Hypocenter‐Based 3D Imaging of Active Faults: Method and Applications in the Southwestern Swiss Alps.

Despite the fact that earthquake occurrence can be strongly influenced by the architecture of pre‐existing faults, it remains challenging to obtain information about the detailed subsurface geometries of active fault systems. Current geophysical methods for studying such systems often fail to resolve geometrical complexities at sufficiently high spatial resolutions. In this work, we present a novel method for imaging the detailed 3D architectures of seismically active faults based on high‐precision hypocenter catalogs, using nearest neighbor learning and principal component analysis. The proposed approach enables to assess variations in fault instabilities and kinematics. We apply the method to the relatively relocated St. Léonard (max. ML = 3.2) and Anzère (max. ML = 3.3) microearthquake sequences in the Southwestern Swiss Alps, revealing strike‐slip fault systems with interconnecting stepovers at depths of 3–7 km and lengths ranging from 0.5 to 2 km. In combination with additional information about fault instabilities and kinematics, we observe significantly reduced earthquake migration velocities and fault locking processes within the stepovers. Understanding such processes and their role in the propagation of strain across stepovers is of great relevance, as these structures can potentially limit earthquake ruptures but also represent possible locations for the nucleation of larger ruptures. Our proposed method is expected to be broadly useful for further applications such as monitoring hydraulic fracture stimulations or geothermal exploration of natural, fluid‐bearing faults. Conducting similar high‐resolution spatiotemporal analyses of microseismic sequences has the potential to greatly enhance our comprehension of how the 3D fault architecture impacts seismogenic fault reactivation. Plain Language Summary: Earthquakes commonly occur on planar geological structures, called faults. Profound knowledge of the presence and geometries of earthquake‐generating faults is crucial to understand the regional earthquake hazards. Especially for regions with distributed and rather small earthquakes, it is however challenging to detect such faults, because of the fact that these faults often do not reach the surface. Here, we present a new method that uses the locations of small earthquakes to resolve the geometries of earthquake‐generating faults at depth. This is only possible due to the recent advances in earthquake location methods, which can reduce the uncertainties of the locations from the kilometers down to some tens of meters for precise measurements. With our method, we image the complex geometries of two fault systems in the Southwestern Swiss Alps at high resolution. The knowledge obtained by applying our method cannot only help to detect previously unknown, potentially hazardous earthquake‐generating faults but might also improve our understanding of earthquake processes in general. Key Points: We present a novel approach that uses relocated hypocenters to image 3D geometries and instabilities of active faultsApplication to two earthquake sequences in the Southwestern Swiss Alps provides detailed insights into these strike‐slip faults systemsThis study documents the dynamics of earthquake‐migration processes across stepover faults at high resolution [ABSTRACT FROM AUTHOR]