Massive Microearthquake Relocation: Technique Development and Application to Seismotectonics

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9814994RubinThis research involves the extension of a precision earthquake relocation method to large numbers of events and to more diverse tectonic frameworks within volcanoes and active fault zones. Work has begun on events following the 1989 Loma Prieta, California, earthquake. Southeast of the main shock, along the transition to the creeping section of the San Andreas to the south, relocations comprising over 60% of the earthquake population show that much of the seismicity is concentrated in linear, nearly horizontal bands separated by regions that are presumably slipping largely aseismically. This coincides with earlier indications that streaks of microseismicity aligned in the slip direction are a common feature of creeping faults. Closer to the main Loma Prieta slip zone, focal mechanisms are more diverse and fault bends are encountered. Scientific issues to be addressed by the relative earthquake relocation in this region include searching for fault-bend folding earthquakes, discriminating between the nodal planes of the anomalous focal mechanisms to determine the actual fault planes, and generally refining what is known of the distinction between aftershocks that occur beyond the main shock perimeter and those that occur within or adjacent to the area of mainshock slip. This research is a component of the National Earthquake Hazard Reduction Program.