Post-seismic investigations repeatedly indicate that structures having frequencies close to foundation soil frequencies exhibit significantly heavier damages (Caracas 1967; Mexico 1985; Pujili, ...Ecuador 1996; L’Aquila 2009). However, observations of modal frequencies of soils and buildings in a region or within a current seismic risk analysis are not fully considered together, even when past earthquakes have demonstrated that coinciding soil and building frequencies leads to greater damage. The present paper thus focuses on a comprehensive numerical analysis to investigate the effect of coincidence between site and building frequencies. A total of 887 realistic soil profiles are coupled with a set of 141 single-degree-of-freedom elastoplastic oscillators, and their combined (nonlinear) response is computed for both linear and nonlinear soil behaviors, for a large number (60) of synthetic input signals with various PGA levels and frequency contents. The associated damage is quantified on the basis of the maximum displacement as compared to both yield and ultimate post-elastic displacements, according to the RISK-UE project recommendations (Lagomarsino and Giovinazzi in Bull Earthq Eng 4(4):415–443,
2006
), and compared with the damage obtained in the case of a similar building located on rock. The correlation between this soil/rock damage increment and a number of simplified mechanical and loading parameters is then analyzed using a neural network approach. The results emphasize the key role played by the building/soil frequency ratio even when both soil and building behave nonlinearly; other important parameters are the PGA level, the soil/rock velocity contrast and the building ductility. A numerical investigation based on simulation of ambient noise for the whole set of 887 profiles also indicates that the amplitude of
H
/
V
ratio may be considered as a satisfactory proxy for site amplification when applied to measurements at urban scale. A very easy implementation of this method, using ambient vibration measurements both at ground level and within buildings, is illustrated with an example application for the city of Beirut (Lebanon).
Graphical abstract
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The H/V-noise technique is now widely used to estimate site effect parameters (fundamental frequency and sometimes the associated soil amplification), and many surveys using this technique have ...provided convincing results. However, a general agreement on a methodology for data acquisition, data processing and result interpretation has yet to be found. H/V measurements from ambient noise recordings imply both reliability of the results and rapidity of data collection. It is therefore important to understand which experimental conditions (1) influence data quality and reliability, and (2) can help speeding up the recording process. Within the framework of the SESAME European project, a specific task was defined to investigate the reliability of the H/V spectral ratio technique in assessing the site effects. The aim of WP02, one specific Work Package of the SESAME project, is to study the effects of experimental conditions on both stability and reproducibility of H/V results. This study has been conducted in a purely experimental way, by testing the possible influence of various experimental conditions on H/V results both on the frequency peak value and on its amplitude. WP02 results help setting up the experimental conditions under which ambient noise recordings have to be performed in order to provide reproducible, reliable and meaningful H/V results. In this paper we present the results of the WP02 SESAME project concerning the evaluation of the influence of experimental conditions of ambient noise recording on H/V results.
The morphological boundary between the Himalayas and the foreland plain is well expressed and most often corresponds to the frontal emergence of the Main Himalayan Thrust (MHT). This boundary is ...affected by surface ruptures during very large Himalayan earthquakes (Mw > 8) that regularly induce (with a recurrence of the order of 500 to 1200 years) the uplift of the foothills relative to the plain.
However, a thrust-fold system is hidden beneath the plain and is displayed by the seismic profiles of oil companies in east/central Nepal and by H/V passive geophysical techniques in Darjeeling. Its long-term kinematic evolution is slow, with a tectonic uplift of the hanging wall that is lower than the subsidence rate of the foreland basin, that is, less than approximately half a millimetre per year. During phases of low sedimentation controlled by climatic fluctuations, the morphological surfaces of the piedmont are incised by large rivers for several tens of metres; therefore, structures hidden under the sediments emerge slightly in the plain.
The evolution of the hidden structures corresponds to an embryonic thrust belt mainly affected by a long-term shortening rate of 1.4 +2.5/−1.2 mm·yr−1, that is, 2–20% of the shortening rate of the entire Himalayan thrust system. Nonetheless, the details of the deformation associated with the embryonic thrust belt are still poorly understood. Several deformation components could affect the central Himalayan and Darjeeling piedmonts. i) Any slow steady-state deformation, such as layer parallel shortening (LPS) is not detected by Global Navigation Satellite System (GNSS) data, and such deformation would therefore absorb less than 0.5 mm·yr−1. The geodetic data that suggest the aseismic growth of some of the structures are highly controversial. ii) For the rest of the deformation of the embryonic thrust wedge, it is yet to be proven whether deformation occurs during rare great earthquakes affecting the piedmont during medium earthquakes and/or during post-seismic deformation related to great earthquakes. The amplitude of this long-term low deformation is too limited to significantly reduce the seismic hazard in the seismic gaps of the Himalayan belt. iii) In some portions of the Himalayan front, such as Darjeeling (India), the thrust deformation related to great earthquakes propagates several tens of kilometres south of the morphological front in the zone previously affected by the long-term low deformation. It induces multi-metre surface ruptures in the piedmont and a mean shortening of 8.5 ± 6.2 mm·yr−1. iiii) Pre-existing faults in the bedrock of the Indian craton, often oblique to the Himalayan structures, are locally reactivated beneath the foreland plain with low deformation rates.
Environmental seismology consists in studying the mechanical vibrations that originate from, or that have been affected by external causes, that is to say causes outside the solid Earth. This ...includes for instance the coupling between the solid Earth and the cryosphere, or the hydrosphere, the anthroposphere and the specific sources of vibration developing there. Environmental seismology also addresses the modifications of the wave propagation due to environmental forcing such as temperature and hydrology. Recent developments in data processing, together with increasing computational power and sensor concentration have led to original observations that allow for the development of this new field of seismology. In this article, we will particularly review how we can track and interpret tiny changes in the subsurface of the Earth related to external changes from modifications of the seismic wave propagation, with application to geomechanics, hydrology, and natural hazard. We will particularly demonstrate that, using ambient noise, we can track 1) thermal variations in the subsoil, in buildings or in rock columns; 2) the temporal and spatial evolution of a water table; 3) the evolution of the rigidity of the soil constituting a landslide, and especially the drop of rigidity preceding a failure event.
•Ambient seismic noise based monitoring reveals feeble perturbations of the subsurface.•Mechanical perturbations are related to temperature, stress, hydrological changes, or to damage.•Seismic noise from rivers and glaciers reveal new sources of seismic activity.•Glacier and fluvial seismology shed a new light on the mechanical process at work.
The estimation of seismic damages and debris at the urban scale—at a building-by-building level- is challenging for several reasons. First, commonly used methodologies for seismic damage estimation ...rarely take into account the local site effects, precisely at the building-level. Second, the available methods for debris estimation fail to estimate at the same time the quantity of debris generated per building according to its damage level and the distribution of the debris (extent and height) around buildings. Finally, the lack of comprehensive data on the building stock and the relevant building properties and their taxonomy further increases the complexity of assessing possible earthquake consequences at an urban scale. This paper addresses these challenges and proposes improvements to the assessment of seismic damages and debris from building-level simulations, along with the development of a 3D building model based on satellite images and heterogeneous data. These developments, applied to the city of Beirut, Lebanon, highlight the control of the site effects on the seismic damage’s spatial distribution throughout the city and the large volume and extent of debris to be expected in the city for a strong earthquake.
Resonance period is a key parameter in the seismic design of a structure, thus dynamic parameters of buildings in Beirut (Lebanon) were investigated based on ambient vibration method for risk and ...vulnerability assessment. Lebanon is facing high seismic hazard due to its major faults, combined to a high seismic risk caused by dense urbanization in addition to the lack of a seismic design code implementation. For this study, ambient vibration recordings have been performed on 330 RC buildings, period parameters extracted and statistically analyzed to identify correlations with physical building parameters (height, horizontal dimensions, age) and site characteristics (rock sites or soft sites). The study shows that (1) the building height or number of floors (N) is the primary statistically robust parameter for the estimation of the fundamental period T; (2) the correlation between T and N is linear and site dependent: T ≈ N/23 for rock sites and N/18 for soft sites; (3) the measured damping is inversely proportional to the period: the taller the building the lower is the damping; (4) a significant overestimation of the period exists in current building codes. However part of the large discrepancy with building code recommendations may be due to the very low level of loading.
Building frequencies (fundamental and higher modes) are a critical parameter especially in the field of structural health monitoring mainly based on the stability of the structural dynamic parameters ...of individual building (frequencies, damping and modes shape). One of the most used methods to find out these parameters is based on the use on ambient vibration analysis. In this work, we study the fluctuations over a month period of the fundamental frequencies (transverse and longitudinal) of a 3.5-story RC-building made of 2 identical units connected by a structural joint. Time independent building frequencies is a strong assumption; as illustrated by our experiment showing that over an observation period of a month, building frequencies fluctuate of about 3.5 %. A clear correlation is found between the building frequency fluctuations and temperature variations, with a phase-shift interpreted as the characteristic time of heat diffusion within the walls. This allows: (1) determining the thermal diffusivity of the structure, (2) inferring its relative stiffness variations, and (3) showing that its Young modulus varies linearly with temperature.
For an optimal analysis of the H/V curve, it appears necessary to check the instrument signal to noise ratio in the studied frequency band, to ensure that the signal from the ground noise is well ...above the internal noise. We assess the reliability and accuracy of various digitizers, sensors and/or digitizer-sensor couples. Although this study is of general interest for any kind of seismological study, we emphasize the influence of equipment on H/V analysis results. To display the impact of the instrumental part on the H/V behavior, some series of tests have been carried out following a step-by-step procedure: first, the digitizers have been tested in the lab (sensitivity, internal noise...), then the three components sensors, still in the lab, and finally the usual user digitizers-sensors couple in lab and outdoors. In general, the digitizer characteristics, verified during this test, correspond well to the manufacturer specifications, however, depending on the digitizer, the quality of the digitized waveform can be very good to very poor, with variation from a channel to another channel (gain, time difference etc.). It appears very clearly that digitizers need a warming up time before the recording to avoid problems in the low-frequency range. Regarding the sensors, we recommend strongly to avoid the use of “classical” accelerometers (i.e., usual force balance technology). The majority of tested seismometers (broadband and short period, even 4.5 Hz) can be used without problems from 0.4 to 25 Hz. In all cases, the instrumentation should be checked first to verify that it works well for the defined study aim, but also to define its limit of use (frequency, sensitivity...).
The study of the variation over time of both spectral amplitudes and H/V curves, has been performed on three different sites, two close to cities and one in the countryside, during periods varying ...from week to over a month. It demonstrates the robustness of the H/V technique to give consistent peak frequency values. In particular, H/V peak frequencies, either fundamental (f
0
) or natural (f
x
, x*1), are not affected by weather nor the level of human activity. However, while fundamental H/V peak amplitudes are stable, they proved rather unstable for natural (secondary) peak. Spectral amplitude curves are very variable but follow human activity cycles from week-week end and day-night variations down to a very small scale, such as lunch breaks. Finally, the frequency limit between anthropic noise and natural noise, commonly taken at 1 Hz, is not straightforward and is varying from site to site from 0.7-0.8 Hz up to 2-3 Hz.
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