Ascending fluids and overpressure pulses on faults drive aseismic slip and swarm seismicity

Weiqiang Zhu, Yuyun Yang, Kali L. Allison, & Eric M. Dunham

Submitted July 30, 2020, SCEC Contribution #10225, 2020 SCEC Annual Meeting Poster #146

It is well established that fault damage zones are conduits for fluids, and that changes in pore fluid pressure and flow alter fault strength. Despite this, most earthquake models prescribe pore pressure and neglect the evolution of pressure and transport properties like permeability. Here we consider the classic 2D antiplane shear earthquake sequence model of a vertical strike-slip fault with rate-and-state friction, but now accounting for along-fault fluid flow and pressure evolution. Fluids are sourced at a constant rate from below the seismogenic zone (a proxy for metamorphic reactions and other fluid sources) and ascend along the fault, with overpressure developing over much of the seismogenic zone (Rice, 1992). Permeability increases with fault slip and decreases over a specified time scale due to healing and sealing processes. Permeability evolution and the coupling to fault strength and slip lead to fault valving (Sibson, 1992): Overpressure at the base of the seismogenic builds during the late interseismic period, when the fault has low permeability, weakening the fault and triggering a large earthquake. The discharge of fluids through the high permeability seismogenic zone then depressurizes the fault. Fault valving is most pronounced when the healing/sealing time is somewhat shorter than the recurrence interval of large earthquakes. As the healing/sealing time is further decreased, fluid-driven aseismic slip and overpressure pulses emerge spontaneously at the base of the seismogenic zone and ascend upward. In some cases, these overpressure pulses continue their ascent through the mid-crust, triggering swarm seismicity. For healing/sealing times of order a few years, the fluid-driven aseismic slip fronts have recurrence intervals and slip/event consistent with long-term slow slip events in subduction zones. For longer healing/sealing times, these aseismic slip fronts manifest as fault unlocking in the late interseismic period, as might be happening in the Cascadia subduction zone (Bruhat and Segall, 2017). We are exploring the relevance of our model to these observations of slow slip, fault unlocking, and swarm seismicity, as well as to rapid pressure transmission along faults in induced seismicity sequences.

Citation
Zhu, W., Yang, Y., Allison, K. L., & Dunham, E. M. (2020, 07). Ascending fluids and overpressure pulses on faults drive aseismic slip and swarm seismicity. Poster Presentation at 2020 SCEC Annual Meeting.


Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)