To understand the convergent characteristics of the westernmost plate boundary between the Philip... more To understand the convergent characteristics of the westernmost plate boundary between the Philippine Sea Plate (PSP) and Eurasian Plate (EP), we have calculated the stress states of plate motion by focal mechanisms. Cataloged by the Harvard centroid moment tensor solutions (Harvard CMT) and the Broadband Array in Taiwan (BATS) moment tensor, 251 focal mechanisms are used to determine the azimuths of the principal stress axes. We first used all the data to derive the mean stress tensor of the study area. The inversion result shows that the stress regime has a maximum compression along the direction of azimuth N299°. This result is consistent with the general direction of the rigid plate motion between the PSP and EP in the study area. In order to understand the spatial variation of the regional stress pattern, we divided the study area into six sub-areas (blocks A to F) based on the feature of the free-air gravity anomaly. We compare the compressive directions obtained from the stress inversion with the plate motions calculated by the Euler pole and the Global Positioning System (GPS) analysis. As a result, the azimuth of the maximum stress axis, σ 1 , generally agrees with the directions of the theoretical plate motion and GPS velocity vectors except block C (Lanhsu region) and block F (Ilan plain region). The discrepancy of convergent direction near the Ilan plain region is probably caused by the rifting of the Okinawa Trough. The deviation of the σ 1 azimuth in the Lanhsu region could be attributed to a southwestward extrusion of the Luzon Arc (LA) block between 21°N and 22°N whose northern boundary may be associated with the right-lateral NE-SW trending fault (i.e. Huatung Fault, HF) along the Taitung Canyon. Comparing the σ 1 stress patterns between block C and block D, great strain energy along HF may not be completely released yet. Alternatively, the upper crust of block C may significantly have decoupled from its lower crust or uppermost mantle.
ABSTRACT Strong motion site responses as well as scenario earthquake studies are important tasks ... more ABSTRACT Strong motion site responses as well as scenario earthquake studies are important tasks for earthquake hazard analysis within the Taipei metropolitan region. Taking the advantage of spectral-element method (SEM), the surface topography and subsurface structures, such as sedimentary basins, the Moho and the subducting plate, can be effectively incorporated in an SEM mesh. While for steep topography or a highly complex subsurface the mesh is seriously distorted, potentially leading to a numerically unstable problem. Here we show new techniques to incorporate topography and complex subsurface structures with successful benchmarks for the Taipei Basin. First, an additional doubling layer with variable grid size along depth axis is needed to have finer mesh configuration near the surface in order to save memory and computing time. A smoothed buffer map is then applied to reduce mesh distortion induced by steep surface topography. Finally, we adjusted the nodes of the basin meshes to coincide with the complex geometry of the basement boundary in the Taipei Basin. With this high resolution model, seismic responses from SEM based D simulation show that the ground motion within the Taipei Basin is strongly depends on the source frequency and basin geometry. The amplification of ground motion is mainly controlled by the depth of the basin. The lateral variations of S wave velocity also play an important role in modeling site amplification phenomena. Our results indicate that a high resolution topography mesh that follows most of the DTM data can be achieved. Large lateral variations in subsurface boundaries can also be careful defined by adjusting the nodes of the affected elements. With these developments, the spectral-element mesh readily deals with the complex subsurface in the Taipei metropolitan region, such as steep topography, complex basin structure, Moho and even the subducting plate beneath northern Taiwan. It will be straightforward to apply these techniques to build a model for all of Taiwan or other to study other regions with complex subsurface structures.
Ž. Ž. Simulation of digital ground penetrating radar GPR wave propagation in two-dimensional 2-D ... more Ž. Ž. Simulation of digital ground penetrating radar GPR wave propagation in two-dimensional 2-D media is developed, Ž. tested, implemented, and applied using a time-domain staggered-grid finite-difference FD numerical method. Three types of numerical algorithms for constructing synthetic common-shot, constant-offset radar profiles based on an actual transmitter-to-receiver configuration and based on the exploding reflector concept are demonstrated to mimic different types of radar survey geometries. Frequency-dependent attenuation is also incorporated to account for amplitude decay and time shift in the recorded responses. The algorithms are based on an explicit FD solution to Maxwell's curl equations. In addition, the first-order TE mode responses of wave propagation phenomena are considered due to the operating frequency of current GPR instruments. The staggered-grid technique is used to sample the fields and approximate the spatial derivatives with fourth-order FDs. The temporal derivatives are approximated by an explicit second-order difference time-marching scheme. Ž. Ž. By combining paraxial approximation of the one-way wave equation A and the damping mechanisms sponge filter , we 2 Ž. propose a new composite absorbing boundary conditions ABC algorithm that effectively absorb both incoming and outgoing waves. To overcome the angle-and frequency-dependent characteristic of the absorbing behaviors, each ABC has two types of absorption mechanism. The first ABC uses a modified Clayton and Enquist's A condition. Moreover, a fixed 2 and a floating A ABC that operates at one grid point is proposed. The second ABC uses a damping mechanism. By 2 superimposing artificial damping and by alternating the physical attenuation properties and impedance contrast of the media within the absorbing region, those waves impinging on the boundary can be effectively attenuated and can prevent waves from reflecting back into the grid. The frequency-dependent characteristic of the damping mechanism can be used to adjust the width of the absorbing zone around the computational domain. By applying any combination of absorbing mechanism, non-physical reflections from the computation domain boundary can be effectively minimized. The algorithm enables us to use very thin absorbing boundaries. The model can be parameterized through velocity, relative electrical permittivity Ž. dielectric constants , electrical conductivity, magnetic permeability, loss tangent, Q values, and attenuation. According to this scheme, widely varying electrical properties of near-surface earth materials can be modeled. The capability of simulating common-source, constant-offset and zero-offset gathers is also demonstrated through various synthetic examples. The synthetic cases for typical GPR applications include buried objects such as pipes of different materials, AVO analysis for ground water exploration, archaeological site investigation, and stratigraphy studies. The algorithms are also applied to iterative modeling of GPR data acquired over a gymnasium construction site on the NCCU campus.
Bulletin of the Seismological Society of America, 2008
We perform full elastic wave-field simulations within the Taipei basin by using a three-dimension... more We perform full elastic wave-field simulations within the Taipei basin by using a three-dimensional (3D) discontinuous finite-difference method. The 3D Taipei basin model is determined from a seismic reflection study. Two major subsurfaces, the Songshan formation (surface soil layer) and the basin basement, are constituted in the model. A parallel-based composite grid technique, a containing scalene grid and a discontinuous grid, is developed in this study to deal with the possible numerical problem of thin depth and low velocity of the Songshan formation. Taking advantage of the composite grid, the resolution of the subsurface structure can be reached to 20 m, and a higher frequency (up to 3 Hz) of the synthetic waveform can be achieved. In our strong ground motion simulations, we assume a constant velocity in each subsurface. Three different types of models are considered in the study: the Songshan formation with a basement structure model, a basin basement model, and a layered half-space model. Results indicate that only the model with both the Songshan formation and the basement structure can produce the apparent basin amplification effects. First, the surface wave generated after the primary S wave is trapped at the shallow part of the basin. Then, when the wave propagates through the deepest part of the basin, most of the energy is reflected from the boundary and focused back into the basin. In addition, part of the seismic wavefront turns and follows the shallow basin edge resulting in further amplification. Our study indicates that the complex Taipei basin geometry and fairly low velocity of the Songshan formation dominate the amplification and wave propagation behavior that result in extraordinary strong shaking patterns in the Taipei metropolitan region.
Bulletin of the Seismological Society of America, 2008
We use the spectral-element method to simulate strong ground motion throughout the Taipei metropo... more We use the spectral-element method to simulate strong ground motion throughout the Taipei metropolitan area. Mesh generation for the Taipei basin poses two main challenges: (1) the basin is surrounded by steep mountains, and (2) the city is located on top of a shallow, low-wave-speed sedimentary basin. To accommodate the steep and rapidly varying topography, we introduce a thin high-resolution mesh layer near the surface. The mesh for the shallow sedimentary basin is adjusted to honor its complex geometry and sharp lateral wave-speed contrasts. Variations in Moho thickness beneath Northern Taiwan are also incorporated in the mesh. Spectralelement simulations show that ground motion in the Taipei metropolitan region is strongly affected by the geometry of the basin and the surrounding mountains. The amplification of ground motion is mainly controlled by basin depth and shallow shear-wave speeds, although surface topography also serves to amplify and prolong seismic shaking.
Terrestrial, Atmospheric and Oceanic Sciences, 2003
This study demonstrates synthesis of seismic responses in general elas tic and viscoelastic fract... more This study demonstrates synthesis of seismic responses in general elas tic and viscoelastic fractured media using a two-dimensional pseudo-spec tral method. From the full waveform simulation approach, different types of source configurations with corresponding wavefields and synthetic records are generated. Viscoelastic wavefield responses, which incorpo rate quality factors for both P-waves (Qp) and S-waves (Qs) through the superposition of relaxation mechanisms and velocities for attenuative and dispersive media, are presented, tested and implemented to assess their potential for practical application. The modeling results show that the more time that elapses, the less the wave amplitudes can be observed in a vis coelastic media than in elastic media. The simulation of both pressure and shear source fully illustrate the corresponding decomposed waves and their corresponding seismograms. Also, the synthetic model of the anticline struc ture embedded in the layered model shows that diffraction responses are directly related to the fracture/fault zone structure. From the prescribed model, transmission, reflection, and converted multi-phase events can readily be identified in both elastic and viscoelastic media. Aside from this, evaluating seismic responses using a specially designed source and receiver configuration, such as horizontal drilling technique, is critical in studies pertaining to reservoirs. It is also demonstrated that by examining such seismic responses, the effects of fracture distributions and the correspond ing time-lapsed monitoring of oil/gas leakage within a reservoir can be evalu ated in advance by the proposed method for numerical studies.
We determine lateral variation of Moho discontinuity, crustal thickness, and Vp/Vs ratios in the ... more We determine lateral variation of Moho discontinuity, crustal thickness, and Vp/Vs ratios in the vicinity of Taiwan by analyzing all the available teleseismic waveform data collected by the Broadband Array in Taiwan for Seismology network from 1998 to 2004. The crustal thickness and the average crustal Vp/Vs ratio beneath each station are obtained by stacking Ps, PpPs and PpSs + PsPs phases coherently. The best estimated crustal thickness of Taiwan from 27 broadband stations is 30 km on average. The thinnest crust (11-15 km) is found east of the Longitude Valley suture zone, as part of the oceanic crust of the Philippine Sea plate. In other places, the crustal thickness varies from 17-19 km in northern Taiwan to 32-39 km in the southwestern island. The deepest Moho is found to be 53 km beneath station SSLB in central Taiwan. The average crustal Vp/Vs ratio in Taiwan is 1.74, with higher values of 1.74-1.99 in the north and lower values of 1.60-1.74 in the south. The crustal thickness variation is supported by gravity measurements in Taiwan and indicates that the collision between the Philippine Sea plate and Eurasian plate in Taiwan involves the whole crust. The thin crust and high Vp/Vs ratios in northern Taiwan are believed to be related to the volcanism in Tatun and Keelung located in the southwestern extent of the Okinawa Trough.
Terrestrial, Atmospheric and Oceanic Sciences, 1999
The relationships among the (scalar) displacement potential, acoustic pressure and particle displ... more The relationships among the (scalar) displacement potential, acoustic pressure and particle displacement is exploited in three-dimensional (3-D) numerical modeling of synthetic multi-component multi-offset seismograms. Using these relations, synthetic one-component pressure and partially resolved three-component (3-C) displacement seismograms may be computed for 3-D models using a finite-difference solution of the (3-D) acoustic wave equation. Compressional-or decoupled-SH and Love wave responses can be used as a complement to elastic response for data interpretation and subsurface structure delineation. This new capability is applicable to a wide range of recording geometries over various complex structures. Simulations include surface sources recorded on a circular array, walkaway survey, multi-component multioffset vertical seismic profiling (VSP), and both offend and off-line shots into a crooked, wide-aperture array. The result indicated that the relationships among source, receiver location and complexity of the subsurface structure will produce different three-component responses. For seismic event identification and subsurface structure discrimination, the uniqueness from forward modeling can be emphasized through integrated approach by considering 3-D scalar, partially resolved 3-C displacement and elastic modeling.
By analysing teleseismic waveform data, we propose a large-scale Taiwan Reference Moho Discontinu... more By analysing teleseismic waveform data, we propose a large-scale Taiwan Reference Moho Discontinuity Model (TRMDM) through coherent stacking and H-κ analysis of receiver functions (RFs). Tectonic structural features are determined from accurate crustal thickness (H) and reliable Vp/Vs ratio (κ) (consequently Poisson's ratio, σ) estimations for Taiwan. The best estimated regional average Moho depth is 33.5 ± 2.9 km. Noted is a westward crustal thinning from 39 ± 3.0 km in eastern Taiwan to 30 ± 1.5 km in southeastern China. The averaged Moho depth is 23.0 ± 2.5 km in northern Taiwan and increases to 33.5 ± 2.0 km in the south. The markedly thin crust and high Vp/Vs ratio in northern Taiwan are mainly associated with backarc opening and magmatic activities. Based on H-κ estimations and associated seismicity patterns beneath Suang-Long (SSLB) and adjacent four stations, we propose two Moho reference models that exhibit distinctive crustal thickening in central Taiwan. The spatial variations of crustal thickness models show strong lateral change in the Moho topography. The thick crust in the south, thin crust in the north and transition in central Taiwan mark the different tectonic units between the subduction wedge and the collision prism onshore. In view of the following list of constraints: thicker crust in the south than in the north; eastward crust thickening; significant Moho depth changes; clear 9-11 s subcrustal boundaries from five stations; the compelling thickening of crust and upper mantle beneath the central mountain belt; seismicity patterns with the absence of mantle earthquake in the Wadati-Benioff zone; regional geological features; the occurrence of the inland Chi-Chi earthquake; the compelling low κ at YULB (1.58 ± 0.13) that may extend to central Taiwan at SSLB (1.59 ± 0.23); relatively thick crust at YULB (39.5 ± 1.6 km) and SSLB (40.5 ± 1.3 km or 52.5 ± 1.5 km), and finally, the apparent horizontal offset between highest Yu Shan Mountain and the deepest Moho at SSLB, we are able to suggest that the initial subduction of the Eurasian Plate occurred in southern Taiwan and most likely extended to the transition zone in central Taiwan. Although the current RF study is limited by large interstation distance, our analysis provides more notable constraints than previous studies to explicitly determine the characteristic features of Moho discontinuity. This detailed approach means that rather than treating the tectonic framework as a whole heterogeneous lithospheric feature, clear Moho boundary geometry variations indicate that active orogenic processes and differential tectonic stress loading create separate and complex crustal and subcrustal-to-upper-mantle structural features caused by the arc-continent collision.
SEG Technical Program Expanded Abstracts 2014, 2014
Imaging both Bottom Simulating Reflectors (BSRs) and ocean fine structure can be achieved through... more Imaging both Bottom Simulating Reflectors (BSRs) and ocean fine structure can be achieved through careful process of long-offset Multi-Channel Seismic (MCS) data. Interpretation and classification on type locality, sedimentation environment and spatial distribution of BSRs with potential methane hydrate production are exploited. Both patchy and continuous distributed BSRs can be identified and may well correspond to different sediment dispersal processes in the frontal and real segment of active accretionary wedge that belongs to active continental margins offshore of SW Taiwan. Our results indicated fluid/gas migration in the active margin may controlled by the tectonic structure control and/or local seismic activity. The image of ocean currents reveals different dynamic processes occurred at different depth. Near-surface current are characterized by continuous, nearly horizontal stratified layers indicate air-sea surface interaction features. More apparent wavy shaped, less continuous with varying in impedance contrast, relative clear top and bottom reflections with low internal amplitude reveal the feature of eddy currents can be identified at the intermediate depth range. Through quantitative analyses, the spatial distribution features of internal waves and others such as eddies, mass movement, turbulence and current mixing are all helpful in promoting our understanding of dynamic ocean processes across different scale constrained by seismic frequency band.
Abstract We derive a velocity field using GPS data between 1993 and 2007 in the Chiayi–Tainan are... more Abstract We derive a velocity field using GPS data between 1993 and 2007 in the Chiayi–Tainan area located in the deformation front of the Taiwan mountain belt. The crustal motion with respect to Penghu shows large velocities of about 33–44 mm/yr in the west-northwest to west directions in the Western Foothills and the velocities decrease westward to 0–5 mm/yr in the coastal area. Significant uplift rates of 5–20 mm/yr are observed at sites to the east of the Jiuchiunken–Muchiliao–Liuchia Fault (JMLF) system. We use a block model, a buried dislocation model, and a two-dimensional fault model to invert for fault geometries and slip rates on major frontal thrust faults. Modeling results from a block model show the inferred long-term slip rate of 42 mm/yr in the direction of 280° and the maximum back-slip rate of 38 mm/yr on a 23° east dipping fault extending to 13 km at depth. On the other hand, the buried dislocation model results in a horizontal decollement at a depth of 8 km with a uniform slip rate of 41.6 mm/yr. If we connect the top edge of decollement to the surface trace of JMLF as a potential future rupture, a 22° east-dipping fault is required. Results from both block model and buried dislocation model suggest the JMLF is nearly fully locked. The results of two-dimensional fault models show the frontal thrust faults have slip rates of less than 2 mm/yr at shallow depths and the inferred decollement is sub-horizontal (5°–7°) at a depth of 10 km with slip rates of 44–46 mm/yr. Results of various approaches show general agreement on fault geometries and slip rates and reveal that the frontal thrust fault system has a high potential for large earthquakes.
This study presents an efficient parallelized staggered grid pseudospectral method for 2-D viscoa... more This study presents an efficient parallelized staggered grid pseudospectral method for 2-D viscoacoustic seismic waveform modeling that runs in a highperformance multi-processor computer and an in-house developed PC cluster. Parallel simulation permits several processors to be used for solving a single large problem with a high computation to communication ratio. Thus, parallelizing the serial scheme effectively reduces the computation time. Computational results indicate a reasonably consistent parallel performance when using different FFTs in pseudospectral computations. Meanwhile, a virtually perfect linear speedup can be achieved in a distributed-memory multi-processor environment. Effectiveness of the proposed algorithm is demonstrated using synthetic examples by simulating multiple shot gathers consistent with field coordinates. For dispersive and attenuating media, the propagating wavefield possesses the observable differences in waveform, amplitude and travel-times. The resulting effects on seismic signals, such as the decreased amplitude because of intrinsic Q and temporal shift because of physical dispersion phenomena, can be analyzed quantitatively. Anelastic effects become more visible owing to cumulative propagation effects. Field data application is presented in simulating OBS wide-angle seismic marine data for deep crustal structure study. The fine details of deep crustal velocity and attenuation structures in the survey area can be resolved by comparing simulated waveforms with observed seismograms recorded at various distances. Parallel performance is analyzed through speedup and efficiency for a variety of computing platforms. Effective parallel implementation requires numerous independent CPU intensive sub-jobs with low latency and high bandwidth inter-processor communication.
We inverted the high-resolution spatiotemporal slip distribution of the 21 September 1999 Chi-Chi... more We inverted the high-resolution spatiotemporal slip distribution of the 21 September 1999 Chi-Chi, Taiwan, earthquake utilizing data from densely distributed island-wide strong motion stations for a three-dimensional (3-D) fault geometry, and 3-D Green's functions calculations based upon parallel nonnegative least squares inversion. The 3-D fault geometry, consistent with high-resolution reflection profile, is determined from GPS inversion and aftershocks distribution. This 3-D fault model presents the dip angle gradually becoming shallower from south to north along the fault and near flat at the deeper portion of the fault. The 3-D Green's functions are calculated through numerical wavefield simulation from three-dimensional heterogeneous velocity structure derived from tomography studies. The Green's functions show significant azimuthal variations and suggest the necessity of lateral heterogeneity in velocity structure. Considering complex fault geometry and Green's functions in full 3-D scale, we invert the spatial/temporal slip distribution of the 1999 Chi-Chi earthquake using the best available and most densely populated strong motion waveform data. We perform the inversion under a parallel environment utilizing multiple-time window to manage the large data volume and source parameters. Results indicate that most slip occurred at the shallower portion of the fault above the decollement. Two major asperities are found, one in the middle of the fault and another one at the northern portion of the fault near the bend in the fault trace. The slip in the southern portion of the fault shows a relatively low slip rate with longer time duration, while the slip in the northern portion of the fault shows a large slip rate with shorter time duration. The synthetics explain the observations well for the island-wide distributed strong motion stations. This comprehensive study emphasizes the importance of realistic fault geometry, 3-D Green's functions, and parallel inversion technique in correctly accounting for both the detailed source rupture process and its relationship with the strong ground motion of this intense earthquake.
Abstract. Various types of noise can obscure reflections and refractions on deep crustal seismic ... more Abstract. Various types of noise can obscure reflections and refractions on deep crustal seismic data. This study presents a simple but elegant approach to true wavefield processing where noise reduction andtrace interpolations are applied implicitly during ...
... used in this study. The receiver function method provides high resolution information down to... more ... used in this study. The receiver function method provides high resolution information down to at least lithospheric depths by using teleseismic waveform data ([Ammon, 1991] and [Langston, 1979]). The interface between crust ...
To understand the convergent characteristics of the westernmost plate boundary between the Philip... more To understand the convergent characteristics of the westernmost plate boundary between the Philippine Sea Plate (PSP) and Eurasian Plate (EP), we have calculated the stress states of plate motion by focal mechanisms. Cataloged by the Harvard centroid moment tensor solutions (Harvard CMT) and the Broadband Array in Taiwan (BATS) moment tensor, 251 focal mechanisms are used to determine the azimuths of the principal stress axes. We first used all the data to derive the mean stress tensor of the study area. The inversion result shows that the stress regime has a maximum compression along the direction of azimuth N299°. This result is consistent with the general direction of the rigid plate motion between the PSP and EP in the study area. In order to understand the spatial variation of the regional stress pattern, we divided the study area into six sub-areas (blocks A to F) based on the feature of the free-air gravity anomaly. We compare the compressive directions obtained from the stress inversion with the plate motions calculated by the Euler pole and the Global Positioning System (GPS) analysis. As a result, the azimuth of the maximum stress axis, σ 1 , generally agrees with the directions of the theoretical plate motion and GPS velocity vectors except block C (Lanhsu region) and block F (Ilan plain region). The discrepancy of convergent direction near the Ilan plain region is probably caused by the rifting of the Okinawa Trough. The deviation of the σ 1 azimuth in the Lanhsu region could be attributed to a southwestward extrusion of the Luzon Arc (LA) block between 21°N and 22°N whose northern boundary may be associated with the right-lateral NE-SW trending fault (i.e. Huatung Fault, HF) along the Taitung Canyon. Comparing the σ 1 stress patterns between block C and block D, great strain energy along HF may not be completely released yet. Alternatively, the upper crust of block C may significantly have decoupled from its lower crust or uppermost mantle.
ABSTRACT Strong motion site responses as well as scenario earthquake studies are important tasks ... more ABSTRACT Strong motion site responses as well as scenario earthquake studies are important tasks for earthquake hazard analysis within the Taipei metropolitan region. Taking the advantage of spectral-element method (SEM), the surface topography and subsurface structures, such as sedimentary basins, the Moho and the subducting plate, can be effectively incorporated in an SEM mesh. While for steep topography or a highly complex subsurface the mesh is seriously distorted, potentially leading to a numerically unstable problem. Here we show new techniques to incorporate topography and complex subsurface structures with successful benchmarks for the Taipei Basin. First, an additional doubling layer with variable grid size along depth axis is needed to have finer mesh configuration near the surface in order to save memory and computing time. A smoothed buffer map is then applied to reduce mesh distortion induced by steep surface topography. Finally, we adjusted the nodes of the basin meshes to coincide with the complex geometry of the basement boundary in the Taipei Basin. With this high resolution model, seismic responses from SEM based D simulation show that the ground motion within the Taipei Basin is strongly depends on the source frequency and basin geometry. The amplification of ground motion is mainly controlled by the depth of the basin. The lateral variations of S wave velocity also play an important role in modeling site amplification phenomena. Our results indicate that a high resolution topography mesh that follows most of the DTM data can be achieved. Large lateral variations in subsurface boundaries can also be careful defined by adjusting the nodes of the affected elements. With these developments, the spectral-element mesh readily deals with the complex subsurface in the Taipei metropolitan region, such as steep topography, complex basin structure, Moho and even the subducting plate beneath northern Taiwan. It will be straightforward to apply these techniques to build a model for all of Taiwan or other to study other regions with complex subsurface structures.
Ž. Ž. Simulation of digital ground penetrating radar GPR wave propagation in two-dimensional 2-D ... more Ž. Ž. Simulation of digital ground penetrating radar GPR wave propagation in two-dimensional 2-D media is developed, Ž. tested, implemented, and applied using a time-domain staggered-grid finite-difference FD numerical method. Three types of numerical algorithms for constructing synthetic common-shot, constant-offset radar profiles based on an actual transmitter-to-receiver configuration and based on the exploding reflector concept are demonstrated to mimic different types of radar survey geometries. Frequency-dependent attenuation is also incorporated to account for amplitude decay and time shift in the recorded responses. The algorithms are based on an explicit FD solution to Maxwell's curl equations. In addition, the first-order TE mode responses of wave propagation phenomena are considered due to the operating frequency of current GPR instruments. The staggered-grid technique is used to sample the fields and approximate the spatial derivatives with fourth-order FDs. The temporal derivatives are approximated by an explicit second-order difference time-marching scheme. Ž. Ž. By combining paraxial approximation of the one-way wave equation A and the damping mechanisms sponge filter , we 2 Ž. propose a new composite absorbing boundary conditions ABC algorithm that effectively absorb both incoming and outgoing waves. To overcome the angle-and frequency-dependent characteristic of the absorbing behaviors, each ABC has two types of absorption mechanism. The first ABC uses a modified Clayton and Enquist's A condition. Moreover, a fixed 2 and a floating A ABC that operates at one grid point is proposed. The second ABC uses a damping mechanism. By 2 superimposing artificial damping and by alternating the physical attenuation properties and impedance contrast of the media within the absorbing region, those waves impinging on the boundary can be effectively attenuated and can prevent waves from reflecting back into the grid. The frequency-dependent characteristic of the damping mechanism can be used to adjust the width of the absorbing zone around the computational domain. By applying any combination of absorbing mechanism, non-physical reflections from the computation domain boundary can be effectively minimized. The algorithm enables us to use very thin absorbing boundaries. The model can be parameterized through velocity, relative electrical permittivity Ž. dielectric constants , electrical conductivity, magnetic permeability, loss tangent, Q values, and attenuation. According to this scheme, widely varying electrical properties of near-surface earth materials can be modeled. The capability of simulating common-source, constant-offset and zero-offset gathers is also demonstrated through various synthetic examples. The synthetic cases for typical GPR applications include buried objects such as pipes of different materials, AVO analysis for ground water exploration, archaeological site investigation, and stratigraphy studies. The algorithms are also applied to iterative modeling of GPR data acquired over a gymnasium construction site on the NCCU campus.
Bulletin of the Seismological Society of America, 2008
We perform full elastic wave-field simulations within the Taipei basin by using a three-dimension... more We perform full elastic wave-field simulations within the Taipei basin by using a three-dimensional (3D) discontinuous finite-difference method. The 3D Taipei basin model is determined from a seismic reflection study. Two major subsurfaces, the Songshan formation (surface soil layer) and the basin basement, are constituted in the model. A parallel-based composite grid technique, a containing scalene grid and a discontinuous grid, is developed in this study to deal with the possible numerical problem of thin depth and low velocity of the Songshan formation. Taking advantage of the composite grid, the resolution of the subsurface structure can be reached to 20 m, and a higher frequency (up to 3 Hz) of the synthetic waveform can be achieved. In our strong ground motion simulations, we assume a constant velocity in each subsurface. Three different types of models are considered in the study: the Songshan formation with a basement structure model, a basin basement model, and a layered half-space model. Results indicate that only the model with both the Songshan formation and the basement structure can produce the apparent basin amplification effects. First, the surface wave generated after the primary S wave is trapped at the shallow part of the basin. Then, when the wave propagates through the deepest part of the basin, most of the energy is reflected from the boundary and focused back into the basin. In addition, part of the seismic wavefront turns and follows the shallow basin edge resulting in further amplification. Our study indicates that the complex Taipei basin geometry and fairly low velocity of the Songshan formation dominate the amplification and wave propagation behavior that result in extraordinary strong shaking patterns in the Taipei metropolitan region.
Bulletin of the Seismological Society of America, 2008
We use the spectral-element method to simulate strong ground motion throughout the Taipei metropo... more We use the spectral-element method to simulate strong ground motion throughout the Taipei metropolitan area. Mesh generation for the Taipei basin poses two main challenges: (1) the basin is surrounded by steep mountains, and (2) the city is located on top of a shallow, low-wave-speed sedimentary basin. To accommodate the steep and rapidly varying topography, we introduce a thin high-resolution mesh layer near the surface. The mesh for the shallow sedimentary basin is adjusted to honor its complex geometry and sharp lateral wave-speed contrasts. Variations in Moho thickness beneath Northern Taiwan are also incorporated in the mesh. Spectralelement simulations show that ground motion in the Taipei metropolitan region is strongly affected by the geometry of the basin and the surrounding mountains. The amplification of ground motion is mainly controlled by basin depth and shallow shear-wave speeds, although surface topography also serves to amplify and prolong seismic shaking.
Terrestrial, Atmospheric and Oceanic Sciences, 2003
This study demonstrates synthesis of seismic responses in general elas tic and viscoelastic fract... more This study demonstrates synthesis of seismic responses in general elas tic and viscoelastic fractured media using a two-dimensional pseudo-spec tral method. From the full waveform simulation approach, different types of source configurations with corresponding wavefields and synthetic records are generated. Viscoelastic wavefield responses, which incorpo rate quality factors for both P-waves (Qp) and S-waves (Qs) through the superposition of relaxation mechanisms and velocities for attenuative and dispersive media, are presented, tested and implemented to assess their potential for practical application. The modeling results show that the more time that elapses, the less the wave amplitudes can be observed in a vis coelastic media than in elastic media. The simulation of both pressure and shear source fully illustrate the corresponding decomposed waves and their corresponding seismograms. Also, the synthetic model of the anticline struc ture embedded in the layered model shows that diffraction responses are directly related to the fracture/fault zone structure. From the prescribed model, transmission, reflection, and converted multi-phase events can readily be identified in both elastic and viscoelastic media. Aside from this, evaluating seismic responses using a specially designed source and receiver configuration, such as horizontal drilling technique, is critical in studies pertaining to reservoirs. It is also demonstrated that by examining such seismic responses, the effects of fracture distributions and the correspond ing time-lapsed monitoring of oil/gas leakage within a reservoir can be evalu ated in advance by the proposed method for numerical studies.
We determine lateral variation of Moho discontinuity, crustal thickness, and Vp/Vs ratios in the ... more We determine lateral variation of Moho discontinuity, crustal thickness, and Vp/Vs ratios in the vicinity of Taiwan by analyzing all the available teleseismic waveform data collected by the Broadband Array in Taiwan for Seismology network from 1998 to 2004. The crustal thickness and the average crustal Vp/Vs ratio beneath each station are obtained by stacking Ps, PpPs and PpSs + PsPs phases coherently. The best estimated crustal thickness of Taiwan from 27 broadband stations is 30 km on average. The thinnest crust (11-15 km) is found east of the Longitude Valley suture zone, as part of the oceanic crust of the Philippine Sea plate. In other places, the crustal thickness varies from 17-19 km in northern Taiwan to 32-39 km in the southwestern island. The deepest Moho is found to be 53 km beneath station SSLB in central Taiwan. The average crustal Vp/Vs ratio in Taiwan is 1.74, with higher values of 1.74-1.99 in the north and lower values of 1.60-1.74 in the south. The crustal thickness variation is supported by gravity measurements in Taiwan and indicates that the collision between the Philippine Sea plate and Eurasian plate in Taiwan involves the whole crust. The thin crust and high Vp/Vs ratios in northern Taiwan are believed to be related to the volcanism in Tatun and Keelung located in the southwestern extent of the Okinawa Trough.
Terrestrial, Atmospheric and Oceanic Sciences, 1999
The relationships among the (scalar) displacement potential, acoustic pressure and particle displ... more The relationships among the (scalar) displacement potential, acoustic pressure and particle displacement is exploited in three-dimensional (3-D) numerical modeling of synthetic multi-component multi-offset seismograms. Using these relations, synthetic one-component pressure and partially resolved three-component (3-C) displacement seismograms may be computed for 3-D models using a finite-difference solution of the (3-D) acoustic wave equation. Compressional-or decoupled-SH and Love wave responses can be used as a complement to elastic response for data interpretation and subsurface structure delineation. This new capability is applicable to a wide range of recording geometries over various complex structures. Simulations include surface sources recorded on a circular array, walkaway survey, multi-component multioffset vertical seismic profiling (VSP), and both offend and off-line shots into a crooked, wide-aperture array. The result indicated that the relationships among source, receiver location and complexity of the subsurface structure will produce different three-component responses. For seismic event identification and subsurface structure discrimination, the uniqueness from forward modeling can be emphasized through integrated approach by considering 3-D scalar, partially resolved 3-C displacement and elastic modeling.
By analysing teleseismic waveform data, we propose a large-scale Taiwan Reference Moho Discontinu... more By analysing teleseismic waveform data, we propose a large-scale Taiwan Reference Moho Discontinuity Model (TRMDM) through coherent stacking and H-κ analysis of receiver functions (RFs). Tectonic structural features are determined from accurate crustal thickness (H) and reliable Vp/Vs ratio (κ) (consequently Poisson's ratio, σ) estimations for Taiwan. The best estimated regional average Moho depth is 33.5 ± 2.9 km. Noted is a westward crustal thinning from 39 ± 3.0 km in eastern Taiwan to 30 ± 1.5 km in southeastern China. The averaged Moho depth is 23.0 ± 2.5 km in northern Taiwan and increases to 33.5 ± 2.0 km in the south. The markedly thin crust and high Vp/Vs ratio in northern Taiwan are mainly associated with backarc opening and magmatic activities. Based on H-κ estimations and associated seismicity patterns beneath Suang-Long (SSLB) and adjacent four stations, we propose two Moho reference models that exhibit distinctive crustal thickening in central Taiwan. The spatial variations of crustal thickness models show strong lateral change in the Moho topography. The thick crust in the south, thin crust in the north and transition in central Taiwan mark the different tectonic units between the subduction wedge and the collision prism onshore. In view of the following list of constraints: thicker crust in the south than in the north; eastward crust thickening; significant Moho depth changes; clear 9-11 s subcrustal boundaries from five stations; the compelling thickening of crust and upper mantle beneath the central mountain belt; seismicity patterns with the absence of mantle earthquake in the Wadati-Benioff zone; regional geological features; the occurrence of the inland Chi-Chi earthquake; the compelling low κ at YULB (1.58 ± 0.13) that may extend to central Taiwan at SSLB (1.59 ± 0.23); relatively thick crust at YULB (39.5 ± 1.6 km) and SSLB (40.5 ± 1.3 km or 52.5 ± 1.5 km), and finally, the apparent horizontal offset between highest Yu Shan Mountain and the deepest Moho at SSLB, we are able to suggest that the initial subduction of the Eurasian Plate occurred in southern Taiwan and most likely extended to the transition zone in central Taiwan. Although the current RF study is limited by large interstation distance, our analysis provides more notable constraints than previous studies to explicitly determine the characteristic features of Moho discontinuity. This detailed approach means that rather than treating the tectonic framework as a whole heterogeneous lithospheric feature, clear Moho boundary geometry variations indicate that active orogenic processes and differential tectonic stress loading create separate and complex crustal and subcrustal-to-upper-mantle structural features caused by the arc-continent collision.
SEG Technical Program Expanded Abstracts 2014, 2014
Imaging both Bottom Simulating Reflectors (BSRs) and ocean fine structure can be achieved through... more Imaging both Bottom Simulating Reflectors (BSRs) and ocean fine structure can be achieved through careful process of long-offset Multi-Channel Seismic (MCS) data. Interpretation and classification on type locality, sedimentation environment and spatial distribution of BSRs with potential methane hydrate production are exploited. Both patchy and continuous distributed BSRs can be identified and may well correspond to different sediment dispersal processes in the frontal and real segment of active accretionary wedge that belongs to active continental margins offshore of SW Taiwan. Our results indicated fluid/gas migration in the active margin may controlled by the tectonic structure control and/or local seismic activity. The image of ocean currents reveals different dynamic processes occurred at different depth. Near-surface current are characterized by continuous, nearly horizontal stratified layers indicate air-sea surface interaction features. More apparent wavy shaped, less continuous with varying in impedance contrast, relative clear top and bottom reflections with low internal amplitude reveal the feature of eddy currents can be identified at the intermediate depth range. Through quantitative analyses, the spatial distribution features of internal waves and others such as eddies, mass movement, turbulence and current mixing are all helpful in promoting our understanding of dynamic ocean processes across different scale constrained by seismic frequency band.
Abstract We derive a velocity field using GPS data between 1993 and 2007 in the Chiayi–Tainan are... more Abstract We derive a velocity field using GPS data between 1993 and 2007 in the Chiayi–Tainan area located in the deformation front of the Taiwan mountain belt. The crustal motion with respect to Penghu shows large velocities of about 33–44 mm/yr in the west-northwest to west directions in the Western Foothills and the velocities decrease westward to 0–5 mm/yr in the coastal area. Significant uplift rates of 5–20 mm/yr are observed at sites to the east of the Jiuchiunken–Muchiliao–Liuchia Fault (JMLF) system. We use a block model, a buried dislocation model, and a two-dimensional fault model to invert for fault geometries and slip rates on major frontal thrust faults. Modeling results from a block model show the inferred long-term slip rate of 42 mm/yr in the direction of 280° and the maximum back-slip rate of 38 mm/yr on a 23° east dipping fault extending to 13 km at depth. On the other hand, the buried dislocation model results in a horizontal decollement at a depth of 8 km with a uniform slip rate of 41.6 mm/yr. If we connect the top edge of decollement to the surface trace of JMLF as a potential future rupture, a 22° east-dipping fault is required. Results from both block model and buried dislocation model suggest the JMLF is nearly fully locked. The results of two-dimensional fault models show the frontal thrust faults have slip rates of less than 2 mm/yr at shallow depths and the inferred decollement is sub-horizontal (5°–7°) at a depth of 10 km with slip rates of 44–46 mm/yr. Results of various approaches show general agreement on fault geometries and slip rates and reveal that the frontal thrust fault system has a high potential for large earthquakes.
This study presents an efficient parallelized staggered grid pseudospectral method for 2-D viscoa... more This study presents an efficient parallelized staggered grid pseudospectral method for 2-D viscoacoustic seismic waveform modeling that runs in a highperformance multi-processor computer and an in-house developed PC cluster. Parallel simulation permits several processors to be used for solving a single large problem with a high computation to communication ratio. Thus, parallelizing the serial scheme effectively reduces the computation time. Computational results indicate a reasonably consistent parallel performance when using different FFTs in pseudospectral computations. Meanwhile, a virtually perfect linear speedup can be achieved in a distributed-memory multi-processor environment. Effectiveness of the proposed algorithm is demonstrated using synthetic examples by simulating multiple shot gathers consistent with field coordinates. For dispersive and attenuating media, the propagating wavefield possesses the observable differences in waveform, amplitude and travel-times. The resulting effects on seismic signals, such as the decreased amplitude because of intrinsic Q and temporal shift because of physical dispersion phenomena, can be analyzed quantitatively. Anelastic effects become more visible owing to cumulative propagation effects. Field data application is presented in simulating OBS wide-angle seismic marine data for deep crustal structure study. The fine details of deep crustal velocity and attenuation structures in the survey area can be resolved by comparing simulated waveforms with observed seismograms recorded at various distances. Parallel performance is analyzed through speedup and efficiency for a variety of computing platforms. Effective parallel implementation requires numerous independent CPU intensive sub-jobs with low latency and high bandwidth inter-processor communication.
We inverted the high-resolution spatiotemporal slip distribution of the 21 September 1999 Chi-Chi... more We inverted the high-resolution spatiotemporal slip distribution of the 21 September 1999 Chi-Chi, Taiwan, earthquake utilizing data from densely distributed island-wide strong motion stations for a three-dimensional (3-D) fault geometry, and 3-D Green's functions calculations based upon parallel nonnegative least squares inversion. The 3-D fault geometry, consistent with high-resolution reflection profile, is determined from GPS inversion and aftershocks distribution. This 3-D fault model presents the dip angle gradually becoming shallower from south to north along the fault and near flat at the deeper portion of the fault. The 3-D Green's functions are calculated through numerical wavefield simulation from three-dimensional heterogeneous velocity structure derived from tomography studies. The Green's functions show significant azimuthal variations and suggest the necessity of lateral heterogeneity in velocity structure. Considering complex fault geometry and Green's functions in full 3-D scale, we invert the spatial/temporal slip distribution of the 1999 Chi-Chi earthquake using the best available and most densely populated strong motion waveform data. We perform the inversion under a parallel environment utilizing multiple-time window to manage the large data volume and source parameters. Results indicate that most slip occurred at the shallower portion of the fault above the decollement. Two major asperities are found, one in the middle of the fault and another one at the northern portion of the fault near the bend in the fault trace. The slip in the southern portion of the fault shows a relatively low slip rate with longer time duration, while the slip in the northern portion of the fault shows a large slip rate with shorter time duration. The synthetics explain the observations well for the island-wide distributed strong motion stations. This comprehensive study emphasizes the importance of realistic fault geometry, 3-D Green's functions, and parallel inversion technique in correctly accounting for both the detailed source rupture process and its relationship with the strong ground motion of this intense earthquake.
Abstract. Various types of noise can obscure reflections and refractions on deep crustal seismic ... more Abstract. Various types of noise can obscure reflections and refractions on deep crustal seismic data. This study presents a simple but elegant approach to true wavefield processing where noise reduction andtrace interpolations are applied implicitly during ...
... used in this study. The receiver function method provides high resolution information down to... more ... used in this study. The receiver function method provides high resolution information down to at least lithospheric depths by using teleseismic waveform data ([Ammon, 1991] and [Langston, 1979]). The interface between crust ...
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