SCEC Award Number 19136 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Detailed Characterization of Stress and Strain Parameters in the Cajon Pass and San Gorgonio Pass Regions from Earthquake Focal Mechanisms
Investigator(s)
Name Organization
Yehuda Ben-Zion University of Southern California
Other Participants Yifang Cheng (USC student)
Niloufar Abolfathian (USC student)
SCEC Priorities 1c, 1d, 1e SCEC Groups SDOT, Seismology, FARM
Report Due Date 03/15/2020 Date Report Submitted 03/18/2020
Project Abstract
 The studies performed in this project provide high-resolution results on stress parameters in Southern California, with a focus on the region around the South Central Transverse Ranges (SCTR), using a refined methodology to invert 1981-2017 declustered and aftershocks focal mechanisms. Comparison between the associated stress parameters provides information on the local dominant loading. The estimated stress parameters are examined in relation to the regional stress regime and local loadings. Over the regional scale, the SHmax trends towards the NNE and the stress ratios vary from transtensional regime near the Eastern California Shear Zone (ECSZ), to shear stress near the SCTR, and towards transpression near the Western Transverse Ranges (Figs. 1 and 2). Detailed analysis of stress parameters near the SCTR indicates deviations from the regional shear stress. The San Bernardino Mountain area shows SHmax direction towards NNW and transpressional stress components likely associated with the relative motion of the San Andreas Fault and ECSZ. The Cajon Pass and San Gorgonio Pass show transpressional regime near the bottom of the seismogenic zones that may reflect the elevated topography. In Crafton Hills, rotation of the principal stress plunges and SHmax and transtensional regime below ~10 km, along with lower estimated apparent friction coefficient suggest a weak fault possibly associated with deep creep (Figs. 1 and 2). The results reveal effects of local loadings resolved by the multi-scale analysis. The observations do not show significant temporal variations of stress variations near the SCTR from the average stress parameters in the analyzed 37 years.
Intellectual Merit The preformed analyses reveal clear spatial variations of stress parameters in different domains of the plate-boundary in southern California (ECSZ, Transverse ranges, region between Cajon Pass and San Gorgonio Pass), different sections of the San Andrea fault and San Jacinto fault, and the region between them. The separate analyses of focal mechanisms of declustered seismicity and aftershocks help to separate regional loading (reflected more clearly in the declustered mechanisms) from local loadings (expressed more clearly by the aftershocks). The observed correlations of stress parameters with topographic variations can provide together with numerical modeling important constraints on absolute and relative stress levels at different locations. The ob-served variations of SHmax orientation near the Crafton Hills may indicate a progressive weakening of the fault with depth and/or aseismic creep at depth in that region.
Broader Impacts The results are relevant to broad issues of crustal and fault dynamics in the complex region of the southern San Andreas system around the transverse ranges. The obtained multi-scale stress inversion results provide useful input for models of dynamic ruptures. The lack of observed significant temporal variations of stress variations provide useful constraints for models of crustal deformation. The project supported directly a female PhD student and contributed to the education of a young female researcher. The techniques developed in the project may be used to derive high resolution results on variations of stress field in other regions.
Exemplary Figure Fig. 1. Variations of the maximum horizontal compressive stress direction (SHmax) and dominant faulting styles in the study area based on declustered focal mechanisms. Note the rotations of SHmax from east to west and in the region between the San Andreas and San Jacinto faults. From Abolfathian et al. (2020)