HR: 0800h
AN: S21A-0241    [Abstracts]
TI: Geophysical Investigations Along the Hayward Fault, Northern California, and Their Implications on Earthquake Hazards
AU: * Ponce, D A
EM: ponce@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025, United States
AU: Graymer, R W
EM: rgraymer@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025, United States
AU: Hildenbrand, T G
EM: tom@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025, United States
AU: Jachens, R C
EM: jachens@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025, United States
AU: Simpson, R W
EM: simpson@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025, United States
AB: Geophysical studies indicate that the Hayward Fault follows a pre-existing basement structure and that local geologic features play an important role in earthquake seismicity. The recent creeping trace of the Hayward Fault extends for about 90 km from San Pablo Bay in the northwest to Fremont in the southeast, and together with its northern extension, the Rodgers Creek Fault, is regarded as one of the most hazardous faults in northern California. The Hayward Fault is predominantly a right-lateral strike-slip fault that forms the western boundary of the East Bay Hills and separates Franciscan Complex rocks on the southwest from Coast Range Ophiolite and Great Valley Sequence basement rocks on the northeast. The Hayward Fault is characterized by distinct linear gravity and magnetic anomalies that correlate with changes in geology, structural trends, creep rates, and clusters of seismicity. These correlations indicate the existence of fault-zone discontinuities that probably reflect changes in mechanical properties. These fault-zone discontinuities may play a role in defining fault segments—locations where recurring seismic ruptures may tend to nucleate or terminate. Along the central part of the Hayward Fault, a prominent gravity and magnetic anomaly correlates with an exposed gabbro body, the San Leandro gabbro. Modeling of these anomalies reveals that the San Leandro gabbro is much more extensive in the subsurface than the outcrop pattern suggests, extending to a depth of about 6-8 km. The inferred extent of the San Leandro gabbro, it's geologic setting, and associated seismicity suggest that the Hayward Fault evolved from a pre-existing basement feature, similar to the ancestral Coast Range Fault. Combined modeling and relocated double-difference seismicity data indicate that the dip of the fault surface varies from near vertical in the north to about 75 degrees in the central part to about 50 degrees in the south near Fremont and ultimately connects with the central Calaveras Fault. A seismicity cluster along the western edge of the San Leandro gabbro and a bend in the fault associated with the gravity and magnetic high along the gabbro body suggests that this mafic body influences fault geometry and behavior, and may serve as a nucleation point for large earthquakes on the fault.
DE: 1219 Gravity anomalies and Earth structure (0920, 7205, 7240)
DE: 1517 Magnetic anomalies: modeling and interpretation
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7299 General or miscellaneous
DE: 8123 Dynamics: seismotectonics
SC: Seismology [S]
MN: 2007 Fall Meeting