HR: 0830h
AN: T11D-0426    [PDF]
TI: Structures in the Hayward and Calaveras Fault Zones Inferred From Relocated Seismicity
AU: * Simpson, R W
EM: simpson@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Graymer, R W
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Jachens, R C
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Ponce, D A
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Wentworth, C M
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AB: Sixteen years of earthquakes recorded by the Northern California Seismic Net (NCSN) between 1984-2000 and relocated using the Double-Difference technique (Ellsworth and others, Eos, 2000) reveal a number of intriguing structures within the Hayward and Calaveras fault zones and a complex relationship between the main fault surfaces and secondary oblique or reverse faults. As suggested by Waldhauser and Ellsworth (2002) and Manaker and Michael (2003), the central Calaveras fault appears to connect to the Hayward fault through a volume bounded on the north by the Mission seismicity trend. As defined by seismicity below 6 km depth, the connection appears quite simple, involving several straight, near-vertical segments forming a restraining bend along the Mission seismicity trend. Above 6 km the connection is complex and not well-defined by seismicity. At the Earth's surface, the mapped creeping trace of the southern Hayward fault deviates from the eastward diverging seismicity along the Mission trend, suggesting that the fault dips to the east in this vicinity to connect with the deeper seismicity. The mapped active central Calaveras fault trace is not very linear, has several strands in many places, and deviates by 2-3 km from the upward projection of a planar fault surface at depth, beautifully delineated by seismicity. The mapped surface traces of the two faults do not connect, but run subparallel about 5 km apart for 25 km. The area of overlap is characterized by mapped reverse and oblique faults. At one location in the overlap region, seismicity suggests a dipping fault that appears to cut and offset horizontally a near-vertical Calaveras fault plane, raising the interesting possibility that seismogenic asperities are being created as the strike-slip and reverse/oblique systems interact. Intersections of this sort between two active fault systems might well explain the linear streaks of repeating microearthquakes that have been noted in a number of studies. One unanswered question is how such complex, interacting fault systems evolve in time, and whether the active creation of asperities might affect the temporal behavior of earthquakes on a fault, as may have occurred if the 1983 Coalinga event led to changes in the geometry of the San Andreas fault surface near Parkfield.
DE: 7230 Seismicity and seismotectonics
DE: 8010 Fractures and faults
DE: 8150 Plate boundary--general (3040)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting