Investigation of the San Gabriel Fault in the Vicinity of Pyramid and Castaic Dams, Los Angeles County, California: Geologic and Seismologic Constraints from Existing
Abstract
Pyramid and Castaic Dams are in an active transpressional deformation zone between the San Gabriel and San Andreas faults. The San Gabriel fault is ~3 km west of both dams, and is an ~80-km long structure. The fault plane geometry exerts a strong control on the calculated ground motion at the dam sites. Geologic studies have characterized the fault activity, geometry, and sense of displacement along the southern San Gabriel fault sections, (i.e., Honor Rancho, Newhall sections), whereas there is more uncertainty along the northern San Gabriel fault section (i.e., Palomas section). In particular, the dip direction and angle of the Palomas section at seismogenic depths are poorly constrained. Existing parameterizations of the Palomas section of the San Gabriel fault geometry range from near vertical orientation with strike-slip displacement to about 60 degree northeast dip with reverse displacement. To better assess the geometry of the San Gabriel fault at seismogenic depths (>5 km), we analyzed existing published oil well logs, available seismicity data, geologic maps, and current fault mapping. Spatial analysis of the seismicity data showed diffuse hypocentral trends that defied discrete fault plane identification. Analyses of focal mechanism solutions indicate fault strike directions in the west-northwest directions, discordant with the strike of the San Gabriel fault near Pyramid Dam. The focal mechanisms better indicate slip transfer to splay structures such as the Holser, Del Valle, and Santa Susana faults that sole into the active San Cayetano fault system to the west. Existing data do not provide sufficient information to refute an easterly dipping San Gabriel fault plane, nor do the data preclude a vertical to near-vertical orientation of the fault near Pyramid Dam. Based on apparent non-deformation of the Pliocene Hungry Valley Formation, the Palomas section of the fault has been proposed as inactive. This study identifies apparent undeformed Quaternary pediment surfaces overlying the mapped San Gabriel fault trace in the vicinity of Pyramid Lake, and an absence of fault-related geomorphic features along strike, consistent with inactivity. Alternatively, the slip rates along this part of the fault simply may be very low (e.g., <0.1mm/yr). Further assessment of the San Gabriel fault in the vicinity of Pyramid Lake will provide significant improvements to the understanding of the seismic hazard in this area.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMNH31C1629P
- Keywords:
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- 7212 SEISMOLOGY / Earthquake ground motions and engineering seismology;
- 4302 NATURAL HAZARDS / Geological