Three-dimensional Seismic Model Research Articles

Prediction of Inter-Well Petrophysical Properties from Seismic Model: A Case of Egbem Field, Niger Delta, Nigeria

We have generated a seismic model for the prediction of inter-well petrophysical properties of Egbem Field; in the Niger Delta, Nigeria. Acoustic Impedances (AI) were calculated from sonic and density logs in the available 24 wells. Reflection Coefficients (RC) determined from the acoustic impedances were convolved with modelled wavelet to produce synthetic seismograms at various well locations. A top structure map of a reservoir from the field was used to define the position of faults, which served as key pillars in gridding the reservoirs of the field into geocellular blocks of 50 x 50 meters each. Petrophysical analysis of the field showed an average porosity of 24.8% and volume of shale of 19.9%. Petrophysical parameters were geostatistically distributed on the generated framework model using Sequential Gaussian Simulation (SGS). The result of the Acoustic Impedance classification reveals four lithologic facies namely, shale, sandy shale, shaley sand and sand facies. The facies model indicates high and low acoustic impedances for sand and shale respectively. This model indicates that sand which is a good reservoir is concentrated in the central part of the field. The result of synthetic seismic generation showed a very good match between acoustic impedance and lithology. Comparison of porosity obtained from the porosity model with conventional well log porosity showed similarities in their histogram distribution. Finally, Three-dimensional seismic model of Egbem Field has been constructed using log data from twenty four wells. The application of geostatistical analyses extrapolated and interpolated these log data to cover the entire field.

Optimization of local scale seismic networks applied to geothermal fields. The case of the Acoculco caldera, Mexico

The exploitation of a geothermal field can be accompanied by both natural and induced seismicity. Hence the installation of a seismic network suitable for also locating low-magnitude earthquakes, is of great interest for geothermal development, especially for monitoring the activity related to the injection or production.Most of the aspects that affect the location of earthquakes are data quality and phase picking, the inverse method chosen to solve the problem, the velocity model, and the seismic network configuration. We focus on the optimization of network configuration and the reduction of the location error is one of the alternatives to finding optimal station positions. Here we propose an improvement of the D-OPTIMAL algorithm (Tramelli et al., 2013) that tries and find optimal station positions minimizing the amount of the error ellipsoid of the event location using the D-criterion. This algorithm is computationally efficient and easy to be implemented. In this version, we also introduced the possibility to account for several prior information that is generally available when implementing a monitoring site permanently or temporarily. These a priori parameters introduced are: i) three-dimensional seismic velocity models, ii) seismic noise levels, iii) topographic gradient, and iv) H/V ratio values. The last three parameters are introduced in the station position selection using a weighted system scheme.We applied this methodology to the Acoculco geothermal field (Mexico) where an injection test was planned and executed in 2021. The comparison between the network defined using the standard approach, which considers a 1D velocity model and the installation costs, and this improved version shows the importance of introducing a priori information during the selection of the network. Installation sites showed better distribution in the region, resulting in an overall increase in the sensitivity and a decreasing of the error location estimation in the target region.The methodology presented here is easy to apply to other study cases such as active volcanoes, anthropogenic activities, or other kinds of location studies at local scale.

Characterizing Concealed Fault Systems by Integrating Field Data Mapping and Laboratory Experiments: A Case Study of the Huawa Area in East China

Concealed faults can be important for understanding the regional structural geology and the subsurface fluid distribution. However, such faults are usually difficult to identify and characterize because of their small size and complex mechanism. To address this issue, we present an integrated approach of three-dimensional seismic data mapping and physical modeling experiments to examine the geometrical and kinematic characteristics of concealed faults and their relationship with the main faults in the Huawa area, east China. Three series of experiments were designed to characterize the differences in the scale of concealed faults and main faults, which also allowed us to examine how the concealed faults grow with the main faults in the area. Through this integrated study, we have demonstrated that: 1) NE-SW-striking concealed faults are below the resolution of the available seismic datasets and not easily recognized in seismic sections and that most of them grew later than the E-W-striking main faults, with some of them having grown at the same time; 2) pre-existing faults, rather than asymmetry of the basin structure, affect the faults that develop during subsequent episodes of extension; 3) E-W-striking pre-existing faults under a NW-SE stress direction of extension are most likely the formation mechanism of concealed faults. This study is of reference value in the interpretation of concealed faults in other regions.

Performance of 3D Wave Field Modeling Using the Staggered Grid Finite Difference Method with General-Purpose Processors

This paper aims to provide a quantitative understanding of the performance of numerical modeling of a wave field equation using general-purpose processors. In particular, this article presents the most important aspects related to the memory workloads and execution time of the numerical modeling of both acoustic and fully elastic waves in isotropic and anisotropic mediums. The results presented in this article were calculated for the staggered grid finite difference method. Our results show that the more realistic the seismic wave simulations that are performed, the more the demand for memory and the computational capacity of the computing environment increases. The results presented in this article allow the estimation of the memory requirements and computational time of wavefield modeling for the considered model (acoustic, elastic or anisotropic) so that their feasibility can be assessed in a given computing environment and within an acceptable time. Understanding the numerical modeling performance is especially important when graphical processing units (GPU) are utilized to satisfy the intensive calculations of three-dimensional seismic forward modeling.

An optimized parallelized SGFD modeling scheme for 3D seismic wave propagation

Large-scale three-dimensional (3D) seismic modeling is considered the foundation of imaging and inversion. Parallelization strategies are crucial to solving such a computationally intensive problem. In this paper, we focus on two factors, decomposition direction and decomposition dimension, that significantly affect the computational performance. The decomposition direction determines the cache hit ratio during register addressing, and the decomposition dimension influences the communication size. We thoroughly analyze these two factors by performing time-space domain staggered-grid finite-difference (SGFD) modeling with a set of decomposition strategies. Four metrics, including computation time, speedup ratio, strong scaling property, and memory usage, are introduced to evaluate the computational performance of each trial. After theoretical analysis and metrics testing, we conclude that the optimized domain decomposition strategy is: decomposing models at two dimensions, the decomposition directions are perpendicular to the fastest and the second fastest dimensions, here we refer the dimension in which data are continuously saved as the fastest dimension. Three examples further verify the feasibility and efficiency of the optimized parallel scheme. Considering that domain decomposition-based 3D seismic parallel simulation packages are seldom available in the public domain, we provide a program template for the optimized domain decomposition strategies as an open-source package.

Crustal structure of the northern Dinarides and southwestern part of the Pannonian basin inferred from local earthquake tomography

We present the results of local earthquake tomography (LET) analysis to investigate the crust and uppermost mantle structure in the northern Dinarides and southwestern Pannonian basin. Datasets of P-wave travel times are inverted to recover a three-dimensional P-wave velocity model of the survey area. Two data subsets were used in this study: (1) data recorded on 15 temporary seismic stations, which were deployed in Croatia in the framework of ALPASS-DIPS project, and (2) travel time datasets from the Croatian Seismological Survey and ORFEUS databases. The data enabled to achieve a resolution of less than a hundred kilometres in horizontal directions and a few kilometres in vertical direction in the area with good ray coverage, as is documented by the resolution tests. Velocity variations are computed on a grid using the three-dimensional nonlinear tomographic inversion method. Our study provides the first crustal three-dimensional seismic model of the studied area, and it is correlated with previous results in the survey area allowing us to infer the main crustal structures with high confidence. The velocity model reveals crustal thickening beneath the Dinarides and significant crustal thinning beneath the Pannonian basin. The Moho surface was determined on the basis of the highest velocity gradients in the vertical cross-sections. We find relatively high velocities below the northern Dinarides at shallow depths (< 10 km), and low velocities caused by deep local depressions in the Pannonian basin. A very pronounced high-velocity body is present in the transitional part between the Dinarides and the Pannonian basin at a depth range of 5–15 km. The strong velocity increase at depth of about 20 km indicates that the Dinaridic crust could be interpreted as two-layered, while the Pannonian crust is probably one-layered.

Geoscience Australia’s contribution to AusArray - Passive seismic imaging of Australia

Geoscience Australia (GA), as a part of the Exploring for the Future (EFTF) Programme, is aiming to create a high-resolution three-dimensional (3D) seismic model of Australia to infer physical properties of the lithosphere from depths of few meters to hundreds of kilometres. This work is based on new data collected from National Seismological Network and a new movable seismic array complimented by legacy seismological data obtained by universities. GA has deployed a movable array of 115 broadband seismic stations for one year between Mount Isa and Tennant Creek arranged in a grid pattern with interstation distance of approximately 55 kilometres in order to attain horizontal resolution of at least 20 kilometres. This dense network is reinforced by fifteen semi-permanent higher sensitivity broadband seismic stations located predominantly in the Northern Territory and Western Australia in order to increase imaging resolution within the array and within areas where the National Seismological Network has gaps. Multiple seismological methods are being combined together to obtain robust constraints on 3D lithospheric architecture. For the first time, particular attention is focused on shallow structures located at depths of less than 1 kilometre.

Modeling of Long-Period Ground Motions in the Nankai Subduction Zone: Model Simulation Using the Accretionary Prism Derived from Oceanfloor Local S-Wave Velocity Structures

The accretionary prism in the subduction zone, which consists of thick low-velocity oceanic sediments, significantly affects the propagation of seismic waves for shallow, offshore earthquakes, including large interplate earthquakes. In order to simulate long-period (> 5 s) ground motions in the Nankai subduction zone, we constructed a three-dimensional (3D) seismic velocity structure model of the accretionary prism by interpolation/extrapolation of local S-wave velocity structures beneath 46 oceanfloor seismic stations (DONET), which are deployed just above the accretionary prism off the southern Kii and eastern Shikoku regions. We modeled local S-wave velocity structures using a simple two-parameter depth-varying velocity function. To investigate the effects of the accretionary prism on ground and seafloor motions, we conducted numerical simulations of seismic wave propagation for three local earthquakes that occurred in southwestern Japan. The simulations reasonably reproduced the observed seismograms, not only for the period ranges of the moment tensor inversion (~ 50 s), but also for the strong, long-period ground motions in the sedimentary basins (~ 5 s), especially in the region where DONET stations are densely deployed. Since depth-varying, local S-wave structures significantly improve the reproducibility of long-period ground motions, our modeling procedure is useful for modeling long-period ground motions of local and regional offshore subduction zone earthquakes.

4D modelling of fault reactivation using complete paleostress tensors from the Cooper–Eromanga Basin, Australia

ABSTRACTDetermining fault activity through time has typically utilised high-resolution seismic data to identify stratigraphic thickness changes or displacement vs distance plots; however, this approach is not possible in regions with low-resolution seismic data. We present a new approach for determining fault reactivation (tensile and shear) through time by integrating three-dimensional seismic data, geomechanical modelling and complete paleostress tensors from calcite twin stress inversion. The Cooper–Eromanga Basin is used as a case study to model the stress conditions present during six tectonic events that have affected the basin and, in doing so, constrain the effective paleostress magnitudes through time. Results show that the likelihood of dilation and shear reactivation of individual fault sets varies through time, with N–S- and E–W-striking faults likely to have been open to fluid flow after the critical moment in the hydrocarbon system. These results have substantial implications for hydrocarbon migration pathway models and structural and stratigraphic models for the Cooper–Eromanga Basin. This approach would benefit other provinces with low-resolution seismic data preventing fault growth analysis, or in regions where hydrocarbon migration pathways are poorly defined.

Shale gas sweet spot identification and precise geo-steering drilling in Weiyuan Block of Sichuan Basin, SW China

Based on drilling and production test data and the geological, geochemical, geophysical and rock mechanics achievements of Weiyuan Block of Sichuan Basin, “sweet spots” of shale gas were determined and geosteering technology was tested in drilling the sweet spots under complex structural background. The high quality Silurian Longmaxi Formation shale (with TOC≥2%) in the study area, thick (36.0−44.5 meters), is divided into four sublayers, from bottom to top, which differ widely in TOC (2.0%−8.1%), gas content (1.2−12.6 m3/t), porosity (0.66%−11.80%), and brittle mineral content (17.5%−98.5%). Considering gas content and fracability of shale, the layer Long 11a in Longmaxi Formation is the best “sweet spot” layer in vertical direction; horizontally, the areas with folding structure in shale reservoir have more natural fractures, and thus have better storage quality and permeability, are “sweet spots”. During drilling of the sweet spots, three-dimensional seismic and geological modeling was used to predict the depth of “sweet spot”, which was corrected in time with drilling data, and by controlling well trajectory this way, the drilling rate of “sweet spots” has been improved.

Structural inversion, imbricate wedging, and out-of-sequence thrusting in the southern Junggar fold-and-thrust belt, northern Tian Shan, China

Accurate definition of structural style in subsurface interpretation is critically important for understanding the deformation history of fold-and-thrust belts, as well as assessing the petroleum prospectivity of structural traps. Using two- and three-dimensional seismic reflection surveys, well data, field mapping, forward models, and balanced cross sections, we describe the structural styles across the actively deforming southern Junggar fold-and-thrust belt in northwestern China, a basin undergoing petroleum exploration and development operations. Subsurface interpretations indicate several folds in the basin overlie Jurassic normal faults that were tectonically inverted in the Late Jurassic to Early Cretaceous. Following inversion, multiple detachment levels propagated northward from the Tian Shan and formed a series of imbricated fault-related folds. The most prominent fold trend in southern Junggar consists of the Tugulu, Manas, and Huoerguosi anticlines, which trap hydrocarbons in clastic Eocene reservoirs. These structures exhibit complex internal geometries, with coeval forethrusts and backthrusts forming imbricated structural wedges. In the latest stages of deformation, and continuing at present, the uppermost thrust sheet, the Southern Junggar Thrust (SJT), truncated the backlimbs of these structural traps, implying the SJT is a tectonically active, out-of-sequence thrust. From these interpretations, we present a model for how the southern Junggar fold-and-thrust belt developed from Jurassic to present. Moreover, we detail how fold growth, fault activity, and structural style affected charge histories, trap formation, and reservoir compartmentalization. Our results have direct implications for assessment of the southern Junggar petroleum system as well as other complex fold-and-thrust belts.

Three-dimensional seismic model of crustal structure in Southern Norway

Three-dimensional seismic model of crustal structure in Southern Norway

A cloud-based synthetic seismogram generator implemented using Windows Azure

Synthetic seismograms generated by solving the seismic wave equation using numerical methods are being widely used in seismology. For fully three-dimensional seismic structure models, the generation of these synthetic seismograms may require large amount of computing resources. Conventional high-performance computer clusters may not provide a cost-effective solution to this type of applications. The newly emerging cloud-computing platform provides an alternative solution. In this paper, we describe our implementation of a synthetic seismogram generator based on the reciprocity principle using the Windows Azure cloud application framework. Our preliminary experiment shows that our cloud-based synthetic seismogram generator provides a cost-effective and numerically efficient approach for computing synthetic seismograms based on the reciprocity principle.

Deepwater canyons: An escape route for methane sealed by methane hydrate

Three-dimensional seismic imaging and modelling of gas hydrates from offshore of west Africa (Mauritania) shows that submarine canyons on stable continental slopes can capture and release methane that is sealed by methane hydrate. We demonstrate this by focussing on a canyon which is ~200km long, 2.4 to 3.1km wide, up to 550m deep and has canyon walls that dip at 25°-30°. Incision of the canyon causes cooling of the surrounding sediment and deepening of the base of the methane hydrate stability zone. The base of the hydrate deepens by up to 550m and also dips at 25°–30°, parallel to the canyon margins. It forms a continuous or semi-continuous wall of lower permeability sediment that can be mapped along the canyon margins on the basis of aligned high amplitude reflection terminations. Theoretically these methane barriers could extend for 10s to 100s of kilometres parallel to both canyon walls. Several free gas accumulations are sealed laterally in this way in a canyon margin free gas zone. Large failures of the sides of the canyon, remove the lower permeability hydrate, allowing free methane to leak. Globally, submarine canyons and marine gas hydrates occur in similar places on continental margins and canyons incise to depths that are comparable with the position of base of the methane hydrate stability zone. Therefore deepening of the base of the hydrate as a result of the cooling effect of canyons should be common and this mechanism for methane trapping and release could be generally applicable to present and past marine methane hydrates.

Mapping the Tohoku forearc: Implications for the mechanism of the 2011 East Japan earthquake (Mw 9.0)

To investigate the generation mechanism of the 2011 great East Japan (GEJ) earthquake (M 9.0), we determined a three-dimensional (3-D) seismic model under the NE Japan forearc region using a large number of P and S wave arrival times from local earthquakes. Our results show that the GEJ main-shock occurred in a high-velocity (Vp, Vs) zone with higher Poisson's ratio (σ). The aftershocks relocated with a master-event location (MEL) procedure indicate that most of them are located at the corner of the mantle wedge along the upper boundary of the subducting Pacific slab, and their focal depths are slightly shallower than the background seismicity. The tomographic images and spatial distribution of the aftershocks imply strong interplate coupling (asperity) in the main-shock area and weak coupling in the surrounding areas of the megathrust zone. We think that fluids in the crust and uppermost mantle in the subduction zone could have affected the earthquake generation through the physical role of fluid pressure and a variety of chemical effects. We conclude that the 2011 GEJ earthquake initiation and the rupture processes of the aftershock sequence were influenced by fluid extrusion into the rupture zone due to the dehydration of the subducting Pacific slab. Fluid-bearing structural heterogeneities with thermal–petrologic variations in the megathrust zone played a key role in the initiation of the 2011 GEJ earthquake and its aftershocks.

A probabilistic seismic model for the European Arctic

[1] The development of three-dimensional seismic models for the crust and upper mantle has traditionally focused on finding one model that provides the best fit to the data while observing some regularization constraints. In contrast to this, the inversion employed here fits the data in a probabilistic sense and thus provides a quantitative measure of model uncertainty. Our probabilistic model is based on two sources of information: (1) prior information, which is independent from the data, and (2) different geophysical data sets, including thickness constraints, velocity profiles, gravity data, surface wave group velocities, and regional body wave traveltimes. We use a Markov chain Monte Carlo (MCMC) algorithm to sample models from the prior distribution, the set of plausible models, and test them against the data to generate the posterior distribution, the ensemble of models that fit the data with assigned uncertainties. While being computationally more expensive, such a probabilistic inversion provides a more complete picture of solution space and allows us to combine various data sets. The complex geology of the European Arctic, encompassing oceanic crust, continental shelf regions, rift basins and old cratonic crust, as well as the nonuniform coverage of the region by data with varying degrees of uncertainty, makes it a challenging setting for any imaging technique and, therefore, an ideal environment for demonstrating the practical advantages of a probabilistic approach. Maps of depth to basement and depth to Moho derived from the posterior distribution are in good agreement with previously published maps and interpretations of the regional tectonic setting. The predicted uncertainties, which are as important as the absolute values, correlate well with the variations in data coverage and quality in the region. A practical advantage of our probabilistic model is that it can provide estimates for the uncertainties of observables due to model uncertainties. We will demonstrate how this can be used for the formulation of earthquake location algorithms that take model uncertainties into account when estimating location uncertainties.

Seismicity and earthquake hazard analysis of the Teton–Yellowstone region, Wyoming

Earthquakes of the Teton–Yellowstone region represent a high level of seismicity in the Intermountain west (U.S.A.) that is associated with intraplate extension associated with the Yellowstone hotspot including the nearby Teton and Hebgen Lake faults. The seismicity and the occurrence of high slip-rate late Quaternary faults in this region leads to a high level of seismic hazard that was evaluated using new earthquake catalogues determined from three-dimensional (3-D) seismic velocity models, followed by the estimation of the probabilistic seismic hazard incorporating fault slip and background earthquake occurrence rates. The 3-D P-wave velocity structure of the Teton region was determined using local earthquake data from the Jackson Lake seismic network that operated from 1986–2002. An earthquake catalog was then developed for 1986–2002 for the Teton region using relocated hypocenters. The resulting data revealed a seismically quiescent Teton fault, at M L, local magnitude > 3, with diffuse seismicity in the southern Jackson Hole Valley area but notable seismicity eastward into the Gros Ventre Range. Relocated Yellowstone earthquakes determined by the same methods highlight a dominant E–W zone of seismicity that extends from the aftershock area of the 1959 (M S surface wave magnitude) 7.5 Hebgen Lake, Montana, earthquake along the north side of the 0.64 Ma Yellowstone caldera. Earthquakes are less frequent and shallow beneath the Yellowstone caldera and notably occur along northward trending zones of activity sub-parallel to the post-caldera volcanic vents. Stress-field orientations derived from inversion of focal mechanism data reveal dominant E–W extension across the Teton fault with a NE–SW extension along the northern Teton fault area and southern Yellowstone. The minimum stress axes directions then rotate to E–W extension across the Yellowstone caldera to N–S extension northwest of the caldera and along the Hebgen Lake fault zone. The combination of accurate hypocenters, unified magnitudes, and seismotectonic analysis helped refine the characterization of the background seismicity that was used as input into a probabilistic seismic hazards analysis. Our results reveals the highest seismic hazard is associated with the Teton fault because of its high slip-rate of approximately 1.3 mm/yr compared to the highest rate of 1.4 mm/yr in southern Yellowstone on the Mt. Sheridan fault. This study demonstrates that the Teton–Yellowstone area is among the regions highest seismic hazard in the western U.S.

The Prospect of Using Three-Dimensional Earth Models to Improve Nuclear Explosion Monitoring and Ground-motion Hazard Assessment

Research Article| January 01, 2009 The Prospect of Using Three-Dimensional Earth Models to Improve Nuclear Explosion Monitoring and Ground-motion Hazard Assessment John J. Zucca; John J. Zucca 1Lawrence Livermore National Laboratory Search for other works by this author on: GSW Google Scholar William R. Walter; William R. Walter 1Lawrence Livermore National Laboratory Search for other works by this author on: GSW Google Scholar Arthur J. Rodgers; Arthur J. Rodgers 1Lawrence Livermore National Laboratory Search for other works by this author on: GSW Google Scholar Paul Richards; Paul Richards 2Lamont-Doherty Earth Observatory Search for other works by this author on: GSW Google Scholar Michael E. Pasyanos; Michael E. Pasyanos 1Lawrence Livermore National Laboratory Search for other works by this author on: GSW Google Scholar Stephen C. Myers; Stephen C. Myers 1Lawrence Livermore National Laboratory Search for other works by this author on: GSW Google Scholar Thorne Lay; Thorne Lay 3University of California Santa Cruz Search for other works by this author on: GSW Google Scholar Dave Harris; Dave Harris 1Lawrence Livermore National Laboratory Search for other works by this author on: GSW Google Scholar Tarabay Antoun Tarabay Antoun 1Lawrence Livermore National Laboratory Search for other works by this author on: GSW Google Scholar Seismological Research Letters (2009) 80 (1): 31–39. https://doi.org/10.1785/gssrl.80.1.31 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation John J. Zucca, William R. Walter, Arthur J. Rodgers, Paul Richards, Michael E. Pasyanos, Stephen C. Myers, Thorne Lay, Dave Harris, Tarabay Antoun; The Prospect of Using Three-Dimensional Earth Models to Improve Nuclear Explosion Monitoring and Ground-motion Hazard Assessment. Seismological Research Letters 2009;; 80 (1): 31–39. doi: https://doi.org/10.1785/gssrl.80.1.31 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search The past 10 years have brought rapid growth in the development and use of three-dimensional (3D) seismic models of Earth structure at crustal, regional, and global scales. In order to explore the potential for 3D seismic models to contribute to important societal applications, Lawrence Livermore National Laboratory (LLNL) hosted a Workshop on Multi-Resolution 3D Earth Models to Predict Key Observables in Seismic Monitoring and Related Fields on 6–7 June 2007, in Berkeley, California. The workshop brought together academic, government, and industry leaders in research programs developing 3D seismic models and methods for nuclear explosion monitoring and seismic ground-motion hazard assessment.... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Three-dimensional seismic modelling of the crustal structure between East European Craton and the Carpathians in SE Poland based on CELEBRATION 2000 data

SUMMARY In this paper, we present results of 3-D first-arrival tomography applied to data recorded in SE Poland during the CELEBRATION 2000 seismic experiment. The target area covers ca. 500 × 500 km 2 and represents a complex geological setting from old Precambrian platform (East European Craton, EEC), through the crustal blocks (terranes) that form the TransEuropean Suture Zone (TESZ) to the young Alpine orogen—the Carpathians. Contrasting velocity distributions in various geological units makes the tomographic inversion challenging. For this reason, much attention was paid to the inversion methodology. We tested ‘multioffset’ and ‘multiscale’ approaches. By increasing the offset range in the ‘multioffset’ inversion we have independently constrained different depth ranges of our model. We have found that the ‘multiscale’ method, that is, the gradual stepping from bigger to smaller model cells produced the preferred solution. Resolution of the resultant crustal model was determined by performing checkerboard and restoration resolution tests. Lateral resolution of the order of 25 km is inferred down to 10 km depth and 50 km down to 25‐30 km depth. We have also applied a quasi-Monte Carlo analysis to determine the absolute errors of model parameters, which to our knowledge, is the first such attempt in case of a 3-D wide-angle data set. The mean uncertainties of our tomographic velocities are 0.1 km s −1 , but some anomalies are recovered with

A BASIC INTRODUCTION TO QUANTITATIVE SEISMIC HAZARD ASSESSMENT

We provide an overview of some of the issues that need to be considered in the context of quantitative seismic hazard assessment. To begin with, one needs to inventory and characterize the major faults that could produce earthquakes that would impact the region of interest. Next, one needs a seismographic network that continually records ground motion throughout the region. Data from this network may be used to assess and locate seismicity, calibrate ground motion simulations, and to conduct seismic early-warning experiments. To assess the response of engineered structures to strong ground motion, seismographs should also be installed at various locations within such engineered structures, e.g., on bridges, overpasses, dams and in tall buildings. The ultimate goal would be to perform 'end-to-end' simulations, starting with the rupture on an earthquake fault, followed by the propagation of the resulting seismic waves from the fault to an engineered structure of interest, and concluding with an assessment of the response of this structure to the imposed ground motion. To facilitate accurate ground motion and end-to-end simulations, one needs to construct a detailed three-dimensional (3D) seismic model of the region of interest. In particular, one needs to assess the slowest shear-wave speeds within the sediments underlying the metropolitan area. Geological information, and, in particular, seismic reaction and refraction surveys are critical in this regard. In the context of end-to-end simulations, detailed numerical models of engineered structures of interest need to be constructed as well. Data recorded by the seismographic network and in engineered structures after small to moderate earthquakes may be used to assess and calibrate the seismic and engineering models based upon numerical simulations. Once the seismic and engineering models produce synthetic ground motion that match the observed ground motion reasonably well, one can perform simulations of hypothetical large earthquakes to assess anticipated strong ground motion and potential damage. Throughout this article we will use the Los Angeles and Taipei metropolitan areas as examples of how to approach quantitative seismic hazard assessment.