocks within the system of asperities and creepers ar e different. We find that model 3 produces a complex sequence of event s consisting of single, double and multiplet earthquakes. These synthe tic sequences of events are comparable to the earthquake cycle observe d along Nankai trough. Model 3 produces a high percentage of multiplet earthquakes when the creeper is allowed to relax within a small fract ion of the maximum recurrence time of the event sequence. This implies that the details of the frictional law invoked for the creeper blocks does not influence the model results significantly. The time evolutio n of model 3 is compared to the postseismic phase of the loading cycle at Muroto Point, Japan. We suggest that the rapid postseismic deforma tion that occurs immediately after the Nankaido earthquake is due to a rapid creeping zone that is in this region.