Widespread non-double-couple earthquakes revealed by a dense seismic network in Tottori, Japan

Understanding how small earthquakes rupture in fault systems is central to revealing the physical processes that govern faulting in the Earth’s crust. These events are commonly assumed to be dominated by simple shear slip, producing earthquake sources that can be described by classical double-couple models. This assumption underpins many interpretations of seismicity but has rarely been tested using comprehensive source representations for large numbers of small events. Here we demonstrate that small earthquakes in a well-instrumented fault system systematically depart from double-couple behavior. By applying full earthquake source inversions to thousands of events recorded by a dense seismic network in western Japan, we identify widespread non-double-couple rupture patterns that cluster spatially and exhibit combined shear and opening motions. These observations indicate that non-double-couple processes are a common feature of small-earthquake rupture rather than rare anomalies. Our findings suggest that fluid-related processes, structural complexity and rock damage within fault zones could influence earthquake nucleation and rupture growth. These findings provide further insights into the complexity of small earthquakes in fault systems and their implications for earthquake rupture physics.