AN EXPLANATION FOR THE DOUBLE SEISMIC LAYERS NORTH OF THE MENDOCINO TRIPLE JUNCTION

We propose that the gently eastward dipping double planed seismic zone observed at 15-25 km depths in the southern Cascadia subduction zone, just north of the Mendocino triple junction, is a direct consequence of the thermally controlled theology. As the oceanic lithosphere subdu cts to a depth of about 15 km, the temperature regime causes a brittle -plastic transition to occur within the oceanic crust. Thus, a ductile layer forms in the lower oceanic crust, sandwiched between the brittl e upper crust and brittle upper mantle. The very high strain rates nea r the triple junction caused by the northward push of the Pacific plat e on the Gorda plate increase the seismicity and thus accentuate the d ouble seismic zone in this region. This model explains the focal mecha nisms observed in the seismic zone and their spatial change. The doubl e seismic layers clearly define the position of the subducting Gorda p late, previously uncertain in the Cape Mendocino region.