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

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

<p>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 Pétion-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.</p><p>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.</p>

Ground motion and site effect observations in the wellington region from the 2016 Mw7.8 Kaikōura, New Zealand earthquake

2017 ◽  
Vol 50 (2) ◽  
pp. 94-105 ◽  
Author(s):  
Brendon A. Bradley ◽  
Liam M. Wotherspoon ◽  
Anna E. Kaiser
Keyword(s):  
Ground Motion ◽  
Site Response ◽  
Ground Motions ◽  
Site Effect ◽  
Site Amplification ◽  
Design Standards ◽  
Site Class ◽  
Near Surface ◽  
Long Period ◽  
Basin Edge

This paper presents ground motion and site effect observations in the greater Wellington region from the 14 November 2016 Mw7.8 Kaikōura earthquake. The region was the principal urban area to be affected by the earthquake-induced ground motions from this event. Despite being approximately 60km from the northern extent of the causative earthquake rupture, the ground motions in Wellington exhibited long period (specifically T = 1 - 3s) ground motion amplitudes that were similar to, and in some locations exceeded, the current 500 year return period design ground motion levels. Several ground motion observations on rock provide significant constraint to understand the role of surficial site effects in the recorded ground motions. The largest long period ground motions were observed in the Thorndon and Te Aro basins in Wellington City, inferred as a result of 1D impedance contrasts and also basin-edge-generated waves. Observed site amplifications, based on response spectral ratios with reference rock sites, are seen to significantly exceed the site class factors in NZS1170.5:2004 for site class C, D, and E sites at approximately T=0.3-3.0s. The 5-95% Significant Duration, Ds595, of ground motions was on the order of 30 seconds, consistent with empirical models for this earthquake magnitude and source-to-site distance. Such durations are slightly longer than the corresponding Ds595 = 10s and 25s in central Christchurch during the 22 February 2011 Mw6.2 and 4 September 2010 Mw7.1 earthquakes, but significantly shorter than what might be expected for large subduction zone earthquakes that pose a hazard to the region. In summary, the observations highlight the need to better understand and quantify basin and near-surface site response effects through more comprehensive models, and better account for such effects through site amplification factors in design standards.

Integration of Multi-geophysical Approaches to Identify Potential Pathways of Heavy Metals Contamination - A Case Study in Zeida, Morocco

2020 ◽  
Vol 25 (3) ◽  
pp. 415-423
Author(s):  
Ahmed Lachhab ◽  
El Mehdi Benyassine ◽  
Mohamed Rouai ◽  
Abdelilah Dekayir ◽  
Jean C. Parisot ◽  
...  
Keyword(s):  
Heavy Metals ◽  
Seismic Velocity ◽  
Seismic Refraction ◽  
Geophysical Methods ◽  
Low Frequency ◽  
Electrical Current ◽  
P Wave ◽  
Low Resistivity ◽  
Refraction Tomography ◽  
Geophysical Surveys

The tailings of Zeida's abandoned mine are found near the city of Midelt, in the middle of the high Moulouya watershed between the Middle and the High Atlas of Morocco. The tailings occupy an area of about 100 ha and are stored either in large mining pit lakes with clay-marl substratum or directly on a heavily fractured granite bedrock. The high contents of lead and arsenic in these tailings have transformed them into sources of pollution that disperse by wind, runoff, and seepage to the aquifer through faults and fractures. In this work, the main goal is to identify the pathways of contaminated water with heavy metals and arsenic to the local aquifers, water ponds, and Moulouya River. For this reason, geophysical surveys including electrical resistivity tomography (ERT), seismic refraction tomography (SRT) and very low-frequency electromagnetic (VLF-EM) methods were carried out over the tailings, and directly on the substratum outside the tailings. The result obtained from combining these methods has shown that pollutants were funneled through fractures, faults, and subsurface paleochannels and contaminated the hydrological system connecting groundwater, ponds, and the river. The ERT profiles have successfully shown the location of fractures, some of which extend throughout the upper formation to depths reaching the granite. The ERT was not successful in identifying fractures directly beneath the tailings due to their low resistivity which inhibits electrical current from propagating deeper. The seismic refraction surveys have provided valuable details on the local geology, and clearly identified the thickness of the tailings and explicitly marked the boundary between the Triassic formation and the granite. It also aided in the identification of paleochannels. The tailings materials were easily identified by both their low resistivity and low P-wave velocity values. Also, both resistivity and seismic velocity values rapidly increased beneath the tailings due to the compaction of the material and lack of moisture and have proven to be effective in identifying the upper limit of the granite. Faults were found to lie along the bottom of paleochannels, which suggest that the locations of these channels were caused by these same faults. The VLF-EM surveys have shown tilt angle anomalies over fractured areas which were also evinced by low resistivity area in ERT profiles. Finally, this study showed that the three geophysical methods were complementary and in good agreement in revealing the pathways of contamination from the tailings to the local aquifer, nearby ponds and Moulouya River.

scholarly journals Subsurface characterization of a quick-clay vulnerable area using near-surface geophysics and hydrological modelling

Solid Earth ◽  
2019 ◽  
Vol 10 (5) ◽  
pp. 1685-1705
Author(s):  
Silvia Salas-Romero ◽  
Alireza Malehmir ◽  
Ian Snowball ◽  
Benoît Dessirier
Keyword(s):  
Large Scale ◽  
Geophysical Methods ◽  
Hydrological Modelling ◽  
P Wave ◽  
Coarse Grained ◽  
Near Surface ◽  
Reflection Seismic ◽  
Geotechnical Investigations ◽  
Quick Clay ◽  
Near Surface Geophysics

Abstract. Quick-clay landslides are common geohazards in Nordic countries and Canada. The presence of potential quick clays is confirmed using geotechnical investigations, but near-surface geophysical methods, such as seismic and resistivity surveys, can also help identify coarse-grained materials associated with the development of quick clays. We present the results of reflection seismic investigations on land and in part of the Göta River in Sweden, along which many quick-clay landslide scars exist. This is the first time that such a large-scale reflection seismic investigation has been carried out to study the subsurface structures associated with quick-clay landslides. The results also show a reasonable correlation with radio magnetotelluric and travel-time tomography models of the subsurface. Other ground geophysical data, such as high magnetic values, suggest a positive correlation with an increased thickness of the coarse-grained layer and shallower depths to the top of the bedrock and the top of the coarse-grained layer. The morphology of the river bottom and riverbanks, e.g. subaquatic landslide deposits, is shown by side-scan sonar and bathymetric data. Undulating bedrock, covered by subhorizontal sedimentary glacial and postglacial deposits, is clearly revealed. An extensive coarse-grained layer (P-wave velocity mostly between 1500 and 2500 m s−1 and resistivity from approximately 80 to 100 Ωm) exists within the sediments and is interpreted and modelled in a regional context. Several fracture zones are identified within the bedrock. Hydrological modelling of the coarse-grained layer confirms its potential for transporting fresh water infiltrated in fractures and nearby outcrops located in the central part of the study area. The modelled groundwater flow in this layer promotes the leaching of marine salts from the overlying clays by seasonal inflow–outflow cycles and/or diffusion, which contributes to the formation of potential quick clays.

Stratigraphic Analysis with Refraction Tomography

2019 ◽  
Vol 25 (3) ◽  
pp. 245-254
Author(s):  
Peter J. Hutchinson ◽  
Maggie H. Tsai
Keyword(s):  
Seismic Refraction ◽  
High Energy ◽  
P Wave ◽  
Tidal Channels ◽  
Near Surface ◽  
Precambrian Rocks ◽  
Stratigraphic Analysis ◽  
P Wave Velocity ◽  
Refraction Tomography ◽  
Near Shore

ABSTRACT Near-surface seismic refraction tomography imaged the basal contact of the Upper Cambrian silica-rich Mount Simon Formation with that of the underlying Precambrian granite in central Wisconsin. The discrimination between the Mount Simon and underlying non-conformable contact with Precambrian rocks was based upon a p-wave velocity of 1,700 m/s. Refraction tomography imaged deep, broad tidal channels within the Mount Simon consistent with the inference that Mount Simon was deposited in a high-energy near-shore, probably fluvial environment. The Mount Simon is an arenite that has high commercial value.

Application of Site Amplification Factors to Determine the Local Magnitude from Borehole Seismic Stations in Taiwan

2020 ◽  
Author(s):  
Tz-Shin Lai ◽  
Yih-Min Wu ◽  
Wei-An Chao
Keyword(s):  
Site Effect ◽  
Site Amplification ◽  
Force Balance ◽  
Earthquake Catalog ◽  
Local Magnitude ◽  
Broadband Seismometer ◽  
Near Surface ◽  
Amplification Factors ◽  
Attenuation Function ◽  
Borehole Seismic

<p>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 (M<sub>L</sub>) determinations use the attenuation function derived from surface stations. Therefore, the M<sub>L</sub> 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 M<sub>L</sub> at downhole stations correlate well with the M<sub>L</sub> at surface stations. Implement of the downhole station in the M<sub>L</sub> determination, it enhances the ability to detect the micro-earthquake and makes the earthquake catalog more comprehensive in Taiwan.</p>

The effect of quaternary alluvium on strong ground motion in the Coyote Lake, California, earthquake of 1979

1981 ◽  
Vol 71 (4) ◽  
pp. 1333-1349 ◽  
Author(s):  
William B. Joyner ◽  
Richard E. Warrick ◽  
Thomas E. Fumal
Keyword(s):  
Ground Motion ◽  
Strong Ground Motion ◽  
Site Amplification ◽  
Horizontal Velocity ◽  
Surface Velocity ◽  
Near Surface ◽  
Soil Response ◽  
A Site ◽  
Strong Ground ◽  
Good Agreement

abstract The effect of alluvium on strong ground motion can be seen by comparing two strong-motion records of the Coyote Lake, California, earthquake of 6 August 1979 (ML = 5.9). One record at a site on Franciscan bedrock had a peak horizontal acceleration of 0.13 g and a peak horizontal velocity of 10 cm/sec. The other, at a site 2 km distant on 180 meters of Quaternary alluvium overlying Franciscan, had values of 0.26 g and 32 cm/sec, amplifications by factors of 2 and 3. Horizontal motions computed at the alluvial site for a linear plane-layered model based on measured P and S velocities show reasonably good agreement in shape with the observed motions, but the observed peak amplitudes are greater by a factor of about 1.25 in acceleration and 1.8 in velocity. About 15 per cent of the discrepancy in acceleration and 20 per cent in velocity can be attributed to the difference in source distance; the remainder may represent focusing by refraction at a bedrock surface concave upward. There is no clear evidence of nonlinear soil response. Fourier spectral ratios between motions observed on bedrock and alluvium show good agreement with ratios predicted from the linear model. In particular, the observed frequency of the fundamental peak in the amplification spectrum agrees with the computed value, indicating that no significant nonlinearity occurs in the secant shear modulus. Computations show that nonlinear models are compatible with the data if values of the coefficient of dynamic shear strength in terms of vertical effective stress are in the range of 0.5 to 1.0 or greater. The data illustrate that site amplification may be less a matter of resonance involving reinforcing multiple reflections, and more the simple effect of the low near-surface velocity. Application of traditional seismological theory leads to the conclusion that the site amplification for peak horizontal velocity is approximately proportional to the reciprocal of the square root of the product of density and shear-wave velocity.

RMS response of a one-dimensional half-space to SH

1996 ◽  
Vol 86 (2) ◽  
pp. 363-370 ◽  
Author(s):  
Steven M. Day
Keyword(s):  
Travel Time ◽  
Half Space ◽  
Ground Motion ◽  
Elastic Properties ◽  
Site Amplification ◽  
Frequency Resolution ◽  
Elastic Structure ◽  
Earthquake Ground Motion ◽  
Site Classification ◽  
Near Surface

Abstract We examine the extent to which the response of a perfectly elastic half-space to an SH-wave incident from below can be characterized when knowledge about the elastic structure is limited to the near surface. Elastic properties are modeled as piecewise continuous functions of the depth coordinate. It is found that the site amplification function can be determined with a frequency resolution that depends inversely on the depth to which the elastic structure is known. Specifically, certain spectral averages of the site amplification function, concentrated over bandwidth Δƒ, depend only on the elastic structure down to a two-way travel-time depth of 1/Δƒ. These spectral averages are entirely independent of the elastic properties at greater depth. Equivalently, when the incident motion has a bandlimited white power spectrum of bandwidth Δƒ, the site amplification of the root mean square (rms) ground motion depends only on the elastic structure down to a two-way travel-time depth of 1/Δƒ. When the bandwidth is sufficiently large, the following corollary applies: the rms surface ground motion equals the rms incident motion multiplied by 2√Ib/I0, where I0 and Ib are shear impedances at the ground surface and basement depth, respectively. This result provides justification for a procedure conventionally used to correct stochastic estimates of earthquake ground motion to account for local site effects. The analysis also clarifies the limitations of that conventional procedure. The results define specific site-response parameters that can be computed from knowledge of shallow structure alone and may thereby contribute to improved understanding of the physical basis for, and limitations of, site classification schemes that are based on average S-wave velocity at shallow depth. While the analytical results are rigorous only for infinite Q, numerical experiments indicate that similar results apply to models with finite, frequency-independent Q. The practical utility of the results is likely to be limited primarily by the degree of lateral heterogeneity present near sites of interest and the degree to which the sites respond nonlinearly to incident ground motion.

scholarly journals Detection of Cover Collapse Doline and Other Epikarst Features by Multiple Geophysical Techniques, Case Study of Tarimba Cave, Brazil

Water ◽  
2020 ◽  
Vol 12 (10) ◽  
pp. 2835
Author(s):  
Yawar Hussain ◽  
Rogerio Uagoda ◽  
Welitom Borges ◽  
Renato Prado ◽  
Omar Hamza ◽  
...  
Keyword(s):  
Electrical Resistivity ◽  
Seismic Refraction ◽  
Geophysical Methods ◽  
Low Frequency ◽  
Least Square ◽  
P Wave ◽  
Positive Anomaly ◽  
Refraction Tomography ◽  
Collapse Doline

Reliable characterization of the karst system is essential for risk assessment where many associated hazards (e.g., cover-collapse dolines and groundwater pollution) can affect natural and built environments, threatening public safety. The use of multiple geophysical approaches may offer an improved way to investigate such cover-collapse sinkholes and aid in geohazard risk assessments. In this paper, covered karst, which has two types of shallow caves (vadose and fluvial) located in Tarimba (Goias, Brazil), was investigated using various geophysical methods to evaluate their efficiency in the delineation of the geometry of sediments filled sinkhole. The methods used for the investigation were Electrical Resistivity Tomography (ERT), Seismic Refraction Survey (SRS), Seismic Refraction Tomography (SRT) and the Very Low Frequency Electromagnetic (VLF-EM) method. The study developed several (2D) sections of the measured physical properties, including P-wave velocity and electrical resistivity, as well as the induced current (because of local bodies). For the analysis and processing of the data obtained from these methods, the following approaches were adopted: ERT inversion using a least-square scheme, Karous-Hjelt filter for VLF-EM data and time-distance curves and Vp cross-sections for the SRS. The refraction data analysis showed three-layered stratigraphy topsoil, claystone and carbonate bedrock, respectively. The findings obtained from ERT (three-layered stratigraphy and sediment-filled doline), as well as VLF-EM (fractured or filled caves as a positive anomaly), were found to be consistent with the actual field conditions. However, the SRS and SRT methods did not show the collapsed material and reached the limited the depth because of shorter profile lengths. The study provides a reasonable basis for the development of an integrated geophysical approach for site characterization of karst systems, particularly the perched tank and collapse doline.

Foundation Evaluation of a Repeater Installation Building using Electrical Resistivity Tomography and Seismic Refraction Tomography

2019 ◽  
Vol 24 (1) ◽  
pp. 27-38
Author(s):  
B. Butchibabu ◽  
Prosanta K. Khan ◽  
P.C. Jha
Keyword(s):  
Electrical Resistivity ◽  
Electrical Resistivity Tomography ◽  
Seismic Refraction ◽  
P Wave ◽  
S Wave ◽  
Resistivity Tomography ◽  
Near Surface ◽  
Refraction Tomography ◽  
Subsurface Layers ◽  
Geophysical Investigations

Geophysical investigations were carried out for evaluation of damage and to assess the possible causes for repeated occurrence of damage at one of the buildings constructed for oil pumping in the northern part of India. Electrical Resistivity Tomography (ERT) and Seismic Refraction Tomography (SRT) techniques were adopted for studying the subsurface of the area around the building with an objective of ascertaining the cause of damage. High resolution imaging was done using both the techniques in this investigation. ERT delineated the presence of low resistivity (2 ohm-m) water filled voids below the structures and mapped different subsurface layers such as sandy soil, clay and sandstone in the study area. SRT revealed P-wave velocity ( V P ) of the subsurface medium in the range of 400–3,400 m/s. Corresponding densities and S-wave velocities ( V S ) were determined based on Gardner's and Castagna's relationships. Subsequently, the V P , V S and the modulus values were used in estimating compressibility of soil and rock strata. Results showed near surface layers were characterized by high compressibility (26.673 × 10 −5 Pa −1 ), decreases with depth. This paper presents the details of the site, techniques used in the investigation and correlation of geophysical results with lithological information, and the subsequent analysis for understanding the distress in the subsurface of the study area.

scholarly journals Identifying recharge under subtle ephemeral features in flat-lying semi-arid region using a combined geophysical approach

2020 ◽  
Author(s):  
Brady A. Flinchum ◽  
Eddie Banks ◽  
Michael Hatch ◽  
Okke Batelaan ◽  
Luk Peeters ◽  
...  
Keyword(s):  
Wave Velocity ◽  
Arid Region ◽  
Geophysical Methods ◽  
P Wave ◽  
Seismic Survey ◽  
Small Scale ◽  
S Wave ◽  
Near Surface ◽  
Semi Arid Region ◽  
Semi Arid

Abstract. Identifying and quantifying recharge processes linked to ephemeral surface water features is challenging due to their episodic nature. We use a unique combination of well-established near-surface geophysical methods to provide evidence of a surface and groundwater connection under a small ephemeral recharge feature in a flat, semi-arid region near Adelaide, Australia. We use a seismic survey to obtain P-wave velocity through travel-time tomography and S-wave velocity through the multichannel analysis of surface waves. The ratios between P-wave and S-wave velocities allow us to infer the position of the water table. A separate survey was used to obtain electrical conductivity measurements from time-domain electromagnetics and water contents were acquired by downhole nuclear magnetic resonance. The combined geophysical observations provide evidence to support a groundwater mound underneath a subtle ephemeral feature. Our results suggest that recharge is localized and that small-scale ephemeral features play an important role in groundwater recharge. Furthermore, we show that a combined geophysical approach can provide a unique perspective that helps shape the hydrogeological conceptualization of a semi-arid region.

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