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- Volume 19, Issue 2, 2021
Near Surface Geophysics - Volume 19, Issue 2, 2021
Volume 19, Issue 2, 2021
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Geophysical investigations for the identification of active seismic faults below alluvium for seismic hazard assessment
More LessABSTRACTThe existence of active faults hidden below Quaternary alluvium is a common geological scenario for intermontane basins, such as the areas struck by the recent earthquakes in Central Italy, and is of great importance for seismic hazard evaluation. Finding hidden faults is a challenging task from the geophysicist's point of view since the goal is twofold: to identify the seismic bedrock at a certain depth; and to detect lateral variations or dislocations that may indicate the presence of a fault. We propose a mixed approach encompassing at first single‐station seismic noise measurements, to detect sudden lateral variations in the bedrock surface in a fast and cost‐effective way, which might serve as a proxy for the potential identification of fault zones. Then, more accurate electrical resistivity tomography investigations are carried out only at selected sites as indicated by the preliminary noise analysis, as electrical methods cannot effectively be employed at a large scale for time and economic limitations. Surface‐wave dispersion analysis is jointly interpreted together with ambient noise data to improve the seismic characterization of the alluvium, giving further insight on the assessment of the depth to bedrock. The proposed approach can be an effective way to manage and investigate a large portion of the territory within a sensible budget, as commonly needed in seismic hazard assessment and microzonation studies. We present a real‐world application to the San Vittorino Plain (Central Italy) close to the epicentre of the 24 August 2016 Amatrice earthquake, where the geological faulted bedrock is covered by alluvial sediments of the Velino River up to a maximum estimated thickness of 150–200 m. Although engineered for the post‐earthquake reconstruction emergency, the approach employed in our study can be adopted in other areas of similar geology, to ease the application of seismic microzonation in time of seismic silence as a tool for long‐term land planning and management.
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Active and passive seismic surface wave methods for levee assessment in the Sacramento–San Joaquin Delta, California, USA
Authors Mitchell S. Craig, Koichi Hayashi and Özgür KozacıABSTRACTSeismic surveys were performed using a novel application of combined active and passive surface wave methods to evaluate the integrity of levees protecting islands in the Sacramento–San Joaquin Delta, California, USA, from flooding. Delta islands have been undergoing rapid subsidence during the past century due to farming practices that have led to the loss of a surficial peat layer. A large earthquake on any one of several active faults in the region could cause multiple levee failures and extensive flooding in the Delta. Surface wave surveys were carried out along the crest of levees using the active method (2D multichannel analysis of surface waves) and along the base of levees using the passive method (microtremor array method). This allowed 2D seismic S‐wave velocity (VS) profiles to be prepared for each site by combining shallow data from the active survey along the crown of the levee with deeper data from a passive survey along the toe of the levee. TheseVS profiles reveal a low‐velocity peat layer beneath the levee body that was confirmed by geotechnical borehole logs. Lateral variability of the levee was evaluated using average velocity to the base of the levee versus along‐levee distance.
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Ground vulnerability derived from the horizontal‐to‐vertical spectral ratio: Comparison with the damage distribution caused by the 2017 ML 5.4 Pohang earthquake, Korea
Authors Su Young Kang, Kwang‐Hee Kim, Mikyung Choi and Sun‐Cheon ParkABSTRACTThe city of Pohang in South Korea experienced substantial damage following the ML 5.4 earthquake on 15 November 2017. Damage surveys immediately after the earthquake revealed significant spatial variations in damage intensity across the epicentral area. We collected ambient noise data (three‐component seismic data) from 124 locations in Pohang, covering a total area of 17 km × 20 km, and applied the horizontal‐to‐vertical spectral ratio technique to estimate the resonance frequency, amplification factor and vulnerability of the ground to seismic shaking. Spatial variations exhibited by these three parameters are strongly correlated and consistent with variations in local geology. By comparing the distribution of the ground vulnerability index (Kg) with the damage distribution (or intensity) documented by the Korea Meteorological Administration following the earthquake, we confirm that the site effect is the prevailing effect that influences damage intensity in this region. We also demonstrate that Kg is an efficient proxy for identifying relatively fragile zones during ground shaking: in areas with a reported damage intensity of VIII, the proportion of the area with Kg > 20 is as high as 78.2%. Given that the study area has experienced a recent moderate earthquake, with significant economic consequences, our new results and analytical techniques will provide valuable information for future urbanization projects.
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Investigation of coseismic liquefaction‐induced ground deformation associated with the 2019 Mw 5.8 Mirpur, Pakistan, earthquake using near‐surface electrical resistivity tomography and geological data
ABSTRACTAn electrical resistivity tomography survey was conducted to assess the subsurface conditions associated with the coseismic liquefaction phenomenon in the epicentral region following the Mw 5.8 Mirpur earthquake (Pakistan) on 24 September 2019. The Mirpur earthquake produced extensive coseismic liquefaction‐induced surface deformations, including: sand blows, ground failure and lateral spreading along the Upper Jhelum Canal and in the nearby villages. Electrical resistivity data were acquired along three profiles and calibrated with available borehole data. The inverted electrical resistivity tomography profiles reveal three regional geoelectric layers, which consist of an upper 2–‐5‐m‐thick discontinuous zones of medium resistivity values ranging from 25 Ωm to 60 Ωm, underlain by a 7–8‐m‐thick zone of low resistivity (<10 Ωm) and a basal layer of high resistivity (> 100 Ωm). Based on geological and geophysical data, we infer that. (1) disrupted geoelectric layers in the shallow subsurface and spatially extended low electrical resistivity (<8 Ωm) layers document the elevated groundwater table due to sudden increase in pore‐water pressure triggered by the Mirpur earthquake. These lenses of high conductivity may represent potential hazards in the case of future earthquakes in the study area. (2) Fracture azimuths vary between 120° ± 15° and 335°–45° (subparallel and orthogonal to the strike of the Himalayan Frontal Thrust. (3) Common coseismic deformational features (e.g., sand blow and ground fracture) are located within the zone of maximum‐recorded ground shaking (intensity of VI) and underlain by Quaternary alluvial sediments. (4) Mega fractures (1.60 m wide and up to 187 m long) oriented parallel to the canal resulted from lateral spreading. We conclude that high resistivity structures extending from depth to the shallow subsurface resulted from either intrusion of air or eruption of sands from layer three. We suggest that high‐resolution geoelectrical imaging is a valuable complementary tool for evaluating the extent of subsurface liquefaction features and in understanding coseismic deformation during earthquakes, which can help with seismic hazard analysis and mitigation in seismically active regions.
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Geo‐integrated assessment of the landslide zone around Gadora along NH 58 of the Garhwal Himalayas, India
Authors Philips Omowumi Falae, Rajesh Kumar Dash, Manojit Samanta and D.P. KanungoABSTRACTThe Gadora village landslide zone situated along National Highway 58 has become very active in recent years. The landslide zone is causing a lot of damage to property along the highway and endangering the lives of people living uphill and downhill of the slopes. In the present study, a combined geological, geophysical and geotechnical investigation has been carried out to establish the subsurface layering system of the landslide. Undisturbed and disturbed samples were collected along the depth of the boreholes to determine the engineering properties of different layers. The subsurface layering and drainage patterns of the slope were inferred from electrical resistivity tomography conducted at different stretches of the slope. The present paper describes the layering system along with the engineering and physical properties obtained from the integrated geo‐investigation. The established near‐surface geometric, hydrological and material model enabled the stability study of the slope.
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Seismic refraction and electrical resistivity tomographies for geotechnical site characterization of two water reservoirs (El Hierro, Spain)
Authors Fabiola Fernández‐Baniela, Daniel Arias and Álvaro Rubio‐OrdóñezABSTRACTOn El Hierro (Canary Islands, Spain), the company Gorona del Viento El Hierro S.A. carried out the construction of a Wind‐Pumped Hydro Power Station in order to achieve the energy autonomy on the island through 100% renewable energy. The station consists of two man‐made water reservoirs, whose design was adapted to the island topography and emplaced in a valley and an inactive crater. In order to study the subsurface of both reservoirs, the characteristic heterogeneity of volcanic materials was taken into account. Therefore, a research campaign combining different methods was proposed; mechanical investigations traditionally applied in engineering (rotary boreholes, dynamic probing super heavy tests and pits) and geophysical methods (seismic refraction tomography and electrical resistivity tomography) were both utilized. The aims were to determine the geological structure of each site, detect possible anomalies or structures (i.e., faults, volcanic cavities, etc.), identify the different lithologies, determine the soil thickness and depth to bedrock, predict the excavatability of the materials, and obtain data about the physical properties and mechanical behaviour of the ground in an effort to carry out a geotechnical site characterization of both sites. At the site of the upper reservoir, electrical resistivity tomography recorded a high thickness of low resistivity soils, which were subsequently investigated by deep boreholes in order to sample and test them properly, since they could generate high settlements during the construction of the reservoir. At the site of the lower reservoir, the existing palaeorelief in the basaltic bedrock was detected by seismic refraction tomography, which allowed for the selection of the appropriate excavation machinery for the planned earthworks. The geological interpretation of the P‐wave velocity models and electrical resistivity models are shown in this paper, as well as a correlation between geotechnical parameters and geophysical properties of volcanic materials and recent deposits, which will be useful for other civil projects which may be carried out in the future at sites with similar geological conditions.
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Electrical resistivity monitoring of an earthslide with electrodes located outside the unstable zone (Pont‐Bourquin landslide, Swiss Alps)
Authors Grégory Bièvre, Denis Jongmans, Thomas Lebourg and Simon CarrièreABSTRACTIn the past decade, passive seismic methods have shown the possibility to detect significant changes in surface wave velocity up to several days prior to landslide failure, even with sensors located outside the unstable zone. Electrical resistivity tomography has also long been used to monitor hydrological changes in landslides. However, the displacement of electrodes relative to each other during landslide movement induces a modification of the geometric factors and, hence, of the apparent resistivity. The first objective of this work is to evaluate the possibility of monitoring the Pont‐Bourquin landslide (Swiss Alps) with electrodes located outside the unstable zone. The second objective is to monitor both seismic velocity and electrical resistivity to get insights into the evolution with time of mechanical and hydrological parameters, respectively. The sliding mass was first imaged in three dimensions to produce a resistivity starting model for the further inversion of time‐lapse data. Daily time series (235 days from February to November 2015) showed that changes are detected but cannot be spatially localized, in agreement with numerical simulation results. At the seasonal scale, resistivity and seismic time series are positively correlated with temperature and suggest a control by superficial water content. On the scale of a few days, geophysical parameters are negatively correlated with precipitation and suggest rapid infiltration of water into the ground. Although laboratory experiments show that no change in resistivity occurs during fluidization, and since no flow occurred during the monitoring period the evolution of resistivity during a flow event remains an open question.
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4D quantification of alpine permafrost degradation in steep rock walls using a laboratory‐calibrated electrical resistivity tomography approach
Authors Riccardo Scandroglio, Daniel Draebing, Maike Offer and Michael KrautblatterABSTRACTThe warming of rock permafrost affects mechanical stability and hydro‐cryostatic pressures in rock walls. The coincident decrease in slope stability frequently affects infrastructure by creep and subsidence and promotes the generation of rockfalls and rockslides. The increasing hazard posed by warming permafrost rock walls and the growing exposure of infrastructure and individuals create a demand for quantitative monitoring methods. Laboratory‐calibrated electrical resistivity tomography provides a sensitive record for frozen versus unfrozen bedrock, presumably being the most accurate quantitative monitoring technique in permafrost areas where boreholes are not available. The data presented here are obtained at the permafrost‐affected and unstable Steintaelli Ridge at 3100 m a.s.l. and allow the quantification of permafrost changes in the longest electrical resistivity tomography time series in steep bedrock. Five parallel transects across the rock ridge have been measured five times each, between 2006 and 2019, with similar hardware. Field measurements were calibrated using temperature‐resistivity laboratory measurements of water‐saturated rock samples from the site. A 3D time‐lapse inversion scheme is applied in the boundless electrical resistivity tomography (BERT) software for the inversion of the data. To assess the initial data quality, we compare the effect of data filtering and the robustness of final results with three different filters and two time‐lapse models. We quantify the volumetric permafrost distribution in the bedrock and its degradation in the last decades. Our data show mean monthly air temperatures to increase from −3.4°C to −2.6°C between 2005‒2009 and 2015‒2019, respectively, while simultaneously permafrost volume degraded on average from 6790 m3 (±640 m3 rock in phase‐transition range) in 2006 to 3880 m3 (±1000 m3) in 2019. For the first time, we provide a quantitative measure of permafrost degradation in unstable bedrock by using a temperature‐calibrated 4D electrical resistivity tomography. Our approach represents a fundamental benchmark for the evaluation of climate change effects on bedrock permafrost.
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Numerical simulation of ground‐penetrating radar data for studying the geometry of fault zone
ABSTRACTPalaeoseismology studies the footprints of ancient earthquakes to improve the knowledge about the modern seismicity of the territory. A ground‐penetrating radar (GPR), among other geophysical methods, is used for quick determination of shallow stratigraphy – displaced, oblique layers within the fault zone. GPR data interpretation from diverse and complex reflection patterns of the fault zone heavily depends on the interpreter's experience. The range of different fault zone parameters in which this method can be successfully applied has not yet been investigated. We used a numerical simulation of GPR data to determine how GPR images the elements of faults (fault plane, hanging wall, footwall) in comparison with other reflections. Furthermore, we studied which parameters have the most significant impact on GPR wave patterns. We performed a series of numerical models of a fault, changing its geometry with increasing complexity from elementary models to realistic ones. The resulting synthetic profiles allowed finding specific GPR signatures from the fault plane, the hanging wall and the footwall. We collected field GPR data from two different fault zones and examined them for verification.
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Volumes & issues
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Volume 22 (2024)
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Volume 21 (2023)
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Volume 20 (2022)
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Volume 19 (2021)
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Volume 18 (2020)
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Volume 17 (2019)
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Volume 16 (2018)
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Volume 15 (2017)
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Volume 14 (2015 - 2016)
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Volume 13 (2015)
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Volume 12 (2013 - 2014)
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Volume 11 (2013)
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Volume 10 (2012)
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Volume 9 (2011)
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Volume 8 (2010)
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Volume 7 (2009)
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Volume 6 (2008)
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Volume 5 (2007)
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Volume 4 (2006)
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Volume 3 (2005)
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Volume 2 (2004)
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Volume 1 (2003)