Late Quaternary slip history of the Mill Creek strand of the San Andreas fault in San Gorgonio Pass, southern California: The role of a subsidiary left-lateral fault in strand switching

The San Andreas fault has the highest calculated time-dependent probability for large-magnitude earthquakes in southern California. However, where the fault is multistranded east of the Los Angeles metropolitan area, it has been uncertain which strand has the fastest slip rate and, therefore, which has the highest probability of a destructive earthquake. Reconstruction of offset Pleistocene-Holocene landforms dated using the uranium-thorium soil carbonate and beryllium-10 surface exposure techniques indicates slip rates of 24.1 ± 3 millimeter per year for the San Andreas fault, with 21.6 ± 2 and 2.5 ± 1 millimeters per year for the Mission Creek and Banning strands, respectively. These data establish the Mission Creek strand as the primary fault bounding the Pacific and North American plates at this latitude and imply that 6 to 9 meters of elastic strain has accumulated along the fault since the most recent surface-rupturing earthquake, highlighting the potential for large earthquakes along this strand.

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High-Resolution Seismic Images and Seismic Velocities of the San Andreas Fault Zone at Burro Flats, Southern California

Bulletin of the Seismological Society of America ◽

10.1785/0120060252 ◽

2008 ◽

Vol 98 (6) ◽

pp. 2948-2961 ◽

Cited By ~ 1

Author(s):

C. C. Tsai ◽

R. D. Catchings ◽

M. R. Goldman ◽

M. J. Rymer ◽

P. Schnurle ◽

...

Keyword(s):

High Resolution ◽

Fault Zone ◽

Southern California ◽

San Andreas Fault ◽

Seismic Velocities ◽

San Andreas ◽

Seismic Images

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New geodetic constraints on southern San Andreas fault-slip rates, San Gorgonio Pass, California

Geosphere ◽

10.1130/ges02239.1 ◽

2020 ◽

Author(s):

Katherine A. Guns ◽

Richard A Bennett ◽

Joshua C. Spinler ◽

Sally F. McGill

Keyword(s):

Velocity Field ◽

Southern California ◽

San Andreas Fault ◽

Plate Boundary ◽

Slip Rate ◽

Fault Slip ◽

Slip Rates ◽

San Andreas ◽

Fault Slip Rates ◽

Gps Velocity Field

Assessing fault-slip rates in diffuse plate boundary systems such as the San Andreas fault in southern California is critical both to characterize seismic hazards and to understand how different fault strands work together to accommodate plate boundary motion. In places such as San Gorgonio Pass, the geometric complexity of numerous fault strands interacting in a small area adds an extra obstacle to understanding the rupture potential and behavior of each individual fault. To better understand partitioning of fault-slip rates in this region, we build a new set of elastic fault-block models that test 16 different model fault geometries for the area. These models build on previous studies by incorporating updated campaign GPS measurements from the San Bernardino Mountains and Eastern Transverse Ranges into a newly calculated GPS velocity field that has been removed of long- and short-term postseismic displacements from 12 past large-magnitude earthquakes to estimate model fault-slip rates. Using this postseismic-reduced GPS velocity field produces a best- fitting model geometry that resolves the long-standing geologic-geodetic slip-rate discrepancy in the Eastern California shear zone when off-fault deformation is taken into account, yielding a summed slip rate of 7.2 ± 2.8 mm/yr. Our models indicate that two active strands of the San Andreas system in San Gorgonio Pass are needed to produce sufficiently low geodetic dextral slip rates to match geologic observations. Lastly, results suggest that postseismic deformation may have more of a role to play in affecting the loading of faults in southern California than previously thought.

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