Microearthquakes on the Endeavour Segment, Juan de Fuca Ridge: A Near Real Time Earthquake Catalog for the ONC Cabled Observatory and a Reanalysis of Historical Data

University Of Washington, Seattle WA

Investigators

Abstract

Midocean ridge earthquakes, while not as large and damaging as subduction zone earthquakes, are nevertheless important for understanding the geological processes that form ocean crust. These earthquakes arise due to motion on faults associated with tectonic plate extension or from magmatic activity associated with seafloor volcanic processes. Earthquakes are also generated by crustal cracking that results from hydrothermal cooling of ocean crust. Ocean Networks Canada have operated a cabled observatory at the Endeavour midocean spreading center segment of the Juan de Fuca Ridge off the Pacific Northwest coast since 2011. Initially, the cabled observatory supported a variety of instruments in two hydrothermal vent fields, but just a single seismic station. In 2016, the seismic network expanded to four stations and several more are being added in summer 2018, together with more instruments to monitor the hydrothermal vent fields. This study will create an earthquake catalog for the Endeavour cabled observatory that will update in near real time. Seismicity from two prior experiments with autonomous ocean bottom seismometers will be relocated with three-dimensional seismic velocity models from recent a tomography. The scientific goals are to characterize how the seismicity has evolved as extensional stresses have accumulated since the segment last ruptured volcanically two decades ago, improve the interpretation of two complex earthquake swarms in 2005, constrain the linkages between hydrothermal seismicity and heat uptake above the magma chamber, and provide a public earthquake catalog that can be used by researchers to investigate temporal linkages between seismicity and variations in the hydrothermal fields. In terms of broader impacts, this project will train a postdoctoral researcher and foster an international collaboration with Canadian scientists studying the Endeavour hydrothermal fields. Long term observatories are an important tool for capturing the seafloor spreading events that form the upper oceanic crust and for understanding the evolution of hydrothermal systems and the chemosynthetic biological communities they support. About 1 meter of extensional strain has accumulated since the last non-eruptive diking event on the central Endeavour segment in 1999, and the rates of seismicity and level of tidal triggering are likely to evolve as the current spreading cycle approaches its culmination. The two swarms that occurred in 2005 on the northern Endeavour segment and West Valley propagator were difficult to interpret because of large systematic location errors. Relocating the earthquakes with an accurate three-dimensional model and the double-difference technique will allow earthquakes to be associated with morphological features and facilitate a better understanding of the magmatic and tectonic contributions to these swarms. Existing locations for the earthquakes above the axial magma chamber have absolute errors of several hundred meters and relocating these events with an accurate three-dimensional velocity model will contribute to the understanding of hydrothermal heat uptake by constraining the position of earthquakes relative to the axial magma chamber. It may also resolve changes in the configuration of the hydrothermal system as a result of deepening of the hydrothermal system as heat is mined from the axial magma chamber and/or the injection of fresh magma. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.