Oceanic plates store water as hydrous minerals through alteration at mid-ocean ridges and ocean basins, and release that water as these minerals break down in subduction zones. The relatively small Juan de Fuca plate allows an opportunity to completely image this process, from ridge to subduction zone thrust. This project enhances a major active-source seismic cruise of the R/V Langseth that will traverse the Juan de Fuca plate during the summer of 2012, extending the active seismic acquisition towards the shore and deploying 6 additional ocean bottom seismometers and 48 onshore seismometers to record the offshore sources. This will allow us to extend the velocity models landward of the trench, covering the locked zone and the up-dip edge of the part of the plate boundary that is characterized by episodic tremor and slip. The recorded data will overlap spatially with two of the densest receiver function transects across a subduction megathrust and will allow us to conduct an integrated analysis of the short-period and broad-band seismic response of thrust zone structure. By sampling along two corridors, one coming ashore in Washington and one in Oregon, we can investigate the hypothesis that subduction zone thrust properties vary along strike due to variable hydration of the oceanic plate, and explore relationships both in the locked zone and in the region of slow slip that may be affected by variations in hydration of the subducting plate. The integrated analysis of active-source with coincident existing passive-source data will place multiple-wavelength constraints on the extent to which thrust zones are characterized by substantial excess pore pressure or thick metasedimentary subduction channels. In addition, off Oregon we propose to shoot 5 shorter lines to enhance and extend the existing 3D coverage of ray paths in a segment of the margin that displays strong evidence of along-strike heterogeneity, including evidence for subducted seamounts that impact interplate coupling. Paleo-seismic data indicate that this segment is a major transition in the recurrence interval of plate boundary earthquakes. These hypotheses are closely related to many of the "Outstanding Questions" in the EarthScope Science Plan for 2010-2020, and address several motivating questions of the GeoPRISMS Subduction Cycles and Dynamics Initiative.
Non-technical Summary
The largest earthquakes on earth arise from the thrust faults of subduction zones, and the most likely place for a magnitude 9.0 earthquake in the 48 contiguous United States lies in the Cascadia subduction zone, just offshore Oregon, Washington, and northern California. Still, little remains known about the behavior of such fault zones, because their physical characteristics cannot be sampled directly at the depths that earthquakes occur. The scarcity of historical earthquakes on the Cascadia thrust zone further clouds our understanding of it. Seismic waves provide the main tool for examining the nature of these faults, allowing to construct images of these regions in a process that is analogous to a combined CAT scan and ultrasound of the human body. Prior studies have indicated an important role for water and sediment in allowing or regulating large earthquakes, and have been interpreted to show that thrust fault zones are either thick regions of very high pore pressure or regions lubricated by sediment. In this study, we take advantage of a previously-scheduled active-source seismic survey just offshore of the Cascadia subduction zone, focused on imaging the incoming Juan de Fuca plate, to gather additional data to sample the structure of the thrust zone. The project includes both offshore and on land seismograph deployments. We will compare the information we obtain on the structure of the fault zone here with images of the incoming plate being obtained farther offshore to see how the plate changes as it enters the subduction zone. We will also compare our results to images obtained previously using much lower frequency earthquake sources and to images from other subduction zones that have recently experienced very large earthquakes (e.g. NE Japan and central Chile). This study will improve our understanding of the relationship between fault zone structure, incoming and upper-plate structure, and earthquake hazards in the region. The project also emphasizes student training across a broad spectrum of field acquisition and seismic analysis techniques.
