Comprehensive analysis of earthquake source spectra in southern California

Abstract

[1] We compute and analyze P wave spectra from earthquakes in southern California between 1989 and 2001 using a method that isolates source-, receiver-, and pathdependent terms. We correct observed source spectra for attenuation using both fixed and spatially varying empirical Green's function methods. Estimated Brune-type stress drops for over 60,000 M L = 1.5 to 3.1 earthquakes range from 0.2 to 20 MPa with no dependence on moment or local b value. Median computed stress drop increases with depth in the upper crust, from about 0.6 MPa at the surface to about 2.2 MPa at 8 km, where it levels off and remains nearly constant in the midcrust down to about 20 km. However, the results at shallow depths could also be explained as reduced rupture velocities near the surface rather than a change in stress drop. Spatially coherent variations in median stress drop are observed, with generally low values for the Imperial Valley and Northridge aftershocks and higher values for the eastern Transverse ranges and the north end of the San Jacinto fault. We find no correlation between observed stress drop and distance from the San Andreas and other major faults. Significant along-strike variations in stress drop exist for aftershocks of the 1992 Landers earthquake, which may correlate with differences in main shock slip.

Spectral models for ground motion prediction in the L'Aquila region (central Italy): evidence for stress-drop dependence on magnitude and depth

“…Large data sets from different seismotectonic regions usually do not show a breakdown in selfsimilarity (e.g. Shearer et al 2006;Allmann & Shearer 2007;Oth et al 2010), yet studies on single seismic sequences often provide indications that smaller events have lower stress drops than larger ones (e.g. Mayeda & Malagnini 2010;Mereu et al 2013;Malagnini et al 2014).…”

FinDer v.2: Improved real-time ground-motion predictions for M2–M9 with seismic finite-source characterization

“…The size of the beachballs scales with magnitude. We apply a rake-based classification of faulting style (Shearer et al 2006) that produces output values ft from −1 to 1. For most events, the FinDer-determined rupture strike and the strike of one nodal plane of the corresponding focal mechanism agree to within ±25 • .…”

“…In Fig. 11, we apply the rake-based classification algorithm for faulting style described in Shearer et al (2006), to assign continuous values ft from −1 to 1 to the earthquakes, that were real time detected by FinDer during the 8-month trial period in California; we classify all earthquakes with −0.25 ≤ ft ≤ −0.25 as strike slip, with ft < −0.25 as thrust-, and with ft > 0.25 as normal-fault events.…”

Probing failure susceptibilities of earthquake faults using small-quake tidal correlations

“…4) of these values for several different oscillatory amplitudes ranging from 0 to 10% of the typical stress drop (or 0-4% of the critical stress F c ). We chose this amplitude range based on estimates of typical local average stress drops on faults, compared with typical tidal or seasonal amplitudes 35,36 . We performed this analysis for two specific frequencies: one corresponding to a diurnal tidal stress frequency of 2p/ (25 h) (Fig.…”