The theory of plate tectonics introduced a paradigm shift in the way we view and study our planet. Many of the world's plate boundaries, however, are beneath our oceans making data collection, the key to furthering our knowledge about these zones, difficult. Recent advances in research vessels, seismic data acquisition techniques, and equipment built to withstand the temperatures and pressures within the oceans and on the seafloor, have made a huge impact in helping us understand the complicated structure and dynamics of our Earth. In the field of seismology, precise earthquake locations can illuminate regions of active seismic deformation, and help us better understand the orientation, mechanics, and kinematics of plate boundary zones. Although ocean bottom seismometers have been in use since the late 1930s, the instruments did not have the recording capacity and endurance to withstand being placed on the seafloor for a large span of time. Today, ocean bottom seismometers are deployed in densely spaced arrays that record seismic signals for approximately a year. The high-precision seismic data now available can help us redefine plate boundaries and further our understanding of the internal processes and deformation within these zones. In this dissertation, I aim to use ocean bottom seismometer data to explore the Pacific-North America plate boundary within the Gulf of California, and the internal workings of the 9º50'N East Pacific Rise high-temperature hydrothermal system. The first chapter of my dissertation uses data collected from an ocean bottom seismometer array deployed along the plate boundary within the Gulf of California from October 2005 to October 2006. In this study, I detect and locate ~700 earthquakes mainly located on the NW-SE striking oceanic transform faults that delineate the plate boundary. In addition, we calculate regional moment tensors for ~30 of these events, and find that the majority are right-lateral strike-slip events consistent with observed transtensional plate motion. Chapter 2 investigates the relationship between tides and ~3500 microearthquakes recorded on six ocean bottom seismometers deployed in the vicinity of the 9º50'N East Pacific Rise high-temperature hydrothermal vent system from October 2003 to April 2004. I find unequivocal evidence for tidal triggering of microearthquakes with maximum extensional stresses induced by the solid Earth tide at this site. Although tides are not the underlying cause of earthquake nucleation within the region, the modulation of microearthquakes by these small amplitude tidal stresses indicates that the hydrothermal system is a high-stress environment that is maintained at a critical state of failure due to on-going tectonic and magmatic processes. In Chapter 3, I further investigate the role of tides in triggering microearthquake activity at the 9º50'N East Pacific Rise high-temperature hydrothermal vent site, and observe systematic along-axis variations between peak microearthquake activity and maximum predicted tidal extension. I interpret this systematic triggering to result from pore-pressure perturbations propagating laterally through the hydrothermal system, and from this result and a one-dimensional poroelastic model, I provide an estimate of bulk permeability at this site. This observation may allow for more sophisticated investigations into the heat and chemical exchange between the newly formed oceanic crust and hydrothermal fluids, and may provide insight into the plumbing supporting the subsurface biosphere.

This dissertation investigates the link between volcanic unrest and the occurrence of moderate-to-large earthquakes with a specific type of focal mechanism. Vertical compensated-linear-vector-dipole (vertical-CLVD) earthquakes have vertical pressure or tension axes and seismic radiation patterns that are inconsistent with the double-couple model of slip on a planar fault. Prior to this work, moderate-to-large vertical-CLVD earthquakes were known to be geographically associated with volcanic centers and vertical-CLVD earthquakes were linked to a tsunami in the Izu-Bonin volcanic arc and a subglacial fissure eruption in Iceland. Vertical-CLVD earthquakes are some of the largest and most anomalous earthquakes to occur in volcanic systems, yet their physical mechanisms remain controversial largely due to the small number of observations. Five vertical-CLVD earthquakes with vertical pressure axes are identified near Nyiragongo volcano in the Democratic Republic of the Congo. Three earthquakes occur within days of a fissure eruption at Nyiragongo, and two occur several years later in association with the refilling of the lava lake in the summit crater of the volcano. Detailed study of these events shows that the earthquakes have slower source processes than tectonic earthquakes with similar magnitudes and locations. All five earthquakes are interpreted as resulting from slip on inward-dipping ring-fault structures located above deflating shallow magma chambers. The Nyiragongo study supports the interpretation that vertical-CLVD earthquakes may be causally related to dynamic physical processes occurring inside the edifices or magmatic plumbing systems of active volcanoes. Two seismicity catalogs from the Global Centroid Moment Tensor (CMT) Project are used to search for further examples of shallow earthquakes with robust vertical-CLVD focal mechanisms. CMT solutions for approximately 400 target earthquakes are calculated and 86 vertical-CLVD earthquakes are identified near active volcanoes. Together with the Nyiragongo study, this work increases the number of well-studied vertical-CLVD earthquakes from 14 to 101. Vertical-CLVD earthquakes have focal depths in the upper ~10 km of the Earth's crust, and ~80% have centroid locations within 30 km of an active volcanic center. Vertical-CLVD earthquakes are observed near several different types of volcanoes in a variety of geographic and tectonic settings, but most vertical-CLVD earthquakes are observed near basaltic-to-andesitic stratovolcanoes and submarine volcanoes in subduction zones. Vertical-CLVD earthquakes are linked to tsunamis, volcanic earthquake swarms, effusive and explosive eruptions, and caldera collapse, and approximately 70% are associated with documented volcanic eruptions or episodes of volcanic unrest. Those events with vertical pressure axes typically occur after volcanic eruptions initiate, whereas events with vertical tension axes commonly occur before the start of volcanic unrest. Both types of vertical-CLVD earthquakes have longer source durations than tectonic earthquakes of the same magnitude. The isotropic and pure vertical-CLVD components of the moment tensor cannot be independently resolved using our long-period seismic dataset. As a result, several physical mechanisms can explain the retrieved deviatoric vertical-CLVD moment tensors, including dip-slip motion on ring faults, volume exchange between two reservoirs, the opening and closing of tensile cracks, and volumetric sources. An evaluation of these mechanisms is performed using constraints obtained from detailed studies of individual vertical-CLVD earthquakes. Although no single physical mechanism can explain all of the characteristics of vertical-CLVD earthquakes, a ring-faulting model consisting of slip on inward- or outward-dipping ring faults triggered by the inflation or deflation of a shallow magma chamber can account for their seismic radiation patterns and source durations, as well as their temporal relationships with volcanic unrest. The observation that most vertical-CLVD earthquakes are associated with volcanoes with caldera structures supports this interpretation.

Thesis advisor: Michael J. Naughton / Thesis advisor: John E. Ebel / Determining accurate source parameters of small magnitude earthquakes is important to understand the source physics and tectonic processes that activate a seismic source as well as to make more accurate estimates of the probabilities of the recurrences of large earthquakes based on the statistics of smaller earthquakes. The accurate determination of the focal depths and focal mechanisms of small earthquakes is required to constrain the potential seismic source zones of future large earthquakes, whereas the accurate determination of seismic moment is required to calculate the sizes (best represented by moment magnitudes) of earthquakes. The precise determination of focal depths, moment magnitudes and focal mechanisms of small earthquakes can help greatly advance our knowledge of the potentially active faults in an area and thus help to produce accurate seismic hazard and risk maps for that area. Focal depths, moment magnitudes and focal mechanisms of earthquakes with magnitudes Mw 4.0 and less recorded by a sparse seismic network are usually poorly constrained due to the lack of an appropriate method applicable to find these parameters with a sparse set of observations. This dissertation presents a new method that can accurately determine focal depths, moment magnitudes and focal mechanisms of earthquakes with magnitudes between Mw 4.0 and Mw 2.5 using the broadband seismic waveforms recorded by the local and regional seismic stations. For the determination of the focal depths and the moment magnitudes, the observed seismograms as well as synthetic seismograms are filtered through a bandpass filter of 1-3 Hz, whereas for the determination of the focal mechanisms, they are filtered through a bandpass filter of 1.5-2.5 Hz. Both of these frequency passbands have a good signal-to-noise ratio (SNR) for the small earthquakes of the magnitudes that are analyzed in this dissertation. The waveforms are processed to their envelopes in order to make the waveforms relatively simple for the modeling. A grid search is performed over all possible dip, rake and strike angles and as well as over possible depths and scalar moments to find the optimal value of the focal depth and the optimal value of the scalar moment. To find the optimal focal mechanism, a non-linear moment-tensor inversion is performed in addition to the coarse grid search over the possible dip, rake and strike angles at a fixed value of focal depth and a fixed value of scalar moment. The method of this dissertation is tested on 18 aftershocks of Mw between 3.70 and 2.60 of the 2011 Mineral, Virginia Mw 5.7 earthquake. The method is also tested on 5 aftershocks of Mw between 3.62 and 2.63 of the 2013 Ladysmith, Quebec Mw 4.5 earthquake. Reliable focal depths and moment magnitudes are obtained for all of these events using waveforms from as few as 1 seismic station within the epicentral distance of 68-424 km with SNR greater or equal to 5. Similarly, reliable focal mechanisms are obtained for all of the events with Mw 3.70-3.04 using waveforms from at least 3 seismic stations within the epicentral distance of 60-350 km each with SNR greater or equal to 10. Tests show that the moment magnitudes and focal depths are not very sensitive to the crustal model used, although systematic variations in the focal depths are observed with the total crustal thickness. Tests also show that the focal mechanisms obtained with the different crustal structures vary with the Kagan angle of 30o on average for the events and the crustal structures tested. This means that the event moment magnitudes and event focal mechanism determinations are only somewhat sensitive to the uncertainties in the crustal models tested. The method is applied to some aftershocks of the Mw 7.8, 2015 Gorkha, Nepal earthquake which shows that the method developed in this dissertation, by analyzing data from eastern North America, appears to give good results when applied in a very different tectonic environment in a different part of the world. This study confirms that the method of modeling envelopes of seismic waveforms developed in this dissertation can be used to extract accurate focal depths and moment magnitudes of earthquakes with Mw 3.70-2.60 using broadband seismic data recorded by local and regional seismic stations at epicentral distances of 68-424 km and accurate focal mechanisms of earthquakes with Mw 3.70-3.04 using broadband seismic data recorded by local and regional seismic stations at epicentral distances of 60-350 km. / Thesis (PhD) — Boston College, 2019. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Twelve earthquakes off the Oregon and Northern California coasts were used in this research to study whether faulting associated with these quakes was of the San Andreas type as observed in California. The first motions of the incident P and S waves recorded at various seismic stations in the Pacific Northwest were used to obtain a fault plane solution for these earthquakes. Nine earthquakes exhibited faulting which could be consistent with San Andreas type of motion, where one of these motions was based on first P and S data, six others were consistent with either northwesterly right-lateral or northeasterly left-lateral motions, and two were recorded at so few stations that faulting could not be defined through a fault plane solution. One earthquake was located in the region of the Gorda Escarpment west of Cape Mendocino, California, and displacements appear to be associated with that zone, one earthquake had Garlock-type displacements as defined through P and S motions, and one earthquake was not associated with any of the above types of faults. / Graduation date: 1964

Thesis (Ph. D.)--University of Washington, 1982. / Vita. Bibliography: leaves [270]-277.

The objective of this study is to develop methodologies to estimate and predict the fragility of deteriorating reinforced concrete (RC) bridges, and to identify the effect of design and construction parameters on the reliability of RC bridges over time to assist in the design and construction process. To accurately estimate the fragility of deteriorating bridge, probabilistic capacity and demand models are developed. In addition, to simplify the calculation cost maintaining accuracy, fragility increment functions are developed. The proposed fragilities account for model uncertainties in the structural capacity, demand models, corrosion models. Furthermore, proper account is made of the uncertainties in the environmental conditions, material properties, and structural geometry. To identify the effect of design and construction parameters on the reliability of RC bridges, a sensitivity and importance analysis is conducted. Sensitivity analysis for an example bridge subject to corrosion is carried out to identify which parameters have the largest impact on the reliability over time. This dissertation considers different combinations of chloride exposure condition, environmental oxygen availability, water-to-cement ratios, and curing conditions, which affect the reliability of bridges over time. The developed models are applicable to both existing and new RC bridges and may be employed for the prediction of service-life and life-cycle cost analysis of RC bridges.

Bridges play an important role in society, especially during the post-earthquake period that enables emergency vehicles and traffic for safe egress and ingress to minimize the loss of property and life. However, some past earthquakes have resulted in large horizontal displacements on the superstructure that have lead to unseating of bridge spans and unexpected pounding forces that damaged critical components such as bearings and anchor bolts. To this end, a new bearing system, referred to as a stopperbearing system (SBS), is proposed as one solution to address the vulnerability of bridge bearings and other components. The horizontal displacement of a deck can be limited to a desired range using the SBS. The nonlinear load-deformation behavior of the SBS is obtained from ABAQUS and used to define the SBS within reinforced concrete analytical bridge models developed in SAP2000, which are subjected to the 1999 Chi- Chi, Taiwan earthquake ground motion (1.01g - E-W component and 0.43g - N-S component). The results from the nonlinear time history analyses show that the SBS is effective in limiting bridge deck displacements and pounding effects. Preliminary analytical modeling of the SBS shows promise as a solution to displacement control of bridge decks for overall enhancement of bridge performance during seismic events.

Thesis (Ph. D.)--University of Queensland, 2002. / Includes bibliographical references.

Recent technological advances have led to dramatic improvements in seismic resolution which now enable seismic attenuation to be measured on a routine basis. The usefulness of seismic attenuation as an interpretation parameter is restricted at present by our incomplete knowledge of the relationships between attenuation and important reservoir parameters such as porosity, permeability and the nature of the pore fluid. In order to redress this imbalance, P-wave and S-wave velocities and attenuations were measured in the laboratory at a frequency of about 0.8 MHz and at an effective pressure of 60 MPa on a set of reservoir rocks comprising twenty-seven shaly sandstones and two sandy shales. It was discovered that both P-wave and Swave attenuation increase with increasing percentage of intrapore minerals (especially clay, but also micrite) from the composition of a clean sandstone to a value of 50%, and then they decrease with increasing clay content as the rock approaches the composition of a pure shale. Porosity plays a subsidiary role to pore fill. More work is needed to predict permeability from its complex relationships with porosity and pore filling minerals. Ultrasonic P-wave and S-wave velocities (Vp and Vs) were shown to be strongly dependent on the amount of pore fill: velocity is reduced by increasing amounts of clays and micrite, but it is increased by increasing amounts of sparry calcite cement. Contrary to popular understanding, the effect on velocity of the percentage of pore filling minerals is much stronger than that of porosity on velocity, although porosity is still an important parameter. There is a strong correlation between Vp and Vs, and between P-wave quality factor (Qp) and S-wave quality factor (Qs). Interesting relationships were also discovered between Qp and Vp, and especially between Qs and Vs. However, high frequency laboratory measurements may not be representative of the low frequencies used in seismic exploration. The frequency dependence of velocity and attenuation in reservoir sandstones was investigated by exploiting the inverse relationship between frequency and pore fluid viscosity predicted by the Biot theory. P-wave and S-wave velocities and attenuations were measured at a frequency of about 0.8 MHz and at an effective pressure of 50 MPa on shaly sandstones saturated with pore fluids of viscosities 0.3 cP to 1000 cP (equivalent frequency range 2.6 MHz to 780 Hz). The Biot theory accounts for the very high Q values encountered in clean sandstones, but not the very low Q values observed in clay-rich sandstones. The observed velocity dispersion in both clean and clay-rich sandstonesim plies a local fluid flow mechanism which predicts the opposite frequency-viscosity dependence of the Biot theory. Qp and Qs in clay-rich sandstones remain constant over nearly four decades in equivalent frequency. This implies a range of relaxation times which may be attributed to the wide distribution of pore sizes of clays and other pore filling minerals. Moreover, this constant Q behaviour and the magnitude of the laboratory Qp and Qs values tie in well with those obtained from field studies. This suggests that Qp and Qs are broadly independent of frequency from seismic frequencies to ultrasonic frequencies in clay-rich sandstones.

No description available.