GMEG - Geophysics Unit of Menlo Park, CA- (GUMP)
David A. Ponce
Geometry and Evolution of the Hayward Fault
The Hayward Fault, regarded as one of the most hazardous faults in northern California, extends for about 90 km from the Warm Springs District of Fremont in the southeast to San Pablo Bay in the northwest (Fig. 1--Geologic map). Beneath San Pablo Bay the Hayward Fault steps over to the east and continues along the Rogers Creek Fault. The Hayward Fault is a NW trending, predominantly right-lateral strike-slip fault zone that forms the western boundary of the East Bay Hills.
A number of on-going and important issues related to the earthquake hazard potential of the San Francisco Bay area remain unresolved and the Hayward Fault itself is of special interest because the combined earthquake probability of 27% along the Hayward-Rodgers Creek Fault System is the greatest in the region. The Hayward Fault itself is enigmatic in that it both creeps to a depth of about 5 km along its entire length and is capable of producing large earthquakes, evidenced by the October 21, 1868 M6.8 earthquake that was known as the "Great San Francisco Earthquake" prior to the 1906 earthqauke.
It is because of these emerging and sometimes conflicting findings that a study of the Hayward Fault in the vicinity of a gabbro body near San Leandro, a known and quantifiable feature, provides a natural laboratory to advance our knowledge on the geometry and evolution of a major strike-slip fault. Our study deals with the relationship of the active recent trace of the Hayward Fault and the long-term Hayward Fault Zone. Geophysical data along the Hayward Fault are used to investigate the nature and origin of a large gabbro body, hereafter referred to as the San Leandro gabbro. Gravity and magnetic methods are particularly well suited to the study of these rocks because of their unusual physical properties that are, in general, strongly magnetic and more dense than surrounding rocks.
Modeling Subsurface Geology and Structure
Geophysical modeling suggests a much greater horizontal and vertical extent of the San Leandro gabbro than previously reported (Fig. 2). Modeling indicates that the gabbro extends from the westernmost exposures west of the Hayward Fault to the Chabot Fault on the east. The Chabot Fault appears to preferentially follow the eastern boundary of the San Leandro gabbro body. Higher magnetizations and lower densities to the northwest suggest that the causative body may be more altered to the northwest. Modeling also suggests that the uppermost parts of the gabbro may be fractured or weathered, reflected in a lower model density for these parts of the gabbro. Our models reveal that the San Leandro gabbro dips about 75 to 80 degrees to the east and extends to a depth of about 6 to 8 km. Magnetic models also indicate that several moderately magnetic and probably ultramafic rock bodies occur west of the Hayward Fault Zone and below the offshore San Leandro basin. These smaller tabular bodies are probably serpentinite rather than gabbro because they do not have an associated gravity high.
Evolution of the Hayward Fault ZoneThe shape and extent of the San Leandro gabbro, its geologic relationships, and associated seismicity all suggest that the Hayward Fault zone evolved from a pre-existing feature, possibly an ancestral, low-angle roof thrust that ultimately brought oceanic crust to the surface. We recognize that other mechanisms could have produced this pre-exsiting structure that the Hayward Fault preferentially followed, like attenuation-related faults or subduction-related thrusts. However, of these possible mechanisms, only a low-angle roof thrust mechanism requires a relatively flat-lying section of oceanic crust at depth that is observed in the geophysical data.
In late Jurassic time (Fig. 3a), choking of the subduction zone along the western margin of North America with island arc terranes probably caused it to step west of the future site of the Coast Ranges, leaving behind or stranding a relatively flat-lying section of oceanic crust. Alternatively, this oceanic crust may have been obducted onto the North American plate . The westward stepping subduction zone may have moved in a series of steps rather than a single step further complicating and disrupting the stranded oceanic crust . By Late Cretaceous time the Franciscan Complex and the Great Valley sequence were already juxtaposed and the subduction-accretion of the Franciscan Complex resulted in major tectonic crustal thickening. In Paleocene to Eocene (Fig 3b) time this over-thickened crust was attenuated and extended by crustal collapse that may have been related to a change in the velocity or angle of the subducting slab. By Early Tertiary time (Fig. 3c), one or more low-angle wedge thrusts developed and scooped-up ophiolitic and Great Valley Sequence rocks, possibly porpoising in and out of the oceanic crust, forming a discontinuous belt of ophiolitic rocks. Ultimately, these ophiolitic rocks were brought to higher levels in the crust as a result of transpressional plate motions. After repeated episodes of extension and attenuation, our model suggests that the roof thrust was brought to a near-vertical position and combined with thrusting, folding, and strike-slip faulting created the present structure (Fig. 3d). Thus, the Hayward Fault along the westernmost edge of the San Leandro gabbro may once have been similar to the ancestral Coast Range Fault that has since been reactivated as a near-vertical strike-slip fault.
ConclusionsBecause of the geophysical properties of mafic and ultramafic rocks and their occurrence along the Hayward Fault Zone, gravity and magnetic investigations afford us a unique opportunity to resolve the nature, spatial relationship, and evolution of the Hayward Fault Zone. Improved understanding of the three-dimensional geometry and physical properties of the Hayward Fault Zone will provide additional constraints on seismic hazard probability, earthquake modeling, and fault interactions that are applicable to other major strike-slip faults around the world. Geophysical modeling indicates that the San Leandro gabbro is much more extensive in the subsurface than the outcrop pattern suggests. The gabbro body is tabular, dips steeply to the northeast, and extends to a depth of about 6 km or greater. The western boundary of the gabbro possibly represents the westernmost part of the Hayward Fault Zone and the eastern boundary of the gabbro coincides with the Chabot Fault. The seismic cluster pattern along the southwest edge or lower bounding surface of the San Leandro gabbro and a salient in the recent trace of the Hayward Fault along the maximum gravity high associated with the gabbro suggest that the gabbro body plays an important role in the release of seismic energy. The western edge of the San Leandro gabbro probably defined a wedge or roof thrust that ultimately, after multiple episodes of extension and attenuation, evolved into the present near vertical fault. Thus, the three-dimensional geometry of the gabbroic body and its relation to earthquake seismicity suggest that the strike-slip Hayward Fault has preferentially followed a pre-existing feature. The earthquake cluster at the southwestern edge of the San Leandro block at depths between about 3 and 9 km suggests that the block acts as an elastically stiff intrusion with stress build-ups around its periphery. This model might be tested by calculating perturbations to the stress field caused by the gabbro body. In addition, the model implies perturbations to the ambient strain field that might be compared to geodetic measurements and geologic deformation. Is it conceivable that this local stressed volume could be the nucleation or termination point of a new rupture related to a large earthquake? Although the answer to this question is enigmatic, the epicenter of the 1868 earthquake, although poorly constrained is in the region of the San Leandro block.
For Additional Information See The Following Articles:Ponce, D.A., Hildenbrand, T.G., and Jachens, R.C., 2003, Gravity and magnetic expression of the San Leandro gabbro with implications for the geometry and evolution of the Hayward fault zone, northern California: Bulletin of the Seismological Society of America, v. 93, no. 1, 11 p. PDF (13.4 Mb).
Ponce, D.A., Simpson, R.W., Graymer, R.W., and Jachens, R.C., 2004, Gravity, magnetic, and high-precision relocated seismicity profiles suggest a connection between the Hayward and Calaveras Faults, northern California: Geochemistry Geophysics Geosystems, v. 5, no. 7, 39 p. PDF (5.1 Mb).
Or contact the author directly: Dave Ponce, U.S. Geological Survey.
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