Seismic Evaluation of a Profile of Volcanic Ash From the South American Andes

Although soils derived from volcanic ash are only a small portion of the soils that cover the world's surface, cities have developed on some of these soils. This makes their study interesting, especially since soils derived from volcanic ash have special characteristics including a high void ratio, liquid limits that can be greater than 100%, and shear wave velocities close to 150 m/s. Cities on these soils in the South American Andes are in areas with frequent and intense seismic activity making an understanding of how earthquakes affect these soils a matter of importance. This paper analyzes site response by the equivalent linear method, taking into account the variability of these soils. The Monte Carlo method was used for degradation of the shear modulus and damping curves. The results of this study show factors that amplify spectral acceleration can cause this measurement to become greater than four. In addition, the authors found that maximum spectral accelerations are functions of high plasticity indexes whose influence increases when shear wave velocities are between 175 and 225m/s.

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Mapping Fundamental-Mode Site Periods and Amplifications from Thick Sediments: An Example from the Jackson Purchase Region of Western Kentucky, Central United States

Bulletin of the Seismological Society of America ◽

10.1785/0120200300 ◽

2021 ◽

Author(s):

Yichuan Zhu ◽

Zhenming Wang ◽

N. Seth Carpenter ◽

Edward W. Woolery ◽

William C. Haneberg

Keyword(s):

Shear Wave ◽

Wave Velocity ◽

Shear Wave Velocity ◽

Fundamental Mode ◽

Site Response ◽

Velocity Profiles ◽

Wave Velocities ◽

Shear Wave Velocities ◽

Jackson Purchase ◽

Western Kentucky

ABSTRACT V S 30 is currently used as a key proxy to parameterize site response in engineering design and other applications. However, it has been found that VS30 is not an appropriate proxy, because it does not reliably correlate with site response. Therefore, the VS30-based National Earthquake Hazards Reduction Program site maps may not capture regional site responses. In earthquake engineering, site resonance, which can be characterized by the fundamental mode with a site period (Tf) and its associated peak amplification (A0), is the primary site-response concern. Mapping Tf and A0 is thus essential for accurate regional seismic hazard assessment. We developed a 3D shear-wave velocity model for the Jackson Purchase Region of western Kentucky, based on shear-wave velocity profiles interpreted from seismic reflections and refractions, mapped geologic units, and digital-elevation-model datasets. We generated shear-wave velocity profiles at grid points with 500 m spacing from the 3D model and performed 1D linear site-response analyses to obtain Tf and A0, which we then used to construct contour maps for the study area. Our results show that Tf and A0 maps correlate with the characteristics of regional geology in terms of sediment thicknesses and their average shear-wave velocities. We also observed a strong dependency of A0 on bedrock shear-wave velocities. The mapped Tf and A0 are consistent with those estimated from borehole transfer functions and horizontal-to-vertical spectral ratio analyses at broadband and strong-motion stations in the study area. Our analyses also demonstrate that the depth to bedrock (Zb) is correlated to Tf, and the average sediment shear-wave velocity (VS-avg) is correlated to A0. This implies that Zb and VS-avg may be considered as paired proxies to parameterize site resonance in the linear-elastic regime.

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Mapping Sediment Thickness in Shillong City of Northeast India through Empirical Relationship

Journal of Earthquakes ◽

10.1155/2015/572619 ◽

2015 ◽

Vol 2015 ◽

pp. 1-8 ◽

Cited By ~ 6

Author(s):

Rajib Biswas ◽

Saurabh Baruah ◽

Dipak K. Bora

Keyword(s):

Resonant Frequency ◽

Shear Wave ◽

Site Response ◽

Empirical Relationship ◽

Empirical Relation ◽

Northeast India ◽

Sediment Thickness ◽

Soil Layers ◽

Wave Velocities ◽

Shear Wave Velocities

Modified form of Nakamura method, H/V ratio, is used to assess the site response through estimation of fundamental resonant frequency at 70 sites using three component digital seismographs in Shillong city, capital of Meghalaya in northeast India. With available borehole information, an attempt is made to develop an empirical relationship between sediment thickness and resonant frequency estimated from H/V ratio technique. Simultaneously, shear wave velocities are computed entailing resonant frequency and sediment thickness for these boreholes. We also endeavored building another empirical relation between sediment thickness and VS. With the help of this, the probable VS values for other sites were also evaluated. It is observed that shear wave velocities range from 200 to 550 m/s while sediment thickness ranges from 10 to 80 m, implicating the heterogeneity prevailing in the soil layers of the Shillong city.

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