Investigating the Effects of Different Pore Pressure Perturbation Scenarios on Short- and Long-term Fault Response

Abstract:

Recent rapid increase in induced seismicity is shown to be closely related to fluid injection into the subsurface. In this study, through numerical modeling, we aim to address the effect of various injection parameters on both the short- and long-term responses of a fault. We use a 2-D fully dynamic rate-and-state frictional model of a fault with a velocity-weakening region embedded inside a large velocity-strengthening region to simulate long-term seismic cycles of over 2000 years, with fluid injection emulated as pore pressure perturbations on the fault. We test the response of the fault under different injection scenarios by varying the duration, rate of pressure change, and magnitude of the imposed pore-pressure perturbation.

Preliminary results show intricate fault responses that vary from entirely aseismic to a complex sequence of triggered events and aseismic transients. In particular, we find several intriguing observations: (1) The extent of perturbation on the timing and sources of triggered events is more sensitive to the maximum value of pore pressure reached on the fault and the rate of pressure change than the cumulative pore pressure. (2) Pore-pressure perturbation tends to trigger a cluster of events with a wide range of magnitude that occur very close in time, with the biggest event preceded by smaller precursors. (3) In some cases, a long quiescence occurs after the triggered cluster, up to a few times of the average recurrence interval before the next earthquake occurs. (4) The effect of perturbation is apparent in subsequent seismic cycles after the quiescence, with the next few earthquakes being advanced or delayed. Occasionally the interseismic period is changed permanently.

Our current work focuses on analyzing the characteristics of triggered events in our simulations, including their magnitude, stress drop and slip distribution inside the seismogenic region. We will also quantify the relative importance of maximum pore pressure, the rate of pressure change, and the presence of aseismic slip. Our goal is to provide better constraints on the maximum possible earthquake magnitude considering all the factors above and the potential interactions among triggered events.