SEISMOLOGICAL STUDIES AT PARKFIELD-VI - MOMENT RELEASE RATES AND ESTIMATES OF SOURCE PARAMETERS FOR SMALL REPEATING EARTHQUAKES

Waveform data from a borehule network of broadband seismographic stati ons have been used to study microearthquakes along the Parkfield segme nt of the San Andreas fault (SAF). Analysis of almost 10 years of such data demonstrates that much of the seismicity in this region consists of repeating sequences, quasiperiodic sequences of earthquakes that a re essentially identical in terms of waveform, size, and location. Sca lar seismic moments have been estimated for 53 of these repeating sequ ences and combined with equivalent estimates from 8 similar but larger event sequences from the Stone Canyon section of the fault and the ma in Parkfield sequence. These estimates show that seismic moment is bei ng released as a function of time in a very regular manner. Measuremen ts of the moment release rate, combined with an assumed tectonic loadi ng rate, lead to estimates of the seismic parameters source area, slip , and recurrence interval. Such parameters exhibit a systematic depend ence upon source size over a range of 10(10) in seismic moment that ca n be described by three simple scaling relational-Lips. Several implic ations of these scaling relationships are explored, including the repe at time of earthquakes, average stress drop, strength of the fault, an d heat generated by earthquakes, What emerges from this analysis of mo ment release rates is a quantitative description of an earthquake proc ess that is controlled by small strong asperities that occupy less tha n 1% of the fault area. This means that the fault is highly heterogene ous with respect to stress, strength, and heat generation. Such hetero geneity helps to explain many of the apparent contradictions that are encountered in the study of earthquakes, such as why faults appear wea k, why significant heat flow is not observed, how significant high fre quencies can be generated by large earthquakes, and, how various,geolo gic features such as pseudotachylytes might form.