Spatial Variations of Stress Patterns Near the South Central Transverse Ranges in Southern California

Date: 4/26/2019

Time: 04:15 PM

Room: Vashon

We analyze high-resolution spatial variations of the stress patterns in South Central Transverse Ranges (SCTR) in southern California and their correlation with topography and the fault system in the area. The analysis is based on the refined stress inversion methodology employing a declustered focal mechanism catalog for the years 1981-2017. The stress inversion provides the orientation of the three principal stress axis, from which we derive the maximum horizontal compressive stress direction (SHmax) and the stress ratio parameter R=(σ1-σ2)/(σ1-σ3). The obtained spatial distribution of stress parameters is generally in agreement with the tectonic setting, showing strike-slip faulting type. The SHmax orientation is generally to the north with some variations toward NEN in the eastern and western sections of the SCTR. The SHmax orientation shows a significant clockwise rotation of about ~23º with depth near the Crafton Hills (CH). Over regional scale, the stress ratios R vary from transtensional stress regime in the east towards transpressional in the west. Sharp changes of R are observed near CH with significant transtensional components, and near Cajon Pass and San Gorgonio Pass with transpressional components. The stress patterns estimated from aftershocks amplify the sharp changes in stress ratios relative to those seen with the background seismicity. The seismicity distribution suggests significant variations of seismogenic depth in the region between the San Andreas Fault (SAF) and San Jacinto Fault (SJF). Initial results from separate analyses of focal mechanisms in sub-regions with different seismogenic thicknesses indicate changes of stress patterns between the eastern part of the SAF, the western part of the SJF, and the region between them, where the stress pattern no longer follows the regional stress variations.

Presenting Author: Niloufar Abolfathian

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