![]() The overlapping seismic waveforms of these two events raised many questions, including (1) how was slip partitioned between these two ruptures, (2) what were the relative orientations of the two faults, and (3) what was the mechanism for triggering rupture on the second fault? With the recent improvements in geophysical monitoring and observations, we can begin to untangle such complex earthquakes.Īlthough aftershock distributions can be used to infer faulting geometry, this geometry is largely obscured because of uncertainties in routine locations. ![]() Routine amplitude-based magnitudes indicated the onshore rupture was larger, and therefore the classically defined “mainshock”, but paradoxically, moment-tensors indicated that the event’s overall style of slip matched the MF orientation. However, analysis by the Northern California Seismic Network (NCSN) and the NEIC quickly identified that the waveforms contained two distinct P-wave arrivals, best explained by two separate ruptures: one on the offshore, interplate MF followed ~11 s later by slip ~30 km east on an onshore fault within the subducting Gorda slab (Fig. The tectonic complexity of the region was illustrated by the December 20, 2021, event, first reported by the USGS National Earthquake Information Center (NEIC) as a single moment magnitude (Mw) 6.2 earthquake. Together these features create a zone of active and complex seismicity. In fact, the Pacific Plate’s northward motion relative to the Gorda Plate creates substantial internal deformation within the Gorda Plate itself, largely accommodated by offshore strike-slip faulting 2. The nature of deformation at the MTJ defies the simplifying assumption of plate tectonic theory that relative motion of the Earth’s crust is accommodated by slip on the boundaries between otherwise rigid, undeforming plates. This intersection, known as the Mendocino Triple Junction (MTJ), connects two of the largest and highest-hazard fault systems in North America 1 the Cascadia subduction zone to the north and the San Andreas transform fault system to the south. At its eastern terminus, three tectonic plates meet: the Gorda, North America, and Pacific plates. The MF is a roughly east-west right-lateral strike-slip fault that demarks the boundary between the Gorda Plate to the north and the Pacific Plate to the south. On December 20, 2021, regional and national seismic networks reported an M6.2 earthquake near the Mendocino Fault (MF), a strike-slip fault offshore of Ferndale, California. ![]() ![]() This event illustrates that moderate-sized earthquakes can exhibit similar complexity to that more commonly documented for large earthquakes. Thus, this rupture combines features of complex ruptures usually considered to be single earthquakes, and triggered ruptures considered as multiple earthquakes. We show that rupture was dynamically triggered, yet concurrent, on two distinct faults roughly 30 km apart. We combine dense seismic and geodetic measurements to reveal an enigmatic rupture in late 2021 at the Mendocino Triple Junction in northern California. Recent investments in geophysical instrumentation allow us to resolve increasingly complex, multi-fault ruptures for even moderate-sized earthquakes. Earthquake science relies on earthquake catalogs and therefore how complex ruptures get documented and cataloged has important implications. A central question of earthquake science is how far ruptures can jump from one fault to another, because cascading ruptures can increase the shaking of a seismic event.
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