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Fine-scale complex fault structure and slip along the San Andreas Fault: Contrasting seismic behaviors in localized and diffuse fault zones

Yifang Cheng, Roland Bürgmann, & Richard M. Allen

Submitted September 11, 2022, SCEC Contribution #12417, 2022 SCEC Annual Meeting Poster #256

The central San Andreas Fault exhibits diverse modes of seismic and aseismic deformation along a highly localized fault zone. Although the main fault trace appears as a simple planar fault structure, small earthquakes reveal a highly complex fault geometry and slip motions. Here, we investigate the focal mechanisms, seismicity patterns, and repeating earthquakes along a 1.5-km-wide, 180-km-long segment of the central San Andreas Fault, including its main creeping section, from 1984 to 2015 to understand the focal mechanism complexity in relation to fault geometry and aseismic slip rate. We show that focal mechanism heterogeneity increases with fault-zone width but decreases with slip rate. During high-slip-rate time periods, most focal mechanisms are strike-slip events having P-axis azimuths at about 45° from the main fault orientation. During the low-slip-rate periods, there are more transpressional and transtensional events with P-axis azimuths smaller and larger than 45° from the main fault orientation, respectively. These observations along the localized fault zone show totally opposite spatiotemporal correlations to the observations in Eastern California Shear Zone and the San Jacinto fault zone in southern California, which we attribute to fault maturity. We suggest that, when the fault slips, localized fault zones have most of the seismic activities near the main fault while diffuse fault zones have a considerable portion of small earthquakes distant to the main faults (>1km), causing the observed opposite seismic behaviors. Our observations and conceptual model provide a more complete understanding of fault zone dynamics, link fault zone properties at different spatiotemporal scales, and suggest that seismic risk estimation using microseismicity should be different in diffuse and localized fault zones.

Cheng, Y., Bürgmann, R., & Allen, R. M. (2022, 09). Fine-scale complex fault structure and slip along the San Andreas Fault: Contrasting seismic behaviors in localized and diffuse fault zones. Poster Presentation at 2022 SCEC Annual Meeting.

Related Projects & Working Groups
San Andreas Fault System (SAFS)