Effective stress drop and aseismic deformation

Tomas Fischer, & Sebastian Hainzl

Published August 15, 2018, SCEC Contribution #8639, 2018 SCEC Annual Meeting Poster #079

The concept of effective stress drop of a seismic sequence is based on the cumulative seismic moment and area activated by seismic ruptures. The analysis of end-member cases of clustered seismicity shows that the estimated effective stress drop of a cluster is only in agreement with the stress drop of a single event rupturing the same area if no aseismic deformation takes place and rerupturing of asperities occurs during the sequence. The evolution of the cumulative seismic moment release as function of the cluster radius can be used to discriminate different processes: the exponent of seismic moment scaling with radius indicates if the ruptured area is uniformly loaded or whether external loading takes place.

Our analysis of 13 sequences ranging from injection-induced activity to natural swarm and aftershock activity shows standard cubic scaling of the total seismic moment in most cases. Slightly higher exponents in the case of injection-induced sequences are indicative of the ongoing local forcing related to the massive fluid injections during the cluster evolution, while lower exponents down to 1 in the case of creeping events might be related to a decreasing/fractal asperity density.

Three seismicity groups can be distinguished: a normal-stress-drop group of geothermal injections, swarms and mainshock-aftershock sequences, a low-stress-drop group of shale and gas fracking, and the very low-stress-drop case of creeping events. The small effective stress drop can be interpreted by small shear modulus of the rocks, or alternatively, by a large portion of creep in the total slip. Then the seismic events with normal static stress drop would account only for a small portion of deformation.
This is probably the case of hydraulic fracturing. Considering the rigidity of sands and shales is of the same order as of other types of rocks, it appears that during hydraulic fracturing of these formations a high portion of strain is released aseismically.

Key Words
earthquake source; seismic moment; stress drop; aseismic slip

Fischer, T., & Hainzl, S. (2018, 08). Effective stress drop and aseismic deformation. Poster Presentation at 2018 SCEC Annual Meeting.

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