Tectonic tremors in Alaska (USA) are associated with subduction of the Yakutat plateau, but their origins are unclear due to lack of depth constraints. We have processed tremor recordings to extract low-frequency earthquakes (LFEs), and generated a set of six LFE waveform templates via iterative network matched filtering and stacking. The timing of impulsive P (compressional) wave and S (shear) wave arrivals on template waveforms places LFEs at 40-58 km depth, near the upper envelope of intraslab seismicity and immediately updip of increased levels of intraslab seismicity. S waves at near-epicentral distances display polarities consistent with shear slip on the plate boundary. We compare characteristics of LFEs, seismicity, and tectonic structures in central Alaska with those in warm subduction zones, and propose a new model for the region's unusual intraslab seismicity and the enigmatic Denali volcanic gap (i.e., an area of no volcanism where expected). We argue that fluids in the Yakutat plate are confined to its upper crust, and that shallow subduction leads to hydromechanical conditions at the slab interface in central Alaska akin to those in warm subduction zones where similar LFEs and tremor occur. These conditions lead to fluid expulsion at shallow depths, explaining strike-parallel alignment of tremor occurrence with the Denali volcanic gap. Moreover, the lack of double seismic zone and restriction of deep intraslab seismicity to a persistent low-velocity zone are simple consequences of anhydrous conditions prevailing in the lower crust and upper mantle of the Yakutat plate. iii Lay Summary We apply a match-filtered technique to 13 3-component, broadband stations from central Alaska and recover 6 low-frequency earthquake (LFE) templates. These LFEs are located at depth ranges from 40 to 58 km, near the upper envelope of intraslab seismicity and immediately updip of increased levels of intraslab seismicity. We further compare the characteristics of LFEs, seismicity, and tectonic structures in central Alaska with those in warm subduction zones, and propose a new model for the region's unusual intraslab seismicity and the enigmatic Denali volcanic gap. We argue that fluids in the Yakutat plate are confined to its upper crust, and that shallow subduction leads to hydromechanical conditions at the slab interface in central Alaska akin to those in warm subduction zones where similar LFEs and tremor occur. These conditions lead to fluid expulsion at shallow depths, explaining strike-parallel alignment of tremor occurrence with the Denali volcanic gap. iv Preface The data presented in this thesis were analyzed by Yuling Chuang, under the supervision of Dr. Michael Bostock. A version of this thesis has been published. Lindsay Chuang, Michael Bostock, Aaron Wech, and Alexandre Plourde. (2017) Plateau subduction, intraslab seismicity, and the Denali (Alaska) volcanic gap. Geology, G38867.1,

We use seismic waveform data from the Mendocino Experiment to detect low‐frequency earthquakes (LFEs) beneath Northern California during the April 2008 tremor‐and‐slip episode. In southern Cascadia, 59 templates were generated using iterative network cross correlation and stacking and grouped into 34 distinct LFE families. The main front of tremor epicenters migrates along strike at 9 km d−1; we also find one instance of rapid tremor reversal, observed to propagate in the opposite direction at 10–20 km h−1. As in other regions of Cascadia, LFE hypocenters from this study lie several kilometers above a recent plate interface model. South of Cascadia, LFEs were discovered on the Maacama and Bucknell Creek faults. The Bucknell Creek Fault may be the youngest fault yet observed to host LFEs. These fault zones also host shallow earthquake swarms with repeating events that are distinct from LFEs in their spectral and recurrence characteristics.

Receiver functions, calculated by deconvolving P (or vertical) component records of teleseismic waveforms from the corresponding SV (or radial) components, have been widely used to obtain receiver‐side Green's functions in an approximate form. Conventional receiver function methods, however, often fail due to numerical instability of the deconvolution and strong multiples on the P components. These problems become severe when analyzing in high frequency and using data from ocean bottom seismometers (OBSs). We present a novel technique to estimate Green's functions of receiver‐side structure from teleseismic P waveforms. In this method, two components of Green's functions, which are assumed to form a series of pulses, are directly related in a single equation without explicit deconvolution. Based on the equation, we construct posterior probability distributions regarding the number of pulses, their timing, and amplitudes within a transdimensional Bayesian framework. A reversible‐jump Markov chain Monte Carlo method is used for this purpose, and we further utilize a parallel tempering method to achieve rapid convergence. Synthetic tests and application to an OBS installed at the Yamato Basin, the Sea of Japan, show that the proposed method can estimate radial‐component Green's functions more accurately than conventional receiver function methods. We suggest that the high‐frequency Green's functions estimated by the new method can be used to reveal fine‐scale (in order ~100 m) structure of the seafloor sediment.

A pair of small earthquakes (MN 2.4 and 2.6, Earthquakes Canada) hit the city of Dartmouth, Nova Scotia, Canada, in early March 2020. The events were recorded by three seismic stations within 200 km, but only one station (HAL, <10  km) is close enough to offer high-quality broadband signals. In this study, we explore their source parameters using the nearest station through waveform modeling. A nearby quarry blast (MN 2.0) with known Global Positioning System coordinates is adopted as a reference for regional velocity model building and location calibration. We first build a half-space velocity model by estimating the P-S travel-time difference of the blast and determine the near-surface velocity through full-waveform modeling (i.e., comparing a set of synthetic waveforms with the observed blast). The velocity model is then used to evaluate the pair of earthquakes, in which waveform fitting and Rg/S amplitude ratios suggest source depths of ∼0.7  km. The epicenters of these two earthquakes are situated in a recently constructed commercial development. Lastly, single-station template matching finds no similar earthquakes near the hypocenters of the two events in the past decade and only three aftershocks in the following four months. Taking advantage of a ground-truth blast and waveform modeling, our study demonstrates the potential to construct a detailed regional velocity model and determine accurate earthquake source parameters in regions where only a single station is available.

SUMMARY We introduce a new relative moment tensor (MT) inversion method for clusters of nearby earthquakes. The method extends previous work by introducing constraints from S-waves that do not require modal decomposition and by employing principal component analysis to produce robust estimates of excitation. At each receiver, P and S waves from each event are independently aligned and decomposed into principal components. P-wave constraints on MTs are obtained from a ratio of coefficients corresponding to the first principal component, equivalent to a relative amplitude. For S waves we produce constraints on MTs involving three events, where one event is described as a linear combination of the other two, and coefficients are derived from the first two principal components. Nonlinear optimization is applied to efficiently find best-fitting tensile-earthquake and double-couple solutions for relative MT systems. Using synthetic data, we demonstrate the effectiveness of the P and S constraints both individually and in combination. We then apply the relative MT inversion to a set of 16 earthquakes from southern Alaska, at ∼125 km depth within the subducted Yakutat terrane. Most events are compatible with a stress tensor dominated by downdip tension, however, we observe several pairs of earthquakes with nearly antiparallel slip implying that the stress regime is heterogeneous and/or faults are extremely weak. The location of these events near the abrupt downdip termination of seismicity and the low-velocity zone suggest that they are caused by weakening via grain-size and volume reduction associated with eclogitization of the lower crustal gabbro layer.