Cascadia low frequency earthquakes along the base of an overpressured subduction shear zone
Abstract
In the Cascadia subduction zone, a landward dipping region of low S wave velocity and elevated Poisson's ratio extends to approximately 45 km depth, and is interpreted to be all or part of the subducting igneous oceanic crust. This crust is considered to be overpressured, because fluids within it are trapped beneath an impermeable seal along the overlying inter-plate boundary. In the region of slow slip, however, there are discrepancies up to 10 km in estimates of the depth of the plate boundary from migration of teleseismic receiver functions and active source surveys, raising uncertainty about whether some features lie in the subducting or overriding plate. To reconcile the results from different seismic methods, we present them in a common reference fame, i.e. depth, using consistent 3D P and S wave velocity models developed for southern Vancouver Island by double-difference tomography, in which the local seismicity and LFEs are also relocated. We show that relocated LFEs, which directly indicate part of the inter-plate boundary, lie immediately below a regionally extensive reflective shear zone that includes the landward-dipping zone of elevated Poisson's ratio previously interpreted to be metamorphosed subducting sediments or over-pressured upper oceanic crust of the subducting Juan de Fuca plate. Consequently, the region of elevated Poisson's ratio previously associated with the subducting oceanic plate either lies within the overriding plate or forms part of a plate boundary zone a few km thick immediately above, and including, the LFEs. We also present inversions of magnetotelluric (MT) data, showing that a conductivity anomaly consistent with increased fluid-filled porosity exists close to the plate boundary in this area. Near its landward termination, the conductor rises into the North American plate and may indicate fluid release from the shear zone at this point, which is close to the downdip termination of the zone of high Poisson's ratio. We suggest that the available geophysical data are consistent with both a thin inter-plate boundary and a vertically distributed inter-plate boundary in the zone of slow slip. We favour the model with a vertically distributed boundary, because active slip is likely able to generate and maintain the 3-5 km thick zone of anomalous elastic properties that are observed.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMT052...05C
- Keywords:
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- 1242 Seismic cycle related deformations;
- GEODESY AND GRAVITY;
- 3613 Subduction zone processes;
- MINERALOGY AND PETROLOGY;
- 7240 Subduction zones;
- SEISMOLOGY;
- 8170 Subduction zone processes;
- TECTONOPHYSICS