A Long-Term Earthquake Catalog for the Endeavour Segment: Constraints on the Extensional Cycle and Evidence for Hydrothermal Venting Supported by Propagating Rifts

We use ocean bottom seismometer data from the Endeavour segment of the Juan de Fuca ridge to construct a long-term earthquake catalog for an intermediate spreading rate mid-ocean ridge. We present > 50,000 new earthquake locations for 2016-2021 from the Ocean Networks Canada NEPTUNE cabled observatory and relocate earthquakes from two autonomous networks in 1995 and 2003-2006. The catalog comprises > 85,000 earthquakes located using three-dimensional segment-scale P and S wave velocity models from a prior tomography experiment. Despite the small footprints of networks near the segment center, locations show good agreement with geologic features at segment ends. The improved locations show that the northern Endeavour segment ruptured southwards from 48.3(?)degrees N to 48.05 degrees N during two diking events in early 2005, possibly accompanied by diking on the West Valley (WV) propagator. Persistent off-axis seismicity near the segment center appears to be related to the WV and Cobb propagating rifts which we infer extend similar to 10 km closer to the Endeavour segment center than is apparent in bathymetry. We suggest that the proximity of the propagators to the Endeavour vent fields (VFs) contributes to the localization, vigor, and longevity of the fields by focusing permeability through ongoing fracturing and by limiting extrusive magmatism through degassing of the axial magma lens. Increasing rates of seismicity beneath the VFs beginning in late 2018 and a deepening of earthquakes in 2020 indicate that the central portion of the segment may be entering the later stages of the eruptive cycle. Plain Language Summary Seafloor spreading centers are boundaries where tectonic plates move apart and ocean crust is created by volcanism. Small earthquakes in these settings result from extensional faulting, magma injection, and cooling of the crust by circulating fluids. Seafloor spreading is cyclical: strain steadily builds up as the plates move apart and is relieved episodically when magma is injected into the crust. Earthquake activity must be observed on the order of decades to fully capture this cycle. Until recently, however, batteries have limited earthquake observations from autonomous seafloor seismometers to about a year. We combine observations from a new long-term undersea cabled observatory and older experiments to present an earthquake catalog that spans 20 years for the Endeavour segment of the Juan de Fuca ridge. We improve earthquake locations relative to past studies and relate evolving earthquake activity to seafloor features. Our results suggest that the complicated geometry of nearby plate boundaries, through focused fracture creation, affects how fluids circulate through the crust. Earthquake patterns preceding the last observed magmatic event in 2005 indicate that the increases in earthquake rates and depths we observe in 2018-2020 may be signs of the center of the segment nearing the next spreading event.