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S.S. Schulz

Publications and source records attributed to S.S. Schulz.

5 recordsLinked to original sources
USGS1988

The response of creeping parts of the San Andreas fault to earthquakes on nearby faults: Two examples

Rates of shallow slip on creeping sections of the San Andreas fault have been perturbed on a number of occasions by earthquakes occurring on nearby faults. One example of such perturbations occurred during the 26 January 1986 magnitude 5.3 Tres Pinos earthquake located about 10 km southeast of Hollister, California. Seven creepmeters on the San Andreas fault showed creep steps either during or soon after the shock. Both left-lateral (LL) and right-lateral (RL) steps were observed. A rectangular dislocation in an elastic half-space was used to model the coseismic fault offset at the hypocenter. For a model based on the preliminary focal mechanism, the predicted changes in static shear stress on the plane of the San Andreas fault agreed in sense (LL or RL) with the observed slip directions at all seven meters; for a model based on a refined focal mechanism, six of the seven meters showed the correct sense of motion. Two possible explanations for such coseismic and postseismic steps are (1) that slip was triggered by the earthquake shaking or (2) that slip occurred in response to the changes in static stress fields accompanying the earthquake. In the Tres Pinos example, the observed steps may have been of both the triggered and responsive kinds. A second example is provided by the 2 May 1983 magnitude 6.7 Coalinga earthquake, which profoundly altered slip rates at five creepmeters on the San Andreas fault for a period of months to years. The XMM1 meter 9 km northwest of Parkfield, California recorded LL creep for more than a year after the event. To simulate the temporal behavior of the XMM1 meter and to view the stress perturbation provided by the Coalinga earthquake in the context of steady-state deformation on the San Andreas fault, a simple time-evolving dislocation model was constructed. The model was driven by a single long vertical dislocation below 15 km in depth, that was forced to slip at 35 mm/yr in a RL sense. A dislocation element placed in the seismogenic layer under XMM1 was given a finite breaking strength of sufficient magnitude to produce a Parkfield-like earthquake every 22 years. When stress changes equivalent to a Coalinga earthquake were superposed on the model running in a steady state mode, the effect was to make a segment under XMM1, that could slip in a linear viscous fashion, creep LL and to delay the onset of the next Parkfield-like earthquake by a year or more. If static stress changes imposed by earthquakes off the San Andreas can indeed advance or delay earthquakes on the San Andreas by months or years, then such changes must be considered in intermediate-term prediction efforts. ?? 1988 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗
USGS1987

Temporal changes in microseismicity and creep near Parkfield, California

The 25-km-long section of the San Andreas fault near Parkfield, California ruptured in similar magnitude-6 earthquakes in 1881, 1901, 1922, 1934 and 1966. On the basis of a number of lines of seismological evidence, a section of the San Andreas fault, now termed the Parkfield preparation zone, has been identified as the locus of the next Parkfield earthquake 1,3 . Here we describe coincident changes in surface creep rates and deep seismicity near the Parkfield preparation zone following the 2 May 1983 Coalinga earthquake, and suggest that both respond to the same stimuli. These changes were concentrated near the point of initiation of the magnitude-6 characteristic Parkfield earthquakes, lending credence to the hypothesis that this section of the San Andreas fault is characterized by a unique set of physical properties 4–6 which make it ideal for earthquake prediction studies and which may also be useful for identifying hypocentral regions in other areas.

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