Research report submitted to the Faculty of Engineering and the Built Environment, University of Witwatersrand, Johannesburg, in partial fulfilment of the requirements for the degree of Master of Science in Engineering Johannesburg 2017 / This report investigates the seismic response of catenary vaults. Through a series of tests, the inherent seismic resilience of catenary vaults was assessed and a number of reinforcement strategies were investigated to improve this. An analytical model, based on the virtual work method, was developed by Ochsendorf (2002) for the assessment of circular voussoir arches. This model was adapted for catenary vaults. This model is used to calculate the minimum lateral acceleration required to cause the collapse of a catenary vault (λmin) for any catenary profile. The model indicates that there is a linear relationship between cross sectional depth of the arch and λmin until the depth to ratio passes approximately 0.3, where the change in λmin becomes exponential. Using the model, it is also predicted that λmin decreases exponentially with an increase in the height to width ratio up to a value of approximately 1.6. After this point λmin linearly decreases with increased height to width ratios and approaches zero. The first series of tests involved subjecting unreinforced catenary vaults to seismic loading. In these tests the frequency of vibration was varied and the stroke was kept constant. From the results of the tests, it was found that there was no frequency at which the vaults underwent excessive vibration due to resonance. It was observed that during seismic loading, hinges form at locations where pre-existing cracks occur despite the higher computed λmin values for these positions. The tests also indicate that the vaults’ behaviour changes drastically with each hinge that forms. In the next series of tests the frequency was set and the stroke was increased. The vaults were subjected to seismic loading at 2 Hz and 6 Hz, representative of low and high frequencies respectively. The tests indicated that the collapse acceleration of arches subjected to vibration at 2 Hz was lower than that of the vaults subjected to vibrations at 6 Hz. Despite this, the stroke, representing ground movement, required to cause collapse at 2 Hz was substantially higher than that of the 6 Hz tests. This indicates that the duration of load cycles has an effect on the collapse acceleration. In comparing the computed collapse acceleration, λmin, with the actual collapse accelerations, it was found that the computed values are highly conservative. Yet this is expected as the model is based on an infinite duration of lateral loading. It was found that the analytical model was more accurate for low frequency tests as compared to high frequency tests in terms of the predicted hinge locations. Finally, three reinforcement strategies were investigated using basalt fibre geogrid. This was found to be an economical and viable reinforcement material. The first strategy consisted of laying the geogrid over the arch and securing it at the arch base. The second was the same as the first with the addition of anchors which held the geogrid down. The final strategy involved prestressing the arch using the geogrid. The latter 2 methods were found to be the most effective, with observed collapse accelerations being over 60% higher than that of the same unreinforced arch. The anchorage solution was found to be the most viable due to the substantially higher technical input required for the prestressing solution. / MT2017

Thesis advisor: John E. Ebel / Studies of aftershock sequences in the Central Virginia Seismic Zone (CVSZ) provide critical details of the subsurface geologic structures responsible for past and (possibly) future earthquakes in an intraplate setting. The 23 August 2011 MW 5.8 Mineral, Virginia, earthquake, the largest magnitude event recorded in the CVSZ, caused widespread damage and generated a lengthy and well-recorded aftershock sequence. Over 1600 aftershocks were recorded using a dense network of seismometers in the four months following the mainshock, offering the unique opportunity to study the fault structure responsible for the post-main event seismicity. Previous work has not accurately determined the geometry of the fault structure or the migration of post-mainshock seismicity and association of the 2011 event with a known fault has been unsuccessful. In this study, relative locations of recorded aftershocks were calculated using a version of the double-difference location method outlined in Ebel et al. (2008) to generate an accurate model of the fault structure. The moment tensor inversion technique of Ebel and Bonjer (1990) was used to generate focal mechanisms of dozens of the aftershocks at various locations on the fault structure. Results from the double-difference and moment tensor inversion methods were used to map the structure responsible for the aftershock sequence in high resolution. The calculated fault structure has planes with similar strikes and dips as known faults and geologic structures in the CVSZ. In-depth analysis of this aftershock sequence provides seismologists with the opportunity to better understand the seismic hazards present in poorly understood intraplate seismic zones. / Thesis (MS) — Boston College, 2016. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

3 offprints in back pocket / 186 [26] xiv leaves : ill. ; 26 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.) from the Dept. of Physics, University of Adelaide, 1973

High seismic activity occurs along the Caribbean and the North American Plate boundary beneath the eastern part of Hispaniola. A large number of intermediate to deep earthquakes are clustered between the Puerto Rico Trench to the north and the Muertos Trench to the south suggesting the possibility of concurrent subduction from both north and south. The body wave inversion technique was used to analyze nine earthquakes, the largest teleseismically recorded events since the establishment of WWSSN (World Wide Standardized Seismograph Network) in 1963 in the geographic region between 72°W and 66°W latitude and 16°N and 21°N longitude. Their body-wave magnitude ranges from 5.6 to 6.1. Each event was inverted for strike, dip and slip of the two possible fault planes, as well as for the centroid depth, the total seismic moment and the source time function. In order to optimize the crustal structure parameters used in the body wave inversion method, a two-dimensional geophysical cross-section across Hispaniola was constructed by forward modeling of gravity and magnetics data. The inversion results are consistent and can be divided into groups according to the depth and the epicentral location of the events. The shallow events, with depths of 6 to 12 km, represent crustal deformation and show thrust mechanisms with large strike-slip component. The intermediate depth events range from 42 to 107 km in depth and occur to the south of eastern Hispaniola. They show clear thrust mechanisms with a consistent dip of the compressional P-axis at about 30° to the north and approximately north-south P-axis strike. The deep earthquakes occur between 110 and 177 km depth, have a steep-dipping tensional T-axis, and define another slab, possibly originating at the Puerto Rico Trench to the north. One m[subscript b]=6.l event, which occurred on 6/24/84, shows opposite orientations of the P- and T-axes from the surrounding intermediate events. It is interpreted as an interface event in the upper mantle. The southern subduction zone is well defined and indicates that the Muertos Trench is active, with the subducting plate dipping to the north beneath eastern Hispaniola. At a depth of about 110 km, the northward dipping slab collides with the almost vertical segment of the other slab. This deep vertical slab segment, extending to at least 200 km in depth, may be a remnant of an earlier subduction zone associated with the Puerto Rico Trench. Alternatively, it may be connected with a more gently dipping part of the slab towards the north or, even in some way, with subduction from the south. / Graduation date: 1990

In this thesis, we present the first detailed, long-term seismotectonic studies of oceanic ridge-transform systems. The proximity of the Juan de Fuca plate to a network of broadband seismic stations in western North America provides a unique synergy of interesting tectonic targets, high seismicity, and recording capabilities. Our main tools are earthquake source parameters, determined by robust waveform modeling techniques, and precise earthquake locations, determined by joint epicenter relocation. Regional broadband data are used to invert for the source moment tensors of the frequent, moderate-sized (M ≥ 4) earthquakes; this analysis began 1994. We include Harvard centroid moment-tensors available since 1976 for larger (M ≥ 5) earthquakes. Two studies comprise the main part of this thesis. In the first, we determine the current tectonics of Explorer region offshore western Canada. Earthquake slip vector azimuths along the Pacific-Explorer boundary require an independent Explorer plate. We determine its rotation pole and provide a tectonic model for the plate's history over the last 2 Ma. Plate motion changes caused distributed deformation in the plate's southeast corner and caused a small piece in the southwest corner to transfer to the Pacific plate. Capture of the plate fragment indicates that preserved fragments not necessary represent entire microplates. In the second study, we investigate seismicity and source parameters along the Blanco Transform Fault Zone (BTFZ). The deformation style-- strike slip and normal faulting-- correlates well with observed changes in BTFZ's morphology. We infer that Blanco Ridge probably consists of two fault segments, that several parallel faults are active along BTFZ's west part, and that Cascadia Depression possibly is a short spreading center. The slip distribution along the BTFZ is highly variable, although seismicity could account for the full plate motion rate along the entire BTFZ. The final part is a short study where we locate an earthquake in the tectonically active Mendocino triple junction region offshore northern California using land and offshore data. The precise location may be useful as a master event for relocating other earthquakes. / Graduation date: 1999

Graduation date: 1987 / Best scan available for figures.

The crust and upper mantle seismic structure beneath southwestern Mexico was investigated using several techniques including teleseismic tomography using 3D raytracing, a joint tomographic inversion of teleseismic and regional data that included relocation of regional seismicity, and a P to S converted wave study. The data used in these studies came from a broadband seismic deployment called MARS. The seismic deployment lasted 1.5 years from January 2006 to June 2007 and the stations covered much of Jalisco and Colima states as well as the western part of Michoacan states. At depth less than 50 km, P-wave receiver function images show a clear dipping slow velocity anomaly above a fast velocity layer. The slow anomaly convertor seen in receiver functions is directly above a fast dipping seismic anomaly seen in regional tomography results. The slow velocity with high Vp/Vs ratio is interpreted as a high pore fluid pressure zone within the upper layer of subducting oceanic crust. Regional seismicity was located using the double difference technique and then relocated in a tomography inversion. The seismicity is located very close to the slow dipping boundary to depths of 30-35 km and thus along the plate interface between the subducted and overlying plate. Deeper events are below the slow layer and thus are intraplate. Receiver function results also show a weaker continental Moho signal above the dipping slab that I interpret as a region of mantle serpentinization in the mantle wedge. Inland of the subduction zone, a clear Moho is observed with a maximum thickness of near 42 km although it thins to near 36 km depth towards the north approaching the Tepic-Zacoalco Rift. Using H-K analysis to examine Vp/Vs ratios in the crust, I find a band of very high Vp/Vs along the Jalisco Volcanic lineament as well as beneath the Michoacan-Guanajuato volcanic field. These observations suggest the continental crust is warm and possibly partially molten over broad areas associated with these two magmatic regions and not just locally beneath the volcanoes. I also found seismicity associated with the Jalisco Volcanic Lineament but it was trenchward of the volcanoes. This may indicate extension in this region is part of the explanation for this magmatic activity. At depths below 100 km, the tomography results show clear fast anomalies, about 0.3 km/s faster than the reference model, dipping to the northeast that I interpret as the subducting Rivera and Cocos plates. Tomography models show that the Rivera slab is dipping much steeper than the Cocos plate at depth. Below 150 km depth, the Rivera plate shows an almost vertical dip supporting the interpretation that the slab has steepened through time beneath Jalisco leading to a coastward migration of young volcanism with mixed geochemical signatures. The location of the young volcanism of the Jalisco Volcanic Lineament is just at the edge of the steeply dipping slab seen in the tomography. The magmatism is thus likely a nascent arc. The models also display evidence of a gap between the Rivera and Cocos plates that increases in width with depth marking the boundary between the two plates. The gap lies just to the west of Colima graben and allows asthenosphere to rise above the plates feeding Colima volcano. Another interesting finding from this study is a possibility of a slab tear along the western edge of the Cocos plate at a depth of about 50 km extending 60 km horizontally. The tear is coincident with a lack of seismicity in this region although there are events below and above the tear. / text

The Nazca-South America convergent margin is marked by the presence of the Andean mountain belt, which stretches along the 8000-km long western margin of the South American plate. The subduction zone is characterized by significant along-strike changes in both upper plate structure and slab geometry that make it an ideal region to study the relationship between the subducting slab, the surrounding mantle, and the overriding plate. This dissertation summarizes the results of three finite frequency teleseismic tomography studies of the central Nazca-South America subduction zone which improve our understanding of how along-strike variations in the Andean mountain belt and the subducting Nazca plate interact with each other and with the surrounding mantle. This is accomplished by first focusing on two smaller adjacent regions of the central Andes to explore upper mantle variations and then by using a combined dataset, which covers a larger region, to image the deeply subducted Nazca slab to investigate the fate of the slab. The first study focuses on the central Andean upper mantle under the Altiplano-Puna Plateau where normally dipping subduction of the Nazca plate is occurring (18° to 28°S). The shallow mantle under the Eastern Cordillera is generally fast, consistent with either underthrusting of the Brazilian cratonic lithosphere from the east or a localized "curtain" of delaminating material. Additional evidence for delamination is seen in the form of high amplitude low velocities under the Puna Plateau, consistent with proposed asthenospheric influx following lithospheric removal. In the second study, we explore the transition between normal and flat subduction along the north edge of the Altiplano Plateau (8° to 21°S). We find that the Peruvian flat slab extends further inland along the projection of the Nazca Ridge than was previously proposed and that when re-steepening of the slab occurs, the slab dips very steeply (~70°) down through the mantle transition zone (MTZ). We also tentatively propose a ridge parallel tear along the north edge of the Nazca Ridge. Both of these observations imply that the presence of the Nazca Ridge is at least locally influencing the geometry of the flat slab. The final study investigates along-strike variations in the deeply subducted Nazca slab along much of the central Nazca-South America subduction zone (6° to 32°S). Our results confirm that the Nazca slab continues subducting into the lower mantle rather than remaining stagnant in the MTZ. Thickening of the slab in the MTZ north of 16°S is interpreted as folding or buckling of the slab in response to the decreased slab sinking velocities in the lower mantle.

No description available.

No description available.