Forearc Basins Complexity and Evolution in Controlling Earthquake Rupture Segmentation of the Cascadia Subduction Zone

Abstract

The Cascadia subduction zone is a prominent seismic risk for North America. The primary objective is to comprehend the long-term behavior of the subduction zone by examining the structural and stratigraphic changes documented in the Cascadia forearc basin. I utilized long-offset multichannel seismic reflection profiles of the Cascadia Seismic Imaging Experiment 2021 (CASIE21) to interpret stratigraphic units and structural features in the region. The CASIE21 dataset comprises approximately 5,500 km of seismic data collected in a quasi-regular grid encompassing margin-crossing and parallel profiles. I interpreted the seismic sections and identified key geological horizons, including the base of the forearc basin, a major unconformity, strike-slip faults, and normal faults, through analysis of reflection characteristics and integration with legacy seismic data. I focused on studying the extent and inactive periods of the forearc basin by analyzing a regional upper Miocene angular subaerial erosional unconformity, associated significant faults, and the tectonic evolution of the basins. The results cover multiple forearc basins with a maximum thickness of ~4.85 km, and the basins formed after the upper Miocene unconformity are ~1.9 km thick. The greatest depth for this unconformity is near the Newport syncline, with the northwest portion reaching ~2.5 km. I propose that the upper plate’s geological complexity, compositional heterogeneity, and structures, such as the Siletz terrane, influence how the forearc crust reacts to subduction, ultimately affecting segmentation and coupling zones, rupture lengths, and propagation.