Multiparameter two‐dimensional inversion of scattered teleseismic body waves 3. Application to the Cascadia 1993 data set

Abstract

This is the third paper in a three‐part series that examines formal inversion of the teleseismic P wave coda for discontinuous, two‐dimensional (2‐D) variations in elastic properties beneath dense arrays of three‐component, broadband seismometers. In this paper, the method is applied to data from the Incorporated Research Institutions for Seismology‐Program for Array Seismic Studies of the Continental Lithosphere (IRIS‐PASSCAL) Cascadia 1993 experiment undertaken across central Oregon. Two major features are imaged in the resulting model. The continental Moho becomes evident ∼150 km from the coast beneath the Western Cascades and extends through the eastern end of the profile at 35–40 km depth. In the western portion of the model, oceanic crust of the subducting Juan de Fuca plate dips shallowly (12°) at the coast and more steeply (27°) below the Willamette Valley and is evident to depths of >100 km beneath the High Cascades. The abrupt increase in plate dip at ∼40 km depth coincides with an apparent thickening of the oceanic crust followed by a diminution in its signature. Building on previous work, we argue that these results are consistent with the consequences of prograde metamorphic reactions occurring within the oceanic crust. Progressive dehydration at lower‐grade facies conditions culminates in the transformation to eclogite, producing a pronounced increase in the seismic velocity, density and dip of the subducting plate, and structural complexity in the overlying wedge.