Group B, Poster #106, Earthquake Geology

Evidence of Active Faulting through the Big San Bernardino Mountains

Frank F. Jordan, Jr., Miles H. Wagner, & Jeff Fitzsimmons
Poster Image: 

Poster Presentation

2024 SCEC Annual Meeting, Poster #106, SCEC Contribution #13941 VIEW PDF
The potential for active faulting within and beneath the Big San Bernardino Mountains in San Bernardino and Riverside Counties, California, has been evaluated to understand the risk of seismic hazard faced by mountain communities. The evaluation utilized earthquake focal mechanisms and moment tensors available through the U.S. Geological Survey’s earthquake mapping program. The evaluation included the plotting of over 1,000 earthquake epicenters with corresponding focal mechanism or tensor moment calculations, noting their relative location to known geographic and geomorphic surface features, hypocentral depths, magnitudes, dates, and orientations of fault plane rupture. This mapping was ...conducted in conformance with the California Building Code (CBC). Earthquake epicenters with calculated focal mechanism or moment tensor solutions within the footprint of the Big San Bernardino Mountains between 1981 and 2024 were plotted on the Google Earth Pro GIS platform. Google Earth does not allow for subsurface plotting of the Z coordinate, therefore the hypocentral coordinate could not be depicted. The focal mechanism and moment tensor solutions are used to plot the two unique fault planes associated with each earthquake. Eighty-five percent of the focal mechanisms are primarily strike-slip motions. The strike of the solutions, the dip of each fault plane, and the depth are used to project the fault to the ground surface. Previous mapping identified lineaments in the San Bernardino Mountains (Jordan and Wagner, 2023). This lineament mapping is used to link the hypocenters to potentially active faults recognized at the ground surface. In addition to the recognized Alquist-Priolo method of determining active faults based on subsurface logging and ground surface rupturing, the 2022 CBC §1613A.2 defines active faulting as faults associated with an earthquake (i.e. historic subsurface rupture) or those faults exhibiting a potential for generating an earthquake. The sheer number of earthquakes plotted by the USGS, NEIC, ANSS and Caltech indicated the presence of active faulting in the San Bernardino Mountains. Previous earthquake mapping documented the thousands of earthquakes that have occurred within the footprint of the mountains over various time periods. The addition of earthquake motion solutions to a significant number of epicenters allows for the correlation of active faulting at depth to potentially active faulting mapped at the ground surface.
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Citation: Jordan, Jr., F. F., Wagner, M. H., & Fitzsimmons, J. (2024, 09). Evidence of Active Faulting through the Big San Bernardino Mountains. Poster Presentation at 2024 SCEC Annual Meeting.
Keywords: focal mechanism, moment tensor, California Building Code
Relevant SCEC Themes:
Improving Observations and Closing Data Gaps
Improving Predictive Analyses of Seismicity
Applied Science Implementation