Tectonic activity in Southeast Alaska and Northwest Canada is a consequence of the relative motion between the North American and Pacific plates. In this dissertation we present the results of three geodetic and seismic studies that investigate earthquake processes in the northernmost section of the Cordilleran orogenic belt within the Alaskan and Northwest Canadian convergent margins. Chapter 2 focuses on geodetic coupling of the Aleutian trench outboard of the Alaska Peninsula. Geodetic coupling is a measure of how elastic strain accumulates along a subduction interface between megathrust (M >9.0) earthquakes and is a critical tool for monitoring earthquake hazards in subduction zones. We present evidence that the subduction interface in this area is comprised of a sequence of discrete segments and show how these correspond with observed rupture characteristics of the 2021 M 7.8 Simeonof earthquake. Chapter 3 explores how the segments identified in Chapter 2 have been updated in successive work and how the coupling segments first presented in Chapter 2 are shown to closely correlate with the 2021-2023 Alaska Peninsula earthquake sequence.Chapter 4 transitions some 1500 km to the northwest into the Northern Canadian Cordillera (NCC). This complex tectonic setting is undergoing active tectonic deformation characterized by diffuse patterns of seismicity. We use broadband seismic data from Mackenzie Mountain Earthscope Project (MMEP), USArray Transportable Array (TA), and permanent Canadian National Seismic Network stations to present a local earthquake catalogue with high sensitivity to small regional earthquakes. Deep learning techniques are adopted for both seismic phase detection and event association. Event relocations are performed to provide well constrained estimates of earthquake depth distributions. Clusters of microseismicity spanning the upper crust are located in the central Richardson Mountains, in the vicinity of the Tintina fault, and in the northeast Selwyn Basin. These clusters suggest that the core of the Richardson Anticlinorium is tectonically active and that the Tintina fault is a locus for low levels of active deformation. We interpret seismicity in the northeast Selwyn Basin as partially occurring along the Plateau thrust at depth or in its hanging wall. We suggest that some combination of localized duplex structures and lithological strength contrasts both within the Selwyn Basin and between the Selwyn Basin and abutting Paleozoic shelf sequences may be responsible for seismicity in the Mackenzie Mountain foreland.Chapter 5 is also situated within the NCC and investigates GPS site velocities in order to place constraints on measurable geodetic deformation patterns occurring within the mountain belt. First order observations of site velocities are on the order of 1-3 mm/yr. We use these to distinguish between 3D predictions of GIA models for the effects of both post-Last Glacial Maximum deglaciation on the North American continent as well as post-Little Ice Age deglaciation in Southeast Alaska. When GPS site velocities are corrected for the 3D effects of GIA a clear effect of strain accumulation at the level of 3-5 mm/yr is observed which extends at least as far inboard as the Mackenzie Mountain foreland. We use these new site velocities to constrain a regional tectonic block model and evaluate whether the Tintina fault and Southern Richardson Mountains could plausibly define present-day block boundaries. Our findings suggest that inclusion of the Tintina fault represents a significant improvement in fit to GPS site velocities and that maximum allowable strike slip and tensile deformation rates on the Tintina fault are 1 mm/yr. We also find that GPS site velocities in the Richardson Mountains favor a more southward directed block motion relative to sites further south, however the inclusion of a separate Richardson block does not represent a significant improvement in model fit based on currently available data.
Read
