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Crustal Stress and Faulting in the Pacific Northwest: Insights From an Updated Focal Mechanism Catalog

Description: 

Characterizing seismic hazard in the Pacific Northwest (PNW) is complicated by a lack of observed moderate magnitude seismicity. This makes it difficult to understand the relationship between mapped crustal faults and the present-day stress field. To address this, we use deep-learning-derived first-motion polarities and S/P amplitude ratios to build an updated catalog of over 10,000 high-quality focal mechanisms from regional microearthquakes. This catalog, spanning Washington, Oregon, and southern British Columbia, reveals several distinct tectonic domains with diverse faulting styles. We use these mechanisms to invert for regional stress orientation, developing high-resolution maps of the crustal stress field along the Cascadia margin. These new maps align with sparse stress observations and GPS-derived strain fields from previous studies. They demonstrate that transpression, characterized by abundant reverse and strike-slip faulting, dominates the stress field in the North American crust. Although the maximum compressive stress is generally oriented north-south, local variability suggests complex interactions between crustal blocks along known structures, such as the southern Whidbey Island fault in Washington State. Finally, comparisons between the focal-mechanism-derived stress and GPS-derived strain fields indicate that locking of the Cascadia subduction zone exerts little influence on contemporary crustal seismicity. These findings clarify how mapped crustal faults accommodate tectonic deformation and provide new insights into the interplay between PNW crustal faults and the Cascadia subduction zone, which is critical for understanding regional seismic hazard.

Session: Linking Subduction Zone Processes and Cascading Hazards in Alaska, Cascadia, Chile and Beyond [Poster]

Type: Poster

Date: 4/16/2026

Presentation Time: 08:00 AM (local time)

Presenting Author: Ian Stone

Student Presenter: No

Invited Presentation: 

Poster Number: 133

Authors