Fault Coupling Between Seismicity Streaks on Strike-Slip Faults: Using the Separation Distance Between Streaks as an Indicator of Coupling

Many strike-slip faults that are partially coupled exhibit lineations of microseismicity that are oriented nearly parallel to the slip direction of fault movement, referred to as seismicity streaks. The observed streaks are up to a few hundred meters wide in the direction perpendicular to fault movement and anywhere from one to ten kilometers long in the direction parallel to fault movement. Parallel streaks are typically separated by a quiescent zone that is devoid of microseismicity. It has been proposed that streaks represent a linear trend of strong asperities embedded on an otherwise creeping fault, or they delineate the discrete boundary between locked and creeping zones. Many of these zones are too narrow (<2 km) to be resolved from geodetic inversions. Thus direct imaging of their kinematic state is often not possible. We assess the coupling of these quiescent zones by considering the stress accumulation on a narrow fault patch. If a quiescent zone is coupled, then this patch would eventually fail in a time frame dependent on the loading rate and the dimensions of the patch. We focus our analysis on a quiescent zone on the San Jaun Bautista segment of the San Andreas fault. This patch has a down-dip width of 2 km, based on the double-differenced relocated seismicity. Assuming a 20 MPa failure threshold, we calculate that the fault patch should fail within 70 years if it is fully coupled. Historical seismicity records extending back to 1932 suggest that this patch has not failed in a major earthquake. Therefore, for this particular patch on the San Juan Bautista segment of the SAF, seismicity streaks are separated by an uncoupled patch that is creeping. The opposite scenario is exemplified on the Parkfield segment of the San Andreas fault where a large quiescent zone, separated by a pair of seismicity streaks, ruptured in the 2004 Parkfield earthquake. Thus we find evidence for both proposed models of coupling between seismicity streaks. Our calculations demonstrate that the down-dip width of the quiescent zone may be an important diagnostic as to whether it is coupled or uncoupled.