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1.
The crustal structure in Myanmar can provide valuable information for the eastern margin of the ongoing Indo-Eurasian collision system. We successively performed H–k stacking of the receiver function and joint inversion of the receiver function and surface wave dispersion to invert the crustal thickness (H), shear wave velocity (VS), and the VP/VS ratio (k) beneath nine permanent seismic stations in Myanmar. H was found to increase from 26 ?km in the south and east of the study area to 51 ?km in the north and west, and the VP/VS ratio was complex and high. Striking differences in the crust were observed for different tectonic areas. In the Indo-Burma Range, the thick crust (H ?～ ?51 ?km) and lower velocities may be related to the accretionary wedge from the Indian Plate. In the Central Myanmar Basin, the thin crust (H ?= ?26.9–35.5 ?km) and complex VP/VS ratio and VS suggest extensional tectonics. In the Eastern Shan Plateau, the relatively thick crust and normal VP/VS ratio are consistent with its location along the western edge of the rigid Sunda Block.  相似文献   

2.
Deep unconsolidated sediments in the Mississippi embayment will influence ground motions from earthquakes in the New Madrid seismic zone. Shear wave velocity profiles of these sediments are important input parameters for modeling wave propagation and site response in this region. Low-frequency, active-source surface wave velocity measurements were performed to develop small-strain shear wave velocity (VS) profiles at eleven deep soil sites in the Mississippi embayment, from north of New Madrid, Missouri to Memphis, Tennessee. A servo-hydraulic, low-frequency source was used to excite surface wave energy to wavelengths of 600 m, resulting in VS profiles to depths of over 200 m. The average VS profile calculated from the eleven sites is in good agreement with common reference VS profiles that have been used in seismic hazard studies of this region. The variability in VS profiles is shown to be associated with changes in formation depth and thickness from site-to-site. Using lithologic information at each site, average formation velocities were developed and compared to previous studies. We found average VS values of about 193 m/s for alluvial deposits, 400 m/s for the Upper Claiborne formations, and 685 m/s for the Memphis Sand formation.  相似文献   

3.
The existence of uncoupled shear (S) and compression (P) wave velocity variations in Earth's mantle is a characteristic that might only be explained by the presence of significant chemical and/or phase heterogeneity, with important implications for the dynamics and evolution of Earth's interior. While making a one-to-one comparison between tomographic models for P and S velocity (VP and VS) variations for a particular geographic region is ill-posed, their global statistical distributions reveal several robust characteristics indicative of the nature of uncoupled VP and VS in the deep mantle. We find that all of the VP and VS model distributions at a given depth are Gaussian-like throughout the lowermost mantle. However, a distinct low velocity feature is present in VS distributions below ≈ 2200 km depth that is not present or is relatively weak in VP models. The presence of anomalously low VS material cannot be explained as an artifact, nor can the absence of a similarly strong feature in P models be ascribed to under-resolution. We propose that this feature can be partly explained by laterally variable occurrences of post-perovskite (pPv) lenses in the D″ layer, however, the persistence of significantly slow VS regions at heights up to ≈ 700 km or more above the core–mantle boundary is likely to be incompatible with a pPv origin and might only be explained by the presence of a laterally discontinuous layer of chemically distinct material and/or some other kind of phase heterogeneity. There also exist significant discrepancies between tomographic models with respect to the width of the distributions as well as differences between the modeled peak values. We propose a scheme for comparison between different seismic models in which the widths of the dominant features in their statistical distributions is exploited.  相似文献   

4.
To better understand the relationship between crustal heterogeneity and seismotectonics in the National Capital Region (NCR), Delhi, India, we carried out local P- and S-velocity tomography beneath the NCR. First arrival times of the first P- and S-wave from 275 crustal earthquakes recorded by the Seismic Telemetry Network in and around Delhi of the India Meteorological Department, India, are inverted to obtain crustal P-, S-velocity and V P /V S variations in the region. Our tomographic images of the upper crust reflect well the surface geological and tectonic features. The Delhi fold belt is identified as low V P, high V S and low V P/V S. The Sohna hot spring region is appearing as low V P, low V S and high V P /V S correlating with the possible presence of fluid-filled rocks. The crustal seismicity is distributed in both the high- and low-velocity zones, but most distinctly in the low V P /V S region.  相似文献   

5.
Parameters of seismic waves from clusters of local weak earthquakes that occurred at the upper boundary of the seismofocal zone in depth intervals of 40–60 and 70–90 km along eastern Hokkaido are investigated for the period 1998–2003, including the strong (M = 8.0) Tokachi-Oki earthquake of September 26, 2003. Analysis of data indicates that the distribution of anisotropic properties along Hokkaido is inhomogeneous and parameters of split waves (the azimuth of the fast S wave and the time delay between split S waves) are sensitive to variations in the stress-strain state of the medium. Unstable behavior of split wave parameters and increased values of the ratio V P /V S for clusters of events in the areas of the Hidaka Mountains and Nemuro Peninsula imply that the medium is in a mechanically weakened state (in the regime of intense dilatant deformation). On the contrary, the regions beneath the Tokachi and Kushiro plains are more rigid and are characterized by lower V P /V S values and comparatively stable behavior of wave parameters. Anomalous parameters of split waves from events of clusters in areas of different stations correlate with each other and are related to occurrence times of large earthquakes around Hokkaido, which may point to a redistribution of stresses and strains and fluid migration in the subduction zone.  相似文献   

6.
High-resolution three-dimensional V P and V P /V S images in the Longtan reservoir area were obtained from local earthquake data by using 3,178 events with total 24,153 P-wave and 23,987 S-wave arrivals collected from 23 seismic stations. The tomographic images show that significant V P heterogeneities can be seen at layers of different depth in the Longtan reservoir area. Low-V P anomalies both beneath and around the main rivers in the reservoir area may be related to the composition of rocks which are mainly deposit carbonate and arenaceous shale, which contributes to water saturation. We deduced that the high porosity rocks beneath the main rivers may be fully saturated with water. The phenomenon that V P is relatively high in the area which is 10–20?km away from the rivers indicates that horizontal saturation of water is limited within a small range of area that is about 10–20?km from the main rivers. The characteristic is significant that seismicity in the Longtan reservoir area is coincident with the distribution of the low-V P area. V P /V S tomographic images show that V P /V S ranges from 1.8 to 2.05 in shallow layers above 4?km depth beneath the Longtan reservoir, suggesting the properties of the rocks are limestone and shale. At the depth of 7?km, the distribution of V P /V S image varies quite remarkably, especially in the dam area. This demonstrates that the range of influence by the saturation of water in the media below the reservoir surface can reach 4–7?km depth in the dam area.  相似文献   

7.
A density profile and a modern temperature distribution in the lithosphere of the Voronezh crystalline massif (VCM) are derived through the use of the VP(z), VS(z) seismic velocity models, petrological data, measurements of VP, VS, density (ρ) and mean atomic weight (m) for several groups of rocks and minerals of different composition and genesis, as well as from pressure and temperature derivatives for different thermodynamic regimes.  相似文献   

8.
On May 21, 2021, an MS6.4 earthquake occurred in Yangbi, Yunnan province, China, which exhibited typical foreshock-mainshock-aftershock characteristics. To better understand the velocity structure of the focal area and adjacent fault zones, Pg/Sg travel times at 12 seismic stations for the local earthquakes with ML ≥ 1.5 from 2009–2019 and the Yangbi sequence in May of 2021 were used to invert the three-dimensional (3D) structures for both vP and vP/vS. The obtained structure extends deeply to 15 km for area (25°N–26.5°N, 99.5°E–101°E) at a horizontal resolution of 10× 10 km, and the accuracy of the vP velocity was verified using airgun signals excited by the Binchuan Airgun Transmitting Seismic Station (BATSS). The resulting vP and vP/vS images correlate with existing fault zones and the Yangbi sequence, including: (1) The shallow velocity structure at 0 km agrees with local topography, where the Binchuan basin exhibits low-vP and high-vP/vS values. From 3–15 km, vP and vP/vS show variations, and the boundaries are consistent with the main faults (e.g., the Weixi-Qiaohou-Weishan, Honghe, and Chenghai faults). (2) The largest foreshock (MS5.6), mainshock (MS6.4), and largest aftershock (MS5.2) occurred near the boundaries where both vP and vP/vS have clear contrasts. (3) Small earthquakes are also concentrated in the transition zone between high- and low-vP and vP/vS anomalies, and are biased toward low-vP/vS zones. (4) Boundaries in vP and vP/vS are observed at 20 km west of the Weixi-Qiaohou-Weishan fault, indicating that there may exist one hidden fault.  相似文献   

9.
Utilizing shear-wave (S-wave) data acquired with compressional waves (P-waves) is becoming more common as joint imaging and inversion techniques improve. Interest in S-waves radiated from vertical sources and buried explosives exploits conversion to P-waves as primary reflections (SP-waves) for reducing acquisition costs and for application to legacy data. However, recent investigations overstate the extent of SP-wave illumination and show isotropic processing results with narrow bandwidth frequency and wavenumber data. I demonstrate that illumination with SP-waves is limited in general to near vertical polar angles up to around 30° or 35° for VP/VS of 2 or 3, respectively. At greater angles, S-waves are typically in the P-wave evanescent range and cannot excite SP-wave reflections. Contrary to recent claims, these sources for P-wave do not radiate SH-waves polarized in horizontal planes in all azimuths. I show these properties for isotropic media with radiation expressions for amplitude derived in vector slowness coordinates. Also, I extend these expressions to transversely isotropic media with a vertical symmetry axis to show agreement with synthetic seismic data that only quasi SV-waves are radiated and become more narrowly focused towards 45°. Furthermore, in orthorhombic media, synthetic data show that fast S1- and slow S2-waves polarized parallel and perpendicular to fractures may appear as SV- and SH-waves. For the partially saturated fracture model studied here, S1-wave radiation has broader azimuthal illumination than slow S2-waves, which are more narrowly focused in azimuth. These produce SP-wave splitting signatures on vertical component reflection data that are nearly identical to PS-wave signatures on radial horizontal component data. Separating these fast and slow SP-waves is an additional processing challenge.  相似文献   

10.
The seismic hazard and risk analysis for the onshore Groningen gas field requires information about local soil properties, in particular shear-wave velocity (VS). A fieldwork campaign was conducted at 18 surface accelerograph stations of the monitoring network. The subsurface in the region consists of unconsolidated sediments and is heterogeneous in composition and properties. A range of different methods was applied to acquire in situ VS values to a target depth of at least 30 m. The techniques include seismic cone penetration tests (SCPT) with varying source offsets, multichannel analysis of surface waves (MASW) on Rayleigh waves with different processing approaches, microtremor array, cross-hole tomography and suspension P-S logging. The offset SCPT, cross-hole tomography and common midpoint cross-correlation (CMPcc) processing of MASW data all revealed lateral variations on length scales of several to tens of metres in this geological setting. SCPTs resulted in very detailed VS profiles with depth, but represent point measurements in a heterogeneous environment. The MASW results represent VS information on a larger spatial scale and smooth some of the heterogeneity encountered at the sites. The combination of MASW and SCPT proved to be a powerful and cost-effective approach in determining representative VS profiles at the accelerograph station sites. The measured VS profiles correspond well with the modelled profiles and they significantly enhance the ground motion model derivation. The similarity between the theoretical transfer function from the VS profile and the observed amplification from vertical array stations is also excellent.  相似文献   

11.
It is noticed that few geophysical studies have been carried out to decipher the crustal structure of southwestern part of the Northeast India comprising of Tripura fold belt and Bengal basin as compared to the Shillong plateau and the Brahmaputra basin. This region has a long history of seismicity that is still continuing. We have determined first-order crustal features in terms of Moho depths (H) and average VP/VS ratios (κ) using H-κ stacking technique. The inversion of receiver functions data yields near surface thick sedimentary layer in the Bengal basin, which is nearly absent in the Shillong plateau and Tripura fold belt. Our result suggests that the crust is thicker (38–45 km) in the Tripura fold belt region with higher shear-wave velocity in the lower crust than the Shillong plateau. The distribution of VP/VS ratio indicates heterogeneity throughout the whole region. While low to medium value of Poisson’s ratio (1.69–1.75) indicates the presence of felsic crust in the Shillong plateau of the extended Indian Archean crust. The medium to high values of VP/VS ratio (> 1.780) in the Bengal basin and the Tripura fold belt region represent mafic crust during the formation of the Bengal delta and the Tripura fold belt creation in the Precambrian to the Permian age. The depth of the sediments in the Bengal basin is up to 8 km on its eastern margin, which get shallower toward its northeastern and southeastern margins.  相似文献   

12.
本文收集了若干甘肃及邻区中强震前地震波速度比异常的资料,並初步总结了该区1970年至1983年发生的中强震前地震波速度异常的时空特性。也讨论了该区和其它地区地震波速度比异常特性的区别。十几年的工作表明,V_S/V_P观测资料应用到该区的地震预报中是有可能的。最后,简单地讨论了速度异常研究中的一些问题。  相似文献   

13.
The forward and inverse problems of studying crustal stress state from breakout data of inclined boreholes are concisely stated. Direction of the maximum horizontal principal stress (compressive) and relative magnitudes of the horizontal stresses to the vertical stress in the upper crust in two regions of the Jizhong depression, the North of China, are obtained by analyzing the breakout data of 6 inclined wells. To get stable results in the analysis we searched for the unknown parameters both forwardly and inversely. The results show that the maximum azimuths of the horizontal principal compressive stresses in the central and southern part of the Jizhong depression are N86°E and N77°E, respectively, while the relative magnitudes of the three principal stresses in the upper crust (about 1000–4000 m) of the depression are variable. In the central part of the Jizhong depression we have foundS H :S V :S h =1.38:1.00:0.57, whereS H ,S V andS h are the maximum horizontal, vertical and minimum horizontal stress, respectively. This indicates that the present stress regime in this area is of strike-slip faulting type. In the southern part of the depression we have obtainedS H :S V :S h =0.80:1.00:0.62, indicating a normal faulting stress regime in the shallow part of the crust.  相似文献   

14.
A practical method is presented for determining three‐dimensional S‐wave velocity (VS) profile from microtremor measurements. Frequency–wave number (f–k) spectral analyses of microtremor array records are combined, for this purpose, with microtremor horizontal‐to‐vertical (H/V) spectral ratio techniques. To demonstrate the effectiveness of the proposed method, microtremor measurements using arrays of sensors were conducted at six sites in the city of Kushiro, Japan. The spectral analyses of the array records yield dispersion characteristics of Rayleigh waves and H/V spectra of surface waves, and joint inversion of these data results in VS profiles down to bedrock at the sites. Conventional microtremor measurements were performed at 230 stations within Kushiro city, resulting in the H/V spectra within the city. Three‐dimensional VS structure is then estimated from inversion of the H/V spectra with the VS values determined from the microtremor array data. This reveals three‐dimensional VS profile of Kushiro city, together with an unknown hidden valley that crosses the central part of the city. The estimated VS profile is consistent with available velocity logs and results of subsequent borings, indicating the effectiveness of the proposed method. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

15.
The macroscopic modelling of two-phase flow processes in subsurface hydrosystems or industrial applications on the Darcy scale usually requires a constitutive relationship between capillary pressure and saturation, the Pc(Sw) relationship. Traditionally, it is assumed that a unique relation between Pc and Sw exists independently of the flow conditions as long as hysteretic effects can be neglected. Recently, this assumption has been questioned and alternative formulations have been suggested. For example, the extended Pc(Sw) relationship by Hassanizadeh and Gray [Hassanizadeh SM, Gray WG. Mechanics and thermodynamics of multiphase flow in porous media including interphase boundaries. Adv Water Resources 1990;13(4):169–86] proposes that the difference between the phase pressures to the equilibrium capillary pressure is a linear function of the rate of change of saturation, thereby introducing a constant of proportionality, the coefficient τ. It is desirable to identify cases where the extended relationship needs to be considered. Consequently, a dimensional analysis is performed on the basis of the two-phase balance equations. In addition to the well-known capillary and gravitational number, the dimensional analysis yields a new dimensionless number. The dynamic number Dy quantifies the ratio of dynamic capillary to viscous forces. Relating the dynamic to the capillary as well as the gravitational number gives the new numbers DyC and DyG, respectively. For given sets of fluid and porous medium parameters, the dimensionless numbers Dy and DyC are interpreted as functions of the characteristic length and flow velocity. The simulation of an imbibition process provides insight into the interpretation of the characteristic length scale. The most promising choice for this length scale seems to be the front width. We conclude that consideration of the extended Pc(Sw) relationship may be important for porous media with high permeability, small entry pressure and high coefficient τ when systems with a small characteristic length (e.g. steep front) and small characteristic time scale are under investigation.  相似文献   

16.
We made an attempt to assess the shear wave velocity values V S and, to a lesser extent, the V P values from ambient noise recordings in an array configuration. Five array sites were situated in the close proximity to borehole sites. Shear wave velocity profiles were modeled at these five array sites with the aid of two computational techniques, viz. spatial autocorrelation (SPAC) and H/V ellipticity. Out of these five array sites, velocity estimates could be reliably inferred at three locations. The shear wave velocities estimated by these methods are found to be quite consistent with each other. The computed V S values up to 30 m depth are in the range from 275 to 375 m/s in most of the sites, which implies prevalence of a low velocity zone at some pocket areas. The results were corroborated by evidence of site geology as well as geotechnical information.  相似文献   

17.
High-frequency (≥2 Hz) Rayleigh wave phase velocities can be inverted to shear (S)-wave velocities for a layered earth model up to 30 m below the ground surface in many settings. Given S-wave velocity (VS), compressional (P)-wave velocity (VP), and Rayleigh wave phase velocities, it is feasible to solve for P-wave quality factor QP and S-wave quality factor QS in a layered earth model by inverting Rayleigh wave attenuation coefficients. Model results demonstrate the plausibility of inverting QS from Rayleigh wave attenuation coefficients. Contributions to the Rayleigh wave attenuation coefficients from QP cannot be ignored when Vs/VP reaches 0.45, which is not uncommon in near-surface settings. It is possible to invert QP from Rayleigh wave attenuation coefficients in some geological setting, a concept that differs from the common perception that Rayleigh wave attenuation coefficients are always far less sensitive to QP than to QS. Sixty-channel surface wave data were acquired in an Arizona desert. For a 10-layer model with a thickness of over 20 m, the data were first inverted to obtain S-wave velocities by the multichannel analysis of surface waves (MASW) method and then quality factors were determined by inverting attenuation coefficients.  相似文献   

18.
土体剪切波速是进行土层地震反应分析的动力学参数，对场地地震动参数确定具有重要意义。基于地质地貌分析，将大同盆地划分为5类典型地质单元。对盆地1429个钻孔剪切波速资料进行分析，探讨VS30与VS20的相关性，研究土体埋深、岩性、地质单元、标贯击数及密实度等地质特征对VS的影响，并基于地质单元、剪切波速比、密实度系数及第四系上部覆盖层厚度相关性分析给出土体VS30预测模型。研究结果表明，基于典型地质特征的VS30预测模型拟合优度R2>0.90，预测精度很高，对于离散性较大、直接拟合估算较差及无剪切波速场地来说，以区分地质单元及土体类型的方式进行VS30分解预测是良好的研究思路。首次在区分地质单元及土体类型的前提下提出剪切波速比及密实度系数，并将其与第四系上部覆盖层厚度综合应用于VS30预测研究。研究结果可为大同盆地城市防震减灾规划、震害预测、区域性地震安全评价提供重要技术支撑。  相似文献   

19.
Three methods that follow the general format of the Seed-Idriss simplified procedure for evaluating liquefaction resistance of soils are compared in this paper. They are compared by constructing relationships between penetration resistance and small-strain shear–wave velocity (VS) implied from cyclic resistance ratio (CRR) curves for the three methods, and by plotting penetration-VS data pairs. The penetration-VS data pairs are from 43 Holocene-age sand layers in California, South Carolina, Canada, and Japan. It is shown that the VS-based CRR curve is more conservative than CRR curves based on the Standard Penetration Test (SPT) and Cone Penetration Test (CPT), for the compiled Holocene data. This result agrees with the findings of a recent probability study where the SPT-, CPT-, and VS-based CRR curves were characterized as curves with average probability of liquefaction of 31, 50, and 26%, respectively. New SPT- and CPT-based CRR equations are proposed that provide more consistent assessments of liquefaction potential for the Holocene sand layers considered.  相似文献   

20.
The aim of this study was to investigate the role of VS30 in site amplifications in the Adapazari region, Turkey. To fulfil this aim, amplifications from VS30 measurements were compared with earthquake data for different soil types in the seismic design codes. The Adapazari area was selected as the study area, and shear-wave velocity distribution was obtained by the multichannel analysis of surface waves (MASWs) method at 100 sites for the top 50 m of soil. Aftershock data following the Mw 7.4 Izmit earthquake of 17 August 1999 gave magnitudes between 4.0 and 5.6 at six stations installed in and around the Adapazari Basin, at Babalı, Şeker, Genç, Hastane, Toyota and Imar. This data was used to estimate site amplifications by the reference-station method. In addition, the fundamental periods of the station sites were estimated by the single station method. Site classifications based on VS30 in the seismic design codes were compared with the fundamental periods and amplification values. It was found that site amplifications (from earthquake data) and relevant spectra (from VS30) are not in good agreement for soils in Adapazari (Turkey).  相似文献   

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