Compaction-induced elevated pore pressure and creep pulsing in California faults

Mostafa Khoshmanesh, & Manoochehr Shirzaei

Published August 1, 2016, SCEC Contribution #6443, 2016 SCEC Annual Meeting Poster #162

The creeping segment of San Andreas Fault (CSAF) is recognized as a weak fault, namely, cannot sustain large earthquake stress drops. Moreover, variable creep rate constrained using kinematic models of geodetic and seismic data implies that the fault frictional strength is both spatially and temporally variable. Intrinsic low friction of fault zone material and locally elevated pore pressure due to ascend of mantle-derived fluid are proposed as possible justifications for CSAF weakness. However, lack of plausible explanation for creep pulsing observed at seismogenic zone in both hypotheses, calls for rethinking of the underlying mechanisms and processes governing the CSAF behavior. Here we provide evidence for the role of pore pressure variation in changing the fault frictional strength, not primarily due to mantle fluids. Using a rate- and state-dependent friction model, we estimate fault frictional properties between 2003 and 2011, and link their apparent temporal variations to undulation of effective normal stress. Since there is no evidence that tectonic stressing rate varies during this study period, we conclude that the variation of effective normal stress is a result of pore pressure change in the fault zone. We show that temporally variable pore pressure and its inferred spatial heterogeneity correlate perfectly with the variation of surface creep rate obtained using InSAR observations. Furthermore, our analysis of microseismicity suggests that the temporal variation of Gutenberg-Richter b-value and released seismic moment has respectively positive and negative correlation with the pore pressure variations. Our results highlight the role of 3D seal-bounded compartments formed through the compaction of intergranular pore spaces, leading to spatially heterogeneous elevated pore pressure and initiation of accelerated creep events. Frictional dilation due to creep acceleration, on the other hand, causes redistribution and reduction of the pore pressure, completing a cycle of creep pulsing. Applying this concept to the creep and microseismicity data along Hayward Fault for period 1995 - 2011, yields similar results, which suggests the observed mechanism is likely a common explanation for creep pulsing in California faults.

Citation
Khoshmanesh, M., & Shirzaei, M. (2016, 08). Compaction-induced elevated pore pressure and creep pulsing in California faults. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Tectonic Geodesy