--> Abstract: New Evidence of Large Surface-Rupturing Earthquakes Along the Northern Hayward Fault, by J. J. Lienkaemper, P. L. Williams, P. Taylor, and K. Williams; #90958 (1995).

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Abstract: New Evidence of Large Surface-Rupturing Earthquakes Along the Northern Hayward Fault

James J. Lienkaemper, Patrick L. Williams, Pamela Taylor, Kenneth Williams

It has been widely assumed that the large earthquake that occurred in the San Francisco Bay area in 1836 did have its source on the Hayward fault. In our trench in Montclair Park in Oakland, we found Physical evidence of a large rupture that occurred soon enough before the 1850s that the fault rupture zone was still extremely porous. A black organic-rich fluvial horizon interpreted to be formed by logging slash in the 1850s percolated its dark coloring easily into the recently fissured and sheared older soil below it. Although a quantitative estimate of recency is not possible, we judge that several decades of infilling of the fault zone pore space would probably have prevented the extreme level of percolation in evidence. Thus, we believe that the most likely interpretat on is that the rupturing event occurred only a few years or decades before the unusual deposition of the 1850s, and the 1836 earthquake is the most reasonable candidate in the early historic period.

At least one and probably a second prior large surface-rupturing event was in evidence in our trench. Unfortunately we found no materials for radiocarbon dating. Evidence of a sizeable amount of 1868 slip is suggested by vertical separations of the 1850s horizon that do not appear in the younger (interpreted to be turn-of-the-century-aged) deposits. The inferred 1868 slip we deduce to be a few decimeters in excess of the expected post-1836 accumulation of creep. The 1868 slip could be either coseismic, triggered or accelerated creep. We cannot distinguish how it occurred, but the existence of a discrete offset in the sediments seems to suggest very rapid slip.

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