Accelerometric Stations Research Articles

Collapse of the Dharahara Tower During the April 25, 2015 Nepal Earthquake: A New Interpretation Based on Video-Camera Footage

ABSTRACT Among the various important monuments impacted by the April 25, 2015 Mw7.8 Nepal earthquake, the Dharahara Tower located in Kathmandu suffered complete collapse. The collapse of the tower, a slender 60-meter-high old brick masonry structure with an almost cylindrical hollow shape, was captured by a low-quality video camera, which nonetheless helped to identify the critical moments. Initially, the tower descended more than 10 m vertically after a rupture at the base. After a few seconds, it overturned, disintegrating upon impact on the ground. At a distance of 1.3 km, an accelerometric station recorded the strong motion which was used as input of a discrete element model of the structure. Mechanical properties were taken from lab tests made on similar building materials. The Dharahara Tower had previously been analysed by other researchers using various methods. However, since they were apparently not aware of the mechanism observed in the video footage, their collapse models do not agree with this new evidence. For the initial phase of the collapse, the present model explains with great accuracy the observations of the video footage, but the final phase is dependent on various details, and modelling is not so robust.

Recognition of the causative fault of the 2017 MW 4.9 Malard (Tehran, Iran) earthquake from directivity analysis of the recorded ground motions

On December 20, 2017, a light shallow earthquake (Mw 4.9) with a purely strike-slip mechanism occurred on a hidden unknown fault, 30 km west of the city of Tehran. The purpose of this study is to determine the causative fault plane based on the directivity effect that was observed during this shallow crustal depth earthquake, referred to as the Malard earthquake. This was achieved by using data from 96 seismic and accelerometer stations, and employing a variety of methods.For the analysis of the directivity effect, we employed methods including empirical Green's function deconvolution in the frequency domain, inversion of corrected ground motions based on empirical models, and comparison of relative peak ground acceleration and velocity between the mainshock and the largest aftershock. All approaches indicate that the Malard earthquake occurred on a previously unknown fault with a strike of 71°, a steep southward dip, and a confirmed left-lateral strike-slip mechanism, as evidenced by the analysis of aftershock distribution. The earthquake exhibited a strong directivity effect, with rupture propagating unilaterally from the hypocenter to the southwest at a velocity of 2.5 km/s.In addition to enhancing our understanding of active earthquake sources in the densely populated areas of Iran, the findings of this study will also contribute to future earthquake risk assessments in the Tehran region. However, it's important to note that Tehran was situated in the backward zone of the Malard earthquake rupture, where the least ground motion was recorded. Future earthquakes on this fault may exhibit different propagation patterns, toward the city. The determination of a preferred propagation direction can only be achieved probabilistically through the study of a substantial number of earthquakes in the region. The absence of recorded ground motion data will lead to significant uncertainty in predicting the propagation direction of future earthquakes, which must be taken into consideration during hazard analysis.

Seismic background noise levels in the Italian strong-motion network

Abstract. The Italian strong-motion network monitors the seismic activity in the region, with more than 585 stations with continuous data acquisition. In this study, we determine the background seismic noise characteristics of the network by using the data collected in 2022. We analyse the spatial and temporal characteristics of the background noise. It is found that most of the stations suffer from anthropogenic noises, since the strong-motion network is designed to capture the peak ground motions in populated areas. Hence, human activities enrich the low periods of noise. Therefore, land usage of the area where the stations are located affects the background noise levels. Stations can be noisier during the day, up to 12 dB, and during the weekday, up to 5 dB, in short periods. In long periods (≥ 5 s), accelerometric stations converge to similar noise levels and there are no significant daily or weekly changes. It is found that more than half of the stations exceed the background noise model designed for strong-motion stations in Switzerland by Cauzzi and Clinton (2013) in at least one of the calculated periods. We also develop an accelerometric seismic background noise model for periods between 0.0124 and 100 s for Italy by using the power spectral densities of the network. The model is in agreement with the background noise model developed by D’Alessandro et al. (2021) using broadband data for Italy in short periods, but in long periods there is no correlation among studies.

Field Investigation on Reinforced Concrete and Masonry Buildings Damages after November 23, 2022 Gölyaka (Düzce) Earthquake

A strong earthquake with a moment magnitude of [Formula: see text].9 according to the Disaster and Emergency Management Authority (AFAD) occurred at 04:08 local time on November 23, 2022, in Gölyaka district of Düzce province that is located in the Western Black Sea region of Turkey. The earthquake with a shallow focus is 0.82 km away from Kemeryani village of Gölyaka district, which is the nearest settlement. According to observation data, the highest acceleration value of the earthquake was measured as 407.76 gal (0.410 g) in the East-West component measured from the 8102 coded Düzce Central accelerometer station, which is one of the five accelerometer stations closest to the earthquake zone. There was no loss of life in the earthquake, in which 93 people were injured. However, damages occurred due to the earthquake in structures that were built before the year 2000 and did not receive good engineering and supervision services. It has been determined that the structures built after the year 2000 received good engineering and supervision services performed better. The purpose of this study is to ensure that the experience gained as a result of the investigations about the earthquake that occurred will be made permanent and usable. For this purpose, damage and collapses that occurred on the buildings due to earthquake are examined based on field investigation. The types and causes of damage that occurred on both reinforced concrete and masonry buildings are evaluated in detail with considering the conditions of the existing Turkey Building Earthquake Design Code. The seismological characteristics of the earthquake are also evaluated. In addition, the earthquake hazard of Düzce Province is evaluated together with the earthquakes experienced in the past years, which caused significant loss of life and property in the region.

Combination of passive and active methods towards site characterization of accelerometer stations in Greece

AbstractThe strong correlation between earthquake damage and the local geological conditions in an area has long been established, and site characterization studies are common practice for realistic estimation of seismic hazard and mitigation of the respective seismic risk. Estimation of the time‐averaged S‐waves velocity of the topmost 30 m (VS30) is recognized as a basic proxy for site characterization and has been incorporated in several building codes. In our work, we examine the inversion of single‐station ambient noise horizontal to vertical spectral ratio (HVSR) curves to estimate the local one‐dimensional (1D) VS(Z) structure. The inversion is performed based on the diffuse field assumption, which allows for the inversion of HVSR, including the contributions of both body and surface waves. Although ambient noise data acquisition is immensely cost‐effective, HVSR curve inversion is subject to the solution non‐uniqueness issue, offering ambiguous results. To provide a possible solution for this matter, additional near‐surface geophysical methods such as the multichannel analysis of surface waves and the electrical resistivity tomography (ERT) were applied to derive information on the shallow subsurface structure. The acquired 1D seismic velocity profile was implemented as initial model in the HVSR inversion to constrain the velocities of the upper meters. Additionally, the inferred stratigraphy from the ERT electrical profile was utilized to constrain the thicknesses of the layers. Tests were conducted with and without a priori information from complementary techniques to explore how the HVSR inversion procedure is facilitated. The proposed methodology was applied at the locations of six accelerometers in the city of Thessaloniki, northern Greece, under different geological conditions. The reliability of the inverted VS(Z) models was checked by performing 1D numerical simulations of ambient noise waveforms generated by distributed sources and by direct comparison between synthetic ambient noise HVSR and single‐station earthquake HVSR curves.

Investigation of site effect of Lake Van region (eastern Turkey) by using strong ground motion records

The site effect plays a very important role in determining the structural damage that may occur in a possible major earthquake. In this study, seventy-nine strong ground-motion records (M > 4.5) from past earthquake events in the Lake Van region, recorded by 14 accelerometer stations located on dissimilar geological units, were analyzed for local soil properties using horizontal-to-vertical spectral ratio (EHVSR) method. In addition, EHVSR results were compared with Standard Spectral Ratio (SSR) results, which is another method used to determine soil amplification. The obtained predominant period, amplification factor, seismic vulnerability index (Kg) and bedrock depth values were examined. The EHVSR results show that the predominant frequencies of the sites and the amplification factor values were estimated as 0.58–5.77 Hz and 1.84–7.5, respectively. The seismic vulnerability index (Kg) values, which are crucial parameter in hazard assessment, change from 1.6 to 30. The bedrock depths range between 11 and 186 m for the study area. From the results of this study it is found that the damage may be high after a possible destructive earthquake in the study area where the predominant period, amplification, Kg values and bedrock depth are high.

Seismic Station Installations and Their Impact on the Recorded Signals and Derived Quantities

Abstract The role of local geology in controlling ground motion has long been acknowledged. Consequently, increasing attention is paid to the assessment of the geophysical properties of the soils at the seismic stations, which impact the station recordings and a series of related quantities, particularly those referring to seismic hazard estimates. Not the same level of attention is commonly dedicated to the seismic station installation, to the point that it is generally believed that housings and shelters containing seismic instruments are of no interest, because they can only affect frequencies well above the engineering range of interest. Using examples from seismometric and accelerometric stations, we describe the (1) housing, (2) foundation, and (3) pillar effects on the seismic records. We propose a simple working scheme to identify the existence of potential installation-related issues and to assess the frequency fidelity range of response of a seismic station to ground motion. Our scheme is developed mostly on ambient noise recordings and, thus, surface waves. The hope is that, besides the parameters that start to be routinely introduced in the seismic archives (VS30, soil classes, etc.), the assessment of the maximum reliable frequency, under which no soil–structure interaction is expected, also becomes a mandatory information. In our experience, for some installation sites, the maximum reliable frequency can even be less than a very few hertz.

Seismic site classification of the Costa Rican Strong-Motion Network based on VS30 measurements and site fundamental period

In this paper, a new seismic site classification for the Costa Rican Strong-Motion Network (CRSMN) is proposed. The soil profile classification of the Costa Rican Seismic Code based on the average shear-wave velocity of the top 30 m (VS30) is used as a reference. The site fundamental period (Tf) is included as a parameter to complement the existing characterization. For this, the VS30 measurements from 52 accelerometric stations are related to the site fundamental period obtained through horizontal-to-vertical spectral ratios (HVSR) using ground motion records from the Costa Rican Strong-Motion Database. The H/V ratios are estimated with 5% damped acceleration response spectra and with traditional Fourier amplitude spectra from the S-wave window. From the relation between VS30 and Tf, different ranges of Tf are assigned to the existing soil profile classification and a graph with three-lines and four-areas is proposed to classify the stations of the CRSMN.

Semi-empirical estimation of the Zagreb ML 5.5 earthquake (2020) ground motion amplification by 1D equivalent linear site response analysis

The 22 March 2020 Zagreb ML 5.5 earthquake ground shaking resulted in damage to buildings and infrastructure. The most affected buildings were older and cultural heritage buildings (built before 1963) in the old city centre with significant damage extent in the epicentral zone (southeastern foothills of Medvednica Mt.). This study presents site response analysis on the realistic site profiles from the epicentre towards the accelerometric stations QUHS and QARH and comparison with strong motion data recorded during the Zagreb 2020 earthquake. Semi-empirical estimation of the ground motion amplification (i.e., peak ground acceleration at surface) showed that modelled and recorded values are comparable. Moreover, we present 2D model of peak ground acceleration at surface (PGAsurf ) variation for the superimposed site profile from the epicentre towards two accelerometric stations. Ground motion amplification for the Zagreb ML 5.5 earthquake scenario showed that PGAsurf is larger by a factor of 2 than the bedrock value (approx. 0.35 g in the epicentre and 0.20 g on the 12 km distant accelerometric station). This study is a contribution to better understanding of the Zagreb ML 5.5 earthquake effects and significance of local site effects in the damage extent, something that combined with older and heritage buildings resulted in high economic consequences. Therefore, it is important that site-specific ground motion simulation and seismic microzonation of the Zagreb continues with installation of an accelerometric array. This is very important for earthquake retrofitting and resilience of the low, mid- and high-rise buildings with particular care of cultural and historical buildings as well for the further urban planning.

The 2017 Ischia Earthquake (Southern Italy): Source Mechanism and Rupture Model From the Inversion of a Near-Source Strong Motion Record

With the aim to investigate the rupture complexity and the radiated wave field of the 2017, Mw 3.9, Ischia earthquake, south-west of Naples (Italy), we used finite-fault modeling to invert the near-source (< 1-km epicentral distance) horizontal-component velocity records of the accelerometric station (IOCA) and searched for the best-fit kinematic rupture parameters. This analysis showed that the rupture nucleated at about 600 m west of IOCA and 1.1-km depth, along a 1 km, NW-SE striking fault (i.e., thrust with right-lateral component), with a rupture velocity of about 0.7 km/s. The retrieved rupture model coupled with multipath reverberations effects related to a thin, low-velocity near-surface volcanic sedimentary layer, well explains the observed long ground motion duration and the large amplitudes recorded all over the island. Finally, the apparent source time function (STF), obtained from inverse modeling using a theoretical Green’ function (GF), is validated by implementing an empirical GF (EGF) analysis.

Improved correlation between foundation and free‐field ground motions through strong motion recordings and kinematic soil–structure interaction analyses

AbstractThe present study explores the effect of soil–structure interaction (SSI) on the foundation motion as recorded at accelerometric stations located at the basement of buildings through a large dataset employed from sites where both foundation and free‐field ground motion recordings are available. The significance of such instrumentation is highlighted, in terms of assessing the high frequency filtering that occurs at the foundation level and its potential implication on ground motion prediction models (GMPMs) that are produced with these datasets. Based on the recorded data, analytical expressions relating foundation, and free‐field motion intensity measures, which have been produced in a previous work, are verified. A sub‐structure analysis procedure is extended to include the site and building characteristics of the recorded data. Nonlinear regression analyses are performed, utilizing numerous analysis results, to derive improved, robust analytical expressions, as well as, to include building characteristics. Residual analysis is performed to assess possible bias due to other variables, as well. The produced analytical expressions are validated against the recorded data and compared to the existing expressions in terms of prediction errors. The produced analytical expressions can be utilized for correcting motions recorded at the basement level of buildings to obtain estimates of free‐field ground motions. The latter being “building‐free” is most appropriate for the development of new generation GMPMs that are not biased by the influence of kinematic and inertial interaction. The impact of correcting ground motion data through the proposed analytical expressions is discussed through their implementation on recently published strong motion dataset.

A GIS procedure for the topographic classification of Italy, according to the seismic code provisions

In this study we present a procedure for the topographic classification of Italy, taking advantage of existing high-resolution digital elevation models (DEM), with the support of routines embedded in Geographic Information Systems. The proposed method is based on morphometric analyses of a DEM, allowing for hilltop ridge detection, slope computation, identification of reliefs with potential topographic site effects, and topographic classification according to the indications of the current European and Italian seismic codes. The developed procedure can be applied worldwide and has a potential engineering interest for a fast and accurate topographic classification of a site, for both scientific and application purposes. The topographic classification of Italy is tested on several well-known cases of topographic amplification and it is adopted for the topographic classification of Italian accelerometric stations.

Back-analysis of the Collapse of a Tetrastyle Canopy during the April 25, 2015 Nepal Earthquake

ABSTRACT A monumental tetrastyle canopy (four columns topped by a hollow cap overlaying a statue), built in 1948 at the centre of Kathmandu, Nepal, has suffered full collapse during the April 25, 2015 Mw7.8 earthquake. Two video camera recordings let us recover the most important movements of the structure. At a distance of 2.9 km from the monument, an accelerometric station (KATNP) recorded the motion which we used as input of a model structure made with DEM. Geometry and dimensions of main elements were mostly obtained from satellite images. Concrete and steel in columns were considered as the common practice at that time. We performed some sensitivity analyses varying the mechanical properties due to the possibility of aging alterations in concrete and steel and to incorporate uncertainties on the geometric characteristics. Albeit the response has a significant variability, results show that it is possible to reproduce well the video images, attesting the importance of video cameras to capture the dynamic performance of structures, providing insight into their behaviour and data to support numerical investigations.

Case Study of a Heavily Damaged Building during the 2016 MW 7.8 Ecuador Earthquake: Directionality Effects in Seismic Actions and Damage Assessment

In this work, the directionality effects during the MW 7.8 earthquake, which occurred in Muisne (Ecuador) on 16 April 2016, were analyzed under two perspectives. The first one deals with the influence of these effects on seismic intensity measures (IMs), while the second refers to the assessment of the expected damage of a specific building located in Manta city, Ecuador, as a function of its azimuthal orientation. The records of strong motion in 21 accelerometric stations were used to analyze directionality in seismic actions. At the closest station to the epicenter (RRup = 20 km), the peak ground acceleration was 1380 cm/s2 (EW component of the APED station). A detailed study of the response spectra ratifies the importance of directionality and confirms the need to consider these effects in seismic hazard studies. Differences between IMs values that consider the directionality and those obtained from the as-recorded accelerograms are significant and they agree with studies carried out in other regions. Concerning the variation of the expected damage with respect to the building orientation, a reinforced concrete building, which was seriously affected by the earthquake, was taken as a case study. For this analysis, the accelerograms recorded at a nearby station and detailed structural documentation were used. The ETABS software was used for the structural analysis. Modal and pushover analyses were performed, obtaining capacity curves and capacity spectra in the two main axes of the building. Two advanced methods for damage assessment were used to obtain fragility and mean damage state curves. The performance points were obtained through the linear equivalent approximation. This allows estimation and analysis of the expected mean damage state and the probability of complete damage as functions of the building orientation. Results show that the actual probability of complete damage is close to 60%. This fact is mainly due to the greater severity of the seismic action in one of the two main axes of the building. The results are in accordance with the damage produced by the earthquake in the building and confirm the need to consider the directionality effects in damage and seismic risk assessments.

The 2016 Central Italy seismic sequence: linear and non-linear interpretation models for damage evolution in S. Agostino’s church in Amatrice

An extensive numerical investigation of the structural behavior of the masonry church of Sant’Agostino in Amatrice (Italy) during the 2016–2017 seismic sequence is presented. The seismic performance of the church is studied in relation to the most energetic shocks of the sequence, occurred on August 24th, 2016, and October 30th, 2016, whose seismic parameters (PGA, Trifunac duration, Arias intensity, destructiveness potential factor) are analyzed in detail. In particular, a comparison between ground motion data recorded by different stations is presented in order to evaluate the influence of local amplification effects on the structural behavior. To achieve this goal, the ground motion data recorded by the permanent accelerometric station (AMT) located in Amatrice on type B soil and belonging to the National Accelerometric Network (RAN) are firstly considered. Then, they are critically compared with those provided by an accelerometric station temporarily installed after the first main shock in the downtown area close to the church site, showing a clear amplification due to the site effects. The dynamic behavior of S. Agostino’s church is numerically investigated by carrying out a set of non-linear dynamic analyses on the 3D finite element model of the church developed in Abaqus CAE and considering as seismic input both the accelerograms recorded by the AMT fixed station and those including the local amplification effects. Non-linear properties of materials are taken into account by defining an elasto-plastic constitutive law with reasonable damage parameters in tension. Numerical results are evaluated in terms of displacement time-history of some relevant control points located on the masonry walls and maps of tensile damage cumulated at the end of the simulations. Numerical results are in good agreement with the evidence based behavior of the structure, thus validating numerical simulations as effective tools for seismic analysis of masonry buildings. Furthermore, a hypothetical strengthening intervention concerning the improvement in masonry mechanical properties is implemented in the model to investigate its positive role in preventing collapse. In the latter case, the approach provides useful hints in interpreting the behavior of the church under the application of repeated accelerograms.

Intensity and damage statistics of the September 19, 2017 Mexico earthquake: Influence of soft story and corner asymmetry on the damage reported during the earthquake

A destructive intraslab earthquake occurred in Mexico City on September 19, 2017 (Mw 7.1), causing significant damage and hundreds of human losses not only in the epicentral area, but also in the States of Morelos, Puebla, Mexico and in Mexico City. Only in Mexico City itself, around 230 people died, and more than 40 buildings collapsed. The intensities recorded in some lakebed areas of the city, especially in zones with soil periods around 1.5 s, were relatively high, even surpassing spectral values of 1.0 g; the vertical component, due to the proximity of the earthquake, was unusually high for Mexico City. The 2017 earthquake raised questions critical to understanding the city’s seismic vulnerability and resilience, and they are partly answered in this article. Using 77 accelerometric stations, the amplification pattern of the seismic intensities is characterized as well as the correlations of buildings structural characteristics with the site effects. A comprehensive statistical analysis of the damages is shown to analyze and understand the structural behavior of damaged buildings. It is including not only the structural types and the year of construction, but also the main structural problems identified (structural pathologies), such as irregularities, both in elevation and plan, soft story, and corner effect. The building damage database was constructed with 2125 reports of buildings carried out by universities and engineering associations after the earthquake, of which 543 had severe damage. It is also included the information of all buildings with no damage in the city thanks to the cadastral information provided by the Mexico City government, and post-earthquake inspections and visual inspections using Google Street View. A full study of selected neighborhoods, which compares similar buildings with and without damage, is included, yielding relevant statistical information on which pathologies cause more damage and even collapses.

The achieved result of the introduction of the universal accelerometric control station and further prospects for determining railcar wheel thread defects

The achieved result of the introduction of the universal accelerometric control station and further prospects for determining railcar wheel thread defects

Subsoil Characteristics of Mexico City, acceleration and hysteretic energy spectra for the Mexico Earthquake of September 19, 2017

The September 19, 2017 intraslab earthquake (Mw7.1), whose epicenter was located near the limits between the states of Puebla and Morelos at approximately 120km from Mexico City, caused severe damage in these regions. In Mexico City more than 40 buildings collapsed, and hundreds had moderate to severe damage; dozens of them are to be demolished. This article analyzes the spectral ratios of accelerometric stations in the lake-bed of Mexico City with respect to the average Fourier spectra at hill zone sites in order to study and compare over time the changes in the behavior of local effects and their relationship with the damage presented during this earthquake; these ratios exhibit also the settlement problem in some places in the city due to over exploiting the aquifer for water supply purposes. Finally, pseudoacceleration and hysteretic energy maps for Mexico City with a discussion with a possible correlation with reported damages are presented.

Analytical expressions relating free-field and foundation ground motions in buildings with basement, considering soil-structure interaction

The present study investigates the effect of soil-structure interaction (SSI) on foundation motion recorded at accelerometric stations at the lowest storey of buildings. Towards this, four sites of instrumented buildings, having foundation and free-field ground motion recordings available, are studied in the frequency domain. The significance of such instrumentation is highlighted, especially in terms of assessing the way foundation tends to filter high frequency components of free-field motions. The well-known kinematic and inertial interaction decoupling is implemented for the sites studied and is assessed on the basis of its capability to reproduce the empirical findings and measurements. Parametric analyses are performed for each site in order to develop analytical expressions relating intensity and frequency content measures between the two motions. The expressions derived are verified through comparison with recorded data. The analysis procedure is then extrapolated to sites with various soil conditions as defined by a generic ground classification as per Eurocode 8. The produced analytical expressions can be utilized, under the conditions studied herein, for correcting motions recorded at the basement level of buildings, in order to estimate the “original” free-field ground motions. The latter being “building-free”, are most appropriate for the development of new generation GMPEs that are not biased by the influence of foundation presence and structural oscillation during ground motion recordings.

On the modal response of an instrumented steel water-storage tank including soil-structure interaction

The modal response of liquid-storage steel tanks and the associated prevailing frequencies in the horizontal impulsive mode of vibration are explored by means of earthquake recordings. To this end, a well-documented case study is employed, referring to an instrumented steel cylindrical tank which is a property of the HEL.PE. Oil Company in the Kalochori area, west of Thessaloniki in Northern Greece. The 28.5m-diameter tank has a self-supporting dome roof fixed to the tank walls and it is partially filled with water, corresponding to a ratio of the liquid free surface height to the tank's radius (H/R) at 0.74. Two accelerometric stations are mounted close to the center of the tank roof and on the ground surface at free-field conditions. Seventy four (74) acceleration records from low-intensity earthquake motions are processed to identify the fundamental frequencies of the tank and of the foundation soil from pertinent transfer functions in the frequency domain. The experimental findings are compared with predictions from theoretical formulae recommended by seismic design codes for fixed-base and flexibly-supported tanks, thus allowing the assessment of soil-structure interaction effects on the fundamental period of the tank. A simplified three-dimensional finite-element model of the fluid-tank system resting on flexible supports is also analyzed to provide further insight on the recorded response, as affected by critical structural features of the tank. Results highlight a dominant rigid-body mode which is associated to the lateral fundamental period of the flexibly-supported system, in agreement with relevant code provisions for tanks with H/R below unity.