Empirical Relations between Elastic Wavespeeds and Density in the Earth’s Crust (original) (raw)
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This article presents new empirical compressional and shear-wave velocity (Vp and Vs) versus depth relationships for the most common rock types in northern California. Vp versus depth relations were developed from borehole, laboratory, seismic refraction and tomography, and density measurements, and were converted to Vs versus depth relations using new empirical relations between Vp and Vs. The relations proposed here account for increasing overburden pressure but not for variations in other factors that can influence velocity over short distance scales, such as lithology, consolidation, induration, porosity, and stratigraphic age. Standard deviations of the misfits predicted by these relations thus provide a measure of the importance of the variability in Vp and Vs caused by these other factors. Because gabbros, greenstones, basalts, and other mafic rocks have a different Vp and Vs relationship than sedimentary and granitic rocks, the differences in Vs between these rock types at depths below 6 or 7 km are generally small. The new relations were used to derive the 2005 U.S. Geological Survey seismic velocity model for northern California em-ployed in the broadband strong motion simulations of the 1989 Loma Prieta and 1906 San Francisco earthquakes; initial tests of the model indicate that the Vp model generally compares favorably to regional seismic tomography models but that the Vp and Vs values proposed for the Franciscan Complex may be about 5% too high.
Journal of Geophysical Research, 2010
1] Surface wave dispersion measurements from ambient seismic noise and array-based measurements from teleseismic earthquakes observed with the EarthScope/USArray Transportable Array (TA) are inverted using a Monte Carlo method for a 3-D V S model of the crust and uppermost mantle beneath the western United States. The combination of data from these methods produces exceptionally broadband dispersion information from 6 to 100 s period, which constrains shear wave velocity structures in the crust and uppermost mantle to a depth of more than 100 km. The high lateral resolution produced by the TA network and the broadbandedness of the dispersion information motivate the question of the appropriate parameterization for a 3-D model, particularly for the crustal part of the model. We show that a relatively simple model in which V S increases monotonically with depth in the crust can fit the data well across more than 90% of the study region, except in eight discrete areas where greater crustal complexity apparently exists. The regions of greater crustal complexity are the Olympic Peninsula, the MendocinoTriple Junction, the Yakima Fold Belt, the southern Cascadia back arc, the Great Central Valley of California, the Salton Trough, the Snake River Plain, and the Wasatch Mountains. We also show that a strong Rayleigh-Love discrepancy exists across much of the western United States, which can be resolved by introducing radial anisotropy in both the mantle and notably the crust. We focus our analysis on demonstrating the existence of crustal radial anisotropy and primarily discuss the crustal part of the isotropic model that results from the radially anisotropic model by Voigt averaging. Model uncertainties from the Monte Carlo inversion are used to identify robust isotropic features in the model. The uppermost mantle beneath the western United States is principally composed of four large-scale shear wave velocity features, but lower crustal velocity structure exhibits far greater heterogeneity. We argue that these lower crustal structures are predominantly caused by interactions with the uppermost mantle, including the intrusion and underplating of mafic mantle materials and the thermal depression of wave speeds caused by conductive heating from the mantle. Upper and middle crustal wave speeds are generally correlated, and notable anomalies are inferred to result from terrane accretion at the continental margin and volcanic intrusions.
Shear wave velocity modelling in crustal rock for seismic hazard analysis
Soil Dynamics and Earthquake Engineering, 2005
P-wave velocity data along with the thickness of sedimentary and crystalline layers within bedrock were collected from all global regions and presented in the Global Crustal Model CRUST2.0, published in 2001. This well-organised database provides invaluable potential contributions towards future seismic hazard modelling, particularly for stable continental regions (SCRs), where there is a scarcity of representative strong motion records for conventional modelling purposes. The P-wave velocity information presented in CRUST2.0 has been converted herein to S-wave velocity information. The latter is especially important for purposes of seismic hazard modelling. The value of the CRUST2.0 model has therefore been greatly enhanced by the important findings presented and further developed in this paper. By making the best use of available information on crustal conditions, the amplification behaviour of seismic waves affecting a region, an area or a site for any given earthquake scenario may be predicted. The developed methodology, which is intended for worldwide applications, has been illustrated by case studies in which model S-wave velocity profiles were developed for different geological regions within North America. The model profiles were found to be in excellent agreement with field measurements reported for each respective region. q
Lateral Variations in Compressional/Shear Velocities at the Base of the Mantle
Science, 1999
Observations of core-diffracted P ( P diff ) and SH ( SH diff ) waves recorded by the Missouri-to-Massachusetts (MOMA) seismic array show that the ratio of compressional ( P ) seismic velocities to horizontal shear ( SH ) velocities at the base of the mantle changes abruptly from beneath the mid-Pacific ( V P / V S = 1.88, also the value predicted by reference Earth models) to beneath Alaska ( V P / V S = 1.83). This change signifies a sudden lateral variation in material properties that may have a mineralogical or textural origin. A textural change could be a result of shear stresses induced during the arrival at the core of ancient lithosphere from the northern Pacific paleotrench.
Radial variation of compressional and shear velocities in the Earth's lower mantle
Geophysical Journal International, 1978
This paper extends an earlier study (Sengupta & Julian) of travel times of P waves of deep-focus earthquakes to include shear waves. Primary advantage of deep-focus earthquakes is the reduction of anomalies caused by complex structures near the source. The standard deviations of travel times and station anomalies of this study are about half as large as those determined from the data of shallow-focus earthquakes (e.g. Herrin et ul.; Hales & Roberts). Spherically-symmetric velocity models derived from the travel times by a linearized inverse technique have resolving lengths of about 70 km for standard errors in velocity of about 0.02 km/s. No pronounced reversal of either compressional or shear velocity was required at the base of the mantle to satisfy the data, though a small velocity decrease could not be entirely ruled out. Some anomalous rapid changes in compressional velocity gradient were, however, found centred around the depths of 2400 and 2600 km. The models derived in this study agree most closely with that of Herrin er ul. for compressional velocity and the model 1066B of Gilbert & Dziewonski for shear velocity.
The use of compressional (longitudinal) and shear wave velocity measurements have become increasingly popular not only for water, oil, gas and other subsurface energy/geological probing and exploration but also for generating essential design parameters for civil engineering and marine structures such as dams, tunnels, highways, bridges, skyscrapers, platforms, reclamation reticulation systems including on-shore/off-shore structures and sub-surface development. In order to realize Performance Based - Value Engineering (PB-VE) designs and precise construction quality control of the foundations of such mega-structures, it is essential that the foundation bearing ground is rigorously investigated. Geophysical surveys have proven to be some of the most reliable methods of measurement in such cases. Nevertheless, in-situ testing can be prohibitive in terms of the engineering parameters that can be derived for carrying out comprehensive characterization of the subsurface/foundation geomaterials. This situation requires the development of models that can effectively correlate the reliable parameters that are measured in the field to those measured in the laboratory where the testing conditions can be varied to simulate various loading, environmental and critical state conditions. Useful correlations that can appreciably achieve these objectives are reported in this and other publications by the same author. The sequel publications are cited in this paper.
Bulletin of the Seismological Society of America, 2013
Baffin Island is one of the several seismically active regions in the far north of Canada. In 1933, a strong earthquake with M w 7.3 occurred in the region. On 7 July 2009, a relatively strong earthquake with M w 6.0 occurred in the same area. This earthquake was very well recorded by many modern seismic stations. We systematically organized the Rayleigh-wave displacement records, measured Rayleighwave dispersion data at 28 stations surrounding the epicenter, and retrieved the S-wave velocity models. We used a previous model for the Western Quebec seismic zone as the initial model in our analyses and found that the velocities of all models at the shallow depths (< 15 km) were obviously slower than those of the initial model. In the middle crust, the velocities in most models were close to those of the initial model. In the directions of azimuths 171°∼ 218°and 241°, the velocities in the middle crust were faster than those of the initial model. In the directions of 263°∼ 279°, the velocities in the middle crust were slower than those of the initial model. The slowest S-wave velocities in the top layers occurred in the directions of azimuths 90°and 218°. These findings indicate differences in the existing crustal structures.
SP12RTS: a degree-12 model of shear- and compressional-wave velocity for Earth's mantle
Geophysical Journal International, 2015
We present the new model SP12RTS of isotropic shear-wave (V S) and compressional-wave (V P) velocity variations in the Earth's mantle. SP12RTS is derived using the same methods as employed in the construction of the shear-wave velocity models S20RTS and S40RTS, and the same data types. SP12RTS includes additional traveltime measurements of P-waves and new splitting measurements: 33 normal modes with sensitivity to the compressional-wave velocity and 9 Stoneley modes with sensitivity primarily to the lowermost mantle. Contrary to S20RTS and S40RTS, variations in V S and V P are determined without invoking scaling relationships. Lateral velocity variations in SP12RTS are parametrised using spherical harmonics up to degree 12, to focus on long-wavelength features of V S and V P and their ratio R. Largelow-velocity provinces (LLVPs) are observed for both V S and V P. SP12RTS also features an increase of R up to 2500 km depth, followed by a decrease towards the core-mantle boundary. A negative correlation between the shear-wave and bulk-sound velocity variations is observed for both the LLVPs and the surrounding mantle. These characteristics can be explained by the presence of post-perovskite or large-scale chemical heterogeneity in the lower mantle. 2 Koelemeijer et al.
Geophysical Journal International, 2009
In an earlier study, Bensen et al. measured surface wave dispersion curves from ambient noise using 203 stations across North America, which resulted in Rayleigh and Love wave dispersion maps from 8-70 s period and 8-20 s period, respectively. We invert these maps in a two-step procedure to determine a 3-D shear wave velocity model (V S ) of the crust and uppermost mantle beneath much of the contiguous US. The two steps are a linearized inversion for a best fitting model beneath each grid node, followed by a Monte Carlo inversion to estimate model uncertainties. In general, a simple model parametrization is sufficient to achieve acceptable data fit, but a Rayleigh/Love discrepancy at periods from 10 to 20 s is observed, in which simple isotropic models systematically misfit Rayleigh and Love waves in some regions. Crustal features observed in the model include sedimentary basins such as the Anadarko, Green River, Williston Basins as well as California's Great Valley and the Mississippi Embayment. The east-west velocity dichotomy between the stable eastern US and the tectonically deformed western US is shown to be abrupt in the crust and uppermost mantle, but is not coincident in these regions; crustal high velocity material tends to lap over the high velocities of the uppermost mantle. The Rayleigh/Love discrepancy between 10 and 20 s period is crustal in origin and is observed in a number of regions, particularly in extensional provinces such as the Basin and Range. It can be resolved by introducing radial anisotropy in the lower or middle crust with V SH > V SH by about 1 per cent.