Constraining maximum event magnitude during injection-triggered seismicity

Abstract

Understanding mechanisms controlling fluid injection-triggered seismicity is key in defining strategies to ameliorate it. Recent triggered events (e.g. Pohang, Mw 5.5) have exceeded predictions of average energy release by a factor of >1000x, necessitating robust methodologies to both define critical antecedent conditions and to thereby constrain anticipated event size. We define maximum event magnitudes resulting from triggering as a function of pre-existing critical stresses and fluid injection volume. Fluid injection experiments on prestressed laboratory faults confirm these estimates of triggered moment magnitudes for varied boundary conditions and injection rates. In addition, observed ratios of shear slip to dilation rates on individual faults signal triggering and may serve as a measurable proxy for impending rupture. This new framework provides a robust method of constraining maximum event size for preloaded faults and unifies prior laboratory and field observations that span sixteen decades in injection volume and four decades in length scale.

Fig. 1. Typical observed fault displacement and cumulative moment over time as induced by fluid injection.

The reactivated laboratory faults are under boundary conditions of (a) constant shear stress and (b) zero displacement. Measured variables and axes are color-coded to show the time evolution of fault fluid pressure, injection volume, fault slip and dilation, instantaneous strength, and cumulative moment magnitude.

Fig. 2. Shear stress drop/relaxation during fluid pressurization.

a Shear stress drop vs. slip displacement for tests ZD1 and ZD2. Experiment ZD1 shows a constant slope during fluid pressurization, while ZD2 features slope changes for the 0.4, 0.7, and 0.9 MPa pressurization steps. Pressurization step at 0.9 MPa for ZD1 (b) and ZD2 (c) are shown for comparison.

Fig. 3. Maximum seismic moment versus total injection volume for fluid-injection-induced earthquakes.

Black line defines the upper limit of the seismic moment for assumed average stress. The black dashed lines represent solution with c values as shown in the lower-right inset. Gray solid lines represent maximum seismic moment and are added for completeness with two different γ values.

Fig. 4. Cumulative moment magnitude versus total injection volume for all fluid pressurization steps in our experiments.

Symbol colors from hot to cold represent an increasing ratio of slip displacement to fault dilation. Specifically, in CSS1, the cumulative moment magnitude exceeds the classic limit when this ratio is >2. Initial stress ratios τ/τp are shown as percentages.