Fluids and seismicity in subduction zone systems

• Main Author: Plourde, Alexandre P.
• Format: Text
• Language: English
• Published: University of British Columbia 2020
• Subjects: Yakutat; Alaska
• Online Access: https://dx.doi.org/10.14288/1.0388207; https://doi.library.ubc.ca/10.14288/1.0388207

I investigate earthquakes and slow-slip phenomena in subduction zones and their overlying forearcs, with a particular focus on how metamorphism and fluid-migration affect the locations and character of earthquakes. I first focus on low-frequency earthquakes (LFEs) associated with episodic tremor-and-slip (ETS) down-dip of the megathrust zone on the subduction interface. I search for LFEs in southern Cascadia and find 34 distinct families. As in other regions of Cascadia, LFE hypocenters suggest that the plate interface is several kilometers shallower than reported by a recent model. South of Cascadia, I discover LFEs on the Maacama and Bucknell Creek faults, and compare them to shallow repeating earthquakes with distinct spectral and recurrence characteristics. After LFEs I turn to regular earthquakes. I develop new methodologies to estimate earthquake source properties that can better inform us of the physical conditions leading to brittle failure. In particular, I develop a multichannel deconvolution algorithm for recovering apparent source-time functions and apply it to a set of earthquakes offshore Japan. The method relates to empirical Green's function deconvolution but makes fewer assumptions of the target earthquakes. I then introduce a new relative moment tensor inversion method that relies on principal component analysis on aligned P-waves of nearby earthquakes to measure relative P amplitudes, and on aligned S-waves to produce coefficients that linearly relate three earthquakes. I apply the method to intraslab earthquakes in the subducted portion of the Yakutat terrane, in eastern Alaska, that appear to be related to eclogitization of oceanic plateau crust. Nearby earthquakes with antiparallel slip imply extremely weak faults and possibly a heterogeneous stress regime. Finally I consider multiple potential causes of enigmatic low Vp/Vs anomalies in the Cascadia forearc and subducting mantle, and explore effective medium models that consider both mineralogy and porosity. Based on their correlation with dense seismicity, high attenuation, and rapid ETS recurrence, I suggest that low Vp/Vs anomalies relate to upwelling slab fluids above regions of high permeability in the subducting plate, which are caused by high slab-curvature.