The role of afterslip in the stress interaction between repeating earthquakes and microseismicity in Parkfield

Earthquake can be triggered by small stress changes from local to distant earthquakes, seasonal forcing, and human activities. While the calculated magnitude and sign of stress change greatly varies with the assumption of source stress drop, receiver fault geometry, and consideration of aseismic slip, the near-field stress triggering can be easily misinterpreted. With a large number of repetitive occurrence times, small repeating earthquakes provides a unique opportunity to examine and model the extent to which fault interaction in the form of static stress changes and transient postseismic fault creep produces the observed aperiodicity in the occurrence of these events. Using the 655 repeating earthquakes (repeater) and M>1 4499 earthquakes during the period of 1984 – 2004 (before the M6 Parkfield event), the significant triggering between small earthquakes were previously documented as the increased rate of events producing(incurring) higher stress changes during the days preceding(following) a repeater. However, how to describe the stress in the vicinity of earthquakes has been a challenge especially that (1) the negative static shear stresses could be mistakenly resolved on the closely-located receiver and (2) the role of afterslip induced by very small earthquakes on the stress interactions could be largely underestimated. In this study, we propose the stress model that properly represents the relative three-dimensional location of the events and determine the instantaneous static stress fields associated with each event as well as the time-dependent contributions from afterslip. We hope to provide a better interpretation of the short-term triggering between the closely-spaced small earthquakes on the creeping strands of the SAF.