Seismic Attenuation Monitoring of a Critically Stressed San Andreas Fault

Abstract

We show that seismic attenuation ( QS^-1) along the San Andreas fault (SAF) at Parkfield correlates with the occurrence of moderate-to-large earthquakes at local and regional distances. Earthquake-related QS^-1 anomalies are likely caused by changes in permeability from dilatant static stress changes, damage by strong shaking from local sources, and pore unclogging/clogging from mobilization of colloids by dynamic strains. We find that, prior to the 2004 M6 Parkfield earthquake, prefailure conditions for some local events of moderate magnitude correspond to positive anomalies of QS^-1 on the Pacific side, with local and regional earthquakes producing sharp attenuation reversals. After the 2004 Parkfield earthquake, we see higher QS^-1 anomalies along the SAF, but low sensitivity to local and regional earthquakes, probably because the mainshock significantly altered the permeability state of the rocks adjacent to the SAF, and its sensitivity to earthquake-induced stress perturbations. Plain Language Summary We discovered that along the San Andreas fault, the damping of seismic energy through the Earth's crust is modulated by the state of stress in the crust, especially when the fault is close to rupture. The phenomenon depends on the density of cracks that permeate the rock, their interconnection, and the degree that they are filled with fluids. We show examples where attenuation, and thus permeability, is modulated by (i) damage from local earthquake shaking, (ii) dilatation imposed by local earthquakes, and (iii) clogging and unclogging of cracks induced by ground motion from distant earthquakes. These examples correlate with observed water well level changes. We note significant changes to the attenuation signal after the 2004 M6.0 Parkfield earthquake, with less sensitivity to local and distant earthquakes.