Thickness Of Soft Sediments Research Articles

Imaging subsurface geological complexity (2D/3D) beneath the Greater Srinagar region of the Kashmir basin, Northwest Himalaya

ABSTRACTA high‐resolution microtremor measurement in Greater Srinagar city of the Kashmir valley has been analysed to image 2D and 3D subsurface geological complexities. This region is located in the highly seismogenic Himalayan belt and sits atop a deep sedimentary lake bed with a laterally varying thickness of soft sediments. Srinagar region is a major economic centre and the capital city of the Kashmir valley with 2 million inhabitants living at high seismic risk. To assess the subsurface complexity beneath the city, we present: (1) high‐resolution subsurface shear wave velocity Vs structure using the horizontal‐to‐vertical spectral ratio inversion; (2) shear wave velocity for top 30 metres of soil column (VS30) map with National Earthquake Hazards Reduction Program site classification; (3) comparison of VS30 maps calculated from horizontal‐to‐vertical spectral ratio inversion and topographic slope methods; and (4) azimuthal behaviour of horizontal‐to‐vertical spectral ratio peaks, all of which unravel the subsurface spatial heterogeneity and suitability for the building of engineering structures in the study area. In addition, a new matlab code is applied to generate 3D subsurface Vs slices in the study region in different directions using its pre‐generated 2D Vs profile data. The presented potentiality of microtremor horizontal‐to‐vertical spectral ratio technique in Srinagar region, which lies on the eastern edge of the basin with significant topographic irregularities, indicates an uneven distribution of local site effects (primary and secondary) in the case of strong ground motion. The comprehensive results can be promising in engineering analyses of local ground and structural responses in order to mitigate the impact of earthquake occurrence and seismic risk in the city and adjoining regions.

Liquefaction in Palu: the cause of massive mudflows

The 7.5 Mw tectonic earthquake that hit Palu City on 28 September 2018 was followed by tsunami and liquefaction, triggered massive mudflows in Balaroa, Petobo, and Jono Oge areas. This study focuses on the generating factors of liquefaction such as the condition of soil lithology, depth of water table, the distance to the focal mechanism, and the thickness of soft sediment. Microtremor data, including the Horizontal Vertical Spectral Ratio (HVSR), geological condition, and borehole data, were examined to conduct the liquefaction analysis. The analysis results based on the microtremor data showed that the distribution of ground shear strain values in Palu City ranged from 0.75 × 10–4 to 2.56 × 10–4. The distribution of the locations of the liquefaction was correlated to the distribution of ground shear strain values. High ground shear strain values and a shallow groundwater level were discovered in Palu City valley, which indicates that liquefaction in Palu City will undoubtedly occur. The semi-empirical method confirmed that Balaroa, Petobo, and Jono Oge had undergone large-scale liquefaction at a maximum depth of 16 m below the ground level. The average peak of water runoff that generated the mudflow was estimated to be at 11.31 cm3/s. Since the soil has loose soil grain with high water content, the soil will turn into a massive amount of mud during the liquefaction.

Establishing site response-based micro-zonation by applying machine learning techniques on ambient noise data: a case study from Northern Potwar Region, Pakistan

Several major earthquakes have jolted Pakistan during the last 30 years, destroyed infrastructure and severe damage to the economy. Despite advancement in data sciences and its linkages with other domains, no such study has been conducted in Pakistan, which incorporates the application of machine learning techniques on ambient noise data. This study presents the application of machine learning techniques on the ambient noise data to establish the micro-zones within the urban settlements of northern Potwar region of Pakistan, based on the site response parameters and seismic vulnerability. The ambient noise data from 148 sites in the study area, are acquired, processed and interpreted. Different clustering techniques namely, K-mean, fuzzy c-means and hierarchical clustering have been applied to the interpreted ambient noise data set. Arc GIS maps of the study area have been developed by making use of the interpretation of the ambient noise data and the resulted clustering solutions. The results showed that the fundamental frequency f0 ranges between 0.5 and 15 Hz, the H/V spectral amplification factor ranges between 0.8 and 5.9; the soft sediment thickness ranges from 1.6 to 316 m, whereas, the soil vulnerability index is observed between 0.1 and 63. These site response parameters indicated that the study area is moderate to highly vulnerable to site amplification, and any seismic event can lead to catastrophe within the study area. The clustering techniques also detected three groups from the interpretation of the ambient noise data set by separating the locations according to their vulnerability due to site amplification. The quality of cluster solutions was evaluated using cluster validity indexes and the results of these techniques were compared. These results present the similarities and dissimilarities among different sites and indicate the sites which are geographically distant but have very similar vulnerability characteristics or vice versa. The Arc GIS tool showed the spatial distribution of site response parameters and three zones were established as zone 1, zone 2 and zone 3 with low, intermediate and high values, respectively. The spatial distribution maps showed that the northeastern and northwestern parts of the study area are more vulnerable to site amplification.

Role of site effect for the evaluation of attenuation characteristics of P, S and coda waves in Kinnaur region, NW Himalaya

The site effect and attenuation studies are carried out for Kinnaur region of northwest Himalaya, India. A total of 109 local events happened in Kinnaur region of magnitude range 1.6–4.5, are utilized for present work. The earthquake records are influenced by the site effect depending on soft sediment thickness beneath the recording sites. Therefore, in the present study, records are corrected for site effects to estimate P (Qp), S (Qs) and coda (Qc) wave quality factor. The regional frequency dependent attenuation relations, i.e., $$ Q_{p} \left( f \right) = \left( {29 \pm 1} \right)f^{(1.01 \pm 0.05)} $$ , $$ Q_{s} \left( f \right) = \left( {38 \pm 5} \right)f^{(1.1 \pm 0.06)} $$ and $$ Q_{c} \left( f \right) = \left( {74 \pm 11} \right)f^{(1.17 \pm 0.01)} $$ are established for the Kinnaur region. The Kinnaur Himalaya mainly belongs to Higher Himalaya Crystalline (HHC) and Tethys Himalaya, where these two geological units are differentiated by the South Tibetan Detachment System (STDS). The resonance frequencies and attenuation characteristics are estimated for both regions, i.e., HHC and Tethys Himalaya. A comparison is made between HHC and Tethys Himalaya in the form of resonance frequencies and attenuation properties. The low value resonance frequency and high rate of attenuation towards the northern side of STDS, i.e., Tethys Himalaya support the presence of low-grade metasedimentary rocks. It suggests that Tethys Himalaya has high seismic hazard potential zone compared to HHC.

New relationship between fundamental site frequency and thickness of soft sediments from seismic ambient noise

In this contribution, a new relationship between the fundamental site frequency and the thickness of soft sediments is obtained for many sites in Egypt. I validated and compared new formula results with the results obtained from other formulas of earlier scientists. The obtained results indicate that the calculated depth and geometry of the bedrock surface using new formula are in a good agreement with well logs data and previously published seismic refraction surveys in the investigated sites.

The uncertainty of estimating the thickness of soft sediments with the HVSR method: A computational point of view on weak lateral variations

The use of the ratio of microtremor spectra, as computed by the Nakamura's technique, was recently proved successful for the evaluating the thickness of sedimentary covers laying over both shallow and deep rocky bedrocks thus enabling bedrock mapping. The experimental success of such application and its experimental uncertainties are today reported in many publications. To map bedrock, two approaches exist. The first is to assume a constant shear wave velocity profile of the sediments. The second, and most preferable, is Ibs-von Seht and Wohlenberg's, based on correlating Nakamura's curves main peak and wells information. In the latter approach, the main sources of uncertainty addressed by authors, despite the lack of formal proof, comprise local deviations of the subsurface from the assumed model. I first discuss the reliability of the simplified constant velocity approach showing its limitations. As a second task, I evaluate the uncertainty of the Ibs-von Seht and Wohlenberg's approach with focus on local subsurface variations. Since the experimental basis is well established, I entirely focus my investigation on numerical simulations to evaluate to what extent local subsurface deviations from the assumed model may affect the outcome of a bedrock mapping survey. Further, the present investigation strategy suggests that modeling and inversion, through the investigation of the parameters space around the reference model, may reveal a very convenient tool when lateral variations are suspected to exist or when the number of available wells is not sufficient to obtain an accurate frequency-depth regression.

Characterizing site response in the Attock Basin, Pakistan, using microtremor measurement analysis

The Attock Basin is situated close to the northwest of Pakistan. Recent seismic event of October 2015 (7.5Mw) near the Pakistan Afghanistan border has proved that the area of interest is seismically active and triggered a series of aftershocks of magnitude even greater than 6.5Mw. This seismic activity has posed danger to the future of the people and infrastructure especially to the northwestern part of the country. Therefore, site response analysis is essential for estimating local site conditions in response to seismic events. Ambient noise recordings were made at 50 sites within urban and semi-urban settlements in the Attock Basin to analyze the site response of the small but densely populated basin. At each of these sites, the fundamental frequency of the soft sediments (f 0), the amplitude (A 0) of corresponding H/V spectral ratios, the thickness of soft sediment (H) lying over competent lithology, and the soil vulnerability index (K g) were studied. Results were correlated with sparsely available borehole data to enhance the credibility of the study conducted for microzonation and predicting the site response to earthquake seismicity in the Attock Basin. The soil vulnerability index was found to range from moderate to high. Results clearly showed that the study area exhibits low to moderate fundamental frequency with greater soft sediment thicknesses distributed throughout the study area. Moreover, higher impedance contrasts were found at most of the sites within the central part of the Attock Basin, thus reflecting a moderate to high susceptibility of damage in those regions in response to seismic events.

Soft sediment thickness and shear-wave velocity estimation from the H/V technique up to the bedrock at meteorite impact crater site, Sao Paulo city, Brazil

To obtain shear wave velocity profile for near-surface application many geophysical techniques is used. The most recently used approach is the joint inversion of H/V spectral ratio curve and dispersion curve extracted from the multi-channel analysis of surface waves (MASW) survey. However, in some cases, it is difficult to acquired dispersion curve in engineering application frequency range 1–30Hz. In that situation, dispersion curve can be indirectly inverted from all the available points of the frequency-VRayleigh curve to shear wave velocity and depth. This result a series of (s-wave velocity and depth) values through which a best-fit trend can be passed. The best fit gives us an idea of the rate of shear wave velocity increase with depth and shear wave velocity at one-meter depth. This information can later be utilized in an inversion of Rayleigh wave ellipticity curve.

Erratum to: Mapping sediment thickness of Islamabad city using empirical relationships: Implications for seismic hazard assessment

Soft sediments make an important component of the subsurface lithology, especially in areas underlain by river/stream basins. Occupying a position directly above the bedrock up to the land surface, these soft sediments can range in thickness from few centimeters to hundreds of meters. They carry a special nuisance in seismic hazards, as they serve as a source of seismic amplification that may enhance the seismic shaking of many folds. Determination of the thickness of the soft sediments is therefore crucial in seismic hazard analysis. A number of studies in recent years have demonstrated that frequency and amplitude spectrum obtained from the noise measurements during the recording of natural seismicity can be used to obtain thickness of soft sediments covering the bedrock. Nakamura (1989) presented a technique to determine such spectrum using ratio of horizontal to vertical components of the Rayleigh waves. The present study is based on an extensive set of microtremor measurements carried out in the Islamabad city, Pakistan. Fundamental frequencies were obtained from weak motion sensors and Tromino Engy Plus instruments to show that the correlation is clearly valid for a wide range of sediment thickness. A simple formula was derived for the investigated area to determine directly the thickness of sediments from the main peaks in the H/V spectrum for seismometer and Tromino data separately. A comparison is made between sediment thicknesses derived from empirical relations developed in this study with those given in literature to demonstrate a positive correlation. The correlation of instrumental resonant frequencies with calculated resonant frequencies (theoretical) suggests that the relation derived from the noise measurements mostly depends on the velocity depth function of the shear wave. The fundamental frequency of the main peak of spectral ratio of H/V using the both instruments correlates well with the thickness of sediments at the site obtained from the borehole data. It is found out that there is a wide variation in soft-sediment thickness in the Islamabad area, but as a general rule, soft sediments are thicker adjacent to stream courses compared to the areas intervening the streams. The distribution of sediments in the studied area is illustrated by means of cross sections constructed from results of the microtremor analyses and available borehole data, which provides a visual distribution of the soft sediments underlying the Islamabad city.

Mapping sediment thickness of Islamabad city using empirical relationships: Implications for seismic hazard assessment

Soft sediments make an important component of the subsurface lithology, especially in areas underlain by river/stream basins. Occupying a position directly above the bedrock up to the land surface, these soft sediments can range in thickness from few centimeters to hundreds of meters. They carry a special nuisance in seismic hazards, as they serve as a source of seismic amplification that may enhance the seismic shaking of many folds. Determination of the thickness of the soft sediments is therefore crucial in seismic hazard analysis. A number of studies in recent years have demonstrated that frequency and amplitude spectrum obtained from the noise measurements during the recording of natural seismicity can be used to obtain thickness of soft sediments covering the bedrock. Nakamura (1989) presented a technique to determine such spectrum using ratio of horizontal to vertical components of the Rayleigh waves. The present study is based on an extensive set of microtremor measurements carried out in the Islamabad city, Pakistan. Fundamental frequencies were obtained from weak motion sensors and Tromino Engy Plus instruments to show that the correlation is clearly valid for a wide range of sediment thickness. A simple formula was derived for the investigated area to determine directly the thickness of sediments from the main peaks in the H/V spectrum for seismometer and Tromino data separately. A comparison is made between sediment thicknesses derived from empirical relations developed in this study with those given in literature to demonstrate a positive correlation. The correlation of instrumental resonant frequencies with calculated resonant frequencies (theoretical) suggests that the relation derived from the noise measurements mostly depends on the velocity depth function of the shear wave. The fundamental frequency of the main peak of spectral ratio of H/V using the both instruments correlates well with the thickness of sediments at the site obtained from the borehole data. It is found out that there is a wide variation in soft-sediment thickness in the Islamabad area, but as a general rule, soft sediments are thicker adjacent to stream courses compared to the areas intervening the streams. The distribution of sediments in the studied area is illustrated by means of cross sections constructed from results of the microtremor analyses and available borehole data, which provides a visual distribution of the soft sediments underlying the Islamabad city.

Ambient noise measurements in Rawalpindi–Islamabad, twin cities of Pakistan: a step towards site response analysis to mitigate impact of natural hazard

Earthquakes are one among the highly dangerous geological hazards. The damage from earthquakes is greatly influenced by the local geological conditions. The history of Pakistan is full of severe earthquakes that caused the mass destruction at national scale. Major earthquake-affected areas in Pakistan are mainly concentrated in the north and western sections of the country. Site response analysis is an initial step towards seismic risk assessment. The present study aims at local site effects by acquiring data along 105 sites of Rawalpindi–Islamabad, the twin cities of federal capital of Pakistan, by applying H/V spectral ratio method. Out of the data acquired for 105 sites, data for 88 sites were considered reliable after observing reliability conditions determined by SESAME guidelines devised in 2004. Fundamental frequency f 0 of soft sediments, amplitude A 0 of corresponding H/V spectral ratios, thickness of soft sediments residing on the bedrock and soil vulnerability index K g over 88 reliable data sites within the study area were estimated and analysed. The results show that the fundamental frequency of the sediments reflects variation and lies within a wide range between 0.6 and 14.4 Hz. Similarly, amplification factor A 0 up to 5.5 was observed at a site. Overburden thickness of soft sediments H also indicates high variation and has been calculated in the range of 1.7–316.5 m. As far as K g is concerned, it is also highly variable and is lying in the range of 0.30–62.7. These results will help in designing a policy to mitigate the impact of seismic hazard in the study area.

Site effect evaluation for Yanbu City urban expansion zones, western Saudi Arabia, using microtremor analysis

Recently, Yanbu City has been greatly growing; great developmental projects, urban community settlements, and petrochemical industries have been established. Historical and instrumental earthquakes were felt by this city, such as the earthquake on 19 May 2009 which was a moderate earthquake (moment magnitude (M w) 5.7) whose ground motions have affected some building structures. Yanbu City has been divided by a grid of points separated by about 500 m, and microtremor measurements have been conducted at 141 measuring sites. The acquired data have been processed using worldwide Geopsy software package to calculate the fundamental frequency peaks and their corresponding amplification factors. The natural origin of these peaks has been confirmed. Based on fundamental frequency (f 0), Yanbu City has been classified into three zones—from 0.13 to 1.0 Hz in the first zone, from 1.0 to 4.0 Hz in the second zone, and from 4.0 to 7.6 Hz in the third zone. The coastal zone of Yanbu illustrates smaller values of f 0 that reflect great thickness of soft sediments while the eastern zone presents an opposite phenomenon. Accordingly, high-rise buildings in the coastal zone will be affected greatly by low frequencies originating from distant earthquakes. Most of Yanbu City has f 0 values in the range of 1.0–4.0 Hz. Furthermore, bedrock ground motion could amplify as much as three times. This indicates that one- to three-story buildings in Yanbu City are vulnerable to hazardous resonant shaking from local and near earthquakes.

Basement topography of the Kathmandu Basin using microtremor observation

Kathmandu Valley, an intermontane basin of the Himalaya, has experienced many destructive earthquakes in the past. The observations of the damage pattern during the 1934 Earthquake (Mw=8.1), in particular, suggest that the spectral ground amplification due to fluvio-lacustrine sediments plays a major role in intensifying the ground motion in the basin. It is, therefore, imperative to conduct a detailed study about the floor variation of sediments in the basin. In this paper, a preliminary attempt was made to estimate the thickness of soft sediment in the Kathmandu Basin using microtremor observations. The measurements of microtremors were carried out at 172sites spaced at a grid interval of 1km. The results showed that the predominant frequency varies from 0.488Hz to 8.9Hz. A non-linear regression relationship between resonance frequency and sediment depth was proposed for the Kathmandu Basin. The thickness of lacustrine sediments at various points in the basin was estimated using the proposed equation, and then the estimated thickness was used to plot a digital elevation model of the basement topography and cross profiles of the sediment distribution in the basin. The results were validated by correlating the estimated sediment thickness with geology and geomorphology of the study area.

Microtremor study of site effects and soil-structure resonance in the city of Ljubljana (central Slovenia)

The city of Ljubljana is located in one of the three areas with the highest seismic hazard in Slovenia, and it is also the most densely populated. Site effects due to Quaternary sediments, which fill the up to 200 m-deep basin, are characteristic of the whole city area, but they can be especially strong in the southern part of Ljubljana, which is built on very soft lacustrine deposits. Existing microzonation studies of the city are inadequate, since there is a lack of borehole, geophysical and earthquake data. The microtremor horizontal-to-vertical spectral ratio (HVSR) method was therefore applied to a 200 m dense grid of free-field measurements over an area of 45 km2 (1,223 measuring points) in order to assess the fundamental frequency of the sediments. The main difficulties in microtremor measurement arose from high levels of traffic and industrial noise, and from underground structures. Experimental conditions which can influence data quality, such as strong wind and water saturation of soil, were analysed. Very clear HVSR peaks were obtained in the entire southern part of the city, whereas in the northern part the site response is in general lower due to lower impedance contrast of gravel with the bedrock. The iso-frequency map of sediments shows a distribution in the range of 0.9–10 Hz. In the southern part of Ljubljana, sediment frequency correlates well with the thickness of soft sediments known from geophysical investigations and sparse drilling. Average amplitude of the HVSR peaks is considerably higher in the southern part (6.7 ± 2.4) than in the northern part (4.0 ± 2.0) of the city, indicating a high impedance contrast of lacustrine sediments with the bedrock. Microtremor measurements were also performed inside 122 buildings of various heights. We focused on important public buildings and selected blocks of flats and houses. To assess the longitudinal and transverse fundamental frequencies of each building, amplitude spectra and the spectral ratio between the upper floor and the basement were analysed for both directions. When one of these frequencies is close to a nearby free-field fundamental frequency, a potential soil-structure resonance is present. This was found in 12 of the measured buildings. Three of them are tall residential buildings (from 10 to 15 floors) with a fundamental frequency of 2–3 Hz, and nine of them are low-rise buildings (from 3 to 5 floors) with a fundamental frequency ranging from 3 to 4.5 Hz. Using the relationship between fundamental frequency and height, the typical height of buildings that might cause soil-structure resonance can be estimated at a given sediment frequency obtained from free-field measurements.

Site effects on earthquake ground motion based on microtremor measurements for metropolitan Beijing

A campaign of microtremor measurements was conducted in summer 2007 for assessing local site effects of seismic strong ground motion in metropolitan Beijing. Using the measurements from over 600 sites with approximately 1–2 km spacing covering the entire area inside Beijing’s 5th Beltway, we present the analysis results in the form of contours of the predominant resonant frequency, the thickness of the uppermost soft sediments, and the ground motion amplification factor. The microtremorderived soft sediment thickness is generally in agreement with previous results based on much sparser borehole data, with the revealing of more short-wavelength undulations, which coincide with major geomorphological and neotectonic expressions in metropolitan Beijing. This study provides additional valuable information to the earthquake-resistant design of civil infrastructures and seismic hazard response in metropolitan Beijing. It also provides a feasible geophysical approaching to explore the 3-D structure beneath metropolitan cities.

Tsunami Hazard for the Auckland Region and Hauraki Gulf, New Zealand

The Hauraki Gulf is a semi-enclosed sea next to the largest population centre in New Zealand, the Auckland metropolitan region. The potential tsunami hazard is of concern to regional and local planners around the Hauraki Gulf. The Hauraki Gulf has recorded 11 tsunamis and one meteorological tsunami (rissaga) since 1840. The historical tsunami data are relatively sparse, particularly for the largest events in 1868 and 1883. Moreover, local sources may produce damaging tsunamis but none has occurred during recorded history. Therefore numerical modelling of potential tsunami events provides a powerful tool to obtain data for planning purposes. Three main scenarios have been identified for numerical modelling: 1. A teletsunami event from an earthquake off the West Coast of South America. Historically this region has produced the largest teletsunamis in the Hauraki Gulf. 2. A tsunami generated by a local earthquake along the Kerepehi Fault. This fault bisects the Gulf, has been active during the last century at the southern inland end, and is overlain by a considerable thickness of soft sediment that may amplify the seismic waves. 3. A tsunami generated by a volcanic eruption within the Auckland Volcanic Field. This field has involved a series of mainly monogenetic basaltic eruptions over the last 140,000 years. Many of these eruptions have involved phreatomagmatic eruptions around the coastal margins, or within the shallow waters close to Auckland.

Microtremor measurements used to map thickness of soft sediments

AbstractThe observations about the behavior of microtremor spectra presented here show that noise measurements can be used as a powerful tool to determine the thickness of soft cover layers. The most suitable method for this determination is Nakamura's technique, which is the ratio of the horizontal-component noise spectrum and that of the vertical component (H/V spectrum). The frequency of the main peak in these spectral ratios correlates well with the sediment thickness at the site. Using an extensive database of microtremor measurements carried out in the western Lower Rhine Embayment (Germany), it was possible to show that this correlation is clearly valid for a wide range of thickness, namely, from tens of meters to more than 1000 m. A simple formula was derived that, for the sediments to be found in the area investigated, directly calculates the cover thickness from the frequency of the main peak in the H/V spectrum. A comparison with calculated resonant frequencies suggests the relation derived from the noise measurements depending on the velocity depth function of the shear wave. Classical spectral ratios are shown to be strongly influenced by the noise level and are therefore less reliable in determining the resonant frequency of the subsoil. The practical relevance of the investigation is illustrated by means of cross sections, constructed from results of the microtremor analyses, which provide a convincing image of the surficial structure of the areas investigated.