Measurements of the strain field associated with episodic creep events on the San Andreas Fault at San Juan Bautista, California

Over the past 6 years, repeated strain steps recorded on a tensor strainmeter installed close to the San Andreas fault trace at San Juan Bautista in California correlated closely with episodic creep events registered on a nearby creepmeter. The strain events were remarkably similar in character, were of about 1 hour duration, and were followed within hours to days by creep events of 2–6 days duration. The episodic slip of a source region from 200 m to 500 m in depth and at most a few kilometers in length is consistent with the observed strain and creep data. We postulate that the additional stress transferred to this region by the Loma Prieta earthquake is giving rise to an increased rate of accumulation of shear strain around a locked patch beneath the creep source region, thus accounting for the increased frequency of the creep/strain events following the Loma Prieta earthquake.