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Continuous strain measurements during and preceding episodic creep on the San Andreas Fault


  • M. J. S. Johnston,

  • A. C. Jones,

  • W. Daul


Continuous strain measurements from 3 three-component invar wire strainmeters installed 1200, 1500, and 1700 m from the San Andreas fault indicate no observable strain change at the instrument resolution (<10−8) during 10 episodic creep events on the fault. These strain observations indicate that the slip area responsible for the creep observations is near surface and of quite limited extent. The episodic creep character probably results from the failure properties of near-surface materials rather than general fault behavior, which is better indicated perhaps by averaged creep. Deeper slower slip apparently loads the surface material. Longer-term strain changes (∼10−7) do occur, but the form of the signal is not what would be expected from simple models, nor is it consistent, for successive events. The amplitude does not increase with creep event amplitude, and similar changes occur without creep events. Deeper slip on the San Andreas fault apparently is smoother than would be inferred from the duration of episodic creep observations. Unfortunately, signal discrimination capability gets worse at longer periods and needs improvements if slow deformation waves are to be detected at strain levels below 10−7.

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