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1.
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
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3.
Paul Tammetta 《Ground water》2013,51(5):723-734
The height of complete groundwater drainage above subsided longwall panels (referred to as H) at underground mines is determined using a data base of hydraulic head measurements made with multiple devices down the depth profile at each of a number of sites worldwide. H is shown to be relatively independent of most parameters except the geometry of the mined void and the overburden thickness. An empirical equation linking H to these parameters is developed using hydraulic head data, and confirmed using an independent data base of ground movement. H is shown to be the same as the height of the zone of major ground movement above a panel. H for special cases (above longwall chain pillars, above pillar extraction panels, and underneath significant water bodies) is invariably smaller than H above center panel for ordinary cases. A new caving model, from a groundwater perspective, is proposed for continuously sheared longwall panels at ordinary locations. It removes complexity and reduces the uncertainty in estimating H. The derived equation for H applies to a variety of strata types. 相似文献
4.
Throughout their long history, the towns of Lod and Ramle have been severely affected by strong earthquakes. The last destructive
earthquake occurred on July 11, 1927 and caused the destruction of large parts of these cities, reaching a seismic intensity
of VIII–IX on the MSK scale. Such a high intensity from a relatively distant earthquake (about 70 km) of magnitude 6.2 is
likely to be the result of local site effects of the sedimentary layers that may have significantly enhanced earthquake ground
motions.
This study is focused on estimating the seismic hazard to Lod and Ramle by implementing a three-step process (1) detailed
mapping of the characteristics of the H/V spectral ratios from ambient noise, (2) incorporating geological information and well data to construct subsurface models
for different sites within the investigated area and (3) estimating the seismic hazard in terms of uniform hazard site-specific
accelerations.
The horizontal-to-vertical spectral ratios of ambient noise were used to approximate the fundamental resonance frequencies
of the subsurface and their associated amplitudes. About 360 sites in Lod and Ramle were instrumented for varying periods.
The soil sites exhibits H/V peak amplitudes ranging from 4 to 6 in the frequency range 0.5–2.5 Hz. These data were used to constrain 1-D subsurface models
that were developed using geological data and borehole information. H/V spectral ratio observations were checked against theoretical subsurface transfer functions at locations where borehole information
is available farther constraint the range of possible Vs velocities of the different layers and thus, by means of trial an error it was possible to conclude a systematic spatial
distribution of the Vs velocity and thickness in the substrata that are also consistent with the spatial distribution of the fundamental resonance
frequencies of the soft sediments obtained by means of the H/V spectral ratios, and other geological and geophysical information available at different locations in the study area.
The evaluated subsurface models are introduced using the SEEH procedure of Shapira and van~Eck [(1993) Natural Hazards 8, 201–205] to assess Uniform Hazard Site-Specific Acceleration Spectra for different zones within the towns of Lod and Ramle.
These evaluations are very important for realistic assessment of the vulnerabilities of all types of existing and newly designed
structures and for urban and land use planning. 相似文献
5.
James Gaiser 《Geophysical Prospecting》2019,67(9):2414-2431
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
6.
Despite the occurrence of riffle–pool sequences in many rivers there are few data concerning riffle–pool unit morphology. Of many criteria proposed to identify riffle–pool units, only three methods can be regarded as objective and robust. These are the ‘zero‐crossing’, the ‘spectral analysis’ and the ‘control‐point’ methods. In this paper statistics are developed using the first two of these methods to describe the streamwise morphology of 275 riffles and 285 pools which form a continuous 32·1 km reach of the bed of the River Severn in Shropshire, England. Yalin's theoretical relationship between the average riffle:pool unit length ( λ p) and channel width ( W ), λ p = 3 W , applies to the River Severn. Reach‐average riffle height ( H ) is a constant proportion of bankfull depth ( h ); typically H ≅ 0·16 h . Riffle height is a positive function of riffle length. Pool depth is a positive function of pool length. However, both riffle length and pool length increase more rapidly than the bed‐level amplitude, such that long riffles or pools are relatively ‘flat’. As channel gradient reduces, bedforms flatten and become more asymmetric as riffle stoss sides and the proximal slope of pools lengthen at the expense of riffle lee sides and pool distal slopes. The statistical relationships between riffle steepness (H/L) and water depth are similar to those for equilibrium subaqueous dunes. The Severn data are consistent with Yalin's theoretical analysis relating riffle bedform length (L r) to water depth, i.e. L r = α2π h, wherein α ≅ 1 for steep near‐equilibrium bedforms but α ≅ 2 to 3 as the relative depth decreases and riffles become long, low features. Theoretical consideration and turbulence data indicate that the frequency of coherent turbulent‐flow structures associated with the riffle–pool mixing length in the Severn should be of the order of 50 to 100 s. The morphological similarity of the steepest River Severn riffles with dunes raises intriguing questions with respect to self‐similar, convergent organization of periodic alluvial bedforms and to bedform dynamic classification particularly. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
7.
本文根据Aki等人提出的尾波理论,导出了地方震尾波水平分量与垂直分量的持续时间比的具体表达式: τ_H/τ_V=I_H/I_V(Q_H/Q_V)~(1/4)·B_H/B_V。该式表明,τ_H/τ_V的变化主要反映了震源体一定范围内,由于介质的各向异性而引起的尾波在不同方向上的激发及衰减能力的差异。本文还讨论了地震前τ_H/τ_V短临异常的物理机制,认为异常的产生与孕震期间介质内裂隙的出现和闭合有关。1986年门源6.4级地震和1975年海城7.3级地震前,τ_H/τ_V都有不同程度的短期低值异常及临震高值突跳。门源地震前后门源台记录的直达S波的最大振幅比A_(mH)/A_(mv)也有与尾波持续时间比类似的异常。最后通过对一些震例的分析,初步得到震级与异常时间的关系为M=0.657lnT+3.44。 相似文献
8.
We estimated three-dimensional P- (Vp) and S-wave velocity (Vs) and Vp/Vs structures in and around the Onikobe volcanic area, northeastern Japan, by local travel time tomography. We used travel time data from source and receiver pairs located within and outside the study area, which plays an important role in obtaining the optimum ray coverage and in elucidating the deeper structure more accurately. Detailed information on deeper structures is essential for imaging the complete volcanic system from the magmatic source zone through areas of shallow hydrothermal circulation. More than 50 000 travel time data for the P-waves and 35 000 for the S-waves were used to image the velocity structure. Our results show the following dominant features: (1) two conduits in the upper crust with low Vp and low Vs indicative of H2O-rich fluid pathways: one lying beneath Naruko volcano, the other beneath the focal area of the 1962 Northern Miyagi earthquake (M6.5); (2) an underlying broad region in the lower crust with low Vp, low Vs and high Vp/Vs, suggestive of a zone of partial melt, from which the fluids in (1) are derived; and (3) low Vp/Vs areas near the surface of the Sanzugawa and Onikobe calderas, suggesting a diffuse vapor-saturated cap. 相似文献
9.
Ronald D. Andrus Paramananthan Piratheepan Brian S. Ellis Jianfeng Zhang C. Hsein Juang 《Soil Dynamics and Earthquake Engineering》2004,24(9-10):713
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
10.
A prediction model for horizontal run-out distance of landslides triggered by Wenchuan earthquake 总被引：1，自引：0，他引：1
The peak ground acceleration (PGA), the volume of a sliding mass V, the height of a mountain H L and the slope angle θ of a mountain are four important parameters affecting the horizontal run-out distance of a landslide L. Correlations among them are studied statistically based on field investigations from 67 landslides triggered by the ground shaking and other factors during the Wenchuan earthquake, and then a prediction model for horizontal run-out distance L is developed in this study. This model gives due consideration to the implications of the above four parameters on the horizontal run-out distance L and the validity of the model is verified by the Donghekou and Magong Woqian landslides. At the same time, the advantages of the model are shown by comparing it with two other common prediction methods. The major findings drawn from the analyses and comparisons are: (1) an exponential relationship exists between L and log V, L and log H L, L and log PGA separately, but a negative exponential relationship exists between L and log tanθ, which agrees with the statistical results; and (2) according to the analysis results of the relative relationship between the height of a mountain (H) and the place where the landslides occur, the probabilities at distances of 2H/3-H, H/3-2H/3, and 0-H/3 are 70.8%, 15.4%, and 13.8%, respectively, revealing that most landslides occurred at a distance of H/2-H. This prediction model can provide an effective technical support for the prevention and mitigation of landslide hazards. 相似文献
11.
Geomorphologically mapped data form a primary set of observations that can be used to infer former environmental conditions. Thus, objective and consistent mapping of landforms from remotely sensed data (e.g. satellite imagery, digital elevation models (DEMs)) is paramount for reconstructing palaeoenvironments. This paper proposes a technique, ‘residual relief separation’, to enhance landforms in DEMs prior to visualization and digital mapping. This is applied to a ～600 km2 region surrounding Lough Gara, Ireland, where drumlins (～200 m wide) overlie a regional relief of hills (～10 km wide). Here, residual relief separation uses this difference in width‐scale. Regional relief is approximated by a 1 km wide median filter, then subtracted to leave the drumlins in a ‘residual’ topography. In a second step, the residual relief is normalized to allow for amplitude variations in the drumlins across the area (～5–40 m high). Finally, visualization uses a simple black‐to‐white colour scale for height. Although not numerically outperforming other visualization techniques, this method performs equally well, and as the data are not ‘illuminated’ there is no azimuthal bias. Additional benefits include the relatively simple calculation, intuitive visual comprehension, no emphasis of noise, and the possibility of using any desired visualization technique after the landscape has been topographically manipulated. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
12.
This paper aimed to examine the site dependence and evaluate the methods for site analysis of far-source ground motions. This was achieved through the examination of frequency content estimated by different methods based on strong ground motions recorded at twelve far-source stations in Shandong province during the Wenchuan earthquake. The stations were located in sites with soil profiles ranging from code classes Ⅰ to Ⅲ. Approaches used included the Fourier amplitude spectrum (FAS), the earthquake response spectrum (ERS), the spectral ratio between the horizontal and the vertical components (H/V), the spectral ratio between the spectra at the site and at a reference site (SRRS), and coda wave analysis (CWA). Results showed that major periods of these ground motions obtained by FAS, ERS and H/V ratio methods were all evidently larger than site dominant periods; the periods were also different from each other and mainly reflected the frequency content of long period components. Prominent periods obtained by the SRRS approach neither illuminated the long period aspect nor efficiently determined site features of the motions. The CWA resulted in a period close to site period for stations with good quality recordings. The results obtained in this study will be useful for the evaluation of far-source effect in constructing seismic design spectra and in selecting methods for ground motion site analysis. 相似文献
13.
The study of the coastal landscapes of hotspot oceanic islands through comprehensive structural metrics and ecological estimators represents an opportunity to explore geomorphological transformations and broad spatiotemporal scale features of coastal evolution. As part of this approach, a new metrical comparative analysis is presented in this study, comprising four islands in different evolutionary stages. They belong to the Cape Verde archipelago, which forms a double insular chain in which an east–west gradient in age and evolution is particularly evident across the southern chain. A space-for-time (SFT) substitution approach is applied to the coasts of (1) Fogo, in the shield stage; (2) Santiago, in the early post-erosional stage; (3) São Vicente, in the advanced post-erosional stage; and (4) Boa Vista, in the last erosional stage. From the obtained spatial distributions and frequencies of landforms, the coastal landscapes of these islands are compared in relation to their (i) geomorphic composition, using similarity indices (Whittaker, βw, Sorensen, Cs) and nestedness estimators (NOFD, WNODF), (ii) geomorphic abundance, using morpho-assembling densities (Dgm), and (iii) geomorphic diversity, using six alpha-diversity indices (Richness, S, Menhinick, DMN, Simpson, D, Shannon, H', Berger-Parker, d, and Brillouin, HB). An advanced geomorphological taxonomy is implemented for areas with limited open-access data, including a set of planform features captured through scale-frequency decomposition. Photographic, cartographic and fieldwork data are used for landform identification at 1200 random sampling points, empirically determined by a bootstrap method. The results show a chronological ordering of the compared variables and a possible co-evolution towards an increase in organizational geomorphic complexity of coastal systems at broad space-timescales. The method proposed in this study can contribute, from a metrical perspective, to finding new long-term evolutionary features and constitutes an advance in the development of an integrated model of coastal evolution in oceanic islands. © 2019 John Wiley & Sons, Ltd. 相似文献
14.
When electric soundings are made over an irregular terrain, topographic effects can influence the values of apparent resistivity and lead to erroneous 1D interpretation. A 3D finite-element method has been applied to study the topographical effect of a slope on Schlumberger soundings parallel to the strike. When the resistivity survey is performed at the top of the slope, the apparent resistivity values can be two times higher than in the flat-earth case, depending on the angle (α) and height (H) of the slope, and on the distance (X) between the sounding and the slope top. The results are presented as nondimensional curves which can be used for evaluating topographic anomalies for any value of the parameters α, H and X. It is numerically shown that the topographic effects can be removed from measurements on horizontally layered structures with an irregular earth surface. Real measurements were performed in different geological conditions over an irregular terrain. The correction method based on the nondimensional curves has been applied to the data and has enabled the determination of the correct layered ground configuration using 1D interpretation. 相似文献
15.
Statistical methods are available which predict the maximum response of simple oscillators given the peak acceleration (Ap), peak velocity (Vp) or peak displacement (Dp) of seismic ground motions. An alternative parameter, namely an ordinate (or ordinates) of the Fourier amplitude spectrum of ground motion acceleration, FS(f), may in fact be a preferred predictor of peak response, especially in a frequency range close to f. Other statistical methods (attenuation laws) use distance R and other parameters such as magnitude (M), Modified Mercalli epicentral Intensity (Io) and Modified Mercalli site Intensity (MMI or Is) to predict spectral velocity (Sv(f)), etc. In using such approaches, it is most desirable to know the total uncertainty in the predicted peak response of the system given the starting parameter values. An extensive strong motion data set is used to study these questions, The most direct prediction models are found to be preferable (have lower prediction dispersion) but data may not be available in all regions to permit their use. 相似文献
16.
A. V. Gorbatikov M. Yu. Stepanova G. E. Korablev 《Izvestiya Physics of the Solid Earth》2008,44(7):577-592
Experiments and numerical model studies have shown that heterogeneities of the Earth’s crust distort the spectrum of the low
frequency microseismic field, decreasing spectral amplitudes of a specific frequency f at the Earth’s surface over high velocity heterogeneities and increasing them above low velocity heterogeneities. The frequency
f is connected with the depth of a heterogeneity H and the velocity of the fundamental mode of Rayleigh waves V
R
(f) through the relation H = 0.5 V
R
(f)/f. The low frequency microseismic field is considered as the superposition of trains of Rayleigh fundamental modes with different
frequency spectra. The paper proposes an experimentally tested technology enabling the determination of the deep structure
of complex geological objects using data on the microseismic background field. 相似文献
17.
Elastic rock properties can be estimated from prestack seismic data using amplitude variation with offset analysis. P‐wave, S‐wave and density ‘reflectivities’, or contrasts, can be inverted from angle‐band stacks. The ‘reflectivities’ are then inverted to absolute acoustic impedance, shear impedance and density. These rock properties can be used to map reservoir parameters through all stages of field development and production. When P‐wave contrast is small, or gas clouds obscure reservoir zones, multicomponent ocean‐bottom recording of converted‐waves (P to S or Ps) data provides reliable mapping of reservoir boundaries. Angle‐band stacks of multicomponent P‐wave (Pz) and Ps data can also be inverted jointly. In this paper Aki‐Richards equations are used without simplifications to invert angle‐band stacks to ‘reflectivities’. This enables the use of reflection seismic data beyond 30° of incident angles compared to the conventional amplitude variation with offset analysis. It, in turn, provides better shear impedance and density estimates. An important input to amplitude variation with offset analysis is the Vs/Vp ratio. Conventional methods use a constant or a time‐varying Vs/Vp model. Here, a time‐ and space‐varying model is used during the computation of the ‘reflectivities’. The Vs/Vp model is generated using well log data and picked horizons. For multicomponent data applications, the latter model can also be generated from processing Vs/Vp models and available well data. Reservoir rock properties such as λρ, μρ, Poisson's ratio and bulk modulus can be computed from acoustic impedance, shear impedance and density for pore fill and lithology identification. λ and μ are the Lamé constants and ρ is density. These estimations can also be used for a more efficient log property mapping. Vp/Vs ratio or Poisson's ratio, λρ and weighted stacks, such as the one computed from λρ and λ/μ, are good gas/oil and oil/water contact indicators, i.e., pore fill indicators, while μρ mainly indicates lithology. μρ is also affected by pressure changes. Results from a multicomponent data set are used to illustrate mapping of gas, oil and water saturation and lithology in a Tertiary sand/shale setting. Whilst initial log crossplot analysis suggested that pore fill discrimination may be possible, the inversion was not successful in revealing fluid effects. However, rock properties computed from acoustic impedance, shear impedance and density estimates provided good lithology indicators; pore fill identification was less successful. Neural network analysis using computed rock properties provided good indication of sand/shale distribution away from the existing wells and complemented the results depicted from individual rock property inversions. 相似文献
18.
Huey‐Chu Huang 《地震工程与结构动力学》2002,31(10):1815-1829
The relationships between the spectral characteristics of earthquake ground motions and those of micro‐tremors are investigated using the observed data from a dense strong‐motion network consisting of 108 stations in the Yun‐Li, Chia‐Yi and Tai‐Nan areas in southwestern Taiwan. Many high‐quality recordings, including those of the 921 Chi‐Chi earthquake (Mw=7.6), the 1022 Chia‐Yi mainshock (ML=6.4), the 1022 major aftershock (ML=6.0), as well as some weak motion events are selected to evaluate site responses. Microtremor measurements are also performed at most ground motion stations. With many stations in the area located on an alluvium structure, however, it is difficult to find good reference stations on rock sites, which therefore necessitates the calculation of single‐station H/V ratios. The predominant frequencies obtained from H/V ratios are consistent with those from spectral ratios. The site characteristics between the strong and weak events are different, however. This implies that a nonlinear effect probably occurred with the strong‐motion events. The main peak in the H/V spectra of the microtremors is in good agreement with the first peak obtained from the spectra of earthquake ground motions. It is reasonable to claim that the main peak reflects the deep underground structure. On the basis of the H/V ratios of the microtremors, it is concluded that the lower predominant frequencies appear in the plain area, while the higher values are near the mountainous region. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
19.
The Kumaon Himalaya region in India has accumulated considerable seismic risk potential from moderate to high seismicity due to ongoing tectonic evolutionary processes. To assess conditions of high seismic risk arising from local site effects at populated locations, we applied the empirical horizontal to vertical (H/V) spectral amplitude ratio method (Nakamura in Quarterly Reports of the Railway Technical Research Institute Tokyo, 30:25–33, 1989) using ambient seismic noise recorded by a network of 32 digital broadband seismographs from June 2005 to June 2008. The data and the estimated parameters were subjected to stability tests to assess the effect of seasonal variations. Seasonal variations in the ambient noise data seemed to show a lesser effect on fundamental frequency estimates and a slightly greater effect on the peak H/V amplitudes. Validation of the ambient noise results was done by complementary tests using H/V ratios of local and regional earthquakes. The ‘peak’ corresponding to the fundamental resonance frequency is prominently present in both the ambient noise and the earthquake datasets. The study locations showed distinct H/V curve topologies, corresponding to the effects of both velocity contrast at well-defined frequencies and characteristic topographic effect around a certain frequency range. The clearly indicated ‘peaks’ in the H/V curves possibly correspond to velocity contrasts created by weathered sediments overlying hard rock basements in rocky hills. Our study indicates high site responses at many populated locations near the surface trace of the seismically active Main Central Thrust (MCT) and other fault boundaries. The fundamental resonance frequencies estimated from the site response studies at the 32 locations could be useful in preliminary site characterization, ground motion prediction and seismic hazard analysis. 相似文献
20.
James E. Pizzuto 《地球表面变化过程与地形》1984,9(3):199-207
Cross-sections of 16 straight sandbed streams in Minnesota, Iowa, and Nebraska were surveyed. Two stratigraphic horizons were found in the banks at each site, an upper cohesive unit usually composed of silt and clay and a lower unit composed of sand. Bank erosion on these rivers occurs when the upper cohesive unit is undercut by scour at bends. The overhanging cohesive block fails by beam or cantilever failure. As upper bank failure is a direct result of undercutting, the stability and rate of retreat of the bank are largely determined by erosion of the sandy part of the bank. The cohesive layer has little influence on bank retreat and width adjustment on the rivers studied here. A quantitative lateral sediment transport model developed by Parker (1978a) is used to calculate the steady-state geometry of the sandy part of the bank. Results are obtained for the shape, length, and height of the sandy part of the bank. The model predicts the length of the bank fairly well, and the theoretical equation for the height of the bank is of the correct form. The model, however, overestimates the slope of the bank. The height of the sandy part of the bank (Db) is approximately equal to the depth of the mean annual flow. Since Db is determined by the lateral sediment transport model, the width (W) may be obtained from the equation of continuity (Q = WDbV), published flow (Q) data, and a resistance equation for the mean velocity, V. The calculated widths are similar to those measured in the field. 相似文献