Source: Dissertation Abstracts International, Volume: 31-09, Section: B, page: 5432. / Thesis (Ph.D.)--The Florida State University, 1970.

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This thesis presents a 3-D Common Conversion Point (CCP) image to examine the lithospheric structure in the Mendocino Triple Junction (MTJ) region. I have used 186 teleseismic earthquakes recorded in the Flexible Array Mendocino Experiment (FAME) to generate the stacked CCP image of PdS receiver functions. The resulting image not only shows the existence of a slab window beneath the northern Coast Ranges, as proposed by the slab window hypothesis, but also reveals the slab window structure in detail for the first time. The slab window is identified as a complex low velocity body with its top connected to the Gorda lithosphere-asthenosphere boundary (LAB). The asthenospheric mantle might upwell as shallow as ∼50 km near Clear Lake and Lake Pillsbury, resulting in high heat flow and basalt intrusions respectively. This work can help develop a dynamic model of the three plates involved in the MTJ's migration.

<p>The mapping of induced seismicity in enhanced geothermal systems presents the best tool available for understanding the resulting hydro-fractured reservoir. In this thesis, two geothermal systems are studied; one in Krafla, Iceland and the other in Basel Switzerland. The purpose of the Krafla survey was to determine the relation between water injection into the fault system and the resulting earthquakes and fluid pressure in the subsurface crack system. The epicenters obtained from analyzing the seismic data gave a set of locations that are aligned along the border of a high resistivity zone ~2500 meters below the injection well. Further magneto-telluric/seismic-data correlation was seen in the polarity of the cracks through shear wave splitting. The purpose of the Basel project was to examine the creation of a reservoir by the initial stimulation, using an injection well bored to 5000 meters. This stimulation triggered a M3.4 event, extending the normal range of event sizes commonly incurred in hydro-fractured reservoirs. To monitor the seismic activity 6 seismometer sondes were deployed at depths from 317 to 2740 meters below the ground surface. During the seven-day period over 13,000 events were recorded and approximately 3,300 located. These events were first located by single-difference techniques. Subsequently, after calculating their cross-correlation coefficients, clusters of events were relocated using a double-difference algorithm. The event locations support the existence of a narrow reservoir spreading form the injection well. Analysis of the seismic data indicates that the reservoir grew at a uniform rate punctuated by fluctuations which occurred at times of larger events, which were perhaps caused by sudden changes in pressure. The orientation and size of the main fracture plane was found by determining focal mechanisms and locating events that were similar to the M3.4 event. To address the question of whether smaller quakes are simply larger quakes scaled down, the data set was analyzed to determine whether scaling relations held for the source parameters, including seismic moment, source dimension, stress drop, radiated energy and apparent stress. It was found that there was a breakdown in scaling for smaller quakes.</p> / Dissertation

<p> Gravity gradiometry has been used as a geophysical tool to image salt structure in hydrocarbon exploration. The knowledge of the location, orientation, and spatial extent of salt bodies helps characterize possible petroleum prospects. Imaging around and underneath salt bodies can be challenging given the petrophysical properties and complicated geometry of salt. Methods for imaging beneath salt using seismic data exist but are often iterative and expensive, requiring a refinement of a velocity model at each iteration. Fortunately, the relatively strong density contrast between salt and background density structure pro- vides the opportunity for gravity gradiometry to be useful in exploration, especially when integrated with other geophysical data such as seismic. Quantitatively integrating multiple geophysical data is not trivial, but can improve the recovery of salt body geometry and petrophysical composition using inversion. This thesis provides two options for quantitatively integrating seismic, AGG, and petrophysical data that may aid the imaging of salt bodies. Both methods leverage and expand upon previously developed deterministic inversion methods. The inversion methods leverage seismically derived information, such as horizon slope and salt body interpretation, to constrain the inversion of airborne gravity gradiometry data (AGG) to arrive at a density contrast model. The first method involves constraining a top of salt inversion using slope in a seismic image. The second method expands fuzzy c-means (FCM) clustering inversion to include spatial control on clustering based on a seismically derived salt body interpretation. The effective- ness of the methods are illustrated on a 2D synthetic earth model derived from the SEAM Phase 1 salt model. Both methods show that constraining the inversion of AGG data using information derived from seismic images can improve the recovery of salt.</p>

This work presents geophysical investigation of the rock properties of crustal boundaries in Colorado and Wyoming that were established during Proterozoic continental amalgamation. I used multicomponent seismic reflection/refraction data to determine seismic velocities, Poisson's ratios and geometries of shallow subsurface structures across the Cheyenne Belt, an Archean-Proterozoic boundary in southeastern Wyoming, and high-frequency geoid data for modeling density contrasts associated with crustal boundaries in Wyoming and Colorado. I adapted a time-domain-based filtering technique described by Butler and Russell (1993) to filter the multicomponent seismic data because high-amplitude harmonic noise obscured P- and S-wave first arrivals. The travel-times of filtered P-wave first arrivals were inverted to obtain a model of both P-wave velocity and subsurface geometry. Since S-wave data quality was inferior to that of the P-wave data and S-wave ray coverage of the subsurface was discontinuous, I proposed a method to estimate Poisson's ratio using SiO2 concentration and the average atomic weight (AAW) of a formation with known mineral and oxide compositions. Subsequently, the final P-wave velocity model was converted into an initial S-wave model using Poisson's ratios estimated by this method. The S-wave data were inverted for velocities only, keeping the subsurface geometry derived from P-wave inversion constant. The dependence of Poisson's ratio on AAW and SiO2 concentration, and measured mineral Poisson's ratios, permitted estimation of two- or three-mineral compositions of formations in the vicinity of the seismic line from the Poisson's ratio model calculated using final P- and S-wave velocity models. Geoid data were modeled along four north-south profiles with positive density contrasts in the crust compensated by deeper negative density contrasts. The modeled crustal-scale bodies were correlated to regional geological features based on their relative locations. Thus, out of an infinite number of possible models explaining the geoid anomalies, I obtained one that fits both the geoid data and current tectonic models.

Seismic source complexity, which may be due to fault geometry changes or discrete subevents, is difficult to quantify with conventional moment tensor inversion techniques. The time-dependent moment tensor (TDMT) inversion procedure is an attempt to extract a time varying source from teleseismic P-waves. The correctness and the limits of the procedure are tested by synthetic examples. To remove the constant moment tensor assumption of the conventional time-independent moment tensor (TIMT) approach, a higher degree of freedom of the linear inversion system is required. The inversion is performed over a range of depths, and the solution is decomposed into subevents with varying depths and source geometry by the temporal consistency of the individual moment tensor elements and the condition of causality. Analysis of the synthetic examples indicates that the inverted moment tensor is sensitive to the crustal structure, particularly the crustal thickness. The inverted focal mechanism and the shape of the moment tensor elements are stable with respect to the uncertainties of the epicenter locations. Source processes of three recent complex earthquakes were investigated by the TDMT analysis. The results show that the October 18, 1992 Colombia earthquake is a double event with similar focal mechanisms. The first subevent is a smaller initial phase about 8 sec long. The second subevent initiated right after the initial phase. A total of five subevents are recognized for the July 30, 1995 Northern Chile earthquake. Four subevents ruptured in the first 34 sec with similar dip-slip focal mechanisms while the fifth subevent has a strike-slip focal mechanism. The Solomon Islands earthquake on August 16, 1995 is dominated by two shallowly dipping, dip-slip subevents with about the same moment release. Both TIMT and TDMT techniques and the GIS (Geographic Information Systems) are combined to study the tectonics of the Northwestern Colombia region. The focal solutions of the strongest events in this area are recovered and the geological environment is mapped by GIS. The 1992 sequence likely ruptured the Murindo fault system. The prevailing focal mechanism in this area suggests that a northwest-southeast compressional stress regime has dominated in the past 20 years.