Tesis sobre el tema "S-wave velocity"

Results from two-station surface-wave inversions across the Archean Kaapvaal craton of southern Africa are compared with seismic velocities estimated from approximately 100 mantle xenoliths brought to the surface in kimberlite pipes. As the xenoliths represent a snapshot of the mantle at the time of their eruption, comparison with recently recorded seismic data provides an opportunity to compare and contrast the independently gained results. These cratonic xenoliths from the southern Kaapvaal, all less than 100Ma in age, have been analyzed geothermobarometrically to obtain the equilibrium P-T conditions of the cratonic mantle to about 180km depth [James et al 2004]. Seismic velocity-depth and density-depth profiles calculated on the basis of these P-T data and the mineral modes of the xenoliths are used to produce theoretical surface-wave dispersion curves and to generate roughly the upper 200km of a starting/reference model. A regionally-developed crustal structure [Niu and James 2002] was used for the crust and 300km of mantle values taken from PREM filled in down to 500km depth. This composite model was used as the starting/reference model for a Neighbourhood Algorithm surface-wave inversion using fundamental-mode Rayleigh-wave phase velocities for 16 paths within the Kaapvaal Craton from five events. The velocity structures found by that inversion are consistent with those derived from the xenolith data. Hence the velocity structure (i.e. thermal structure) of the mantle to a depth of 180km beneath the Kaapvaal craton is basically the same today as it was 80-90Ma. Further, synthetics runs show that for this surface-wave dataset, there is no strong low-velocity zone at depths shallower than at least 200km.
Master of Science

The velocity structures of southern and central part of Sweden have been derived with the local tomography (LET) method. The region has been divided into two study areas and the datasets come from the P- and S-wave traveltimes recorded by the Swedish National Seismic Network (SNSN). Man-made explosions and earthquakes occurring over the period of 5 years and 10 years, respectively, within the study areas have been used. One-dimensional starting models were derived based on an a priori model obtained from the SNSN, that were later used for starting models in the inversion for the 3-D crustal structures of the study areas. Attempts were also made to invert for Moho topography in the areas. The study areas are found to have an upper-crustal thickness of approximately 20 to 25 km and the Moho boundaries vary from 42 to 46 km in depth. The Vp/Vs ratios varies from about 1.68 to 1.78. The LET method appear to resolve the different between the Sveconorgwegian and Svecofennian orogen regions, but the stations and sources are too sparsely distributed for higher resolution models. The seismicity in the study areas are distributed in two distinctive depth ranges. The focal depth of the SNSN catalogued earthquakes concentrated in approximately 5 km and 15 - 20 km depth. Relocations of the earthquakes using a global search method reduced this tendency. The results also show that using 3-D models produces less biased results than using 1-D models with the same relocation method.

Earthquakes are among the most destructive natural disasters affecting urban populations. Structural damage caused by the earthquakes varies depending not only on the seismic source and propagation properties but also on the soil properties. The amplitude and frequency content of seismic shear waves reaching the earth&rsquo
s surface is dependent on local soil conditions. It is well known that the soft sediments on top of hard bedrock can greatly amplify the ground motion and cause severe structural damage. When the fundamental period of the soil is close to the fundamental period of a structure, structural damage increases significantly. Estimation of the fundamental periods, amplification factors and types of soils is critical in terms of reduction of loss and casualties. For the reasons stated, estimation of dynamic behavior of soils has become one of the major topics of earthquake engineering. Studies for determining dynamic properties of soils depend fundamentally on the estimation of the S-wave velocity profiles, amplification factors and ground response. In this study first, the Multi-Mode Spatial Autocorrelation (MMSPAC) method is used to estimate the S-wave velocity profiles at the sites of interest. This method is different than the other ones in the sense that it works for the higher modes as well as the fundamental mode. In the second part, Horizontal to Vertical Spectral Ratio (HVSR) method will be used on both microtremor and ground motion data. Finally, the amplification factors from alternative methods are compared with each other. Consistent results are obtained in terms of both fundamental frequencies and amplification factors.

Les glissements de terrain argileux affectent de nombreux versants à travers le monde et menacent régulièrement les activités humaines dans les zones urbanisées montagneuses. Ces glissements sont caractérisés par des cinématiques souvent lentes mais ils peuvent brutalement se liquéfier et accélérer de manière imprévisible. Cette transition solide-liquide a été étudiée sur les argiles de la région du Trièves (Alpes Françaises) à l'aide d'études rhéologiques. Elles ont montré le caractère de fluide à seuil thixotrope avec une bifurcation de viscosité importante lors de la fluidification pouvant expliquer le caractère catastrophique de l'accélération observée sur le terrain. Cette perte de rigidité du matériau peut être observée par une chute de la vitesse des ondes de cisaillement (Vs). Des études réalisées en parallèle à la fois sur un modèle analogique de plan incliné et sur le terrain (glissement de Pont-Bourquin, Suisse) ont permis d'observer une chute de Vs précédent à cette fluidification montrant ainsi que Vs pourrait être un bon proxy pour la surveillance des instabilités de terrain argileux
Landslides affect many clay slopes in the world and regularly threaten people in urban areas mountainous. These landslides are characterized by a slow velocity but they may suddenly liquefy and accelerate unexpectedly. The solid-liquid transition on the clay has been studied of Trièves region (French Alps) using rheological experiments. They have shown the yield stress thixotropic behavior with a viscosity bifurcation which can explain the catastrophic fluidization observed in the field. This loss of material stiffness can be followed by a drop in the shear wave velocity (Vs). Inclined plane test and field experiments (Pont-Bourquin landslides in Switzerland) have both shown a precursor drop of Vs indicating that it could be a good proxy for monitoring unstable clay slope

Les réservoirs carbonatés sont exploités pour leur réserve d’eau potable, de ressource géothermique ou le stockage géologique du dioxyde de carbone. Ces réservoirs sont difficiles à caractériser à cause d’une histoire diagénétique souvent complexe. Cette thèse offre de nouvelles perspectives sur la caractérisation des propriétés pétrophysiques et élastiques des calcaires urgoniens de Provence. Une approche intégrée et multi-échelle est proposée pour caractériser les propriétés pétrophysiques et élastiques des carbonates. Cette étude est basée sur des mesures de vitesse d’ondes P (Vp) et S (Vs) à l’échelle du laboratoire (centimètre–décimètre) et du terrain (mètre–décamètre). En laboratoire, les Vp et Vs ainsi que l’anisotropie sont mesurées sur des plugs et sur des carottes, en utilisant différentes fréquences centrales ultrasonores. Sur le terrain, l’approche consiste à mesurer les Vp et Vs entre deux puits distant de 2 m sur une profondeur de 14 m. Les mesures sont ensuite interprétées en fonction de la géologie observée aux échelles macro- et microscopiques. Les principaux résultats montrent que les Vp et Vs moyennes sont indépendantes de l’échelle de mesure, car elles sont dictées par la porosité. L’anisotropie causée par les fractures (15%) et les hétérogénéités (5%) se manifestent par une variabilité autour des vitesses moyennes. L’approche adoptée ici a permis de définir les interactions entre les propriétés de la matrice, les hétérogénéités, les fractures et les propriétés élastiques des roches carbonatées. Elle a montré que les propriétés élastiques tout comme les structures géologiques varient en fonction de l’échelle
Carbonate reservoirs are also exploited for water production, geothermal energy, and carbon geological storage. Their Geophysical characterization remains challenging because of complex diagenetic history. This work offers new insights into the characterization of petrophysical, and elastic properties of the Urgonian limestones in the Provence region. An integrated multi-scale approach is proposed to characterize carbonate rocks petrophysical and elastic properties. This study relies on P- and S-wave velocity (Vp and Vs) measurements carried out at laboratory (centimeter–decimeter) and field (meter–decameter) scales. Laboratory scale Vp, Vs, and anisotropy are measured on plugs and cores, while on the field they are measured between two boreholes (crosshole) over a distance of 2 m and 14 m depth. The measurements are then compared to the geology from the macro- to the microscopic scale. The main results show that the average Vp and Vs are porosity related, and are independent from scale. Anisotropy caused by fractures (15%) and heterogeneities (5%) is responsible for variations around the mean velocities. The approach adopted during this work has enabled to scope out the interplay between matrix properties, heterogeneity, fracturing, and elastic properties in carbonate rocks. It has shown that the elastic properties evolve with scale as well as the geological structures

The aim of this diploma thesis is to assess the applicability of pulse wave propagation monitoring in the cardiovascular system in the field of prediction and early diagnosis of abdominal aortic aneurysm (AAA). The very first part is focused on description of heart and blood vessels with its pathological changes in presence of aneurysm. For this reason, current methods of monitoring and surgical treating of AAA were mentioned. Due to their difficult clinical use widely in the population, new methods based on pulse wave monitoring were presented. Using an analytical approach we estimated the difference in the arrival of the pulse wave at measurable locations between healthy and pathological aorta in the order of miliseconds. By experimental monitoring using photoplethysmographic sensors, we observed significant changes of pulse wave velocity with respect to the mechanical properties of the artery wall (mainly associated with age), which we tried to implement by hyperelastic material models used in computational simulations of pulse wave proagation on simplified geometries by fluid structure interaction method. These analyzes should verify applicability of FSI simulations in further development of diagnostic methods of AAA.

Utilizamos neste trabalho duas metodologias distintas, a função do receptor com ondas P e a função do receptor com ondas S, para mapear variações da crosta e interfaces do manto (litosfera-astenosfera, 410 km e 660 km) em diferentes estações sismográficas na placa Sul-Americana. No estudo da interface litosfera-astenosfera, por ser o primeiro realizado nesta região, utilizamos as estações temporárias do IAG/USP em conjunto com as estações permanentes da rede mundial cobrindo toda a placa Sul-Americana. O estudo para as outras interfaces (Crosta-Manto, 410 km e 660 km) foi feito com caráter regional, buscando detalhar características da crosta e manto na região estável da placa. Para ambos os métodos os traços (sismogramas) foram rotacionados para o sistema LQT, deconvolvidos, agrupados por pontos de perfuração e por estações, e finalmente empilhados. Nos traços empilhados as fases convertidas de interesse (Ps, Ppps, Ppss+Psps e Sp) foram identificadas e interpretadas. Para a parte estável da placa obtivemos um valor médio de espessura da crosta de 39.4±0.6 km, variando desde 31.0±0.5 km para a província Borborema, até 41.3±1.0 km para a bacia do Paraná, onde aplicamos uma correção para descontar o efeito do sedimento. A razão de velocidade para a crosta, Vp/Vs, apresentou valores mais altos para a bacia do Paraná (~1.75±0.08) e região litorânea oriental (>1.74), enquanto que as regiões cratônicas (cráton São Francisco e Amazônico) apresentaram valores de Vp/Vs baixos (<1.72), chegando até 1.68. O valor médio de Vp/Vs para todas as estações analisadas foi de 1.73±0.02. As variações dos tempos para as interfaces do manto mostraram boa correlação com resultados de tomografia sísmica de outros trabalhos, indicando alterações de até 5% na velocidade das ondas sísmicas para o manto superior sob os crátons, uma deflexão de até 15 km na interface de 660 km para a região Sul da bacia do Paraná e se mostraram bem correlacionadas com as médias globais para as outras região estudadas. Por fim, a espessura da litosfera apresentou valores desde ~40 km, sob as regiões de ilhas oceânicas, até ~160 km, sob as regiões mais estáveis. Para as regiões oceânicas a espessura da litosfera se mostra correlacionada com a idade da placa. À medida que adentramos a parte continental, o limite litosfera-astenosfera se torna menos proeminente, atingindo profundidades maiores no interior dos continentes e menores para as regiões marginais. Para a zona de subducção, observamos duas possíveis litosferas, uma oceânica, subduzindo junto com a placa de Nazca, e outra pertencente à parte continental.
Two distinct methodologies, the P- and S-wave receiver functions, are used to map variations in the crustal parameters (thickness and Vp/Vs) and mantle interfaces (lithosphere-asthenosphere, 410 km and 660 km) on a number of different seismograph stations located in the South American plate. The results of the S receiver function for the lithosphere-asthenosphere boundary are the first of this kind ever performed in South American continent and showed the large scale variations of this interface. To perform this study we analyze data from various global permanent stations together with all available data from temporary stations operated by the IAG/USP during the last15 years. For both methods the traces (seismograms) were rotated to the LQT system, deconvolved, grouped by piercing points and stations, and finally stacked. In the stacked traces, the converted phases (Ps, Ppps, Ppss+Psps and Sp) were identified and interpreted. Inside the stable part of the plate we found a mean crustal thickness of 39.4±0.6 km, ranging from 31.0±0.5 km in Borborema Province up to 41.3±1.0 km in the Paraná Basin, where we applied a correction to remove the sediment effects on the crustal estimates. The crustal velocity ratios, Vp/Vs, showed higher values for the Paraná Basin (~1.75±0.08) and Ribeira belt (>1.74), while the cratonic regions (São Francisco and Amazon cratons) showed low values of Vp/Vs (<1.72), down to 1.68. The average Vp/Vs obtained for all stations was equal to 1.73±0.02. The observed times of the converted mantle phases presented a good correlation with other tomographic studies, indicating that the upper mantle for the cratonic roots may be characterized by a variation up to 5% in seismic velocities, a 15 km deflection in the South Paraná 660 km discontinuity (probably due to a decreased temperature caused by the subducted slab); for other regions the converted times were close to the global average. As a final result, the lithospheric thickness presented values ranging from ~40 km under oceanic islands, to ~160 km under the stable continental regions. We found that for the oceanic islands the thickness of the lithosphere is correlated with the age of the plate. When we go further inside the continents, the lithosphere-asthenosphere boundary becomes less sharp, reaching larger depths inside the continents and shallower depths near the continental margin. In the Andean subduction area, we observed two possibles lithospheres, one oceanic, subducting together with the Nazca plate, and another belonging to the Continent, parallel to the crust interface.

Although much progress has been made in determining the three dimensional distribution of seismic wave velocities in the Earth, substantially less is known about the three dimensional distribution of intrinsic attenuation. In this study variations in attenuation and shear velocity of the Earth's mantle are constrained using measurements of differential travel time and attenuation. The data are broadband displacement SH seismograms filtered to have energy in the period range 8 to 20 s. Broadband data are used as they should allow a more accurate estimation of body wave attenuation to be made. The seismograms are obtained from over 600 globally distributed earthquakes of magnitude, Mw, 5.5 or greater. Two new methods for determining differential travel times and differential t* values from multiple S phases are presented. The first of these, referred to as the "waveform fitting method" is used to analyse approximately 4300 SS and S waveforms and around 1000 SSS and SS waveforms resulting in differential SS-S and SSS-SS travel times, and corresponding values of differential attenuation represented by t*. The second method, referred to as the "spectral ratio method" is used to analyse approximately 3200 SS and S and around 900 SSS and SS waveforms. The differential travel times and t* values are inverted to obtain models of the lateral variation of shear velocity and lateral variation of q(mu) where q(mu) =1/Q(mu). The models explain the data well but have limited depth resolution. The velocity models show good correlation with previous studies, in particular, low velocities are observed underlying spreading ridges and convergent margins and high velocities are observed under continental regions. The q(mu) model shows shield regions to be less attenuating than PREM, with ridges appearing as highly attenuating features. Models of shear velocity and attenuation are also obtained by combining the body wave dataset of this study with the surface wave datasets of Van Heijst (1997) and Selby (1998).

L'étude des données Lithoscope permet d'étudier les structures profondes de la lithosphère. Trois méthodes ont été développées. La première porte sur l'analyse des ondes converties à l'interface croûte-manteau afin de contraindre les variations d'épaisseur de croûte. La seconde méthode caractérise l'orientation des structures dans le manteau, par la rnesure de l'anisotropie des ondes S. La troisième méthode mesure l'atténuation des milieux échantillonnés à l'aide de deux méthodes d'analyse fréquentielle et temporelle. Ces trois méthodes ont été appliquées aux enregistrements provenant du Nord Tibet, du Massif Central et de l'Abitibi, L'étude des données du plateau tibétain a permis de mettre en évidence des variations latérales très importantes de vitesse et de structure dans la lithosphère. Un saut de Moho de 20km a été observé sans qu'aucune variation de topographie ne soit observable. Ces travaux montrent que les phénomènes de l'équilibrage dans la lithosphère sont lents comparés aux mouvements horizontaux et verticaux. L'étude de l'anisotropie a permis de révéler l'importance de l'extrusion latérale le long de la faille du Kunlun. Les valeurs d'anisotropie obtenues impliquent que les mouvements horizontaux des blocs lithosphériques orientent les minéraux sur des épaisseurs mantelliques supérieures à 260 km. L'étude de l'atténuation sous le Massif Central a permis de constater qu'il existait une bonne corrélation entre les zones atténuantes et ies édifices volcaniques majeurs de cette région. Les valeurs du facteur de qualité obtenues montrent que les perturbations occasionnées par le volcanisme intéressent l'ensemble de la lithosphère mais sur de zones très étroites. Les modé!isations de perturbation thermique dans la croûte montrent de très bonnes corrélations avec les âges des différentes crises volcaniques. L'application systématique de ces trois analyses sur les enregistrements télésismiques du programme Lithoscope a permis de contraindre et de compléter les images de la lithosphère obtenues par la tomographie en vitesse des ondes P.

碩士
國立中央大學
地球物理研究所
86
The purpose of this study is to investigate the S-wave velocity structure of Taipei Basin using the shallow reflection seismic method.It includes two parts ,the first is to find the overall S-wave velocity structure distribution of Taipei Basin, the other is to assess the S-wave velocity structure at strong motion stations.73 seismic profiles have been collected in this study, among them 22 are near strong motion stations. These S-wave velocity structures can help us to study the strong motion of Taipei Basin, The results revealed in the study are : 1. S-wave reflection seismic method is simple and useful. It is appropriate for this S-wave site condition study. 2. The lower S-wave velocity reflects the higher PGA value. 3. Some useful engineering parameters can be determined by S- wave velocity study such as natural vibration frequency, shear modulus, and Poison''s ratio, etc. 4. The top 30m layers in the Taipei Basin have the S-wave velocities smaller than 360 m/sec. This means that Taipei Basin is quite soft.

碩士
國立臺灣海洋大學
應用地球科學研究所
103
The South China Sea (SCS), located at the junction of Eurasian plate, Indo-Australian plate and Pacific plate, is one of the marginal seas along the western Pacific. In the past, some studies investigated the plate features of the SCS by geophysical explorations using magnetic field, gravity, heat flow and seismic waves. For providing additional evidence in the tectonic evolution, the purpose of this study was to investigate the 3-D S-wave velocity structure in the SCS on the basis of Rayleigh-wave group and phase velocities. Earthquakes with magnitude from 5.5 to 7.0 and the focal depths of less than 100-km occurred from 1995 to 2012 at the region of 88-132E and 4S-32N were used to analyze Rayleigh-wave dispersion curves at periods of 12-150 seconds. At last, we adopted about 7000 Rayleigh-wave paths travelling across the SCS and its adjacent areas. Building a 3-D S-wave velocity structure by using surface waves requested a two-step inversion. First, the study area was divided into 375 sub-regions with each size of 22 in latitude and longitude. A block inversion with smoothing constraints, i.e., a tomographic method, was used to image 2-D maps of Rayleigh-wave group- and phase-velocity. Subsequently, the secondary inversion was to invert the S-wave velocity structure of each sub-region using its group- and phase-velocities. Finally, we combined the S-wave velocity structure of all sub-regions to construct the 3-D velocity model in the SCS. The model showed the lateral heterogeneity up to depths of 200 km, that is, the tectonic structure was complex in the crust and upper mantle under the SCS. The vertical velocity profiles along the EW-direction showed a high-velocity zone at a depth of about 50 km, which can be also found from north to south. This depth indicated the position where it is the lid of upper mantle. In additional, the crustal thickness decreased gradually toward the center of the SCS, where the crust is about 10-km-thick. The lithospheric thickness beneath the SCS is about 45-50 km. Low velocities in the Reed Bank and Dangerous Ground were related to the thick sediments; whereas the low velocity under the Tibet Plateau was in connection with its thicker crust. There are also low velocity in Sulu Sea and Central Philippine Islands, to be associated with high heat flow. The Nan-Uttaradit Suture and Sagaing Fault were also importantly geological boundaries, where the velocity differences can be obviously identified across these geological units. Along the NS-direction vertical profiles, the velocity variation in the Celebes Sea was relatively smooth than that in the Sulu Sea, in which the topography is complicated. This implied that the Sulu Sea has relatively higher action in plate tectonics than the Celebes Sea. The velocity discrepancy between the two sides of the Red-River Fault zone only was down to the crust, not to the lithosphere. For this reason, we inferred the Red-River Fault zone as a crustal fault.

碩士
國立中央大學
地球科學學系
104
Microtremor measurements had been conducted in the Ilan basin, one of the most vulnerable area in Taiwan regarding earthquakes. The Ilan basin is a delta filled with unconsolidated Quaternary alluvium with underlying Miocene basement, which may give great effect in the ground shaking characteristic due to its strong impedance contrast. As the area is prone to earthquakes, detailed velocity structure is important for site effect studies. An approach is proposed in this research, to estimate S-wave velocity structure from microtremor data using horizontal-to-vertical (H/V) spectral ratio simulation. The theoretical transfer function of Haskell-Thomson matrix is calculated and compared to the observed H/V spectrum. The velocity profiles derived from Receiver Function (RF) analysis of the nearby Taiwan Strong Motion Instrumentation Program (TSMIP) stations and borehole data from Engineering Geological Database for TSMIP (EGDT) are used as the initial model. Genetic Algorithm (GA) searching method is applied to find the best fit solution of S-wave velocity profiles on each site. Structure maps are derived from the simulation results for top basement structure and the upper three layers. The basement contour derived in this study has similar characteristic with the basement contour presented by previous studies. Due to a dense and even measurement sites in this study, the structure obtained in this study is smoother and more detail. Based on the analysis, it is identified that the thickness of the uppermost soft soil layer reaches 250 m in the coastline in the east of Ilan with velocity between 150 – 400 m/s. This structure also represents the depth of top Alluvium layer whose velocity ranges from 300 – 700 m/s. The depth of Alluvium – Pleistocene layer boundary varies between 10 – 800 m, where the structure tends to gradually gets deeper to the east. The top basement has depth between 30 – 1300 m, with velocity of 900 – 1500 m/s. The PGA and PGV contour derived by Chang (2009) show that the PGA distribution may not related to the structure but rather affected by the source and/or path effect and that the PGV distribution value is controlled by the basement structure.

碩士
國立中央大學
地球物理研究所
91
The purpose of this study is trying to understand the site effect and to compute near surface S wave velocity structure of the Lan-Yang plain. We used very dense microtremor measurements to study the site effect, and used portable seismic array to estimate the near surface sediment''s S wave velocity structure. In this study, we performed 354 microtremor measurements, and analyzed site effect by applying single station spectral ratio method (Nakamura, 1989). After that, we conducted 9 portable seismic arrays. The analysis procedures can be categorized as (1) Frequency- Wavenumber analysis to get the dispersion curve, (2) to search the good velocity models using genetic algorithm, and (3) to refine the SURF and GA searched models using traditional surface wave inversion technique. From the predominant frequency of microtremor measurement, we can figure out the alluvium thickness become thicker toward the sea. But there were some higher region (thinner alluvium) beside the east of I-lan and Lotung cities, and north of Lan-Yang River cutlet. The portable seismic arrays also showed similar results. This research result was slightly different from previous study.

碩士
國立中央大學
地球物理研究所
92
We conducted array measurements of microtremors in 17 strong motion stations to estimate shallow S-wave velocity structures. These strong-motion stations are distributed in different geological areas including Taipei Basin, Lan-Yang Plain and Longitudinal Valley. First, we use Frequency-Wavenumber method to estimate phase velocities of microtremors and get the dispersion curve, then use (Genetic Algorithms) GA method to find some good S-wave velocity models, which have high fitness. Finally, we take the GA result as initial model and use the traditional surface wave inversion technique (SURF) to do inversion and estimate the best S-wave velocity model. In the near surface part (depth < 100 meter), we compare our S-wave velocity structures with the models estimated from PS-logger and SASW method. The results of array measurement in most stations of Taipei Basin and Lan-Yang Plain are very similar to results of PS-logger and SASW. It exhibit array measurement of microtremors is suitable for estimating velocity structure on alluvium plain. But these results in Longitudinal Valley are very different due to complex topographic and subsurface structure. We also try to improve our array measurement method to estimate well velocity structure in more complex geological area.

博士
國立中正大學
地震研究所
100
The objectives of this study are to estimate the shallow S-wave velocity structures and to understand the bedrock distributions at different areas using the microtremor array measurements and the receiver-function methods. In order to examine the validity of the results, we compare the results with those from the well logging date and the geophysical data. Finally, we do the ground motion simulations based on the inverted S-wave velocity structures using the microtremor array measurements. The above results can be summarized as follows: (1) At the TCDP drilling sites, the inverted S-wave velocity gradually increases from 1.52 km/sec to 2.22 km/sec at depths between 585 m and 1710 m using the microtremor array measurements. This result is similar to those from the velocity logs, which range from 1.4 km/sec at a depth of 597 m to 2.98 km/sec at a depth of 1705 m. According to the results of the seismic reflection method, the depths of the Chinshui Shale, the Chelungpu fault and the Sanyi fault are 900-1200 m, 1100 m and 1800 m, respectively; while the results of this study, its depths are 855-1395 m, 1125 m and 1755-1800 m, respectively. (2) For the blind test of ESG2006, the inverted S-wave velocity structure in the study is very close to the theoretical data of ESG. For the real data analysis, the inverted S-wave velocity structures can match well with the well logging data at the depths 0-200 m. It means that the results from this study are reliable. (3) Based on the inverted results from the microtremor array measurements at 16 sites, the depths of the Quaternary sediments are between 300 m and 870 m in the Puli area if the S-wave velocity of the bedrock is assumed to be 2000 m/sec. We also compare our results with those from the seismic exploration method. Both patterns are similar. The depths of the bedrock gradually increase from the basin’s edge to its center. (4) In order to explore the shallow S-wave velocity structures of the Chiayi area, we conduct microtremor array measurements at 46 sites. If the S-wave velocity of the bedrock is assumed to be 1500 m/sec, the depths of the alluvium are between 560 and 1400 m gradually increasing from east to west. The thickness of the sediments increases from east to west while the S-wave velocities are in the range of 270 and 1500 m/sec; whereas, the thickness of the sediments decreases from east to west while the S-wave velocities are between 1500 and 2370 m/sec. The results are in good agreement with the geological and geophysical information. (5) In order to investigate the shallow S-wave velocity structures in the Hualien area, we analyze the strong-motion data recorded by 16 stations of the SMART2 array using the receiver-function method. According to the inverted results, the shallow velocity structures (0-200 m) are similar to those from the well-logging data and the microtremor array measurements at the Dahan site. The thickness of the sediments at the Hualien area is about 130-660 m if the S-wave velocity of the bedrock is assumed to be 1000 m/sec The S-wave velocity decreases from east to west while the alluvial thickness increases from east to west. (6) In order to detect the distribution of the seismic bedrock, we estimate the receiver functions by adopting the iterative deconvolution method. In the Taipei basin, the stacking receiver functions clearly reveal that the peak signals of the Ps-P time appear at about 0.295-0.925 sec. Both the Ps-P time and the alluvial thickness gradually increase from the southeast to the northwest. The results are consistent with those from the seismic exploration method and the well logging data. In the Chiayi area, the ranges of the Ps-P time are between 1.01 and 1.69 sec while the alluvial thickness increases from east to west. These results are in good agreement with those from the microtremor array measurements at the Chiayi area. (7) We integrate the velocity structure between the crustal velocity structure and the near-surface velocity structure from the microtremor array measurements. In order to simulate the ground motions of the Chiayi earthquake (ML=6.4) in 1999, we use the stochastic Green’s function method based on the velocity structure, the Q values and its source rupture model. According to the simulation results of eight strong motion stations, the waveform amplitudes are obviously underestimated while the stations are located at the northwest of the epicenter. On the contrary, the waveform amplitudes are overestimated while the stations are located at the southeast of the epicenter. This phenomenon can be attributed to the rupture directivity. Moreover, the 1D velocity structure and uniform rupture model used also lead to some discrepancies between the observed and the synthetic data.

碩士
國立臺灣大學
地質科學研究所
101
Surface wave travel-time tomography has been widely used as a powerful strategy to image shear wave velocity structure of the Earth’s crust and upper mantle. Traditionally with either ray theoretical great-circle approximations or 2-D phase kernels, phase velocity maps are first obtained at multiple frequencies. They are then combined to invert for shear wave velocity structure using 1-D depth-varying Frechet derivatives of phase velocity with respect to shear wave speed. Such approach runs short on considering the directional- and depth-dependence of scattering while surface wave propagating through laterally heterogeneous Earth. We here present a fully 3-D finite-frequency method based on the Born scattering theory in conjunction with surface-wave mode summation and apply it to regional fundamental Rayleigh wave tomography in central Tibet. Our data were collected from Hi-CLIMB array in the central Tibet during 2004-2005. Following a standard procedure to obtain empirical Green’s functions of Rayleigh waves from ambient noise cross correlation functions (CCFs) between station pairs, the phase differences between the CCFs and corresponding synthetics are measured by a multi-taper cross-spectral method. We apply the 3-D sensitivity kernels at individual frequencies convolved with the same eigentapers used in the phase measurement to conduct a 3D tomography of shear wave velocity perturbations with respect to a spherically-symmetric earth model suitable for central Tibet. A wavelet-based, multi-scale parameterization is invoked in the tomographic inversion to deal with the intrinsic problem of unevenly distributed data and resolve the structure with data-adaptive spectral and spatial resolutions. The result shows that the crust is generally slower to the north of the Bangong-Nujiang Suture (BNS) in marked contrast to the south with higher speeds. The absence of pervasive low velocity anomalies in the mid-to-lower crust indicates that the ductile channel flow of the lower crust may be inactive beneath southern Tibet. The model resolution in the lithospheric mantle can be improved by integrating longer-period surface data from distant earthquakes, which will yield better constrains on the geodynamic process of the Himalayan-Tibetan orogeny.

碩士
國立中央大學
地球物理研究所
87
Seismic waves are affected by seismic sources, paths of propagation, and structures of underground layers. To understand the seismic sources within a specific region, it is necessary to acquire a more detailed velocity structure. Many studies based on inversions of P-wave arrivals have been made to explain the velocity structure underneath Taiwan area. In this study, a generalized ray theory and finite difference method were applied to analyze seismograms recorded by the strong motion network within the Sanyi-Puli Seismic Zone, to have a closer view of the velocity structure beneath the mid-western Taiwan region. One dimensional velocity structure for each path is constructed by the generalized ray theory with the consideration of the arrival times and amplitudes of the reflected seismic waves, including reflections from the base of the sedimentary layer, Conrad Discontinuity and Moho Discontinuity. Based on these results, finite difference method is then applied to construct a two dimensional velocity structure to analyze the depth distribution of each discontinuity presented in the region. The results from this study are stated below: A. S-wave velocity structure within the mid-western Taiwan region: 1. The depth of the sedimentary layer in the region increased from the western coast towards the Central Range, which is strongly affected by the geological structure near the surface. The average depths at the coastal plane and the western foothill were 2~3 and 3~4 km, respectively. To the east, the depth of the sedimentary layer is at least more than 4 km after encountered the Central Range and the Hsuehshan Range. 2. During the construction of the one dimensional velocity structures, an unknown horizontal discontinuity with depth ranges from 9 to 12 km was necessary to explain most of the observed seismograms. The depths of the discontinuity were deeper at the northwestern end and the southeastern end of the studied region. The average depth is about 10 km. 3. The depth of the Conrad Discontinuity increased from east to west, with the average depth of 18, 18~20 and 20 km beneath the coastal plane, the western foothill, and the Hsuehshan Range, respectively. The depth gradient increased eastwards after encounters the Hsuehshan Range, to a maximum depth as 25 km. As a whole, the Conrad Discontinuity inclined to the Central Range with an average angle of 5 degrees. 4. The inclination of the Moho Discontinuity towards the Central Range is very obvious in the studied region. The minimum depth is 29 km, while the maximum depth is up to 45 km, with an average inclination angle of 10 degrees. The depth contour showed an reversed V shape at the southwestern end of the studied region, with its top point been located near the Puli Basin, which indicated an shallower depth of the Moho Discontinuity beneath the certain area. B. Sanyi-Puli Seismic Zone: The two-dimensional velocity structure indicated a sharp variation of Moho depth at the Sanyi-Puli seismic zone. The depth was shallower at the northeastern side, and deeper at the southwestern side. The variation is obvious beneath the Hsuehshan Range. The depth variation of the Conrad Discontinuity had a similar but smoother trend. From these results, we postulated that the formation of the Sanyi-Puli Seismic Zone might have a close relationship with the sharp variation of the depth. As the oblique convergence of the Eurasia Plate and the Philippine Sea Plate, the thrust wedge at the front of the orogenic zone escaped in a northwestern direction by the affection of the existence of the BeiKang Gravity High in the south. The northward movement of the thicken crust was then induced and produced a sharp variation of the Moho depth.

碩士
國立中央大學
應用地質研究所
84
The purpose of this study is to find the S-wave velocity structure of Sungshan formation in Taipei Basin using shallow reflection seismic method. Taipei Basin is known as a structural basin with unconsolid sediment lying above Tertiary bed rock. Sungshan formation, the uppermost layer, is filled with mud-sand or sand-mud. We proposed a simple and efficient S-wave reflection seismic survey to explore this layer including its sublayer structure and S-wave velocity. Besides these,the method can also provide useful lithological information for engineering application such as shear modulus, stress condition, Poison''s ratio, etc. Basing on the S-wave velocity distribution, we attempt to divide Taipei Basin into four areas with three types of velocity structure. The first velocity layer has S-wave velocity: 165 to 185 m/s and the thickness about 15 m, the second has the velocity: 220 to 260 m/s and the thickness about 20 m, and the third layer has the velocity: 300 to 350 m/s down to the bottom of Sungshan formation. This S-wave velocity structure is found to have close relations with the sedimentation history of Sungshan formation.The influence of three river system: Da-Han, Hsin-Tien, and Kee-Long river on the sediment deposit is very obvious. Finally, the structure of S-wave velocity also reveals close tie with the site effect and natural frequency vibration of Taipei Basin under the impact of earthquake. It could be of great significance in strong motion study of Taipei Basin.

碩士
國立臺灣海洋大學
應用地球科學研究所
103
Study of gas hydrate in Taiwan has focused on the offshore area, especially in southwest off Taiwan. Amounts of sediments with high concentrations of methane-rich gas are provided from the accretionary wedge at Hengchun Ridge and the continental margin in the South China Sea. In addition, bottom-simulating reflectors （BSRs） were observed from seismic reflection profiles, which preferred drilling sites for gas hydrate will be located in this area. This research analyzed Ocean Bottom Seismic (OBS) data collected from Ocean researcher 1 at the Yuan-An Ridge in May, 2014. We built the P-wave and Vp/Vs velocity sedimentary models by using travel-time inversion of OBS data after considering the Poisson's ratio near the active margins from the previous researches. The results show that the P-wave velocity above the BSR is 1.8~2.0 km/s in the Yuan-An Ridge. Furthermore, the Vp/Vs model shows the range of Vp/Vs about 1.7~2.3 above the BSR. Estimation of the P-wave and S-wave velocities suggests that methane gas migrated through the cracks, gathered and then became the gas hydrate at the west side of the ridge.

碩士
國立中正大學
地震研究所
100
Shear-wave velocities have widely been used for earthquake ground-motion site characterization. Thus, the shallow S-wave velocity structures of the Tiehchenshan region in the Tiachung-Miaoli area are investigated using the array records of microtremors at 19 sites. The dispersion curves at these sites are calculated using the f-k method (Capon, 1969); then, the S-wave velocity structures of the Tiehchenshan region are estimated by employing the surface wave inversion technique (Herrmann, 1991). At frequencies lower than about 3 Hz, the propagation directions are concentrated between the northwest and southwest quadrants. The generation of these may be attributed to the ocean waves of the Taiwan Strait. If the S-wave velocity of the bedrock is assumed to be 2200 m/s, the depths of the alluvium are between 1300 m and 2800 m. S-wave velocity gradually decreases from east to west at the depths less than 1200 m. At the depths between 1200m and 4000m, the higher Vs values appear at the north and the south of the region while the lower values are at the east and the west. Our results are in good agreement with the available geological and geophysical information at the Tiehchenshan region.