Collaborative Research: A seismic investigation of slow slip and fault locking along the Alaska-Aleutian subduction zone

University Of Washington, Seattle WA

Investigators

Abstract

Some of the greatest earthquake hazard on the planet is located along the Alaska-Aleutian subduction zone, where the Pacific Plate is diving underneath North America. Understanding seismic hazard in this region is complicated by a phenomenon known as slow slip, in which tectonic plates slide past each other without causing damaging earthquakes. Changes in water content and in rock type along the subduction zone are thought to control whether slow slip or fast earthquakes happen, but these properties are not well understood. In addition, there are gaps in seismic data coverage that make it difficult to understand how water and rock type vary along the plate boundary. This study focuses on one of these gaps, in the Cook Inlet region of Alaska. The research team has deployed two types of seismic instruments to record local seismic events and use data from earthquakes around the world to understand the deep structure of the subduction zone. One is a traditional experiment using broadband instruments deployed on land in the Kenai Peninsula. These instruments are placed on the property of volunteers, including two K-12 schools, enabling community participation and outreach. The other experiment uses Distributed Acoustic Sensing (DAS) technology – using fiber optic cables as seismic sensors – along and across the Cook Inlet. The goal of this project is to combine both types of data to model the structure and speed of seismic waves in the Pacific plate and its interface with the North American plate. From this water content and rock type, and how these properties vary between regions where slow slip does and does not happen, can be understood. Ultimately, this work will help scientists understand the processes that control seismic hazard where multiple modes and rates of slip coexist. Broader Impacts from this work include showcasing project field practices, and facilitating K-12 school outreach and community participation in broadband data collection. Slow slip events and tectonic tremor have been linked to eclogitization in the subduction interface, but the mechanisms of how these properties vary along the subduction zone and how they govern seismicity style are not well understood. This project harnesses novel and cutting-edge seismic data to investigate the processes that govern these phenomena in the Cook Inlet region of the Alaska-Aleutian subduction zone. The study area encompasses two locations where the subduction interface undergoes recurrent, prolonged, Slow Slip Events (SSEs) as well as damaging megathrust earthquakes. Crucially, only one of the SSE-prone regions is associated with tectonic tremor, providing a natural laboratory for isolating tremor and slip mechanisms. In this project, seismic data from a traditional land-based broadband seismometer deployment will be combined with data from a Distributed Acoustic Sensing (DAS) experiment within the Cook Inlet to fill in a crucial resolution gap over a patch of the subduction zone that experiences slow slip but not tremor. A suite of seismic imaging products, including receiver functions, P-wave autocorrelations, and surface wave dispersion from ambient noise correlations, will be generated and jointly inverted to constrain P and S wavespeed within both plates in order to infer variations in fluid properties and eclogite content across slipping and locked sections of the subduction zone, and evaluate the relative importance of these processes. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.