Studies of Fault Fabrics and Earthquake Mechanics from the Precise Relative Locations of Microearthquakes

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Microearthquakes (earthquakes smaller than magnitude ~3) that have similar hypocenters (locations) and focal mechanisms (fault planes and slip directions) produce ground motions at a given seismic station that appear very similar. By comparing the recorded seismic waveforms of these earthquakes, it is possible to obtain very accurate estimates of the relative arrival times of the seismic waves, and ultimately very accurate estimates of the relative locations of the earthquakes. Errors in relative location obtained using this method are tens of meters for events separated by hundreds of meter, and, after correcting for time-dependent station changes, meters for events separated by tens of meters or less. These errors are a factor of 10-100 smaller than those in the published US Geological Survey Northern California Seismic Network (NCSN) catalog. This increase in resolution has led to significant new insights in two different areas of fault studies. First, structures internal to the fault zone that previously could not be seen have been imaged. Second, because the relative location errors between the nearest microearthquakes are much smaller than the rupture dimensions (tens to hundreds of meters), this resolution allows studies of earthquake interaction using datasets of many thousands of events. In the area of fault structure, microearthquake relocation shows that the seismicity along many creeping faults is organized into slip-parallel "streaks", tens to a few hundred meters tall and up to kilometers in length. In the area of rupture interaction, it has been observed that the distribution of aftershocks of microearthquakes along the central San Andreas fault is very asymmetric, with many more of the nearest aftershocks (in space and time) occurring to the northwest of a prior mainshock than to the southeast. This is attributed to the contrast in material properties across the fault and how this contrast affects the dynamics of the mainshock (preferential propagation to the southeast). If this explanation is correct, it is relevant to seismic hazards because directivity of large earthquakes concentrates ground shaking in the direction of propagation.Current research is devoted to developing catalogs of thousands of precisely-located microearthquakes in regions of geologic interest, interpreting these catalogs in terms of their implications for fault mechanics, and improving the relocation method. More time-dependent corrections are being determined for NCSN stations and made available to the seismological community. The more precise locations obtained with the station corrections are being used to search for (1) subtle changes in fault dip associated with the observed microseismic lineations and (2) short lineations on faults believed to have little slip, to distinguish between competing ideas for the origins of the lineations. In the area of asymmetric aftershock distributions, the dataset is being extended to smaller mainshock sizes using the Parkfield HRSN catalog, the relocation code (with time-dependent station corrections) is being modified to routinely determine the local across-fault P-wave and S-wave velocity contrasts, and numerical modeling of rupture on the interface between dissimilar materials is being undertaken to test the explanation proposed for the origin of the symmetry. Finally, the statistics of aftershock sequences of microearthquakes on different faults are being compared to the predictions of analytic models of rate-and-state friction, and to numerical models of rupture interaction on planar faults. The goals are to test models of rate-and-state fault friction and to determine differences in loading rates and/or surface properties of the various faults. Because for some faults the earliest portion of the aftershock sequences might have been "lost" because of the network "blind" time following a triggering event (perhaps a few tens of seconds), the nearly 100 s archived waveforms of all cataloged events are being processed to identify and locate events that would ordinarily have triggered the network. These studies will lead to further insights into the mechanics of earthquakes.