--> Abstract: High Angle Faulting in the Western Sacramento Delta Region, Pittsburg, California, by J. McCarthy, P. E. Hart, and D. Oppenheimer; #90958 (1995).

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Abstract: High Angle Faulting in the Western Sacramento Delta Region, Pittsburg, California

J. McCarthy, P. E. Hart, D. Oppenheimer

In an effort to identify and map the geometry of the faults of the San Francisco Bay Area, the U.S. Geological Survey has been conducting marine seismic reflection surveys in the shallow waters of the Bay and western Sacramento Delta. Both shallow-penetrating acoustic profiles and deeper-penetrating multichannel seismic reflection data have been acquired. These variably penetrating images provide evidence for faulting and deformation beneath the Sacramento River in the vicinity of Pittsburg, California. The shallow-penetration data reveal a 1.0-km-wide zone of 3- to 5-degree west-dipping strata unconformably overlain by a 30- to 40-m-thick package of westward-thickening prograding sediments. The localized nature of these dipping strata and the prominent angular unconformi y that separates them suggest a recent episode of tilting and erosion. The deeper seismic reflection profiles extend this zone of deformation downward to at least 4 km depth.

The faulting and deformation in the vicinity of Pittsburg is also associated with unusually deep seismicity, ranging between 15 and 28 km. This deep seismicity, when linked to the seismically defined fault zone in the upper crust, suggests that the fault is nearly vertical and extends through the entire crust. Across the Sacramento River the seismicity is broadly distributed and may be associated with a small right step in the fault. Focal mechanisms indicate predominantly strike-slip faulting.

The earthquake activity near Pittsburg extends at least 25 km north and ~5 km south of the Sacramento River. To the north, in the vicinity of Kirby and Potrero Hills, the seismicity is coincident with the mapped Kirby Hill fault. To the south it dies out near the northern limit of the Kirker fault at the northern edge of the Diablo Range.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California