Heterogeneous earthquake distributions and stress drop estimates along the Cascadia subduction megathrust and upper plate faults

Slip observed within subduction zones falls within a broad range, with non-volcanic tremor (NVT), slow slip, repeating small earthquakes, to great earthquakes all contained within this spectrum.  The diversity of observed slip suggests diversity in fault zone conditions, which can be affected by a variety of factors, such as fluids, sediment inputs, upper plate structure, and topography on the subducting plate.  The role of subducting topography on seismicity appears to be regionally variable, with some bathymetric features leading to high slip during earthquakes, and others leading to plate creep and small earthquakes associated with upper plate deformation.  Seismicity characteristics in regions of subducting topography can be difficult to assess in many areas however, because traditional land-based seismic networks can easily miss offshore smaller earthquakes associated with this process. We use data from an amphibious seismic network deployed along the Cascadia subduction zone to examine seismic characteristics of thousands of newly detected and located earthquakes along the margin.  Over 5000 earthquakes in the catalog generally agree with asperity locations modeled from geodetic data and the 1700 M9 rupture, along with several clusters of upper plate earthquakes in areas of subducting topography.  Stress drop estimates, based on a spectral ratio technique, vary depending on earthquake location, with higher stress drop earthquakes occurring on the upper plate faults relative to the lower stress drop earthquakes occurring along the megathrust.  These variations may reflect important changes in fault conditions between upper plate splay faults and the megathrust.