Relationship between rupture process complexity and earthquake size

[ 1] We studied the complexity of waveforms for two shallow crustal earthquake sequences in Japan. Using 47 earthquakes of the 2003 northern Miyagi sequence and 95 events of the 2004 mid-Niigata sequence, we estimated the number of complex and simple P waveforms for events with magnitudes from M3.1 to M6.6. Data were recorded at borehole and surface stations with minimal site responses for close hypocentral distances of less than 30 km. The determination of complexity was done by counting the zero crossings of the accelerations waveforms. Our results show that for the smaller earthquakes, there are both simple and complex events but that all of the larger earthquakes are complex. There appears to be a threshold size over which the earthquakes always become complex. For both data sets this threshold is M4.0 to M4.5. We attribute this difference in complexity with earthquake size as an indication that larger earthquake grow with a complex ruptures that may be indicative of stochastic processes. There do not appear to be large earthquakes that grow smoothly to their final size.