Improved Characterization of Seismicity and Fault Structure by Wide Area Event Relocation

The use of waveform cross correlation to improve seismic arrival time measurements, and the use of multiple event location techniques to reduce model errors, have led to substantial improvement in event locations. Specialized studies that typically involve the relocation of up to a few thousands of earthquakes within selected areas of interest have revealed images of the seismicity and fault structure at seismogenic depths with unprecedented detail. We comment on and present results from ongoing efforts to apply these techniques on a massive scale to increase resolution in the seismicity across wide areas and different tectonic environments. The first such effort involves the use of local network data to relocate ∼250,000 events in the NCSN catalog of northern California, and catalogs of intra-plate earthquakes in eastern North America. Phase-pick-based double-difference locations of the entire NCSN catalog show substantial reduction in rms residuals, revealing a much sharper view of the seismicity compared to the routine locations in almost all areas where seismicity is dense enough. A recently completed cross correlation database of 3 billion P- and S-wave differential times indicates that about 90% of the events in northern CA correlate with coefficients above 0.7. The additonal location improvement expected from the use of that data in the relocation process will allow the precise mapping of detailed fault structures and their correlation with the local geology. In the New Madrid and Charlevoix seismic zones fewer events correlate and less-well-defined fault zones are imaged, likely reflecting the more complex faulting processes in these areas. Results are also shown from the relocation of several thousand subduction-zone earthquakes beneath the Central Andes, using global network data. For this purpose the hypoDD software has been extended to accept regional and teleseismic first- and later-arriving phase pairs, with least square adjustments to the hypocentral parameters being computed in a spherical Earth. The new locations map in detail areas along the subducting plate that slip seismically, indicating three ∼50 km-wide sub-horizontal bands of activity that extend for hundreds of km along the strike of the slab at the depths 50, 120, and ∼250 km. These bands are 15-20 km thick, with a generally sharp transition from seismic to aseismic behavior at what is believed to be the interface between the oceanic crust and continental mantle. They may bound areas where large earthquakes occur infrequently, or areas where aseismic slip occurs.