RECOVERING 'FOSSILISED' STRAIN: RECONSTRUCTING THE EVOLVING STRAIN FIELD ABOVE THE LOCKED CASCADIA MEGATHRUST OVER MULTIPLE EARTHQUAKE CYCLES USING ANISOTROPY OF MAGNETIC SUSCEPTIBILITY

In deforming sediments, the preferred alignment of phyllosilicate minerals in response to strain, reflected by the Anisotropy of Magnetic Susceptiblity (AMS), is now known to develop within ~25 years of deposition. This opens new possibilities for measuring contemporary and ancient strain fields in deforming regions with high seismic hazard, such as the Cascadia subduction zone. This study presents preliminary AMS measurements of recently deposited sediments on the Cascadia forearc, that can potentially extend observations of interseismic strain offshore into the most strongly locked region of the megathrust, over multiple earthquake cycles.

The AMS ellipsoid, reoriented using paleomagnetic data, was determined for samples from the upper ~1 m of marine sediment cores along a transect at ~45.5º N, from ~40 km west of the Cascadia trench to the coast. Most samples exhibit magnetic fabrics with clear lineations within the bedding plane, with the average degree of anisotropy increasing from 1% on the coast to 12% close to the trench. Inferred NE-SW shortening azimuths match convergence between the Juan de Fuca and North American plates; additionally, azimuths from the upper 0.4 m of a core from Tillamook Bay are close to the current azimuth of horizontal motion recorded by a nearby UNAVCO GPS station. Within each core, intervals with similar lineation azimuths are separated by abrupt 10-50º jumps in the average bearing; preliminary dating of the cores indicates that these sharp transitions may occur close to periods when earthquakes are inferred in the paleoseismic record.

Overall these results indicate that the fabrics measured by AMS are tectonic in origin and generated by interseismic strain accumulation on the Cascadia megathrust. The data also indicate measurable variations in the strain field on the Cascadia margin over centennial timescales, both within and between earthquake cycles, that can potentially be correlated with the variability in Cascadia rupture characteristics observed in the paleoseismic record. The extension of strain observations in time and space enabled by AMS measurements can help to refine estimates of the future seismic hazard on the Cascadia subduction zone, and other deforming regions worldwide.