Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Science, 1982. / Microfiche copy available in Archives and Science / Bibliography: leaves 68-69. / by Wafik Bulind Beydoun. / M.S.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Sciences, 1979. / Microfiche copy available in Archives and Science. / Vita. / Bibliography: leaves 316-327. / by Michael Fehler. / Ph.D.

Thesis. 1978. Sc.D.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Science. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND LINDGREN. / Vita. / Bibliography: leaves 212-219. / by Cheng, Chuen Hon Arthur. / Sc.D.

Thesis. 1976. M.S. cn--Massachusetts Institute of Technology. Dept. of Earth and Planetary Sciences. / Microfiche copy available in Archives and Science. / Bibliography: leaf 72. / M.S.cn

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Sciences, 1979. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND LINDGREN. / Vita. / Bibliography: leaves 378-399. / by David H. Johnston. / Ph.D.

Thesis. 1978. Ph.D.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Science. / Microfiche copy available in Archives and Science. / Vita. / Bibliography: leaves 257-274. / Ph.D.

Twenty-four blasts from three quarries operating in the central Virginia area were used, first to test the locational capabilities of the Central Virginia - North Anna Network and then to generate relative station delay suites for network stations. Using two different methods of approximating blast origin times, the Closest Station Method (CSM) and the Single Iteration Method (SIM), station delays were derived for different areas within central Virginia. Application of these station delay suites reduced locational errors in the general area from an average of 3.0 ± 1.2 to 1.7 ± 0.6 km (95% confidence level). In both cases, the average equivalent radii, a linear measure of error ellipse size, were 1.3 km. However, this result depends primarily on the improvement at one of the three quarries, where the locational error was reduced from 6.5 km to 2.6 km. Utilizing one of these methods (the SIM), lateral varational patterns in velocity were inferred and determined to be velocity banding similar to that observed in the Piedmont province in Georgia, North and South Carolina. / Master of Science

A model is proposed in which intraplate earthquakes are triggered by the tensile failure of asperities by subcritical crack growth. The principle subcritical crack growth mechanism is fatigue driven by cyclic tensile stresses within an asperity. Cyclic stresses result from cyclic pore pressure variations which propagate from the Earth's surface because of annual variations in the height of the water table. Asperities are postulated to be porous and permeable masses of saturated host rock hydrologically connected to fluids within an open fracture. Porosity and permeability within asperities are assumed to be due to microcracks within the rock matrix. Tensile stresses within an asperity are due partly to mechanical loads, but pore pressure is the primary Inechanism by which tension is developed and fatigue operates primarily in tension. Fatigue crack growth is enhanced by chemical subcritical crack growth mechanisms such as stress corrosion and these mechatusms are proposed to act in unison. Faulting may be initiated when a population of asperities is either driven to failure by these mechanisms alone or when it is weakened to the point at which it is vulnerable to small applied stresses. Numerical modeling of the interaction of pore pressures and stresses within asperities indicates that a small transient increase of pore pressure on the order of a fraction of a megapascal will increase the tensile stresses within an asperity by a fraction of a megapascal. Consequently, it may be possible for a small increase in pore pressure, due to elevated water table levels, to trigger asperity failure and result in seismicity. / Master of Science

Seamounts, or submarine volcanos, have two distinct phases: the formation phase when the seamount is magmatically active and the passive phase when the seamount is transporting on the seafloor until subduction. Being able to understand various aspects of seamounts both in the formation and passive phase can enhance our understanding of volcanic processes and how seamounts can influence other processes such as crustal deformation, plate subduction, and earthquake generation. The purpose of this thesis is to examine seamounts on the Juan de Fuca plate both in the formation and passive phases. The first three chapters of this thesis focuses on understanding the subsurface properties and volcanic processes of Axial Seamount, an active seamount located on the Juan de Fuca Ridge. The fourth and final chapter focuses on understanding how seamounts influence subduction and the seismogenic properties of the Cascadia Subduction Zone. In chapter 1, I analyze multichannel seismic data to characterize the internal crustal structure of the rift zones of Axial Seamount. The new reflectivity images reveal small and discontinuous crustal magma bodies beneath and in the vicinity of the rift zone lava flows from the three most recent eruptions. We also image wide magma bodies within the overlap regions between the rift zones and neighboring segments of Juan de Fuca Ridge. Collectively the new observations indicate that multiple small crustal magma bodies underlie Axial and likely contribute to rift zone magmatism with implications for interpretations of seismicity patterns and lava flow compositions. In chapters 2 and 3, I process over 7 years of continuous seismic noise at Axial Seamount and use cross-correlation functions to calculate the relative seismic velocity (dv/v) changes beneath the caldera. I find a long-term trend of decreasing velocity during rapid inflation, followed by slight increase in velocities as background seismicity increases and inflation rate decreases. I also observe small short-term increases in dv/v which coincide with short-term deflation events. The observations of changes in dv/v and their correlation with other geophysical data provide insights into how the top ~1 km of the crust at Axial Seamount changes in response to subsurface magma movement and capture the transition from a period of rapid reinflation to a period where the caldera wall faults become critically stressed and must rupture to accommodate further inflation. From the relative seismic velocity variations, I also observe a strong annual pattern constrained within the 0.1-0.2Hz filter band. This annual pattern correlates well with timing and location of storm activity within the Pacific Ocean. Through the comparison of annual variation with ocean data, we determine that the annual pattern observed is likely an apparent velocity change due to changes in the seismic noise source. Lastly, in chapter 4, I utilize multichannel seismic data and high-resolution P-wave velocity (Vp) models of the CASIE21 expedition to calculate residual Vp models to examine properties of the sediments relative to seamounts on the incoming plate. At one of the larger seamounts located within 25km of the deformation front, I show evidence consistent with predicted stress effects of buried topography where there is an increase in normal stresses landward and a stress shadow seaward of the seamount, which can alter and impact rupture patterns along the margin. I also show evidence for signatures of enhanced hydrothermal circulation at seamounts near the deformation front which show that seamounts can be potential carriers of additional fluid into the margin when subducted. In additional to looking at the sediments relative to the seamounts, I also evaluate properties of a high Vp basal layer that is prevalent on all of the lines which can provide insights into the earthquake rupture and tsunami potential for the area.

Subduction zones, where one tectonic plate descends beneath another, are the most seismically active regions on Earth and have produced the largest earthquakes and some of the most destructive tsunamis ever recorded. Significant questions remain regarding the roles both the downgoing and overriding plates play in contributing to varying styles of rupture along the main seismogenic contact between the two plates, or megathrust, where such great (Mw > 8) earthquakes are generated. In the last few decades, the scientific community has recognized how different structural and compositional properties of both plates, and in particular the hydration state of the incoming plate can contribute to variations in megathrust slip behaviors. In this thesis, I show how marine multichannel seismic (MCS) and ocean-bottom seismometer (OBS) data can be used to investigate structural controls on megathrust slip behavior including the different styles of great earthquakes and/or the generation of slow slip events. Offshore Alaska and Sumatra, we used long-streamer multichannel seismic data to create a high-resolution P-wave velocity (Vp) model of the upper oceanic crust prior to subduction. Using a differential effective medium theory, we place the first constraints on the amounts pore (free) water contained therein. Our results indicate that the uppermost oceanic crust of the incoming plates in both regions is significantly hydrated. Offshore Alaska, we show that pervasive faulting in the bending area allows seawater to penetrate into the uppermost crust. We propose that high water content in uppermost crust might contribute to observations of low coupling along the shallow plate interface in this area through the expulsion of pore fluids. Geochemical analyses of arc lavas in this segment of the Alaska subduction zone suggests significant fluid release from the downgoing crust compared to adjacent segments. Thus, we propose that during subduction, additional bending and high-temperature circulation of remaining pore fluids could further alter the upper oceanic crust that dehydrates around sub-arc depths. Offshore Sumatra, few bending-related faults are observed; however, evidence for significant and homogeneous hydration within the the uppermost crustal layer 2A (extrusives) suggests that plate bending plays a role in the shallow reopening cracks, facilitating the shallow penetration of seawater. In layer 2B (sheeted dikes) just below, our results suggest heterogeneous, yet significant, hydration that we attribute to the slow and diffuse deformation taking place in the Wharton Basin. We speculate that the large amounts of upper-crustal water carried into the Sumatra subduction zone can influence shallow slip behavior, as evidenced by recent records of a long-lasting slow slip event in the area. To further explore potential structural and compositional controls on spatial varia- tions in megathrust slip behavior in Alaska, we use OBS data to create a 3D Vp model of the Alaska Peninsula Subduction zone within a 500-by-400 km wide area with good resolution down to 20-25 km depths in both the incoming and overriding plates. Our model samples two subduction zone segments that exhibit differences in history and style of megathrust rupture. We interpret reductions in seismic velocities within the incoming plate as evidence for modest hydration of the Pacific oceanic plate resulting from a series of fracture zones and the formation of large seamounts and an associated basement swell, or platform. The bathymetry of the seamounts and platform in part modulates the distribution and lithology of subducting sediments across the margin that we propose might influence shallow slip behavior. Within the overriding North American plate, we see evidence for contrasting styles of deformation and variations in composition (i.e., rigidity) that agrees well with observed changes in plate coupling and great earthquake history. These results emphasize the importance of considering not only one, but several factors related to both the incoming and overriding plates which collectively contribute to along-strike and downdip variations in megathrust slip behavior between segments. Our final study looks at the incoming Cocos plate just before it subducts offshore Mexico beneath the North American plate. Here we jointly inverted 2D OBS and long-offset MCS data acquired parallel to the trench to derive a 270 km-long, high-resolution Vp model of the entire oceanic crust and uppermost mantle. We provide the first constraints on the quantities of both free and structural (i.e., mineral-bound) water contained within the Cocos plate outboard of the Guerrero Gap and adjacent segments of the Mexican subduction zone. The Guerrero gap hosts large slow slip events that are commonly explained through the release of water through the dehydration of altered sediments and upper oceanic crust downdip. Strikingly, our results show that while the Cocos plate is hydrated offshore Mexico, nearly all of the water is contained within the upper oceanic crust. Moreover, we see that most of the water by weight is present as free fluids in the pores and that the upper oceanic crust is only moderately altered (0.3-1.3 wt.%) compared to global averages (> 1.5-3 wt.%). While the upper crust appears hydrated everywhere across our profile, we find that ∼30% more water is subducting outboard the Guerrero seismic gap where large seamounts contribute to a thicker extrusive layer and more alteration. This, along with evidence for the subduction of seamounts in Guerrero might help explain observations of weak shallow plate coupling and a greater propensity for slow slip at greater seismogenic depths compared to adjacent segments. These results provide important new constraints on how much pore and structural water is carried in the Cocos plate offshore Mexico. We propose that global estimates of incoming structural water content are not applicable everywhere, as is commonly assumed by petrologic and thermal models. Much less structural water may be needed within the upper oceanic crust just before subduction to explain the occurrence of slow-slip events downdip in some subduction zones.