[ghsc-seminars] GHSC Seminar *TOMORROW* - Pieter Share, OSU

[Llenos, Andrea L]

​The Tale of Two Faults: The Young and Loud Versus the Old and Quiet

​​​Speaker: Pieter Share
                Oregon State University
Location: online/virtual only
Date/Time: Tuesday, Oct 18, 2022 10-11am MDT

Abstract: In southernmost California, only ~40 km separates the near-parallel San Jacinto fault zone (SJFZ) and Southern San Andreas fault (SSAF) but there is much greater separation from a structural and seismotectonic perspective. The SJFZ transects mostly rocks of the Peninsular Ranges batholith, has an accumulated offset of ~25 km and accommodates abundant and distributed seismicity. In contrast, the SSAF is relatively aseismic on an everyday scale, exhibits frequent shallow creep, has an accumulated offset of 100s of km and now juxtapose crustal blocks of the Peninsular Ranges (southwest) and San Bernardino Mountains (northeast). Both the SJFZ and SSAF have accommodated numerous moderate to large earthquakes and will do so again in future.

In this presentation, I will present the diverse passive geophysical toolkit myself and collaborators use to inform the internal features of the SJFZ and SSAF and relate these to past tectonic activity and their potential effects on the physics of future large earthquake ruptures. In the case of the SJFZ, we detect fault zone head wave (FZHW) signals generated by the distributed seismicity clouds and use these FZHWs to pinpoint a >70 km-long bimaterial spine (interface) within the
complex SJFZ that varies in dip, strike and across-fault P- and S-wave velocity contrasts. Along the SSAF, we use a combination of magnetotellurics (MT), large-N seismic array data and analysis of geodetic signals from shallow creeping events to show that the SSAF likely has a steep northeast dip in the upper crust (at least) over 10s of km along-strike and that strain may be accumulating in more than one location in the ductile crust beneath this fault. These characteristics have important implications for earthquake rupture speed, mode, directivity and subsequent ground shaking along the SJFZ and SSAF as well as other major transform faults around the world.

Bio: Dr. Pieter-Ewald Share is an assistant professor in Geophysics at Oregon State University. His group there focusses on the state of structure and strain along active plate margins in western North America. Dr. Share’s career started with a Bachelors in Geology and Physics in his native South Africa. From there he completed a Masters in Geophysics at the Dublin Institute for Advanced Studies. After moving to the United States in 2013, he started a PhD at USC in Los Angeles followed by a postdoc at Scripps/IGPP as a Green Fellow. The Share group at OSU is a big proponent of integrative studies in the Geosciences and continues to combine old and new methods to achieve a more holistic understanding of Earth processes.

For questions, comments, or speaker suggestions, please contact the GHSC Seminar Committee: Andrea Llenos <allenos at usgs.gov>, Benjamin Murphy <bmurphy at usgs.gov>, Lauren Schaefer <lschaefer at usgs.gov>


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