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Observations show that microseismic events from the same location can have similar source durations but different seismic moments, violating the commonly assumed scaling. We use numerical simulations of earthquake sequences to demonstrate that strength variations over seismogenic patches provide an explanation of such behavior, with the event duration controlled by the patch size and event magnitude determined by how much of the patch area is ruptured. We find that stress drops estimated by typical seismological analyses for the simulated sources significantly increase with the event magnitude, ranging from 0.006 to 8 MPa. However, the actual stress drops determined from the on-fault stress changes are magnitude-independent and similar to 3 MPa. Our findings suggest that fault heterogeneity results in local deviations in the moment-duration scaling and earthquake sources with complex shapes of the ruptured area, for some of which stress drops may be significantly (similar to 100-1,000 times) underestimated by the typical seismological methods.

Plain Language Summary Microseismicity, that is, relatively small earthquake ruptures that occur much more frequently than large, destructive ones, is actively studied to understand properties of seismogenic faults in the Earth's crust. The properties of interest include earthquake durations, sizes, and stress drops that describe how much fault loading an earthquake has relieved. Observations show that microseismic ruptures from the same fault area can have similar source durations but different sizes, violating the commonly assumed scaling between the duration and size. Our numerical simulations of a sequence of ruptures on a fault patch explain such behavior by heterogeneous fault patch properties. We also compare the stress drops for the simulated ruptures obtained directly from our modeling and inferred from the ground motion produced on the surface, as done for natural earthquakes, find significant discrepancies between them, and explain the discrepancies by the complex shapes of the rupture areas of the simulated events.