The Impact of a Buried High‐Velocity Layer in the Seismic Site Amplification of the City of Llolleo, Chile

High‐velocity layers (HVLs) often hinder seismic imaging of deeper reflectors using conventional techniques. A major factor is often the unusual energy partitioning of waves incident at an HVL boundary from lower‐velocity material. Using elastic physical modeling, I demonstrate that one effect of this factor is to limit the range of dips beneath an HVL that can be imaged using unconverted P‐wave arrivals. At the same time, however, partitioning may also result in P‐waves outside the HVL coupling efficiently with S‐waves inside. By exploiting some of the waves that convert upon transmission into and/or out of the physical‐model HVL, I am able to image a much broader range of underlying dips. This is accomplished by acoustic migration tailored (via the migration velocities used) for selected families of converted‐wave arrivals.

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SEISMIC DAMAGE SCENARIOS IN KALAMATA (S. GREECE)

Bulletin of the Geological Society of Greece ◽

10.12681/bgsg.11862 ◽

2017 ◽

Vol 50 (3) ◽

pp. 1495

Author(s):

D. Kazantzidou-Firtinidou ◽

I. Kassaras ◽

A. Ganas ◽

C. Tsimi ◽

N. Sakellariou ◽

...

Keyword(s):

Seismic Hazard ◽

Ground Motion ◽

Fragility Curves ◽

Site Amplification ◽

Soil Conditions ◽

Damage Scenarios ◽

Local Soil ◽

Protection Strategies ◽

The Impact ◽

The City

Damage scenarios are necessary tools for stakeholders, in order to prepare protection strategies and a total emergency post-earthquake plan. To this aim, four seismic hazard models were developed for the city of Kalamata, according to stochastic simulation of the ground motion, using site amplification functions derived from ambient noise HVSR measurements. The structural vulnerability of the city was assessed following an empirical macroseismic model, developed for the European urban environment (EMS-98). The impact of the vulnerability due to the seismic hazard potential is also investigated by means of synthetic response spectral ratios at 108 sites of the city. The expected damage grade per building block, is calculated by combining vulnerability with the respective seismic intensities, derived for the four seismic sources. The importance of the followed methodology for implementing microzonation studies is emphasized, since the expected influence of the ground motion amplification due to local soil conditions has been approximated in detail. Moreover, new fragility curves for the main structural types in Kalamata are proposed for each seismic scenario.

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Rift structures and its related unconformities on and adjacent the Dongsha Rise: insights into the nature of the high-velocity layer in the northern South China Sea

Marine Geophysical Research ◽

10.1007/s11001-019-09381-x ◽

2019 ◽

Vol 40 (2) ◽

pp. 99-110 ◽

Cited By ~ 1

Author(s):

Chao Lei ◽

Jianye Ren ◽

Xiong Pang

Keyword(s):

South China Sea ◽

South China ◽

High Velocity ◽

Northern South China Sea ◽

China Sea ◽

High Velocity Layer ◽

Velocity Layer

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Surface‐to‐borehole illumination of a high‐velocity layer using marine VSP

10.1190/1.1822754 ◽

1994 ◽

Author(s):

Colin MacBeth ◽

Enru Liu ◽

Mark Boyd ◽

Karen Sweeney

Keyword(s):

High Velocity ◽

High Velocity Layer ◽

Velocity Layer

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Elastic wave propagation along layers in two-dimensional models

Bulletin of the Seismological Society of America ◽

10.1785/bssa0530030593 ◽

1963 ◽

Vol 53 (3) ◽

pp. 593-618

Author(s):

D. K. Chowdhury ◽

Peter Dehlinger

Keyword(s):

Layer Thickness ◽

High Velocity ◽

Wave Length ◽

Two Dimensional ◽

Layer Model ◽

Low Velocity ◽

Long Period ◽

Low Velocity Layer ◽

High Velocity Layer ◽

Velocity Layer

Abstract Propagation of direct waves and dispersive long-period waves along a layered system was investigated experimentally by means of two-dimensional ultrasonic models. Velocities of direct and head waves were measured within layers or in a medium adjacent to layers as functions of layer thickness to wave length or source-from-interface distance to wave length. Amplitudes of direct longitudinal, direct shear, and long-period waves were measured on three profiles, each perpendicular to the layers. Three models were used: the first consisted of a low-velocity layer between two thick sheets; the second of a high-velocity layer between two sheets; the third of six alternating high- and low-velocity layers between two sheets. The source was a wave train, simulating a wave from a seismic explosion. The frequency was varied so as to obtain different ratios of layer thickness to wave length. In the single low-velocity layer model the direct longitudinal wave contained a larger amplitude than the dispersive long-period wave in the layer at offset distance of 6 to 10 times the layer thickness. In the single high-velocity layer model the direct longitudinal wave was attenuated rapidly and the amplitudes of the long-period waves were negligigble. In the multilayered model, direct waves had negligible amplitudes at the corresponding distances; nearly all of the energy was in the dispersive long-period waves. In this model the low-velocity layer carried 1 1/2 to 3 times the amplitude observed in the high-velocity layers, whether the source was located in the high- or low-velocity layers. Dispersion of the long-period waves in the multilayered model was pronounced within the low-velocity layers and weak in the high-velocity layers, when the source was either in a high- or low-velocity layer. Dispersion was anomalous when the source was in a low-velocity layer and normal when in a high-velocity layer.

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Some improvements in subbasalt imaging using pre-stack depth migration

Solid Earth ◽

10.5194/se-2-1-2011 ◽

2011 ◽

Vol 2 (1) ◽

pp. 1-7 ◽

Cited By ~ 3

Author(s):

I. Flecha ◽

R. Carbonell ◽

R. W. Hobbs ◽

H. Zeyen

Keyword(s):

High Velocity ◽

Velocity Model ◽

Original Model ◽

Depth Migration ◽

Reflection Data ◽

Long Offset ◽

Synthetic Test ◽

Complex Models ◽

High Velocity Layer ◽

Velocity Layer

Abstract. Subbasalt imaging can be improved by carefully applying pre-stack depth migration. Pre-stack depth migration requires a detailed velocity model and an accurate traveltime calculation. Ray tracing methods are fast but, often fail in calculating traveltimes in complex models, specially, when they feature high velocity contrasts. Finitte difference solutions of the eikonal are more stable and can produce a traveltime field for the whole model avoiding shadow zones. A synthetic test was carried out to check the performance of a new pre-stack depth migration algorithm in a model that features a high velocity layer surrounded by lower velocities. The results reasonably reproduce the original model. The same scheme was used to process long-offset reflection data from the Faroe Shelf where conventional techniques (stack) were insufficient to assess the structure under a basalt layer. Pre-stack depth migration produced an improved image which recovered the main features in the stacked section and allowed to identify some subbasalt coherent events.

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