Vertical Component Of Ground Motion Research Articles

A Predictive Equation for Vertical‐to‐Horizontal Response Spectral Ratios in Northern Iran

Abstract The importance of the vertical component of ground motion in damage infliction on a variety of structural systems has been shown in recent years. However, the available models for ground‐motion prediction equations (GMPEs) for this component of earthquakes are limited, and most of the existing GMPEs have been specifically developed for the horizontal component of ground motion. Alternatively, in some cases, it is proposed to scale the horizontal spectrum of the earthquake to the vertical one using vertical‐to‐horizontal response spectral ratios (VHSRs). In this work, the applicability of some global and regional VHSR models for northern Iran are evaluated using several existing statistical approaches. The results of these evaluations show inadequate correlation between these models and the observed VHSRs in this region. Consequently, a new VHSR model has been proposed to convert the horizontal earthquake spectrum to the vertical one in the studied region. In order to determine the regression coefficients for the proposed VHSR model, some 712 three‐component records (with distances less than 300 km from the source) from 79 earthquakes with the magnitude ranging from M w 4.1 to 7.3 were used. Regression coefficients for peak ground acceleration and pseudospectral acceleration (for 5% damped systems) are determined using a random‐effect approach. A series of comparative studies on the proposed model with respect to the existing VHSRs was performed using the median residuals for the spectral periods in the 0.1–3 s range. The results of such investigation show significant reduction of the variability for the proposed model (at least 50%) compared with the other VHSRs. Online Material: Database of strong‐motion records.

Effects of site condition in near-fault area on the nonlinear response of fire-damaged base-isolated structures

Structural damage due to resonance, when the predominant vibration periods of both site and structure are close, is expected in the case of base-isolated structures subjected to near-fault earthquakes. Little is known, however, about the effects of different site conditions on the nonlinear seismic response of fire-weakened base-isolated structures. To analyze this aspect, eight five-storey reinforced concrete (r.c.) office buildings, base-isolated with High-Damping-Laminated-Rubber Bearings (HDLRBs), are designed in line with the Italian seismic code (NTC08) assuming four subsoil types, ranging from rock-site to soil-site, combined with relatively low and high values of the ratio between the vertical and horizontal stiffnesses of the HDLRBs. Three fire scenarios are considered at 45 (i.e. R45) and 60 (i.e. R60)minutes of fire resistance, with the fire compartment confined to the area of the first level (F1), the lower two levels (F1/2) and the fifth level (F5). A finite element thermal model to measure temperature distribution in the cross-section for columns, exposed to fire on one and four sides, and beams, exposed to fire on one and three sides, is considered in combination with the parametric time–temperature fire curve evaluated in line with Eurocode 1. Then, a nonlinear incremental dynamic analysis is carried out which takes into consideration the horizontal and vertical components of near-fault ground motions recorded at different site conditions from rock- to soil-site and normalized with respect to the NTC08 ones in accordance with the Modified Velocity Spectrum Intensity. Plastic conditions are assessed at the potential critical sections of the beams (i.e. end sections of the sub-elements in which a beam is discretized) and columns (i.e. end sections), whose reduced mechanical properties are evaluated in accordance with a thermal mapping and considering the 500°C isotherm method proposed by Eurocode 2. Maximum ductility demand under the horizontal and vertical components of near-fault ground motions depends on the position of the fire compartment and increases moving from rock-site to soil-site.

Acceleration response spectra for Tbilisi city with site effects

The utilization of time histories of earthquake ground motion has grown considerably in the field of earthquake engineering. It is very unlikely, however, that recordings of earthquake ground motion will be available for all sites and conditions of interest. Hence, there is a need for efficient methods for the simulation of strong ground motion for a given region. Due to lack of the real strong ground motion records the objective of this research is to develop a methodology for rapid generation of horizontal and vertical components of earthquake ground motion at any site for Tbilisi city. The model developed in this study provides simulation of ground motion over a wide range of magnitudes and distances at 8 earthquake sources zones of Tbilisi region (within 50 km). The research includes three main topics: (i) the stochastic simulation of earthquake ground motion at a given site of the city of Tbilisi; (ii) the estimation of acceleration time histories at a given site using the direct method of engineering seismology considering soil conditions based on the theory of the reflected waves and (iii) calculation of horizontal and vertical acceleration elastic response spectra for main sites of Tbilisi territory. The simulation procedure typically consists of multiplying deterministic modulating function with a stationary process of known power spectral density. The obtained results in the terms of normalized elastic response spectra can be widely applied in the practice of earthquake engineering in Georgia.

Integrated duration effects on seismic performance of concrete gravity dams using linear and nonlinear evaluation methods

For the assessment of seismic performance of concrete gravity dams, ground motions with horizontal and vertical components are usually selected as seismic excitations. Although various definitions have been proposed to compute strong motion duration from a single component (either horizontal or vertical one) of seismic excitations, duration definitions accounting for the contributions of all components of seismic excitations are still lacking. In order to bridge the gap between duration definitions and multi-component seismic excitations used for simulation, a concept of integrated duration is proposed in this contribution. With the definition of integrated duration, a unified strong motion duration can be calculated for a seismic excitation with multiple components, which facilitates the quantitative evaluation of duration effects. To examine the influence of integrated duration on the seismic performance of concrete gravity dams, both the linear and nonlinear evaluation methods considering the dam-reservoir-foundation interaction are adopted in this study. The linear evaluation method is based on demand capacity ratio, cumulative overstress duration and spatial extent of overstressed regions, whereas the nonlinear seismic analysis of concrete gravity dam-water-foundation systems is on the basis of a Concrete Damaged Plasticity model. 20 real earthquake records with a broad range of durations are selected as seismic excitations to quantify the correlation between the proposed integrated duration and the seismic performance of concrete gravity dams. It is found that the cumulative overstress duration, the accumulative damage and the residual plastic deformation are strongly affected by the integrated duration. Apart from the integrated duration effects, the influence of the vertical component of ground motions is also discussed.

Effects of near-fault ground motions on the nonlinear behaviour of reinforced concrete framed buildings

The design provisions of current seismic codes are generally not very accurate for assessing effects of near-fault ground motions on reinforced concrete (r.c.) spatial frames, because only far-fault ground motions are considered in the seismic codes. Strong near-fault earthquakes are characterized by long-duration (horizontal) pulses and high values of the ratio α PGA of the peak value of the vertical acceleration, PGAV, to the analogous value of the horizontal acceleration, PGAH, which can become critical for girders and columns. In this work, six- and twelve-storey r.c. spatial frames are designed according to the provisions of the Italian seismic code, considering the horizontal seismic loads acting (besides the gravity loads) alone or in combination with the vertical ones. The nonlinear seismic analysis of the test structures is performed using a step-by-step procedure based on a two-parameter implicit integration scheme and an initial stress-like iterative procedure. A lumped plasticity model based on the Haar-Karman principle is adopted to model the inelastic behaviour of the frame members. For the numerical investigation, five near-fault ground motions with high values of the acceleration ratio α PGA are considered. Moreover, following recent seismological studies, which allow the extraction of the largest (horizontal) pulse from a near-fault ground motion, five pulse-type (horizontal) ground motions are selected by comparing the original ground motion with the residual motion after the pulse has been extracted. The results of the nonlinear dynamic analysis carried out on the test structures highlighted that horizontal and vertical components of near-fault ground motions may require additional consideration in the seismic codes.

Analytical case study on the seismic performance of a curved and skewed reinforced concrete bridge under vertical ground motion

Rational representation of seismic demand based on realistic characterization of earthquake ground motion is essential for seismic bridge design. Realization of the impact of vertical ground motion has led to an initiative to better characterize and simulate the seismic scenario for typical bridges in near-fault regions, such that structural design practices may improve. Current structural design provisions either do not account for effects of vertical ground motion or specify a simplistic equivalent-static-load based methodology. An assessment of the effect of vertical ground motions on a horizontally skewed and curved highway bridge is presented for a moderate-to-high seismic region. A numerical model of a skewed and curved, three-span bridge located in Tacoma, Washington is subjected to a suite of horizontal and vertical ground motions using non-linear time-history analysis. The ground motions selected are composed of a range of near-fault earthquake records with varying component characteristics such as site condition, fault distance, and vertical-to-horizontal acceleration ratios. The scenario represents a bridge with a short vibrational period in a moderate seismic zone where vertical ground motion effects may be applicable, yet are not considered by structural codes. The results of the numerical simulations depict a considerable impact from vertical ground motion to the substructure and superstructure, including behavior typically not documented in existing studies. The numerical simulations show amplification from the vertical component of ground motion in the demand on components of the substructure, particularly for vertical-to-horizontal peak ground accelerations exceeding one. The effects of vertical ground motion on skewed and curved bridges are also evaluated against a straight counterpart with comparable overall configuration. For structural designers, the results of this study indicate that excluding the vertical ground motion component from analyses may impose a larger margin of risk than previously perceived. This is particularly the case for moderate-to-high seismic regions where shorter period bridges may resonate with predominant frequencies of vertical ground motion.

Theoretical Investigation of Bridge Seismic Responses with Pounding under Near-Fault Vertical Ground Motions

Vertical earthquake loading is normally regarded not as important as its horizontal components and are not explicitly considered in many seismic design codes. However, some previous severe near-fault earthquakes reveal that the vertical ground motion component can be much larger than the horizontal components and may cause serious damage to the bridge structures. This paper theoretically investigates the vertical pounding responses of a two-span continuous bridge subjected to the severe near-fault vertical ground motions. The bridge is simplified as a continuous beam-spring-rod model. The structural wave effect and the vertical pounding between the bridge girder and the supporting bearing are considered, and the theoretical solutions of bridge seismic responses are derived from the expansion of transient wave functions as a series of eigenfunctions. The effects of vertical earthquake and vertical pounding on the bridge bearing, girder and pier are investigated. The numerical results show that the severe vertical earthquake loading may cause the bridge girder to separate from the supporting bearing and hence result in vertical poundings between them when they are in contact again. These vertical poundings can significantly alter the seismic responses of the bridge structure and may cause severe damage to the bridge components such as bridge girder, supporting bearing and bridge pier. Neglecting the influence of vertical earthquake loading may lead to inaccurate estimation of seismic responses of bridge structures, especially when they are subjected to near-fault earthquake with relatively large vertical motion.

Damping Scaling Factors for Vertical Elastic Response Spectra for Shallow Crustal Earthquakes in Active Tectonic Regions

This paper develops a new model for a damping scaling factor (DSF) that can be used to adjust elastic response spectral ordinates for the vertical component of earthquake ground motion at a 5% viscous damping ratio to ordinates at damping ratios between 0.5% and 30%. Using the extensive NGA-West2 database of recorded ground motions from worldwide shallow crustal earthquakes in active tectonic regions, a functional form for the median DSF is proposed that depends on the damping ratio, spectral period, earthquake magnitude, and distance. Standard deviation is a function of the damping ratio and spectral period. The proposed model is compared to the DSF for the “average” horizontal component. In general, the peak in DSF is shifted toward shorter periods and is farther from unity for the vertical component. Also, the standard deviation of DSF for vertical motion is slightly higher than that observed for the “average” horizontal component.

Plastic Hinge Length of RC Columns under the Combined Effect of Near-Fault Vertical and Horizontal Ground Motions

Plastic hinges are an extension of the ductile design concept used in building seismically resistant structures. The formation of a plastic hinge in an RC column in regions that experience inelastic actions depends on the characteristics of the earthquakes as well as the column details. Recordings from recent earthquakes have proved that in the near-fault region of large earthquakes, the vertical component of the near-fault ground motion has exceeded the horizontal, and damage to the structures has been predominantly by these motions. Therefore, this paper focuses on near-fault ground motions that have been recorded within rather 20 km of the causative fault and 936 inelastic time-history analyses have been performed to predict the nonlinear behavior of RC columns subjected to combined effect of near-fault vertical and horizontal ground motions. The effects of some parameters are evaluated analytically by finite element methods and the results are compared with corresponding experimental data. Findings from this study provide a simple expression to estimate plastic hinge length of RC columns subjected to the combined effect of near-fault vertical and horizontal ground motions.

Predictions of Experimentally Observed Stochastic Ground Vibrations Induced by Blasting

In the present paper, we investigate the blast induced ground motion recorded at the limestone quarry “Suva Vrela” near Kosjerić, which is located in the western part of Serbia. We examine the recorded signals by means of surrogate data methods and a determinism test, in order to determine whether the recorded ground velocity is stochastic or deterministic in nature. Longitudinal, transversal and the vertical ground motion component are analyzed at three monitoring points that are located at different distances from the blasting source. The analysis reveals that the recordings belong to a class of stationary linear stochastic processes with Gaussian inputs, which could be distorted by a monotonic, instantaneous, time-independent nonlinear function. Low determinism factors obtained with the determinism test further confirm the stochastic nature of the recordings. Guided by the outcome of time series analysis, we propose an improved prediction model for the peak particle velocity based on a neural network. We show that, while conventional predictors fail to provide acceptable prediction accuracy, the neural network model with four main blast parameters as input, namely total charge, maximum charge per delay, distance from the blasting source to the measuring point, and hole depth, delivers significantly more accurate predictions that may be applicable on site. We also perform a sensitivity analysis, which reveals that the distance from the blasting source has the strongest influence on the final value of the peak particle velocity. This is in full agreement with previous observations and theory, thus additionally validating our methodology and main conclusions.

The performance of churches in the 2012 Emilia earthquakes

In this study the damage suffered by churches during the 2012 Emilia seismic sequence in Italy is analysed, based on surveys and inspections carried out in the area. Similarly to what was observed after other Italian earthquakes, the damage to churches was severe. However, the Emilia churches present some characteristic features such as the use of unreinforced clay brick masonry. In order to appropriately address the performance of this class of buildings, typical architectural layouts and construction techniques are described. Such techniques are interpreted also in the light of the local seismic catalogue. Fifty churches are then selected and their damage is studied, with reference to typical local-collapse mechanisms of different macro-elements. The study highlights that the damage is often concentrated at the top section of the facade, in the clerestory walls, in the vaults and in the bell towers. Structural analyses are performed to explain some of the observations. The overturning of the top section of the facade is analytically addressed, modelling the friction interlocking. With reference to the case study of San Francesco in Mirandola, non-linear static and dynamic analyses allow us to correlate the directionality of damage to the higher seismic demand along the NS direction, to point out the negligible role of the large vertical component of ground motion and to emphasise the relevance of the buttresses for the seismic response of the facade.

1D velocity structure of the Po River plain (Northern Italy) assessed by combining strong motion and ambient noise data

Strong ground motions recorded on the sedimentary deposits of the Po River alluvial plain during the Emilia (Northern Italy) Mw 5.7 earthquake of May 29, 2012 are used to assess the vertical profile of shear-wave velocity above the limestone basement. Data were collected by a linear array installed for site effect studies after the Mw 5.9 mainshock of May 20, 2012. The array stations, equipped with both strong and weak motion sensors, are aligned in the South–North direction, at distances ranging from 1 to 26 km from the epicenter. The vertical components of ground motion show very distinctive, large-amplitude, low-frequency dispersive wave trains. Wavelet analysis yields group-velocity dispersion curve in the 0.2–0.7 Hz frequency band. The availability of a long ambient noise record allows estimates of the site resonance frequency along with its stability among stations. The joint inversion of dispersion of surface waves and ellipticity curves derived from ambient noise H/V allows extending investigations down to the sediment-limestone interface, at a depth of about 5,000 m. Our results add new information about the velocity structure at a scale that is intermediate between the local scale already investigated by other authors with small-aperture arrays using ambient noise and the regional scale inferred from modeling of seismogram waveforms recorded at hundreds of kilometers from the source.

Mode misidentification in Rayleigh waves: Ellipticity as a cause and a cure

The surface wave method is a popular tool for geotechnical characterization because it supplies a cost-effective testing procedure capable of retrieving the shear wave velocity structure of the near-surface. Several acquisition and processing approaches have been developed to infer the Rayleigh wave dispersion curve which is then inverted. Typically, in active testing, single-component vertical receivers are used. In most cases, the inversion is carried out assuming that the experimental dispersion curve corresponds to a single mode, mostly the fundamental Rayleigh mode, unless clear evidence dictates the existence of a more complex response, e.g., in presence of low-velocity layers and inversely dispersive sites. A correct identification of the modes is essential to avoid serious errors. Here we consider the typical case of higher-mode misidentification known as “osculation” (“kissing”), where the energy peak shifts at low frequencies from the fundamental to the first higher mode. This jump occurs, with a continuous smooth transition, around a well-defined frequency where the two modes get very close to each other. Osculation happens generally in presence of strong velocity contrasts, typically with a fast bedrock underlying loose sediments. The practical limitations of the acquired active data affect the spectral and modal resolution, making it often impossible to identify the presence of two modes. In some cases, modes have a very close root and cannot be separated at the osculation point. In such cases, mode misidentification can create a large overestimation of the bedrock velocity and a large error on its depth. We examine the subsoil conditions that can generate this unwanted condition, and the common field acquisition procedures that can contribute to producing data having such deceptive Rayleigh dispersion characteristics. This mode misidentification depends strongly on the usual approach of measuring only the vertical component of ground motion, as the mode osculation is linked to the Rayleigh wave ellipticity polarization, and therefore we conclude that multicomponent data, using also horizontal receivers, can help discern the multimodal nature of surface waves. Finally, we introduce a priori detectors of subsoil conditions, based on passive microtremor measurements, that can act as warnings against the presence of mode osculation, and relate these detectors to the frequencies at which dispersion curves can be misidentified. Theoretical results are confirmed by real data acquisition tests.

수직지반운동의 특성과 3층 RC건물의 응답에 미치는 영향

이 논문은 수직지반운동의 특성과 3층 RC건물의 응답에 수직지반운동이 미치는 영향을 평가한 것이다. 수직지반운동의 주파수 특성, 수평 수직지반운동의 최대지반가속도 비율 (V/H), 수평 수직지반운동의 최대지반가속도 도달시간차를 분석하고, 다양한 V/H에 따라서 수직지반운동이 건물의 응답에 미치는 영향을 평가하였다. 해석결과 수직지반운동을 포함하는 것이 이 연구에서 선정한 구조물의 응답에 중대한 영향을 미치고 있음을 알 수 있었다. 따라서 V/H가 높을 가능성이 있는 단층인접지역에서는 신뢰성 있는 구조해석 및 설계를 위하여 수직지반운동을 포함한 구조해석이 필요하다. The paper presents the characteristics of vertical ground motion and its effect on 3-story RC building. The characteristics of vertical ground motion are discussed including frequency content, vertical-to-horizontal peak ground acceleration (V/H) ratio, and time intervals between the arrival of the vertical and horizontal peaks. The vertical ground motion effects on the RC structure are also investigated considering V/H ratio. It is observed that the inclusion of the vertical component of ground motion has an important effect on the response of the selected RC building. It is therefore concluded that vertical motion should be included in analysis for assessment and design in the near fault area where V/H is likely to be high.

Significant coherence for groundwater and Rayleigh waves: Evidence in spectral response of groundwater level in Taiwan using 2011 Tohoku earthquake, Japan

Summary It was found that groundwater can be fluctuated by a variety of disturbances. For instance, seismic activity inducing the dilatation of earth can disturb groundwater level fluctuation as observed in groundwater wells. In this research, spectral analysis in the frequency domain was used to quantitatively evaluate coherence between groundwater head fluctuations and the ground motions recorded in Taiwan from a distant earthquake, the 2011 Tohoku earthquake in northern Honshu, Japan with magnitude of 9. The relationship between groundwater fluctuations and the decomposed ground motions of Rayleigh waves are clearly identified. By analyzing autospectral density, cross-spectral density, and resultant coherence for the seismograms and groundwater head, it was found that the Rayleigh waves dominated the groundwater fluctuations at period of about 21–32 s for six pair of groundwater and seismograms of broadband distributed around Taiwan; fluctuations of groundwater are highly coherent with the radial and vertical components of ground motions. Our analysis also shows the time from event to station for Rayleigh waves ranged from 780 to 900 s approximately. Wave parameter for seismic event to station of groundwater and seismograms were also identified as 3.0–3.5 km/s and 64–110 km for wave velocity and wave length, respectively. The relationship of groundwater fluctuations and ground motions induced by seismic activity become feasible to assess using spectral analysis.

Modelling and Analysis of Base Isolated Structures with Friction Pendulum System Considering near Fault Events

The seismic behavior of base isolated structures with friction pendulum slide bearings devices subjected to near fault events characterized by significant vertical ground motion components is investigated. In particular, in order to evaluate the effects of vertical components on seismic response, non-linear dynamic analysis, carried out by using several numerical models, have been performed by considering two near-fault seismic events, L’Aquila 2009 and Emilia Romagna 2012. The obtained results show that increasing vertical seismic motion base shear significantly increases while relative displacements increase very little.

Nonlinear Dynamic Response of RC Buildings with Different Base Isolation Systems Subjected to Horizontal and Vertical Components of Near-Fault Ground Motions

Near-fault ground motions are characterized by long-duration horizontal pulses and high values of the peak vertical acceleration, which can become critical for a base-isolated structure. In order to check if current code provisions can be considered adequate for the design of base-isolated structures located in a near-fault area, base-isolated five-storey r.c. framed buildings with elastomeric bearings acting alone (“Base Isolation” system) or combined in parallel or in series with sliding bearings (“Base Isolation and in-Parallel Sliding”, BIPS, or “Base Isolation and in-Series Sliding”, BISS, systems) are studied. The base-isolated structures are designed assuming the same values for the fundamental vibration period and equivalent viscous damping in the horizontal direction. Different values of the stiffness ratio, defined as the ratio between the vertical and horizontal stiffness of the elastomeric bearings, are considered; moreover, different values of the sliding ratio, defined as the global sliding force corresponding to an examined BIPS or BISS system divided by the maximum sliding force (in the case of a sliding bearing under each column), are also assumed. The nonlinear analysis of the test structures subjected to strong near-fault ground motions is performed using a step-by-step procedure based on a twoparameter implicit integration scheme and an initial-stress-like iterative procedure. At each step of the analysis, plastic conditions are checked at the potential critical sections of the girders (i.e. end sections of the sub-elements in which a girder is discretized) and columns (i.e. end sections), where a bilinear moment-curvature law is adopted. The response of an elastomeric bearing is simulated by a model with variable stiffness properties in the horizontal and vertical directions, depending on the axial force and lateral deformation, and linear viscous damping. Finally, a rigid-plastic (with friction variability) law is assumed to simulate the behaviour of a sliding bearing.

Multi-temporal mapping of land subsidence at basin scale exploiting Persistent Scatterer Interferometry: case study of Gioia Tauro plain (Italy)

The temporal and spatial evolution of the pumping-induced ground subsidence in the Gioia Tauro plain (Southern Italy) is investigated exploiting multi-temporal analysis of PSInSAR (Permanent Scatterer Interferometric Synthetic Aperture Radar) data from ERS 1/2 (1992–2001) and ENVISAT (2002–2006) satellites. The PSInSAR derived ground deformation velocity maps and related displacement time series reveal that subsidence rates as high as 10–15 mm/yr are observed in 1992–2006, with the highest velocities (up to 23 mm/yr) occurring between 1992 and 2000 in the central portion of the Plain. Ascending and descending data are combined to extract the vertical and east-west components of ground motions, and reveal the presence of predominant components in the vertical direction due to the compaction of Quaternary sediment in the basin. Aquifer compaction is a consequence of groundwater abstraction. An acceleration index is computed to synthesize the temporal changes between the observed displacements in the two intervals, 1992–2001 and 2002–2006. Deceleration of motions is observed in most of the plain with velocity reductions of about 65% from 1992–2001 to 2002–2006; only a few narrow areas in the southern sector reveal significant increases of motion velocities in the 2002–2006 period, with subsidence velocities exceeding by about 30% those observed in 1992–2001.

EFFECT OF BIDIRECTIONAL EXCITATIONS ON SEISMIC RESPONSE OF RC BUILDINGS

This paper investigates the effect of the horizontal and vertical components of ground motions (HGM and VGM, respectively) on the seismic response of Reinforced Concrete (RC) buildings designed to modern capacity design principles and located in the vicinity of active faults. Fiber-based analytical models are used to monitor the global and local response of twelve reference structures, including verifying the response modification factor and tracing the member shear supply-demand response using a ductility- and axial force-sensitive shear strength approach. The simulation models are subjected to near-field earthquake records with increasing severity up to collapse, including and excluding VGM. The results indicate that the lower the contribution of horizontal seismic forces to the seismic response, the higher is the significance of VGM. The fluctuation of axial forces in vertical structural members significantly increases when including VGM. This not only has direct consequences on tension and compression response but also has impact on shear capacity. The diverse range of buildings and performance criteria and large number of incremental dynamic analyses confirm the importance of including VGM in seismic design and assessment of contemporary RC buildings, and hence cast doubts on the reliability of pre-code structures located in the vicinity of active faults.

Soil damping and site dominant vibration period determination, by means of random decrement method and its relationship with the site-specific spectral decay parameter kappa

We use the random decrement method (RDM) to determine the damping (ξ) and dominant period (Td) of soils using earthquake records and ambient noise, and compare the values of ξ with the spectral decay parameter kappa (κ0) previously estimated at several sites in the San Bernardino Valley, northeastern Sonora, Mexico.The dominant period take values from 0.12s at sites on igneous rocks, to 0.17–0.20s at sites on continental deposits (Tertiary conglomerates). The damping varies between 2.8 and 5.7%.We find the empirical relationship: ξ=(0.50±0.178)×κ0, which predominate in the study area, and is comparable with that obtained for stations on conglomerates (ξ=(0.43±0.130)×κ0). We also find a theoretical equation ξ=(Vs/2H)κ0 useful for any region, that depend on the shear wave velocity (VS), the decay parameter (κ0) and the thickness of the soil-layer considered (H).We calculate the thickness of the surface layer at each site and find that it varies from 80- to −150-m. We conclude that: it is possible to obtain ξ and Td simultaneously with the RDM; consistent estimates of ξ and Td can be obtained using the whole record or parts of it; the RDM avoids the calculation of spectral ratios to estimate Td and ξ as well as any other pre-processing of the signal; the vertical component of ground motion reflects the properties of shallow layers at the analyzed sites.