Illuminating the Cascadia Subduction Zone with ultra-long offset Multi-Channel Seismic and Ocean Bottom Seismometer data: Overview on the CAscadia Seismic Imaging Experiment 2021 (CASIE21).

Cascadia is notable among global subduction zones for exceedingly low levels of recorded seismicity from the plate interface for much of the margin. Given the paucity of historical seismicity and lack of deep penetration seismic imaging little is known of the properties of the megathrust and the transitions in slip behavior that bound the seismogenic zone. Further, along-margin variations in slip during past earthquakes is inferred from paleoseismic studies and in modern-day locking status along the megathrust from onshore geodetic studies but the origin of this segmentation is poorly understood. Geologic structure and properties of the subducting Juan de Fuca plate, of the thick folded and faulted accretionary wedge sediments, and of the megathrust fault rocks, could contribute to these along-margin variations. In summer 2021, ultra-long offset Multi-Channel Seismic (MCS) data were acquired during the CAscadia Seismic Imaging Experiment (CASIE21) aboard the R/V Langseth to provide a regional-scale high-resolution characterization of the incoming Juan de Fuca plate, accretionary wedge and megathrust plate interface at Cascadia. A total of ~5300 km of data were acquired extending from 42N, crossing the Gorda plate, to 49.8N offshore Vancouver Island. The survey included 18 primary dip lines spaced 50-75 km apart and oriented perpendicular to the margin from ~50 km seaward of the deformation front to the shelf over areas inferred to be paleo-rupture patches and their boundary zones. Several strike lines were also acquired, including along the continental shelf to investigate segmentation near the possible down-dip limit of the seismogenic zone, and ~10 km west of the deformation front to sample the incoming plate within this region of plate bending and incipient horizontal compression within the sediment section. MCS data were acquired using Langseths airgun array (target volume 6600 cu in) with a 15-12 km long multi-channel seismic streamer. In addition, wide-angle seismic data were acquired using 120 Ocean Bottom Seismometers (OBSs). The OBSs were deployed from RVs Langseth and Oceanus at a 8-13 km spacing along 10 of the dip lines, recording the Langseths MCS soundings. In this presentation an overview on the CASIE21 experiment and preliminary results will be presented.