Abstract

The static stress drop is a standard measure of the average decrease of shear stress on a fault during an earthquake. It has been observed that stress drop does not vary significantly with earthquake magnitude and may be regarded as an invariant parameter of the rupture process at different scales. Although typical stress drops of earthquakes range between 1 and 10 MPa, much smaller stress drops in fractions of MPa are reported for slow earthquakes and in some cases also for earthquake swarms. For the latter cases, the effective stress drop was introduced as an alternative parameter that makes use of the cumulative seismic moment and total activated area of the seismic cluster. In this article, we test how the effective stress drop is comparable to the static stress drop of a single earthquake rupturing the same fault portion. To this purpose, we compare the spatiotemporal evolution of the seismic moment release and analyze the uncertainties of the estimated stress drops. We show that the effective stress drop is only comparable to earthquake stress drops in specific cases. In particular, the effective stress‐drop values significantly underestimate the earthquake stress drops in the presence of aseismic deformation. The values are only scale independent if prestress and poststress conditions are uniform in space. Our analysis of data from injection‐induced seismicity, natural earthquake swarms, and aftershock sequences shows that in most cases the effective stress‐drop estimate is rather stable during the cluster evolution. However, slightly increasing estimates for injection‐induced seismicity are indicative for the local forcing of the system. Although the effective stress drops of natural and induced seismicity in geothermal projects range from 0.1 to 3 MPa, those related to fracking in hydrocarbon formations are anomalous low, from 0.08 to 1.8 kPa, which hints to the important role of aseismic deformations.

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