3D S-wave Velocity Research Articles

Identification of Engineering Bedrock in Jakarta by Using Array Observations of Microtremors

The bedrock depth is one of the most important parameters in seismic hazard analysis. This parameter has not been identified well in Jakarta. This study was conducted to determine the bedrock depth in Jakarta based on S-wave velocity parameters. Microtremors array method was applied in this study to obtain 1D and 2D S-wave velocity profiles. The spatial autocorrelation (SPAC) method was used to estimate dispersion curves, while S-wave velocity structure was derived by genetic algorithm. Reffering to the geological condition in the study area, microtremors array measurements were conducted for two East-West lines in the northern and southern parts of Jakarta. The result of 2D construction of S-wave velocity structure shows stratigraphy cross sections that consists of four layers, where the bedrock depths in northern Jakarta can be depicted in the range from 519 to 662 m and in the southern part in the range from 353 m to 399 m. It has a positive correlation with the local geological condition i.e the sediment thickness increases to the north.

Analysis of induced seismicity in geothermal reservoirs – An overview

In this overview we report results of analysing induced seismicity in geothermal reservoirs in various tectonic settings within the framework of the European Geothermal Engineering Integrating Mitigation of Induced Seismicity in Reservoirs (GEISER) project. In the reconnaissance phase of a field, the subsurface fault mapping, in situ stress and the seismic network are of primary interest in order to help assess the geothermal resource. The hypocentres of the observed seismic events (seismic cloud) are dependent on the design of the installed network, the used velocity model and the applied location technique. During the stimulation phase, the attention is turned to reservoir hydraulics (e.g., fluid pressure, injection volume) and its relation to larger magnitude seismic events, their source characteristics and occurrence in space and time. A change in isotropic components of the full waveform moment tensor is observed for events close to the injection well (tensile character) as compared to events further away from the injection well (shear character). Tensile events coincide with high Gutenberg-Richter b-values and low Brune stress drop values. The stress regime in the reservoir controls the direction of the fracture growth at depth, as indicated by the extent of the seismic cloud detected. Stress magnitudes are important in multiple stimulation of wells, where little or no seismicity is observed until the previous maximum stress level is exceeded (Kaiser Effect). Prior to drilling, obtaining a 3D P-wave (Vp) and S-wave velocity (Vs) model down to reservoir depth is recommended. In the stimulation phase, we recommend to monitor and to locate seismicity with high precision (decametre) in real-time and to perform local 4D tomography for velocity ratio (Vp/Vs). During exploitation, one should use observed and model induced seismicity to forward estimate seismic hazard so that field operators are in a position to adjust well hydraulics (rate and volume of the fluid injected) when induced events start to occur far away from the boundary of the seismic cloud.

表層部分に注目した地震動シミュレーションのための関東平野の3次元S波速度深部地盤モデル

表層部分に注目した地震動シミュレーションのための関東平野の3次元S波速度深部地盤モデル

Estimating surface-wave dispersion curves from 3D seismic acquisition schemes: Part 1 — 1D models

Surface-wave analysis is based on the estimation of surface-wave dispersion curves, which are then inverted to provide 1D S-wave velocity profiles. Surface-wave dispersion curves can be extracted from P-wave records obtained in seismic exploration and used to characterize the ground structure at a shallow depth. Dispersion curve estimation using 2D wavefield transforms is well-established for 2D acquisition schemes (in-line source and receiver spread). It is possible to extract surface-wave dispersion curves using 2D wavefield transforms from 3D seismic data acquired with any acquisition scheme. In particular, we focus on areal geometry and orthogonal geometry, and we provide a method based on the analysis in the offset domain and the [Formula: see text] multiple signal classification (MUSIC) transform. We assess the performance of the method on synthetic and field data concerning 1D sites.

Evaluation of proxies for seismic site conditions in large urban areas: The example of Santiago de Chile

Characterizing the local site response in large cities is an important step towards seismic hazard assessment. To this regard, single station seismic noise measurements were carried out at 146 sites in the northern part of Santiago de Chile. This extensive survey allowed the fundamental resonance frequency of the sedimentary cover, derived from horizontal-to-vertical (H/V) spectral ratios, to be mapped. By inverting the spectral ratios under the constraint of the thickness of the sedimentary cover, known from previous gravimetric measurements, local S-wave velocity profiles have been retrieved. After interpolation between the individual profiles, the resulting high resolution 3D S-wave velocity model allows the entire area, as well as deeper parts of the basin, to be represented in great detail. Since one lithology shows a great scatter in the velocity values only a very general correlation between S-wave velocity in the uppermost 30 m ( v s 30 ) and local geology is found. Local S-wave velocity profiles can serve as a key factor in seismic hazard assessment, since they allow an estimate of the amplification potential of the sedimentary cover. Mapping the intensity distribution of the 27 February 2010 Maule, Chile, event ( Mw = 8.8) the results indicate that local amplification of the ground motion might partially explain the damage distribution and encourage the use of the low cost seismic noise techniques for the study of seismic site effects.

Upper mantle structure of marginal seas and subduction zones in northeastern Eurasia from Rayleigh wave tomography

The upper mantle structure of marginal seas (the Seas of Japan and Okhotsk) and subduction zones in northeastern Eurasia is investigated, using the three-stage multimode surface wave tomography incorporating finite-frequency effects. Broadband waveform data from 305 events with magnitude greater than 5.5 from 1990 to 2005 recorded at 25 stations of the IRIS network in northeastern Eurasia and Japan and at 8 stations of the broadband seismic network in Far-Eastern Russia from 2005 to 2008 are employed in our analysis. The dispersion curves of the fundamental mode and first two higher modes of Rayleigh waves are simultaneously inverted for the shear-wave velocity structure of the region. The off-great circle propagation due to strong heterogeneities in the region is also taken into account in the construction of intermediary phase velocity models for each mode as a function of frequency. The obtained 3D S-wave velocity model is well resolved down to 200km depth. Checkerboard tests show the average horizontal resolution of 5° in the study region. The subducting Pacific plate is clearly imaged as a high velocity anomaly up to 6%. The mantle wedge above the Pacific plate is associated with low velocity anomalies. The absolute minimum S-wave velocity in the mantle wedge is 4km/s in the Sea of Okhotsk in the depth range from 80 to 160km, probably indicating the presence of partial melt. The anomalous spot with conspicuous low velocity in the southern end of the Sea of Okhotsk may indicate the existence of hot upwelling flow in the mantle. A high velocity anomaly subparallel to the present subduction zone is found in the northwestern Sea of Okhotsk in the depth range from 100 to 200km. The position of this anomaly correlates well with the high velocity anomaly found in the P-wave tomography of Gorbatov et al. (2000), which may be interpreted as a relict of the Okhotsk plate subducted in the past. We also attempted a mapping of azimuthal anisotropy in this region. The fast phase velocity directions near the Pacific plate are observed subparallel to the Kuril and Japan Trenches at all the periods, indicating a strong effect of the subducting Pacific plate on the mantle flow, while the anisotropy appears to be weak in tectonically inactive marginal seas.

Direct nonlinear inversion of multiparameter 1D elastic media using the inverse scattering series

In the direct nonlinear inversion method and in algorithms for 1D elastic media, P-wave velocity, S-wave velocity, and density are depth dependent. “Direct nonlinear” means that the method uses explicit formulas that (1) input data and directly output changes in material properties without the need for indirect procedures such as model matching, searching, optimization, or other assumed aligned objectives or proxies and that (2) the algorithms recognize and directly invert the intrinsic nonlinear relationship between changes in material properties and the recorded reflection wavefields. To achieve full elastic inversion, all components of data (such as PP, SP, and SS data) are needed. The method assumes that only data and reference medium propertiesare input, and terms in the inverse series for moving mislocated reflectors resulting from the linear inverse term are separated from amplitude correction terms. Although in principle this direct inversion approach requires all components of elastic data, synthetic tests indicate that a consistent value-added result may be achieved given only PP data measurements, as long as the PP data are used to approximately synthesize the PS and SP components. Further value would be derived from measuring all components of the data as the method requires. If all components of data are available, one consistent method can solve for all of the second terms (the first terms beyond linear). The explicit nonlinear inversion formulas provide an unambiguous data requirement message as well as conceptual and practical added value beyond both linear approaches and all indirect methods.

Dipping-interface Mapping Using Mode-separated Rayleigh Waves

Multichannel analysis of surface waves (MASW) method is a non-invasive geophysical technique that uses the dispersive characteristic of Rayleigh waves to estimate a vertical shear (S)-wave velocity profile. A pseudo-2D S-wave velocity section is constructed by aligning 1D S-wave velocity profiles at the midpoint of each receiver spread that are contoured using a spatial interpolation scheme. The horizontal resolution of the section is therefore most influenced by the receiver spread length and the source interval. Based on the assumption that a dipping-layer model can be regarded as stepped flat layers, high-resolution linear Radon transform (LRT) has been proposed to image Rayleigh-wave dispersive energy and separate modes of Rayleigh waves from a multichannel record. With the mode-separation technique, therefore, a dispersion curve that possesses satisfactory accuracy can be calculated using a pair of consecutive traces within a mode-separated shot gather. In this study, using synthetic models containing a dipping layer with a slope of 5, 10, 15, 20, or 30 degrees and a real-world example, we assess the ability of using high-resolution LRT to image and separate fundamental-mode Rayleigh waves from raw surface-wave data and accuracy of dispersion curves generated by a pair of consecutive traces within a mode-separated shot gather. Results of synthetic and real-world examples demonstrate that a dipping interface with a slope smaller than 15 degrees can be successfully mapped by separated fundamental waves using high-resolution LRT.

Bedrock Depth Mapping of the Coast South of İstanbul: Comparison of Analytical and Experimental Analyses

Local S-wave velocity-depth profiles and bedrock depth distribution are key factors in assessing seismic hazard and earthquake ground motion characteristics since they allow determination of the amplification potential of geological formations overlying bedrock. In this study, an empirical relationship between the thickness of Tertiary-Quaternary sediments (hereafter referred as cover) overlying Palaeozoic bedrock and their resonance frequencies was calculated for the İstanbul region and the bedrock depth distribution beneath the city was presented. The relationship was investigated by comparing transfer functions obtained from single station microtremor analyses and one-dimensional (1D) S-wave velocity profiles at sites where shallow velocity structure is known. Geotechnical data consisting of standard penetration test (SPT) blow counts and standard soil descriptions were evaluated from 15 boring sites and microtremor measurements were carried out. The bedrock depth of each site was determined by computing analytical transfer functions to fit the resonance frequency and the shape of experimental transfer functions. Based on those results, a relationship between the resonance frequency and the thickness of the cover was derived. Finally, the bedrock distribution beneath populated areas of İstanbul was obtained by applying the derived relationship to 86 strong-motion sites, where the resonance frequencies are known.

福岡市中央区天神地区の表層地盤のＳ波速度構造

We investigate the shallow subsurface S-wave velocity structure across the Kego fault at the area of Tenjin in Chuo Ward, Fukuoka, Japan. We observed microtremors for several minutes by small arrays consisting of 7 sensors at 8 sites using wireless LAN data loggers. These sites were situated at the same locations of observation sites by Yamanaka et al. (2005) for aftershocks of the 2005 north-west off Fukuoka prefecture MJ 7.0 earthquake and with K-NET station FKO006. The micro-tremor array data were processed to obtain Rayleigh wave phase velocities in the period range of 0.04-0.4 s by SPAC analysis. Then the 1D S-wave velocity structures having three layers with a Vs of about 130-170 m/s, 220-260 m/s and 800 m/s are estimated from hybrid inversions at each site. The depth to the top of the layer of Vs 800 m/s is only 5 m at the site west of the Kego fault, while the depth is 25-40 m at the sites east of the fault. The layer with a Vs of 800 m/s has a steep slope with a thickness difference of about 20 m at the fault. These 1D velocity structure models show amplifica-tions characterized by dominant peaks in period range of longer than 0.2 s. However, the distribution of the AVS30s for the profiles shows no correlation to those of the ratios of PGA and PGV and the difference of seismic intensity. This indicates the limit of understanding only by 1D structure model and necessity of the consideration of 2D or 3D subsurface structure. The profile for FKO006 at site 08 is different from previous study, which has a deeper depth of the top of the engineering bedrock in our model. These results must be included to explain the observed strong ground motion records of the 2005 earthquake around the central Fukuoka area in short period range.

Research on the middle-of-receiver-spread assumption of the MASW method

The multichannel analysis of surface wave (MASW) method has been effectively used to determine near-surface shear- (S-) wave velocity. Estimating the S-wave velocity profile from Rayleigh-wave measurements is straightforward. A three-step process is required to obtain S-wave velocity profiles: acquisition of a multiple number of multichannel records along a linear survey line by use of the roll-along mode, extraction of dispersion curves of Rayleigh waves, and inversion of dispersion curves for an S-wave velocity profile for each shot gather. A pseudo-2D S-wave velocity section can be generated by aligning 1D S-wave velocity models. In this process, it is very important to understand where the inverted 1D S-wave velocity profile should be located: the midpoint of each spread (a middle-of-receiver-spread assumption) or somewhere between the source and the last receiver. In other words, the extracted dispersion curve is determined by the geophysical structure within the geophone spread or strongly affected by the source geophysical structure. In this paper, dispersion curves of synthetic datasets and a real-world example are calculated by fixing the receiver spread and changing the source location. Results demonstrate that the dispersion curves are mainly determined by structures within a receiver spread.

Middle and upper crust shear-wave velocity structure of the Chinese mainland

In order to give a more reliable shallow crust model for the Chinese mainland, the present study collected many short-period surface wave data which are better sensitive to shallow earth structures. Different from traditional two-step surface wave tomography, we developed a new linearized surface wave dispersion inversion method to directly get a 3D S-wave velocity model in the second step instead of inverting for 1D S-velocity profile cell by cell. We convert all the regionalized dispersions into linear constraints for a 3D S-velocity model. Checkerboard tests show that this method can give reasonable results. The distribution of the middle-and upper-crust shear-wave velocity of the Chinese mainland in our model is strongly heterogeneous and related to different geotectonic terrains. Low-velocity anomalies delineated very well most of the major sedimentary basins of China. And the variation of velocities at different depths gives an indication of basement depth of the basins. The western Tethyan tectonic domain (on the west of the 95°E longitude) is characterized by low velocity, while the eastern Tethyan domain does not show obvious low velocity. Since petroleum resources often distribute in sedimentary basins where low-velocity anomaly appears, the low velocity anomalies in the western Tethyan domain may indicate a better petroleum prospect than in its eastern counterpart. Besides, low velocity anomaly in the western Tethyan domain and around the Xing’an orogenic belt may be partly caused by high crustal temperature. The weak low-velocity belt along ∼105°E longitude corresponds to the N-S strong seismic belt of central China.

Surface wave tomography for southeastern Asia using IRIS-FARM and JISNET data

Previous seismic studies for southeastern Asia around Indonesia have used global seismic data and thus included few regional broadband seismic stations. To better understand seismic structure of this area, we temporarily deployed Japan–Indonesia Seismic NETwork (JISNET) which was composed of 23 broadband seismic stations in central to western Indonesia since 1998 under the Super Plume Project of Japan. In this study we analyse regional JISNET data and global Incorporated Research Institute of Seismology (IRIS)-Fast Archive Recovery Method (FARM) data, in order to obtain detailed images of 3D S-wave velocity (Vs) of this area. We use vertical component LP seismograms of FARM data recorded from 1989 to 1993 (13471 seismograms) and vertical component of broadband JISNET data recorded from 1998 to 2000 (1782 seismograms). We select seismic events with body wave magnitudes greater than 5.5, and take hypocenters and centroid moment tensor solutions from the Harvard CMT catalogue. Phase velocity anomalies of fundamental mode Rayleigh waves (period from 34 to 197 s) relative to Preliminary Reference Earth Model (PREM) are determined by measuring phase differences between the synthetic waveforms and the observations. Global distribution of Rayleigh wave phase velocities are determined by block inversions using damped least squares with a finer block size of 5°×5° for the Indonesia region than the surrounding area. Firstly, we compare the results obtained from the inversion using the FARM data alone (called “FARM inversion”) and those from the inversion using both the FARM and the JISNET data (called “JISNET + FARM inversion”). For the fundamental Rayleigh waves of short periods, phase velocities obtained from the JISNET + FARM inversion show greater degrees of negative anomaly throughout Indonesia than those from the FARM inversion. The clearest difference is seen for the region around Kalimantan and Java: slower phase velocities of less than 1% relative to PREM at periods of about 70 s are obtained by the FARM inversion, whereas including JISNET data requires lower phase velocity anomalies of 2.5%. Secondly, we compare the resolution kernels of both data set. Around Java, Sumatra, Kalimantan, and Sulawesi the resolution of the JISNET + FARM inversion is substantially better than that of the FARM data alone, with the peaks of resolution kernels up to five times as high as those for the FARM inversion. The best resolution improvement is obtained for west Kalimantan to Java Sea. Thirdly, we invert the phase velocity perturbations for the modes to obtain the upper mantle 3D S-wave velocity (Vs) structure of the study area. The JISNET + FARM inversion tends to give larger (up to four times) negative Vs anomalies when compared with the FARM inversion. The Vs heterogeneities concentrate in the uppermost mantle shallower than 200 km throughout Indonesia. Beneath Sumatra, Java, and Kalimantan, the uppermost 100 km of the mantle is slower than PREM by about 2–3%. A prominent high Vs anomaly (2–4%) around the depths from 100 to 200 km extends from northern Australia, through the Banda Sea, to east Sulawesi and Halmahera. The subducted Indian plate is visible as a nearly 1% high Vs anomaly.

3D S-wave velocity pattern in the upper mantle beneath the continent of Asia from Rayleigh wave data

Group velocities of Rayleigh waves in the 10–150 s period range along about 1200 paths across Central Asia and Siberia are used to obtain group velocity maps of the region. To image the lateral variations of group velocities we used the tomography method developed earlier for spherical surface [PEPI 122 (2000) 19]. The method is supplemented by the opportunity to estimate the resolving power of the data. The locally averaged dispersion curves have been then inverted to vertical S-wave velocity profiles up to depth of 500 km at different sites in the region, which thus image a 3D velocity distribution. In order to reduce the number of unknown parameters we assumed the relationships between S-wave and P-wave velocities and densities to be those of the PREM model. The 3D S-wave velocity pattern is displayed in the form of maps at depths from 50 to 350 km with a step of 50 km, and of 2D vertical velocity sections along some profiles crossing different tectonic units. The most remarkable feature of the 3D velocity distribution is the presence of a high-velocity rise at the depths of 250–350 km beneath southern Siberia and western China, which borders upon the low-velocity asthenosphere along the longitude about 105°E. Collision of the high- and low-velocity substances results in pressing-out the viscous asthenosphere material in the weakened zone of the Baikal rift, where the lithosphere is thinned. The rise of the asthenosphere in this zone probably produces mantle upwelling, and leads to uplift of the Khangay Plateau.

3D S-Wave Velocity Structure of the Crust and Upper Mantle

In this paper, 238 Rayleigh wave path data are selected and processed by the matched-filtering frequency-time analysis technique and the grid dispersion inversion method to obtain the 3D S-wave velocity structure of China mainland and its adjacent sea regions. The results show that the velocity structure relates to geotectonic division, Bouguer gravity anomaly is basically controlled by the relief of Moho discontinuity, the buried depth of LVL in upper mantle concerns the surface heat flow deeply. In this paper, authors indicate the main characteristics of the velocity structure in tectonic active and stable regions.