Collaborative Research: Imaging the Cascadia Subduction Zone: A Ship-to-Shore Opportunity

DESCRIPTION: 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.

OUTCOMES: The R/V Langseth was used to deploy six new ocean bottom seismometers and 48 onshore seismometers to extend the velocity models landward of the trench.