Presentation by Lynn R. Sykes, Higgins Professor Emeritus
Title:
Decadal Forecasts of Large Earthquakes along Northern San Andreas Fault, California: Increased Activity on Nearby Creeping Faults Prior to Major and Great Events
Abstract:
The three largest earthquakes in northern California since 1849 were preceded by accelerating activity for moderate-size shocks. Each also was preceded by decadal earthquake activity on surrounding nearby faults. Accelerating seismicity, double-difference precise locations of earthquakes since 1968, geodetic data and fault offsets for the 1906 great shock are used to re-examine the timing and locations of possible future large earthquakes. The physical mechanisms of regional faults like the Calaveras, Hayward and Sargent, which exhibit creep, differ from those of the northern San Andreas, which is currently locked and is not creeping. Much decadal forerunning activity occurred on creeping faults. Moderate-size earthquakes along those faults slowly became more frequent as stresses in the region increased in the latter part of the cycle of stress restoration for major and great earthquakes along the San Andreas. They may be useful for decadal forecasts. Yearly to decadal forecasts, however, are based on only a few major to great events. Activity along closer faults like that in the two years prior to the 1989 Loma Prieta shock need to be examined for possible yearly forerunning changes to large plate boundary earthquakes. Geodetic observations are needed to focus on identifying creeping faults close to the San Andreas. The distribution of moderate-size earthquakes has not increased significantly since 1990 compared to what took place in the decades prior to the three major historic earthquakes in the region. It is now clear from a re-examination of the 1989 mainshock that the increased level of moderate-size shocks in the one to two preceding decades occurred on nearby East Bay faults. Double-difference locations of small earthquakes provide structural information about faults in the region especially their depths. The northern San Andreas fault is divided into several strongly coupled segments based on differences in seismicity.
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