SCEC Award Number 13040 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Heterogeneity, Rotations, and Volumetric Strain near Faults from Focal Mechanism Data
Name Organization
Yehuda Ben-Zion University of Southern California
Other Participants One SCEC undergraduate intern, One Graduate Student RA
SCEC Priorities 3, 2, 4 SCEC Groups Seismology, FARM, GMP
Report Due Date 03/15/2014 Date Report Submitted N/A
Project Abstract
We performed several observational studies of spatio-temporal patterns in earthquake source mechanisms that accounted for possible effects of rock damage and sampled the range of seismogenic depths in close proximity to faults. The focus was on quantifying large-scale heterogeneities, rotations, and volumetric changes of seismic strain fields around large rupture zones. The research was conducted along two primary directions: analysis of spatio-temporal variations of double-couple-constrained focal mechanisms, and derivation of earthquake source tensors with a waveform inversion procedure that includes isotropic and CLVD components. Analysis of rotations of double-couple-constrained mechanisms in the Eastern California Shear Zone indicates that the most immediate aftershocks have focal mechanisms most unlike like the mainshock. This may be explained by neglecting a small isotropic component (0.03-0.15 of the total moment) in the focal mechanism determination process. Analysis of full source tensors of seven M > 4 earthquakes in the trifurcation area of the San Jacinto fault zone indicates that each event has a small but statistically significant explosive isotropic component. The obtained isotropic components are 0.04-0.23 of the total moment, which is consistent with the estimates from analysis of rotations.
Intellectual Merit The studies established and used state-of-art parameterization of earthquake source tensor properties, along with several types of observational analyses to quantify heterogeneities of earthquake processes and examine possible signatures of rock damage in earthquake source volumes. The various results were rigorously tested using a set of statistical techniques to quantify their uncertainties. Constraining the heterogeneities and damage process that is generated within fault zones by earthquakes with seismological observations is essential to the SCEC science objective 3c. The investigations also contribute directly to SCEC objectives 2d (information for CSM from focal mechanisms) and 4c (extend the CFM to include spatial uncertainties and fault heterogeneity), as well as objective 1b (clarifying the stress state near the bottom of the seismogenic zone).
Broader Impacts The studies contributed to the following long-term research goals of SCEC4: Seismology – “… improving the estimation of source parameters…”, “Investigate near-fault crustal properties, evaluate fault structural complexity…” Earthquake Geology – “…damage in relation to the rupture propagation processes…” The studies are closely related to SCEC interdisciplinary focus areas: FARM – “Determine the relative contribution of on- and off-fault damage to the total earthquake energy budget…”, “…understanding the influence of material properties, geometric irregularities and heterogeneities in stress…”, “Determine the properties of fault cores and damage zones and characterize their variability with depth and along strike…”. SDOT – “Contributions to the development of a Community Stress Model ... a set of spatio-temporal (4D) representations of the stress tensor in the southern California lithosphere”. The studies will also contribute to goals of the SoSAFE project.
The theoretical and code developments contribute to the infrastructure of software for analyzing earthquake source properties. In addtion to SCEC and earthquake physics objectives, improved quantification of isotropic radiation from earthquake sources can contribute to discrimination efforts of small explosions from earthquakes.
Exemplary Figure Figure 2.

Inversion results for the analyzed earthquakes. The blue focal mechanisms are unconstrained full source tensor solutions determined from gCAP waveform inversions. Each earthquake is found to have a small explosive isotropic component. The yellow focal mechanisms are double-couple-constrained and show with red “+” symbols the locations of stations used in the derivations.

From Ross, Ben-Zion and Zhu (2014)