Thesis (M.S.)--Oregon State University, 2007. / Printout. Includes bibliographical references (leaves 111-121). Also available on the World Wide Web.

The Centroid-Moment Tensor (CMT) method allows an optimal point-source description of an earthquake to be recovered from a set of seismic observations, and, for over 30 years, has been routinely applied to determine the location and source mechanism of teleseismically recorded earthquakes. The CMT approach is, however, entirely general: any measurements of seismic displacement fields could, in theory, be used within the CMT inversion formulation, so long as the treatment of the earthquake as a point source is valid for that data. We modify the CMT algorithm to enable a variety of near-field seismic observables to be inverted for the source parameters of an earthquake. The first two data types that we implement are provided by Global Positioning System receivers operating at sampling frequencies of 1,Hz and above. When deployed in the seismic near field, these instruments may be used as long-period-strong-motion seismometers, recording displacement time series that include the static offset. We show that both the displacement waveforms, and static displacements alone, can be used to obtain CMT solutions for moderate-magnitude earthquakes, and that performing analyses using these data may be useful for earthquake early warning. We also investigate using waveform recordings - made by conventional seismometers deployed at the surface, or by geophone arrays placed in boreholes - to determine CMT solutions, and their uncertainties, for microearthquakes induced by hydraulic fracturing. A similar waveform inversion approach could be applied in many other settings where induced seismicity and microseismicity occurs.

No description available.

The Pampean flat slab subduction in west-central Argentina (latitude 30-33S) and the steeply dipping Northern Costa Rica subduction zone (latitude 9-11N) show significant along-trench variations in both the subducting and overriding plates. This dissertation contains the results of three seismological studies using broadband instruments conducted in these subduction zones, with the aim of understanding the structure of the lithosphere and the correlation between the variability observed in the downgoing and the overriding plates. In the Costa Rica region, by analyzing teleseismic receiver functions we investigate the variability in the hydration state of the subducting Cocos Plate and the nature of three distinct crustal terranes within the overriding Caribbean Plate: the Nicoya and Chorotega terranes that display an oceanic character, and the Mesquito Terrane, which is more compatible with continental crust.In the Pampean region of Argentina, we apply a regional-scale double-difference tomography algorithm to earthquake data recorded by the SIEMBRA (2007-2009) and ESP (2008-2010) broadband seismic networks to obtain high-resolution images of the South America lithosphere. We find that most of the upper mantle has seismic properties consistent with a depleted lherzolite or harzburgite, with two anomalous regions above the flat slab: a higher Vp/Vs ratio anomaly consistent with up to 10% hydration of mantle peridotite and a localized lower Vp/Vs ratio anomaly consistent with orthopyroxene enrichment. In addition, we study the geometry and brittle deformation of the subducting Nazca Plate by determining high-quality earthquake locations, slab contours, and focal mechanisms. Our results suggest that the subduction of the incoming Juan Fernandez Ridge controls the slab geometry and that ridge buoyancy and slab pull are key factors in the deformation of the slab. The spatial distribution of the slab seismicity suggests variability in the hydration state of the subducting Nazca Plate and/or in strain due to slab bending. These observations support the hypothesis that the along-trench variability in bathymetric features and hydration state of the incoming plate has profound effects in the subducting slab geometry and the upper plate structure in both flat and steeply dipping subduction zones.

Although there is no obvious reason why seismic 6-value and the spatial fractal dimension of earthquakes, D, should be related, there are several reports of observed empirical correlations between these two quantities. In order to investigate this phenomenon, and attempt to relate it to different types of earthquakes, industrially induced seismicity in The Geysers geothermal area, California and earthquake swarms in Long Valley caldera, California were analysed. Raw seismograms from the Unocal-NEC-Thermal network in The Geysers were processed automatically, calculating magnitudes from coda lengths and locating them using a three-dimensional velocity model. Seismicity correlated with the locations of commercial wells and surface fault locations. The entire Geysers dataset was too complex for clear correlations between b, D, seismicity and injection to be observed. In several cases, short pulses of injection induced bursts of seismicity of either small-magnitude, clustered events or large-magnitude diffuse seismicity, resulting always in a transient anomaly of negative b/D. However, sometimes pulses of injection were not accompanied by b/D transients and sometimes b/D transients were not accompanied by known injection. The latter cases may or may not indicate undisclosed injection activity. A seismic crisis in Long Valley caldera was associated with major b/D anomalies that accompanied migration of the activity from a hydrothermal zone on the south edge of the resurgent dome to the right-lateral, blind, near-vertical South Moat fault to the immediate south. The results indicated that the hydrothermal zone is an inhomogeneous structure whereas the South Moat has a mature, coherent fault plane, capable of generating magnitude M = 6 earthquakes and posing a threat to the town of Mammoth Lakes.

In general, the depths of shallow earthquakes are poorly resolved in current catalogues. Variations in depth of ±10 km can significantly alter the tectonic interpretation of such earthquakes. If the depth of a seismic event is in error then moment tensor estimates can also be significantly altered. In the context of nuclear-test-ban monitoring, a seismic event whose depth can be confidently shown to exceed say, 10km, is unlikely to be an explosion. Surface wave excitation is sensitive to source depth, especially at intermediate and short periods, owing to the approximate exponential decay of surface wave displacements with depth. The radiation pattern and amplitude of surface waves are controlled by the depth variations in the six components of the strain tensor associated with the surface wave eigenfunctions. The potential exists, therefore, for improvements to be made to depth and moment tensor estimates by analysing surface wave amplitudes and radiation patterns. A new method is developed to better constrain seismic source parameters by analysing 100-20s period amplitude spectra of fundamental-mode surface waves. Synthetic amplitude spectra are generated for all double-couple sources over a suitable depth range and compared with data in a grid-search algorithm. Best fitting source parameters are calculated and appropriate bounds are placed on these results. This approach is tested and validated using a representative set of globally-distributed events. Source parameters are determined for 14 moderately-sized earthquakes (5.4 ≤ M_w ≤ 6.5), occurring in a variety of tectonic regimes with depths calculated between 4-39km. For very shallow earthquakes the use of surface wave recordings as short as 15s is shown to improve estimates of source parameters, especially depth. Analysis of aftershocks (4.8 ≤ M_w ≤ 6.0) of the 2004 great Sumatra earthquake is performed to study the depth distribution of seismicity in the region. Three distinct tectonic regimes are identified and depth estimates calculated between 3-61km, including the identification of one CMT depth estimate to be in error by some 27km.

In this thesis I use a multi-disciplinary approach to investigate two areas of active mountain-building within the Alpine-Himalayan belt: the Altai range in western Mongolia, and the Zagros mountains in southern Iran. I begin by studying a clustered earthquake sequence that struck a previously unrecognised fault zone in the NW Altai mountains in 2003. By combining seismology and field observations with satellite radar interferometry (InSAR), I attempt to unravel the detailed history of faulting in time and space. Differences between body-wave and InSAR-based models prevent me from matching individual seismic events with individual fault segments, and I explore the cause of these discrepancies. In the following two chapters, I establish late Quaternary slip-rates on major right-lateral and thrust faults in the eastern part of the Altai. In particular, I explore the use of in situ-produced cosmogenic Be-10 and Optically Stimulated Luminescence (OSL) for dating offset alluvial fans and river terraces. My results suggest that faulting has migrated toward the eastern margin of the range from the high, interior Altai, presumably in response to stresses introduced by topography. In the final, main chapter, I investigate a link between buried reverse faulting and surface folding in the Zagros Simply Folded Belt. Using surface displacements measured with InSAR, I show that a major anticline on Qeshm Island was uplifted during an earthquake in 2005. However, the pattern of uplift is discordant with the growth of neighbouring folds, preventing us from establishing a simple connection between faulting and folding. All in all, my work demonstrates the importance of using several techniques in parallel when studying regions of active continental deformation.

ter have most of their energy held in the region of the water column near to the surface. Tsunamis are extremely long waves with long periods that can range from five minutes to several hours. Due to their very long wavelengths, tsunamis travel at the shallow water wave celerity which is equal to the square root of the gravitational acceleration times the water depth. The speed of a tsunami in the open ocean can reach in excess of 500 miles per hour. Tsunamis are also characterized by low wave height when moving through oceanic depths and are often hard to recognize when seen out in the deep ocean. As tsunamis approach the coastal region, they are affected by the rapid decrease in water depth. / US Navy (USN) author.

Seismology is the study of earthquakes and the Earth’s internal structure using seismic waves. Traditional seismology is constrained by the timing and location of seismic sources, and by the location of seismometers with which energy from the sources are recorded. Improvements in the global seismometer networks have reduced the latter constraint. Furthermore, recent advances into Seismic Interferometry (SI) have enabled detailed information about the Earth’s interior to be obtained using ambient seismic noise, hence even in areas with low natural seismicity. The most common approach to SI is to use the cross-correlation of ambient noise recordings to construct an estimate of the Green’s function between two seismometer locations. The Green’s function estimate is then analysed or inverted for seismic properties of the Earth. This method of noise interferometry is now a popular approach in earthquake seismology as in some situations it renders active seismic sources (earthquakes or synthesised explosions) obsolete, as subsurface information can be obtained even in times of seismic quiescence. This thesis investigates a different method: Source-Receiver Interferometry (SRI). SRI can be used to construct earthquake seismograms on seismometers that were not necessarily deployed when the earthquakes occurred - a form of ‘retrospective seismology’. This might be useful if, for example, we wish to analyse old earthquakes with newly installed seismometers. The application of SRI involves evaluating two interferometric integrals. The first integral is evaluated using ambient noise interferometry: at least 6 months of noise data is cross-correlated to estimate the Green’s functions between pairs of seismometers. These inter-receiver Green’s functions are then used as the “propagators” for SRI. Their role is to project earthquake signals recorded on a backbone array of seismometers to the location of a target sensor at which a new, novel earthquake seismogram is to be constructed - a form of spatial redatuming. To spatially redatum the earthquake data, the second interferometric integral is evaluated using either processes of correlation (resulting in correlation-correlation SRI) or convolution (correlation-convolution SRI). The method used depends on the relative location of the target sensors with respect to both the backbone seismometer array and the earthquake epicentre. The SRI process is completed by integrating (summing) over all projected earthquake signals. To regularise the spatial distribution of the projected earthquake data and to invoke this second interferometric integral more precisely, the backbone seismometers are embedded within 2D spatial Voronoi cells. New seismograms for 87 earthquakes were reconstructed on up to eight target sensors, seven of which were deployed when the earthquakes occurred and are used to test the success of the method by comparing with the SRI results with the directly-recorded seismograms. The seismogram reconstructions on the eighth target sensor are truly novel. The SRI method was developed to operate over two length scales. The first focusses on relatively small length scales in which the inter-station distance between the eight target sensors and the backbone array seismometers is between ~ 210 km and 540 km. Both correlation-correlation SRI and correlation-convolution SRI are used to reconstruct the earthquake seismograms on four of the same target sensors. Applying correlation-convolution SRI is shown to remove spurious signals associated with correlation-correlation SRI. Second, a significantly larger length scale is considered where a second set of target sensors are located up to 2420 km from a second backbone seismometer array. The correlation-correlation and correlation-convolution SRI methods are used in parallel to increase the spatial extent of the study. The quality of the SRI seismograms constructed is shown to depend on the quality of three components: 1) the SRI propagators constructed using ambient noise interferometry, 2) the earthquake signals recorded on the backbone seismometer array, and 3) the correlation (or convolution) functions that are summed in the second interferometric integral to construct the final SRI seismogram. The quality of each component is quantified by its signal-to-noise ratio and root-mean-square value, and criteria are proposed to obtain optimal earthquake seismogram reconstructions using SRI. SRI is most successful when the target sensors are located less than 540 km from the backbone array seismometers. Such SRI seismograms are being used to create a catalogue of new, ‘virtual’ earthquake seismograms that are available to complement real earthquake data for use in future earthquake seismology studies. An alternative approach to noise interferometry is also considered: the recordings from just 15 earthquakes are used to perform multidimensional deconvolution (MDD) to estimate the Green’s functions between pairs of seismometers. This is the first time such data has been used to perform MDD, which is valid in attenuating media and is thus theoretically more valid in earthquake seismology settings than correlational interferometry. The Green’s functions estimated using MDD are compared with those same Green’s functions estimated using ambient noise interferometry and the results are comparable on several occasions, despite using far fewer data for MDD. However, the quality of the results of MDD is significantly affected by the illumination of the receiver array from the earthquake sources. A greater density of earthquakes that sufficiently illuminates all backbone array seismometers is required to obtain accurate Green’s functions by MDD.

An accurate seismic hazard assessment can only be carried out if a homogeneous and sufficiently complete catalogue for the study area is available. Since the catalogue for southern Africa was last updated in the early 1990s for the Global Seismic Hazard Assessment Program (GSHAP), it is necessary that a new updated, homogeneous and complete earthquake catalogue be compiled that includes data acquired during the last two decades. The process of compiling the new earthquake catalogue for southern Africa (South Africa, Lesotho, Swaziland, Botswana, and Namibia) was done as part of the Global Earthquake Modelling (GEM) project. The data from published and unpublished sources, and databases from the South African National Seismograph Network (SANSN), Bulawayo (BUL), the Geological Survey of Botswana, the National Earthquake Information Centre (NEIC), the Advanced National Seismic System (ANSS), and the International Seismic Centre (ISC) were retrieved and evaluated. After the data from the different sources were merged, duplicates and induced earthquakes were removed. The catalogue was unified with all magnitude types converted to moment magnitude (MW). Unifying the southern African catalogue to one magnitude scale had multiple challenges, considering that the catalogue is mostly incomplete, and it was therefore not easy to derive relations between different magnitude scales. The question of which method and relations are most suitable for converting all magnitude scales to MW had to be addressed. To ensure that all the events are independent, several procedures were carried out to decluster the catalogue and most suitable method selected. The final catalogue includes all available events, i.e. historical, and instrumental events from 1690 to December 2011, excluding fore- and aftershocks and induced events. This catalogue has 920 events with MW ≥ 4 whereas GSHAP has only 100 events with MS ≥ 4 in the southern African region. The largest event in the final catalogue occurred in 1952 and is located in the Okavango Delta region in Botswana with magnitude MW = 6.7. The maximum likelihood method was used at each point on a grid covering the study area, to estimate the spatial distribution of the b-value and the activity rate. The maximum curvature method was used for estimating the spatial distribution of magnitude of completeness, which was also substantiated with the Gutenberg-Richter, time-scale and spatial-scale magnitude of completeness graphs.