A new onshore-offshore seismic dataset from the Cascadia subduction zone was used to characterize mantle lithosphere structure from the ridge to the volcanic arc, and plate interface structure offshore within the seismogenic zone. The Cascadia Initiative (CI) covered the Juan de Fuca plate offshore the northwest coast of the United States with an ocean bottom seismometer (OBS) array for four years; this was complemented by a simultaneous onshore seismic array. Teleseismic data recorded by this array allows the unprecedented imaging of an entire tectonic plate from its creation at the ridge through subduction initiation and back beyond the volcanic arc along the entire strike of the Cascadia subduction zone. Higher frequency active source seismic data also provides constraints on the crustal structure along the plate interface offshore. Two seismic datasets were used to image the plate interface structure along a line extending 100 km offshore central Washington. These are wide-angle reflections from ship-to-shore seismic data from the Ridge-To-Trench seismic cruise and receiver functions calculated from a densely spaced CI OBS focus array in a similar region. Active source seismic observations are consistent with reflections from the plate interface offshore indicating the presence of a P-wave velocity discontinuity. Until recently, there has been limited success in using the receiver function technique on OBS data. I avoid these traditional challenges by using OBS constructed with shielding deployed in shallow water on the continental shelf. These data have quieter horizontals and avoid water- and sediment-multiple contamination at the examined frequencies. The receiver functions are consistently modeled with a velocity structure that has a low velocity zone (LVZ) with elevated P to S-wave velocity ratios at the plate interface. A similar LVZ structure has been observed onshore and interpreted as a combination of elevated pore-fluid pressures or metasediments. This new offshore result indicates that the structure may persist updip indicating the plate interface may be weak. To focus more broadly on the entire subduction system, I calculate phase velocities from teleseismic Rayleigh waves from 20-100 s period across the entire onshore-offshore array. The shear-wave velocity model calculated from these data can provide constrains on the thermal structure of the lithosphere both prior to and during subduction of the Juan de Fuca plate. Using OBS data in this period band requires removal of tilt and compliance noise, two types of water-induced noise that affect long period data. To facilitate these corrections on large seismic arrays such as the CI, an automated quality control routine was developed for selecting noise windows for the calculation of the required transfer functions. These corrections typically involve either averaging out transient signals, which requires the assumption of stationarity of the noise over the long periods of time, or laborious hand selection of noise segments. This new method calculates transfer functions based on daily time series that exclude transient signals, but allows for the investigation of long-term variation over the course of an instrument’s deployment. I interpret these new shoreline-crossing phase velocity maps in terms of the tectonics associated with the Cascadia subduction system. Major findings include that oceanic plate cooling models do not explain the velocities observed beneath the Juan de Fuca plate, that slow velocities in the forearc appear to be more prevalent in areas modeled to have experienced high slip in past Cascadia megathrust earthquakes, and along strike variations in phase velocity reflect variations in arc structure and backarc tectonics.

This thesis examines the seismic behavior of sediments in shallow subduction zones. In the traditional view of the seismogenic zone, the upper stability limit is controlled by a transition to velocity-strengthening (frictionally stable) clay-rich sediments at shallow depths in the accretionary prism. However, recent observations have emphasized that these shallow sediments can host a wide range of seismic behaviors. On one end of the seismic spectrum, the March 2011 Mw9.1 Tohoku-oki earthquake demonstrated that peak slip in a megathrust rupture can be hosted at the shallowest depths. At the other end of the spectrum, observations at the Hikurangi trench off the North Island of New Zealand have revealed that spontaneous, periodic slow slip events (SSEs) can nucleate in the shallowest portions of a subduction zone. The Japan Fast Trench Drilling Project (JFAST, IODP Expedition 343) drilled through the plate boundary faults in the Japan Trench to investigate the structure that hosted the Tohoku-oki earthquake. In Chapter 2, I use a trace element-based stratigraphy to identify several large displacement faults within the bottom ~15 m of the JFAST core. This work highlights that there are multiple candidate structures that could host a megathrust rupture and that not all displacement is accommodated along a weak pelagic clay layer recovered in the JFAST core. However, this method is incapable of determining which of these faults experienced significant seismic slip. In Chapter 3, I develop a novel paleoseismic indicator appropriate for faults hosted in seafloor sediments. This tool takes advantage of the fact that organic material (molecular biomarkers) in sediments degrades as a function of time and temperature. In this study, I determine the kinetics of thermal maturation for alkenones (coccolithophore-derived molecules) and n-alkanes (plant leaf wax-derived molecules) found in western Pacific sediments. In Chapter 4, I apply these kinetics to measured biomarker anomalies in JFAST samples to determine which faults recovered in the JFAST core could have hosted a megathrust event such as the Tohoku-oki earthquake. This approach reveals that multiple faults in the plate boundary region have likely hosted megathrust events and that the occurrence of seismic slip is not confined to a particular lithology. This implies that small differences in frictional behavior in subducting sedimentary lithologies are not the primary control on the occurrence of shallow seismic slip. In Chapter 5, I turn to a different type of shallow seismic behavior and focus on SSEs in the shallowest portion of the Hikurangi trench. In this study, I measure friction and velocity-dependence of the input sediments for this subduction zone at a range of pressures and temperatures relevant to the shallow portion of the slab where SSEs have been observed. These experiments demonstrate that the sediment here becomes frictionally weak at effective stresses expected deeper than ~2 km. At the same effective stresses, the sediment becomes less velocity strengthening, and under some conditions exhibits velocity neutral behavior. A plate-rate experiment exhibits velocity-weakening behavior and two spontaneous SSEs, indicating that at slow velocities, the sediment subducting at the Hikurangi trench is capable of unstable frictional behavior required to promote shallow SSEs. These results demonstrate that subducting sediments can exhibit a variety of frictional properties that can support unstable behavior in the shallowest reaches of the subduction zone.

Primitive, mantle-derived magmas provide important clues about the formation and equilibration conditions of magmas at depth. In subduction zones, it is uncommon for primitive magmas to ascend through the shallow mantle and crust without undergoing chemical modification. Instead, magmas commonly differentiate through fractional crystallization, crustal assimilation, or magma mixing. Those rare primitive lavas that do erupt along a volcanic arc are useful for elucidating subduction-related processes within the mantle wedge (~30–80 km depth) and are the focus of this research. I used piston-cylinder apparatuses to investigate the high-pressure, high-temperature, H2O-undersaturated phase equilibria for several primitive compositions that have erupted at volcanic arcs. I aimed to reveal the permissible residual mantle mineralogy, as well as the P-T- H2O conditions over which the putative mantle melts last equilibrated before erupting. My work focuses on the Trans-Mexican Volcanic Belt (TMVB), where primitive compositions span a range of SiO2, total alkalies (K2O+Na2O), magmatic H2O, and incompatible trace element enrichments. Variations among these components are presumed to result from melting heterogeneous mantle that has been affected, to varying degrees, by a subduction component. Chapter III focuses on the phase equilibria of a Mexican basaltic andesite and an Aleutian basalt. Results show that hydrous basaltic andesite equilibrated with harzburgite in the shallow mantle, whereas the basalt equilibrated with lherzolite. The former appears more common in continental arcs and the latter in intraoceanic arcs. Chapter IV focuses on two alkaline lavas of varying K2O content from the TMVB that are transitional between potassic, hydrous minette and H2O-poor intraplate alkali basalt. Experimental phase relations and trace element modeling reveals that melting and/or mixing of peridotite and clinopyroxene-rich veins are likely involved in producing these transitional lava types. These experimental data are integrated with other petrologic and geophysical data to provide an along-arc perspective of mantle-melt equilibration in the TMVB. Primitive melts appear to commonly equilibrate with chemically heterogeneous mantle at depths above the "hot nose" of the mantle wedge. It is apparent that the shallow mantle wedge is a key component for understanding the geochemical complexities of subduction zone magmas. This dissertation includes previously published and unpublished co-authored material.

The analysis of marsh cores from the tidal zones of the Siuslaw, Umpqua, and Coos River systems on the south-central Oregon coast provides supporting evidence of coseismic subsidence resulting from megathrust earthquakes and reveals the landward extent of the zero-isobase. The analysis is based on lithostratigraphy, paleotidal indicators, microfossil paleotidal indicators, and radiocarbon age. Coseismic activity is further supported by the presence of anomalous thin sand layers present in certain cores. The analysis of diatom assemblages provides evidence of relative sea-level displacement on the order of 1 to 2 m. The historic quiescence of local synclinal structures in the Coos Bay area together with the evidence of prehistoric episodic burial of wetland sequences suggests that the activity of these structures is linked to megathrust releases. The distribution of cores containing non-episodically buried marshes and cores that show episodically buried wetlands within this area suggests that the landward extent of the zero-isobase is between 100 km and 120 km from the trench. The zero-isobase has a minimum width of 10 to 15 km. Radiocarbon dating of selected buried peat sequences yields an estimated recurrence interval on the order of 400 years. The apparent overlapping of the landward margin of both the upperplate deformation zone (fold and/or thrust fault belt) and the landward extent of the zero-isobase is interpreted to represent the landward limit of the locked zone. The earthquake magnitude is estimated to be 8.5 based on an arbitrary rupture length of 200 km and a locked zone width of 105 km. The identification of the zero-isobase on the southcentral Oregon coast is crucial to the prediction of regional coseismic subsidence and tsunami hazards, the testing of megathrust dislocation models, and the estimation of megathrust rupture areas and corresponding earthquake magnitudes in the Cascadia Margin.

Thesis (Ph. D.)--University of Washington, 2003. / Vita. Includes bibliographical references (p. 127-141).

No description available.

Thesis (M.S.)--Brigham Young University. Dept. of Geology, 2005. / Includes bibliographical references (p. 19-23).

Thesis (Ph. D.)--University of Alberta, 2010. / Title from pdf file main screen (viewed on June 28, 2010). A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Geophysics, Department of Physics, University of Alberta. Includes bibliographical references.

Thesis (doctoral)--Freie Universität Berlin, 2005. / "April 2006"--P. [2] of cover. Vita. DVD in pocket contains supplementary data. Includes bibliographical references (p. 115-127). Text is also available via the World Wide Web.

Thesis (Ph. D.)--Lehigh University, 2006. / Includes vita. Includes bibliographical references (leaves 187-192).