Earthquake Migrations Along Streaks in Southern California

Improved earthquake relocation techniques have revealed a highly fractured crust in Southern California with many lineaments (or "streaks") of seismicity. In a preliminary survey of about 20 streaks from the new SHLK earthquake catalog (Shearer et al., BSSA, 2005), I discovered two cases of migration of seismicity over distances of up to 8 km and time intervals of 1.5 to 3 years. Both migrating streaks are relatively shallow, at 4 to 5 km depth in the tectonic block between the San Jacinto and Elsinore faults, and are oriented at high angles to major regional trends. The deeper streaks tend to consist of seismicity occurring more uniformly (i.e., randomly) over the 20-year catalog duration. Migrations on this time and space scale are rare in non- volcanic settings, but would be difficult to detect without high-precision relocations. The scale and velocities (1 to 10 km/yr) of the migration suggest an earthquake triggering process driven by migrating fluids.