Transform push, oblique subduction resistance, and intraplate stress of the Juan de Fuca Plate

Abstract

The Juan de Fuca plate is a small oceanic plate between the Pacific and North America plates. In the southernmost region, referred to as the Gorda deformation zone, the maximum compressive stress σ1 constrained by earthquake focal mechanisms is N‐S. Off Oregon, and possibly off Washington, NW trending left‐lateral faults cutting the Juan de Fuca plate indicate a σ1 in a NE‐SW to E‐W direction. The magnitude of differential stress increases from north to south; this is inferred from the plastic yielding and distribution of earthquakes throughout the Gorda deformation zone. To understand how tectonic forces determine the stress field of the Juan de Fuca plate, we have modeled the intraplate stress using both elastic and elastic‐perfectly plastic plane‐stress finite element models. We conclude that the right‐lateral shear motion of the Pacific and North America plates is primarily responsible for the stress pattern of the Juan de Fuca plate. The most important roles are played by a compressional force normal to the Mendocino transform fault, a result of the northward push by the Pacific plate and a horizontal resistance operating against the northward, or margin‐parallel, component of oblique subduction. Margin‐parallel subduction resistance results in large N‐S compression in the Gorda deformation zone because the force is integrated over the full length of the Cascadia subduction zone. The Mendocino transform fault serves as a strong buttress that is very weak in shear but capable of transmitting large strike‐normal compressive stresses. Internal failure of the Gorda deformation zone potentially places limits on the magnitude of the fault‐normal stresses being transmitted and correspondingly on the magnitude of strike‐parallel subduction resistance. Transform faults and oblique subduction zones in other parts of the world can be expected to transmit and create stresses in the same manner.