Stress sensitivity of fault seismicity: A comparison between limited-offset oblique and major strike-slip faults

We present a new three-dimensional inventory of the southern San Francisco Bay area faults and use it to calculate stress applied principally by the 1 989 M = 7.1 Loma Prieta earthquake and to compare fault seismicity rates be fore and after 1989. The major high-angle right-lateral faults exhibit a di fferent response to the stress change than do minor oblique (right-lateral/ thrust) faults. Seismicity on oblique-slip faults in the southern Santa Cla ra Valley thrust belt increased where the faults were unclamped. The strong dependence of seismicity change on normal stress change implies a high coe fficient of static friction. In contrast, we observe that faults with signi ficant offset (>50-100 km) behave differently; microseismicity on the Haywa rd fault diminished where right-lateral shear stress was reduced and where it was unclamped by the Loma Prieta earthquake. We observe a similar respon se on the San Andreas fault zone in southern California after the Landers e arthquake sequence. Additionally, the offshore San Gregorio fault shows a s eismicity rate increase where right-lateral/oblique shear stress was increa sed by the Loma Prieta earthquake despite also being clamped by it. These r esponses are consistent with either a low coefficient of static friction or high pore fluid pressures within the fault zones. We can explain the diffe rent behavior of the two styles of faults if those with large cumulative of fset became impermeable through gouge buildup; coseismically pressurized po re fluids could be trapped and negate imposed normal stress changes, wherea s in more limited offset faults, fluids could rapidly escape. The differenc e in behavior between minor and major faults may explain why frictional fai lure criteria that apply intermediate coefficients of static friction can b e effective in describing the broad distributions of aftershocks that follo w large earthquakes, since many of these events occur both inside and outsi de major fault zones.