RELATIONSHIP BETWEEN THE CASCADIA FOREARC MANTLE CORNER, NON VOLCANIC TREMOR, AND THE DOWNDIP LIMIT OF SEISMIC RUPTURE

Great earthquakes anticipated on the Cascadia subduction fault can potentially rupture beyond the geodetically and thermally inferred locked zone to the depth of non volcanic tremor (NVT) or to the even deeper forearc mantle corner (FMC). We can reasonably assume that NVT and the FMC serve as the extreme lower bounds on rupture during great earthquakes, based on geodetic observations of slow slip in the tremor band and on sparse seismic velocity evidence that the forearc mantle wedge is significantly serpentinized. To evaluate these extreme rupture limits, we mapped the FMC from southern Vancouver Island to central Oregon by combining published seismic velocity structures with a model of the Juan de Fuca plate. These data indicate that the FMC is somewhat shallower beneath Vancouver Island (36–38 km) and Oregon (35–40 km) and deeper beneath Washington (41–43 km). We lack the data needed to determine FMC depth beneath California.

Recent great earthquakes in Indonesia and Japan reveal a correspondence between degree of mantle wedge serpentinization and downdip rupture extent. Resolving similar heterogeneity in serpentinization along the Cascadia subduction margin will require more comprehensive 3D seismic velocity and thermal models than are currently available. In the interim, we assume the occurrence of NVT indicates the availability of slab-derived fluids to serpentinize forearc mantle. The updip edge of tremor follows the same general pattern as the FMC, overlying a slightly shallower Juan de Fuca plate beneath Vancouver Island and Oregon (~30 km) and a deeper plate beneath Washington (~35 km). Similar to the Nankai subduction zone, the best constrained FMC depths correspond to the center of the tremor band suggesting that NVT is controlled by conditions near the FMC, including the relative permeability of forearc mantle and crust, rather than directly by temperature or pressure. Unlike Nankai, a gap as wide as 70 km exists between the downdip limit of the Cascadia locked zone and the FMC. This gap also encompasses a ~50 km wide gap between the locked zone and the updip limit of tremor, perhaps representing a zone of conditional stability. The separation of these features offers a natural laboratory for determining the key controls on rupture limits as well as the relation between slow slip and the onset of great earthquakes.