The Cascadia Initiative (CI) community broadband seismic experiment provides an unprecedented opportunity to evaluate the processes of plate formation and subduction and their impact on great earthquake behavior and volcanism in the Pacific Northwest.    The Cascadia subduction zone is formed where the oceanic Juan de Fuca plate plunges beneath the North American plate beneath Washington, Oregon, and Northern California.  The trench just offshore demarks a major earthquake-producing fault zone that runs from the Canadian-US border to Cape Mendocino, CA, and the subduction also produces the line of active Cascades volcanoes.  Variations in temperature and water content in the subducting Juan de Fuca plate and in the mantle beneath the North America plate may have a profound influence on the earthquake occurrence and eruption behavior in the system.  In particular, the fluid content on fault zones have a dramatic effect on their slip behavior, including the ability to produce great earthquakes.  The temperature structure and fluid content of the overriding plate can place a strong control on earthquake coupling, as well as the ability of the mantle to produce magma.  The speed at which seismic waves travel is directly related to both temperature and water content.  The distribution of the Cascadia amphibious-array seismic stations both onshore and offshore enables a systematic characterization of the seismic velocity attributes of the entire subduction system, allowing for a detailed evaluation of the variations in temperature and water content and their impact of faulting behavior and volcanism.

Intellectual merit.  In this project we utilize a seismic imaging analysis designed to both detect and characterize abrupt velocity transitions with depth associate with a fluid-rich subduction-zone fault, as well as provide broad constraints on the temperature of the downgoing slab and overriding plate.  The velocity images suggest that the Juan de Fuca plate cools very abruptly once moving away from the mid-ocean ridge, relatively to other plates of comparable age.  By analyzing signals recorded at offshore stations near the coast, we have mapped for the first time the offshore Cascadia fault that generates great earthquakes. The fault is characterized by low velocities associated with a thin (few-km-thick) fluid-rich zone.  In the region between the subduction-zone trench and the volcanic arc, the velocity structure of North America crust varies along the plate boundary, with low velocities observed in locations where gravity lows and changes in seismic slip also occur.  We speculate that this correlation is caused by locations of an earthquake-prone locked plate boundary inducing a low-velocity, low-density basin in the overriding plate.  In the arc and backarc we see a large variation in seismic velocity, with velocities to the north being systematically higher than velocities in the south.  This variation correlates with magmatic output of the Cascades volcanoes, with eruptions to the south being systematically large than to the north.  We tentatively attribute this variation to large-scale mantle processes associated with the incursion of Basin-and-Range  extension into the Cascades arc region. 

Broader Impacts.  The results of this study provide a framework for understanding the controls on great earthquakes and explosive volcanic eruptions in subduction zones.  The Cascadia zone represents one of the largest natural hazards in the U.S., capable of hosting a M9 earthquake, large tsunamigenic landslides, and site of some of the best-known recent eruptions.  The latest models of these processes are directly dependent on thermal structure and hydration of the downgoing and overriding plates.  This project comprised the PhD research for a graduate student, who has moved on to an excellent Post Doctoral researcher position.  In the process of doing the project, the graduate student developed a new data analysis package that is particularly useful for ocean-bottom seismic data, and she is now producing a manuscript to advertise and distribute this code to the broader seismology community, most likely through github.

Last Modified: 11/10/2017
Modified by: James B Gaherty