Loading rates in California inferred from aftershocks

C Narteau,M Holschneider,P Shebalin

doi:10.5194/npg-15-245-2008

Abstract

Abstract. We estimate the loading rate in southern California and the change in stress induced by a transient slip event across the San Andreas fault (SAF) system in central California, using a model of static fatigue. We analyze temporal properties of aftershocks in order to determine the time delay before the onset of the power law aftershock decay rate. In creep-slip and stick-slip zones, we show that the rate of change of this delay is related to seismic and aseismic deformation across the SAF system. Furthermore, we show that this rate of change is proportional to the deficit of slip rate along the SAF. This new relationship between geodetic and seismological data is in good agreement with predictions from a Limited Power Law model in which the evolution of the duration of a linear aftershock decay rate over short time results from variations in the load of the brittle upper crust.

Highlights

In the last decades, geodetic measurements have considerably improved the description of spatio-temporal properties of strain accumulation and release along faults (Savage and Burford, 1973; Sauber et al, 1986; Langbein et al, 1990; Bennett et al, 1996; Peltzer et al, 2001; Fialko, 2006)
We estimate the loading rate in southern California and the change in stress induced by a transient slip event across the San Andreas fault (SAF) system in central California, using a model of static fatigue
It is likely that earthquakes are strongly under-reported during early parts of aftershock sequences, and the c value may be significantly influenced by non-physical effects

Summary

Introduction

Geodetic measurements have considerably improved the description of spatio-temporal properties of strain accumulation and release along faults (Savage and Burford, 1973; Sauber et al, 1986; Langbein et al, 1990; Bennett et al, 1996; Peltzer et al, 2001; Fialko, 2006). The aftershock decay rate is a sum of exponential decay rates, and Narteau et al (2002) have shown that various failure rates λ(σ0) and simplified overload distributions N (σ0) result in the same formula that has been called the Limited Power Law (LPL) In this formula, t is the elapsed time since the mainshock, A is a constant, x γ (ρ, x)= τ ρ−1 exp(−τ ) dτ,. The main idea behind our analyzes is that, at a regional length scale, when averaged on a representative sample of aftershock sequences triggered by mainshocks in the same magnitude range, the variation of the upper limit of the overload distribution should reflect variations in the load (i.e. in the mean value of the overload distribution itself) This load being essentially affected by tectonic motions and major seismic or aseismic events, the evolution of a spatially averaged σb value (expressed by either λb or c) could allow for a better understanding of the accommodation of deformation along the plate boundary. Most importantly, using intermediate magnitude mainshocks, aftershocks are distributed in the entire seismic zone and the resulting catalog of aftershocks is the best available sampling of the seismicity of an entire area for a given period of time

Statistical properties of selected aftershocks

Estimation of the time delay before the onset of the power law decay rate

Findings

Discussion and conclusion