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Pacific Coastal & Marine Science Center

Coastal and Marine Earthquake Studies

 

The Cascadia Megathrust and Tectonic Stress in the Pacific Northwest

Overview

Oceanic-COntinental(Figure from online edition of This Dynamic Earth)

The Pacific Northwest (Northern California, Oregon, Washington, and British Columbia) is the site of the Cascadia subduction zone, where an oceanic tectonic plate (the Juan de Fuca plate) is being pulled and driven (i.e., subducted) beneath a continental plate (the North American plate). As a result of the interaction between the two plates, the continent overlying the subduction zone is actively deforming. Earthquakes in the Pacific Northwest are generally thought to occur in three different parts of the Cascadia subduction zone:

  1. The deforming part of the North American plate
  2. The contact between the two plates, termed the interplate thrust or megathrust
  3. Within the down-going Juan de Fuca plate

This project addresses two of the many questions regarding earthquake hazards in the Pacific Northwest:

  1. The state of stress in the Pacific Northwest is generally N-S oriented horizontal compression. Because one would expect that the orientation of compression would be close to the direction that the Juan de Fuca plate is moving toward North America, what is the origin of the N-S compressive stress field in the Pacific Northwest?
  2. A very unusual aspect of the Cascadia subduction zone is that there have been few, if any, earthquakes instrumentally recorded along the Cascadia megathrust. This fact, in combination with the fact that the seismogenic part (where earthquakes would be generated) of the Cascadia megathrust is located offshore, limits our understanding of the mechanics of this important fault. Can the observed stress field in the Pacific Northwest tells us anything about the fault itself? In light of the fact that very few earthquakes occur along the megathrust, is it otherwise mechanically similar to other major plate boundary faults around the world (e.g., the San Andreas fault)?

To address these questions, a modeling study was devised to relate the stress field within the North American plate to tectonic forces acting along the plate boundaries.