Factors influencing stress drops of small earthquakes in southern California

Peter Bird, & Peter M. Shearer

In Preparation 2011, SCEC Contribution #1467

Shearer et al. [2006] determined Brune-type stress drops of 64801 relocated magnitude 1.5–3.1 earthquakes in southern California by using spectral methods with signal/noise control, empirical local Green’s functions, and source and station corrections. They found that stress drop increases with depth when computed assuming uniform shear velocity; however, stress drop is not correlated with magnitude, focal mechanism, or distance to the nearest fault. We recompute stress-drops using hypocentral shear velocities from the Harvard Community Velocity Model; stress drop versus burial depth now shows a broad peak centered at 11 km depth and another peak for apparent hypocenters <2 km deep; this shallow peak is probably an artifact. We use the heat-flow map of Blackwell & Steele [1992] to estimate hypocentral temperatures, and study the separate effects of temperature (at constant depth) and burial depth (at constant temperature). Median stress drop rises with temperature up to 200-250 C, and then above 300 C it declines. Median stress drops on the declining branch are positively correlated with burial depth, but those on the rising branch are independent of depth. We interpret the rising branch as showing that shallow-crustal stress-drops are proportional to the amount of interseismic fault healing expected under rate-and-state friction theory. This is supported by our finding that median stress drops at hypocenters cooler than 200 C are negatively correlated with tectonic strain rates from the model of Bird [2009], varying by a factor of 3~4 across 3 decades in strain rate. This can be described either by a logarithmic formula for strain rates below a critical value of 10-12 s-1 or by a more universal power-law formula with exponent of -0.2. We estimate an activation energy of 47 kJ mol-1 for the fault healing process in the frictional regime, and show that the overall amounts of healing are consistent with laboratory models. In contrast, we interpret the depth- and temperature-dependent median stress-drops for temperatures above 300 C as proportional to the preseismic tectonic shear stress around the future hypocenter, which is controlled by temperature-sensitive dislocation creep in the weakest parts of a heterogeneous crust and by a gradient of mean composition (sialic to mafic) with burial depth. Separating stress drops by geologic terrane has no effect, except in the Sierra Nevada where higher stress drops could be due to either compositional differences or inadequacies in our geotherms and/or hypocentral depths. Combining all of these predictors, we are only able to explain 8% of the variance in log(stress drop). Post-model residual stress-drop factors have approximately a log-normal distribution with standard deviation of 0.48 decades. Mechanisms that might increase some stress-drops above the median include total stress release due to additional slip-weakening mechanisms at low temperature, or boudinage geometry at high temperature. Mechanisms that might decrease some stress-drops below the median include slip restriction by cross-cutting fault offsets and/or patches of previous stress release occurring within the outermost perimeter of a complex rupture.

Bird, P., & Shearer, P. M. (2011). Factors influencing stress drops of small earthquakes in southern California. Journal of Geophysical Research, (in preparation).