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Masters Thesis

Offset of a mid-holocene alluvial fan near Banning, CA: constraints on the slip rate of the San Bernardino strand of the San Andreas fault

Geologic slip-rate studies are essential for understanding the kinematics and estimating the seismic hazard of active faults and fault systems. The southern Big Bend region of the San Andreas fault, centered at San Gorgonio Pass, is one of the most structurally complex region of the fault complexities of the San Andreas fault where, at a contractional left step, the surface trace of the San Andreas fault splays into a family of structures. In addition, other fault systems - the San Jacinto fault and the Eastern California Shear Zone - merge with the San Andreas fault to the northwest and southeast of the Pass region, respectively. How slip transfers through the southern Big Bend region remains an open question. This slip-rate study reports findings from the San Bernardino strand of the San Andreas fault at Bun-o Flats, near Banning, in the center of the southern Big Bend region. Here, the fault offsets a mid-Holocene all uvial fan and has produced a northeast-facing scarp, indicating up-on-the-southwest separation. The fault strikes N50�W and dips moderately SW, making it a right-reverse oblique slip fault. Assuming a N45�W slip vector for the fault, the strike- to dip-slip ratio is estimated to be ~8: 1. Vertical separation of the alluvial fan surface (2.0 � 0.8 m) and a subsurface contact exposed in trenches (2.6 � 0.9 m) can therefore be converted to estimate strike-slip displacements of 16 � 6 m and 20 � 7 m, respectively. Direct estimates for st1ike-slip displacement of 22 � 14 m and 15 � 4 mare obtained from the reconstruction of a subsurface lithologic contact and the lateral offset of a piercing line, the pinch-out of a lithologic unit, respectively. Radiocarbon ages of the alluvial fan units indicate that fan deposition ceased from 4300 to 4000 yrs BP, and yield four independent slip rates of 4.0 � 1.5, 5.5 �1.5, 6.0 � 5.0, and 3.7 � 1.1 mm/yr, respectively. The mid-Holocene to present average slip rate for the San Bernardino strand of the San Andreas fault is therefore about 4.5 � 2.5 mm/yr. This slip rate agrees with current geodetic models for slip on the San Andreas fault at San Gorgonio Pass. If accurate, the geologic and geodetic rates indicate that the San Andreas fault slips 20 to 25% slower than rates determined 60 km to the northwest and southeast, at Cajon Pass and near Indio, respectively. Greatly reduced slip on the San Andreas fault at San Gorgonio Pass, at the center of the southern Big Bend region, requires that the 'missing slip' is somehow transferred to other active structures in the region, like the San Jacinto and the Eastern California Shear Zone. Furthermore, the relatively low slip rate for the San Andreas fault in the Pass region suggests less frequent and/or smaller slip per earthquake for the fault here, reducing the seismic hazard posed by the San Andreas fault in the Pass region to less than previously thought.

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