Collaborative Research: A New Subsurface Framework for the Cascadia Subduction Zone derived from Integrated Analyses of the CASIE21 Long-offset Multi-channel Seismic Experiment

DESCRIPTION: The Cascadia Subduction Zone in the Pacific Northwest is a region of great national interest and growing public awareness because of its high earthquake and tsunami hazard. The Cascadia margin has low levels of recorded seismicity, which reduces what is known of the properties of the earthquake zone. This project aims to identify variations in the structure and properties of the Cascadia margin to better understand the earthquake and tsunami risk to the Pacific Northwest. This project will work with other groups to build a new 3-dimensional picture for the margin.

From June 1 to July 11, 2021, a regional-scale multi-channel seismic study of the Cascadia Subduction Zone was collected using the state-of-the-art long-offset capabilities of the R/V Marcus G. Langseth as part of the CAscadia Seismic Imaging Experiment 2021 (CASIE21). During the expedition, 5,347 km of high quality 12-15 km streamer data were acquired in a 50-75 km spaced grid of seismic lines spanning the margin from the northern Gorda plate at 42°N to the end of subduction offshore Vancouver Island and from ~50 km seaward of the deformation front to near the coastline. This new survey represents the first regional-scale study conducted at Cascadia, spanning most of the subduction zone and filling important gaps in data coverage. This project utilizes the CASIE21 dataset in order to study nearly the whole Cascadia margin and will yield unique insights into three core questions: (1) What is the geometry of the plate interface along the margin, where is significant sediment subduction occurring, and how do plate interface properties including reflectivity and roughness vary along the margin and down dip? How do plate interface properties relate to incoming plate and upper plate structure and megathrust rupture segmentation? (2) How do the structural style and evolution of the upper plate vary along strike and how are they related to megathrust strength, backstops, and fluid migration? (3) How does the lithostratigraphic architecture of the sediment section, and structure, deformation and hydration potential of the shallow-most oceanic crust of the incoming plate associated with bend faulting, distribution of seamounts and propagator shear zones vary along the margin and how do these properties contribute to wedge evolution and megathrust rupture segmentation? This study will result in construction of a new regional-scale structural interpretation of the offshore Cascadia region from seismic images as a primary product along with select studies making use of reflectivity, velocity models, and strain analysis.

BROADER IMPACTS: Because of the high earthquake and tsunami hazard, the Cascadia subduction zone is a region of great societal interest and growing public awareness. The proposed study will contribute to characterization of the earthquake source regions and offshore margin at Cascadia needed for hazard assessment and will directly benefit other aligned research programs and initiatives. These include the Cascadia CoPes Hub to investigate geohazards and increase resilience, plans for geodetic and borehole instrumentation (proposed to IODP), and for offshore instrumentation for a Cascadia earthquake/tsunami early warning system, in addition to a major multi-year research effort by the USGS involving comprehensive mapping and shallow imaging, paleoseismology, and paleogeodesy. The primary products of our study, including a new structural framework for the offshore margin integrated with MCS-derived velocity models for the sediment section, will be of direct use for analysis of the OBS and land seismometer studies conducted coincident with the CASIE21 survey. Collectively these complementary studies will support development of a new onshore-offshore regional velocity model for much of the Cascadia margin derived from consistent acquisition and a new integrating framework for future studies along the margin. Additionally, our program will significantly contribute to the training of four graduate students (LDEO, UTIG, UW, DAL), will support two postdoctoral scientists (LDEO and UTIG), and three early-career scientists (Boston, Han, Beeson).