Geographic correlation between hot spots and deep mantle lateral shear-wave velocity gradients

2004 ◽  
Vol 146 (1-2) ◽  
pp. 47-63 ◽  
Author(s):  
Michael S Thorne ◽  
Edward J Garnero ◽  
Stephen P Grand
Keyword(s):  
Shear Wave ◽  
Wave Velocity ◽  
Shear Wave Velocity ◽  
Hot Spots ◽  
Lateral Shear ◽  
Velocity Gradients ◽  
Deep Mantle

SHEAR‐WAVE VELOCITY VERSUS DEPTH IN MARINE SEDIMENTS: A REVIEW

Geophysics ◽  
1976 ◽  
Vol 41 (5) ◽  
pp. 985-996 ◽  
Author(s):  
Edwin L. Hamilton
Keyword(s):  
Shear Wave ◽  
Wave Velocity ◽  
Marine Sediments ◽  
Shear Wave Velocity ◽  
Shear Waves ◽  
Shear Velocity ◽  
Velocity Versus ◽  
Velocity Data ◽  
Velocity Gradients

The objectives of this paper are to review and study selected measurements of the velocity of shear waves at various depths in some principal types of unlithified, water‐saturated sediments, and to discuss probable variations of shear velocity as a function of pressure and depth in the sea floor. Because of the lack of data for the full range of marine sediments, data from measurements on land were used, and the study was confined to the two “end‐member” sediment types (sand and silt‐clays) and turbidites. The shear velocity data in sands included 29 selected in‐situ measurements at depths to 12 m. The regression equation for these data is: [Formula: see text], where [Formula: see text] is shear‐wave velocity in m/sec, and D is depth in meters. The data from field and laboratory studies indicate that shear‐wave velocity is proportional to the 1/3 to 1/6 power of pressure or depth in sands; that the 1/6 power is not reached until very high pressures are applied; and that in most sand bodies the velocity of shear waves is proportional to the 3/10 to 1/4 power of depth or pressure. The use of a depth exponent of 0.25 is recommended for prediction of shear velocity versus depth in sands. The shear velocity data in silt‐clays and turbidites include 47 selected in‐situ measurements at depths to 650 m. Three linear equations are used to characterize the data. The equation for the 0 to 40 m interval [Formula: see text] indicates the gradient [Formula: see text] to be 4 to 5 times greater than is the compressional velocity gradient in this interval in comparable sediments. At deeper depths, shear velocity gradients are [Formula: see text] from 40 to 120 m, and [Formula: see text] from 120 to 650 m. These deeper gradients are comparable to those of compressional wave velocities. These shear velocity gradients can be used as a basis for predicting shear velocity versus depth.

Shear Wave Velocity Gradients in Near-Surface Marine Sediment

1991 ◽  
pp. 295-304 ◽  
Author(s):  
Michael D. Richardson ◽  
Enrico Muzi ◽  
Bruno Miaschi ◽  
Ferda Turgutcan
Keyword(s):  
Shear Wave ◽  
Wave Velocity ◽  
Shear Wave Velocity ◽  
Marine Sediment ◽  
Near Surface ◽  
Velocity Gradients ◽  
Surface Marine Sediment

The influence of velocity gradients choice in deep alluvial basin seismic site response

2020 ◽  
Author(s):  
Valeria Cascone ◽  
Jacopo Boaga
Keyword(s):  
Seismic Hazard ◽  
Shear Wave ◽  
Wave Velocity ◽  
Shear Wave Velocity ◽  
Urban Areas ◽  
Risk Mitigation ◽  
Site Response ◽  
Velocity Gradients ◽  
Seismic Site Response ◽  
The World

<p align="justify"><span>The characterization of seismic site response represents one of the most important issues of seismic hazard assessment and risk mitigation planning. Characterizing the site conditions involves the measurement of several soil properties such as the shear-wave velocity (Vs), density and damping properties as a function of depth. Therefore, most of the site-effect studies in earthquake ground motions are based on the properties of the upper 30 meters and the anti-seismic building codes propose in most cases a simplified analysis based on shear wave velocity of the shallow subsoil. From a seismological perspective, the upper 30 meters would almost never represent more than 1% of the distance from the source. This should be taken into account especially for large and deep alluvial basins, representing the most inhabited geological environment of the world, where could be difficult to estimate the thickness and the velocity profile of the soft sediment overlying the rigid seismic bedrock. </span></p> <p align="justify"><span>The common approach adopted to characterize greater depths is then an extrapolation of shear wave velocity in depth, considering a selected linear or non-linear velocity gradients till the depth of the considered seismic bedrock (usually set to Vs ≥ 800 m/s). These gradients are generally derived from geological information or from literature, but how much the gradients choice affects the final site response analyses is often a neglected aspect.</span></p> <p align="justify"><span>In this work we try to investigate the generic case of deep alluvial basins. We consider the shallow subsoil as characterized by several <em>in-situ</em> tests in northern Italy. We extrapolate the deeper soil structure considering different literature velocity gradients obtained for deep basins in different geological contests: tectonic basins (Lower Rhine Basin and Po Plain) and Alpine basins (Grenoble and Lucerna Basins). We perform one-dimensional analysis of shear waves with the Linear Equivalent Method. The study demonstrates how relevant can be the role of velocity gradient choice for the ground response scenario. Starting from the same shallower Vs structures, the computed seismic motion at surface can present variation in the order of 50% varying the velocity gradients in depth. The results are of relevant interest for the analysis of seismic hazard in the deep alluvial basins environments, which host the main urban areas around the world. </span></p>

scholarly journals Calibrating a new attenuation curve for the Dead Sea region using surface wave dispersion surveys in sites damaged by the 1927 Jericho earthquake

Solid Earth ◽  
2019 ◽  
Vol 10 (2) ◽  
pp. 379-390 ◽  
Author(s):  
Yaniv Darvasi ◽  
Amotz Agnon
Keyword(s):  
Shear Wave ◽  
Wave Velocity ◽  
Shear Wave Velocity ◽  
Dead Sea ◽  
Attenuation Curve ◽  
Surface Wave Dispersion ◽  
Near Surface ◽  
Moderate Seismicity ◽  
Local Point ◽  
The Dead

Abstract. Instrumental strong motion data are not common around the Dead Sea region. Therefore, calibrating a new attenuation equation is a considerable challenge. However, the Holy Land has a remarkable historical archive, attesting to numerous regional and local earthquakes. Combining the historical record with new seismic measurements will improve the regional equation. On 11 July 1927, a rupture, in the crust in proximity to the northern Dead Sea, generated a moderate 6.2 ML earthquake. Up to 500 people were killed, and extensive destruction was recorded, even as far as 150 km from the focus. We consider local near-surface properties, in particular, the shear-wave velocity, as an amplification factor. Where the shear-wave velocity is low, the seismic intensity far from the focus would likely be greater than expected from a standard attenuation curve. In this work, we used the multichannel analysis of surface waves (MASW) method to estimate seismic wave velocity at anomalous sites in Israel in order to calibrate a new attenuation equation for the Dead Sea region. Our new attenuation equation contains a term which quantifies only lithological effects, while factors such as building quality, foundation depth, topography, earthquake directivity, type of fault, etc. remain out of our scope. Nonetheless, about 60 % of the measured anomalous sites fit expectations; therefore, this new ground-motion prediction equation (GMPE) is statistically better than the old ones. From our local point of view, this is the first time that integration of the 1927 historical data and modern shear-wave velocity profile measurements improved the attenuation equation (sometimes referred to as the attenuation relation) for the Dead Sea region. In the wider context, regions of low-to-moderate seismicity should use macroseismic earthquake data, together with modern measurements, in order to better estimate the peak ground acceleration or the seismic intensities to be caused by future earthquakes. This integration will conceivably lead to a better mitigation of damage from future earthquakes and should improve maps of seismic hazard.

scholarly journals Multi-method site characterization to verify the hard rock (Site Class A) assumption at 25 seismograph stations across Eastern Canada

2021 ◽  
pp. 875529302110010
Author(s):  
Sameer Ladak ◽  
Sheri Molnar ◽  
Samantha Palmer
Keyword(s):  
Shear Wave ◽  
Wave Velocity ◽  
Shear Wave Velocity ◽  
Hard Rock ◽  
Site Characterization ◽  
Eastern Canada ◽  
Site Class ◽  
Paleozoic Rock ◽  
Rock Types ◽  
Site Period

Site characterization is a crucial component in assessing seismic hazard, typically involving in situ shear-wave velocity ( VS) depth profiling, and measurement of site amplification including site period. Noninvasive methods are ideal for soil sites and become challenging in terms of field logistics and interpretation in more complex geologic settings including rock sites. Multiple noninvasive active- and passive-seismic techniques are applied at 25 seismograph stations across Eastern Canada. It is typically assumed that these stations are installed on hard rock. We investigate which site characterization methods are suitable at rock sites as well as confirm the hard rock assumption by providing VS profiles. Active-source compression-wave refraction and surface wave array techniques consistently provide velocity measurements at rock sites; passive-source array testing is less consistent but it is our most suitable method in constraining the rock VS. Bayesian inversion of Rayleigh wave dispersion curves provides quantitative uncertainty in the rock VS. We succeed in estimating rock VS at 16 stations, with constrained rock VS estimates at 7 stations that are consistent with previous estimates for Precambrian and Paleozoic rock types. The National Building Code of Canada uses solely the time-averaged shear-wave velocity of the upper 30 m ( VS30) to classify rock sites. We determine a mean VS30 of ∼ 1600 m/s for 16 Eastern Canada stations; the hard rock assumption is correct (>1500 m/s) but not as hard as often assumed (∼2000 m/s). Mean variability in VS30 is ∼400 m/s and can lead to softer rock classifications, in particular, for Paleozoic rock types with lower average rock VS near the hard/soft rock boundary. Microtremor and earthquake horizontal-to-vertical spectral ratios are obtained and provide site period classifications as an alternative to VS30.

scholarly journals The Polito Surface Wave flat-file Database (PSWD): statistical properties of test results and some inter-method comparisons

2021 ◽  
Vol 19 (6) ◽  
pp. 2343-2370
Author(s):  
Federico Passeri ◽  
Cesare Comina ◽  
Sebastiano Foti ◽  
Laura Valentina Socco
Keyword(s):  
Surface Wave ◽  
Shear Wave ◽  
Wave Velocity ◽  
Shear Wave Velocity ◽  
Active Surface ◽  
Statistical Properties ◽  
Test Results ◽  
Flat File ◽  
Research Fields ◽  
Database Content

AbstractThe compilation and maintenance of experimental databases are of crucial importance in all research fields, allowing for researchers to develop and test new methodologies. In this work, we present a flat-file database of experimental dispersion curves and shear wave velocity profiles, mainly from active surface wave testing, but including also data from passive surface wave testing and invasive methods. The Polito Surface Wave flat-file Database (PSWD) is a gathering of experimental measurements collected within the past 25 years at different Italian sites. Discussion on the database content is reported in this paper to evaluate some statistical properties of surface wave test results. Comparisons with other methods for shear wave velocity measurements are also considered. The main novelty of this work is the homogeneity of the PSWD in terms of processing and interpretation methods. A common processing strategy and a new inversion approach were applied to all the data in the PSWD to guarantee consistency. The PSWD can be useful for further correlation studies and is made available as a reference benchmark for the validation and verification of novel interpretation procedures by other researchers.

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