Assessment of Site Amplification Using Borehole and Surface Data: Variability of Site Effect Estimation from Different Phases of the Accelerogram

Since the inception of 62 borehole seismic arrays deployed by Central Weather Bureau (CWB) in Taiwan until the end of 2018, a large quantity of strong-motion records have been accumulated from frequently occurring earthquakes around Taiwan, which provide an opportunity to detect micro-seismicity. Each borehole array includes two force balance accelerometers, one at the surface and other at a depth of a few ten-to-hundred (30-492) meters, as well as one broadband seismometer is below the borehole accelerometer. In general, the background seismic noise level are lower at the downhole stations than surface stations. However, the seismograms recorded by the downhole stations are smaller than surface stations due to the near-surface site effect. In Taiwan, the local magnitude (ML) determinations use the attenuation function derived from surface stations. Therefore, the ML will be underestimated by using current attenuation function for downhole stations. In this study, we used 19079 earthquakes to investigate the site amplification at subsurface materials between downhole and surface stations. Results demonstrate the amplification factors ranging from 1.11 to 5.74, provide the site effect parameter at shallow layers and have a strong relationship with Vs30. Further, we apply the amplification factors to revise the station local magnitude for downhole station. The revised ML at downhole stations correlate well with the ML at surface stations. Implement of the downhole station in the ML determination, it enhances the ability to detect the micro-earthquake and makes the earthquake catalog more comprehensive in Taiwan.

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Estimation of S-Wave Velocity Profiles at Lima City, Peru Using Microtremor Arrays

Journal of Disaster Research ◽

10.20965/jdr.2014.p0931 ◽

2014 ◽

Vol 9 (6) ◽

pp. 931-938 ◽

Cited By ~ 1

Author(s):

Selene Quispe ◽

◽

Kosuke Chimoto ◽

Hiroaki Yamanaka ◽

Hernando Tavera ◽

...

Keyword(s):

Wave Velocity ◽

Velocity Structure ◽

Site Effect ◽

Site Amplification ◽

Velocity Profiles ◽

S Wave ◽

Phase Velocity Dispersion ◽

Seismic Recording ◽

Phase Velocity Dispersion Curve ◽

Alluvial Material

Microtremor exploration was performed around seismic recording stations at five sites in Lima city, Peru in order to know the site amplification at these sites. The Spatial Autocorrelation (SPAC) method was applied to determine the observed phase velocity dispersion curve, which was subsequently inverted in order to estimate the 1-D S-wave velocity structure. From these results, the theoretical amplification factor was calculated to evaluate the site effect at each site. S-wave velocity profiles at alluvial gravel sites have S-wave velocities ranging from ∼500 to ∼1500 m/s which gradually increase with depth, while Vs profiles at sites located on fine alluvial material such as sand and silt have Swave velocities that vary between ∼200 and ∼500 m/s. The site responses of all Vs profiles show relatively high amplification levels at frequencies larger than 3 Hz. The average transfer function was calculated to make a comparison with values within the existing amplification map of Lima city. These calculations agreed with the proposed site amplification ranges.

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Site effect estimation on two Icelandic strong-motion arrays using a Bayesian hierarchical model for the spatial distribution of earthquake peak ground acceleration

Soil Dynamics and Earthquake Engineering ◽

10.1016/j.soildyn.2019.02.007 ◽

2019 ◽

Vol 120 ◽

pp. 369-385 ◽

Cited By ~ 3

Author(s):

Sahar Rahpeyma ◽

Benedikt Halldorsson ◽

Birgir Hrafnkelsson ◽

Russell A. Green ◽

Sigurjón Jónsson

Keyword(s):

Spatial Distribution ◽

Hierarchical Model ◽

Peak Ground Acceleration ◽

Strong Motion ◽

Bayesian Hierarchical Model ◽

Site Effect ◽

Ground Acceleration ◽

Bayesian Hierarchical ◽

Effect Estimation

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Site effects on Gros Morne Hill, Port-au-Prince, Haïti

10.5194/egusphere-egu2020-2098 ◽

2020 ◽

Author(s):

Hans-Balder Havenith ◽

Sophia Ulysse

Keyword(s):

Ground Motion ◽

Geophysical Methods ◽

Site Effect ◽

Site Amplification ◽

P Wave ◽

Near Surface ◽

Refraction Tomography ◽

Resonance Frequencies ◽

The Hill ◽

Earthquake Data

After the M = 7.0 Haiti earthquake in 2010, many teams completed seismic risk studies in Port-au-Prince to better understand why this not extraordinarily strong event had induced one of the most severe earthquake disasters in history (at least in the Western World). Most highlighted the low construction quality as the main cause for the disaster, but some also pointed to possible soil and topographic amplification effects, especially in the lower and central parts of Port-au-Prince (e.g., close to the harbor). Therefore, we completed a detailed site effect study for Gros-Morne hill located in the district of P&#233;tion-Ville, southeast of Port-au-Prince by using near surface geophysical methods. The horizontal to vertical spectral ratio technique was applied to ambient vibrations and earthquake data, and multichannel analysis of surface waves and P-wave refraction tomography calculation were applied to seismic data. Standard spectral ratios were computed for the S-wave windows of the earthquake data recorded by a small temporary seismic network. Electrical resistivity tomography profiles were also performed in order to image the structure of the subsurface and detect the presence of water.Different site effect components are represented for the entire survey area; we present maps of shear wave velocity variations, of changing fundamental resonance frequencies, and of related estimates of soft soil/rock thickness, of peak spectral amplitudes and of ambient ground motion polarization. Results have also been compiled within a 3D surface-subsurface model of the hill that helps visualize the geological characteristics of the area, which are relevant for site effect analyses. From the 3D geomodel we extracted one 2D geological section along the short-axis of the hill, crossing it near the location of the Hotel Montana on top of the hill, which had been destroyed during the earthquake and has now been rebuilt. This cross-section was used for dynamic numerical modelling of seismic ground motion and for related site amplification calculation. The numerical results are compared with the site amplification characteristics that had been estimated from the ambient vibration measurements and the earthquake recordings. Related results only partly confirm the strong seismic amplification effects highlighted by previous papers for this hill site, which had been explained by the effects of the local topographic and soil characteristics.

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