RELATING SEISMIC STRUCTURE TO SEISMOGENIC BEHAVIOR IN THE CASCADIA FOREARC (Invited Presentation)

Details of the seismic structure around subduction interfaces that experience deep episodic tremor and slip (ETS) remain difficult to quantify due to significant heterogeneity in the seismic properties of these regions. Along the Cascadia margin’s forearc, this slip behavior also correlates with a number of other tectonic phenomena and geophysical observations, including potential field measurements, uplift/exhumation rates, and seismic velocities beneath the downgoing plate and within the forearc crust.

In this study, we analyze the discontinuity and velocity structure of the Cascadia margin to better understand how the seismic structure relates to variations in non-volcanic tremor (NVT), which often represents the seismic signature of ETS. We observe anomalously low shear-wave velocity zones (LVZs) in the forearc lower crust in regions that experience large amounts of NVT. Seismic reflection imaging indicates that these LVZs are primarily composed of (meta)sediments, consistent with a corresponding decrease in Bouguer gravity along the forearc in high NVT regions. These LVZs extend down to ca. 50 km in some places, corresponding well with the projected depth extent of NVT, and likely represent packages of accreted material from the downgoing plate to the base of the overriding plate (basal accretion or “subcretion”). Thermomechanical modeling of subcretion predicts domal uplift above the locus of underplating, explaining increased uplift/exhumation rates along the northern and southern Cascadia forearc. In central Cascadia where NVT is relatively low, evidence for significant subcretion is largely absent. The fundamental control on variations in subcretion in Cascadia may be related to variable fluid release along the margin due to deformation within the downgoing plate imposed by large subslab buoyancy anomalies. These fluids enable the subcretion of weak, sedimentary material, and may relate to NVT distribution due to the rheological characteristics of the material being “subcreted” (phyllosilicate-rich metasediments). Alternatively, rather than having a causative relationship, subcretion and NVT may simply reflect the structural vs. seismogenic expressions of processes enabled by relatively large volumes of fluid near the plate interface.