ATTENUATION RELATIONSHIP FOR LOW MAGNITUDE EARTHQUAKES USING STANDARD SEISMOMETRIC RECORDS

Northwestern Italian weak-motion data were used to study attenuation characteristics of horizontal peak ground acceleration (PGA) and horizontal peak ground velocity (PGV) from earthquakes of local magnitudes (M _l ) up to 5.1. Data have been provided by the RSNI (Regional seismic network of Northwestern Italy) and RSLG (Regional seismic network of Lunigiana-Garfagnana) waveform database. The database consists of more than 14000 horizontal components recorded in the period 1999-2002 by both broadband and enlarged band seismometers. The accuracy of the procedure used to extract PGA values from the velocity recordings was verified comparing observed and derived PGA values at station STV2, which was equipped with both a temporary K2 Kinemctrtcs accelerometer and Guralp CMG40 broadband sensor. The attenuation of both peak ground acceleration and peak ground velocity was found to be logarithmically distributed with a strong attenuation for low distances (less than 50 km) and low M _l values (<3.0). The resulting equations are:

Log(PGA)=?3.19+0.87M?0.042M ²?1.92 Log(R)+0.249S,

Log(PGA)=?4.23+0.76M?0.018M²?1.56 Log(R)+0.230S,

where PGA is expressed in g, PGV is expressed in m/s, M is local magnitude, R is the hypocentral distance in kilometers and S is a dummy variable assuming values of 0 and 1 for rock and soil respectively. For increasing distance and magnitude, both PGA and PGV values show a linear distribution. The validity range of the obtained attenuation relationships is 0–200 km for distances and M _l up to 4.5. Sensitivity studies performed by analysis of residuals, showed that predicted PGA and PGV values are stable with respect to reasonable variations of the model and distances providing the data. Comparisons with attenuation relationships proposed for Italian region, derived from strong motion records, are also presented.     

Correlations Between Frequency Content Indicators of Strong Ground Motions and PGV

The frequency content of ground motions seems to be one of the most important parameters to explain the structural damage experienced during worldwide strong earthquakes. The frequency content of ground motions can be characterized by various stochastic and/or deterministic indicators: the frequency bandwidth indicator ? (Cartwright & Longuet-Higgins) related to the power spectral density function and, respectively, the control (corner) period T_c of the structural response spectra or the mean period T_M . Peak ground velocity (PGV) and the ratio PGA/PGV can be used as either damage potential parameters or frequency content indicators. A comparative analysis of stochastic and deterministic frequency content indicators and of PGV is applied to a set of 30 strong ground motion records having peak ground acceleration (PGA) from 0.2–0.8 g and recorded on 4 continents during the last 70 years.     

Sensitivity of the Earthquake Response of Tall Steel Moment Frame Buildings to Ground Motion Features

The seismic response of two tall steel moment frame buildings and their variants is explored through parametric nonlinear analysis using idealized sawtooth-like ground velocity waveforms, with a characteristic period (T), amplitude (peak ground velocity, PGV), and duration (number of cycles, N). Collapse-level response is induced only by long-period, moderate to large PGV ground excitation. This agrees well with a simple energy balance analysis. The collapse initiation regime expands to lower ground motion periods and amplitudes with increasing number of ground motion cycles.     

Empirical Correlations of Frequency-Content Parameters of Ground Motions with Other Intensity Measures

Empirical correlations between the frequency-content parameters of earthquake ground motions and amplitude-, cumulative-, and duration-based intensity measures (IMs) are examined in this study. Three commonly used scalar frequency-content parameters are considered, namely the mean period (T_m), the average spectral period (T_avg), and the smoothed spectral predominant period (T₀). It is found that the frequency-content parameters have weakly negative correlations with high-frequency IMs such as peak ground acceleration (PGA) and spectral accelerations (SAs) at periods smaller than 0.3 s, low-to-moderate positive correlations with peak ground velocity (PGV) and SA within a period range of 0.5 s–10 s, negligible correlations with cumulative-based IMs, and weakly positive correlations (in the vicinity of 0.1–0.3) with significant durations. Simple piecewise parametric equations are proposed to fit the empirical correlations of T_m, T_avg, and T₀ with SA over the entire period range. The presented correlation results and parametric models enable the frequency-content parameters to be easily used in various applications such as ground-motion selection and vector-based probabilistic seismic hazard analysis.     

A Static Nonlinear Procedure for the Evaluation of the Elastic Buckling of Anchored Steel Tanks Due to Earthquakes

Ground-supported steel tanks experienced extensive damage in past earthquakes. The failure of tanks in earthquakes may cause severe environmental damage and economic losses. This study deals with the evaluation of the elastic buckling of above-ground steel tanks anchored to the foundation due to seismic shaking. The proposed nonlinear static procedure is based on the capacity spectrum method (CSM) utilized for the seismic evaluation of buildings. Different from the standard CSM, the results are not the base shear and the maximum displacement of a characteristic point of the structure but the minimum value of the horizontal peak ground acceleration (PGA) that produces buckling in the tank shell. Three detailed finite element models of tank-liquid systems with height to diameter ratios H/D of 0.40, 0.63, and 0.95 are used to verify the methodology. The 1997 UBC design spectrum and response spectra of records of the 1986 El Salvador and 1966 Parkfield earthquakes are used as seismic demand. The estimates of the PGA for the occurrence of first elastic buckling obtained with the proposed nonlinear static procedure were quite accurate compared with those calculated with more elaborate dynamic buckling studies. For all the cases considered, the proposed methodology yielded slightly smaller values of the critical PGA for the first elastic buckling compared to the dynamic buckling results.     

Seismic Vulnerability of Building Contents for a Given Occupancy Due to Multiple Failure Modes

A methodology is presented for assessing the seismic vulnerability of inventories of contents to multiple failure modes. An ordering method to find out probabilities of failure of a conditional mode upon the survival of the other modes is applied. The procedure considers the statistical correlation of failure modes due to the dynamic response, such as sliding and/or overturning, of contents and also to non structural components. This methodology was applied to inventories of four types of occupancy (house, school, office, and hospital) located in Mexico City, considering that all contents are situated at ground level. Expected damage functions for these inventories show large differences between them, the house and school inventories being the least vulnerable, and the hospital inventory, the most vulnerable, even for low intensities.     

Earthquake-Induced Damage Detection in a Monumental Masonry Bell-Tower Using Long-Term Dynamic Monitoring Data

ABSTRACT

This work investigates the use of an advanced long-term vibration-based structural health monitoring tool to automatically detect earthquake-induced damages in heritage structures. Damage produced in a monumental bell-tower at increasing values of the Peak Ground Acceleration (PGA) of the seismic input is predicted by incremental nonlinear dynamic analysis, using a Finite Element model calibrated on the basis of experimentally identified natural modes. Then, predicted damage effects are artificially introduced in the monitoring data to check for their detectability. The results demonstrate that a very small damage, associated to a low intensity and low return period earthquake, is clearly detected by the monitoring system.     

Impact of Modification on Ground Motion Characteristics and Geotechnical Seismic Analyses for a California Site

The impact of different modification techniques on ground motion characteristics and results of seismic geotechnical analyses is investigated for a site in California. Twenty-eight motions were selected and scaled and also modified using both time domain (TD) and frequency domain (FD) techniques. PGV and PGD of the TD-modified motions are found to be larger than their FD-modified counterparts, but slightly less than the scaled ground motion characteristics. Cyclic stress ratios and amplification factors are similar for all sets of motions. Newmark-type slope displacements caused by the scaled and modified ground motions are similar (within 25%) for a variety of sliding masses.     

Numerical Evaluation of Seismic Soil Pressures on Rigid Walls Fixed to the Bedrock

Seismic soil pressures developed on a 7 m rigid retaining wall fixed to the bedrock are investigated using a finite element model that engages nonlinear soil intended materials available in OpenSees. This allows incorporation of the inelastic behavior of the soil and wave propagation effects in the soil-wall system seismic response. The nonlinear response of the soil was validated using the well-stablished, frequency-domain, linear-equivalent approach. An incremental dynamic analysis was implemented to comprehensively examine the effect of soil nonlinearity and input motion on the induced seismic pressures and to evaluate current code equations/methodologies at different levels of earthquake intensity. The results show that soil nonlinearity and seismic wave amplification may play an important role in the response of the soil-wall system. Therefore, methodologies that rely only on peak ground acceleration may introduce large bias on the estimated seismic pressures in scenarios where high nonlinearity and site amplification are expected.     

Effects of Geometrical Features on the Seismic Response of Historical Masonry Towers

ABSTRACT

Historical masonry structures are often located in earthquake-prone regions and the majority of them are considered to be seismically vulnerable and unsafe. Historical masonry towers are slender structures that exhibit unique architectural features and may present many inadequacies in terms of seismic performance. The seismic protection of such typologies of structures and the design of effective retrofitting interventions require a deep understanding of their behavior under horizontal loads. This paper presents the results of the seismic performance evaluation of historical masonry towers located in Northern Italy. A large set of case studies is considered, comprising a significant number of towers with high slenderness and marked inclination. First, a preliminary assessment of the dynamic behavior of the different towers is carried out through eigenfrequency analyses. Then, non-linear dynamic simulations are performed using a real accelerogram with different peak ground accelerations. A damage plasticity material model, exhibiting softening in both tension and compression, is adopted for masonry. The huge amount of results obtained from the non-linear dynamic simulations allows a comparative analysis of the towers to be performed in order to assess their seismic vulnerability and to show the dependence of their structural behavior on some geometrical characteristics, such as slenderness, inclination, and presence of openings and belfry. The evaluation of different response parameters and the examination of tensile damage distributions show the high vulnerability of historical masonry towers under horizontal loads, mainly in the presence of marked inclination and high slenderness. Some general trends of the seismic behavior of the towers are deduced as a function of the main typological features.     

Probabilistic Seismic Hazard Model of West Bengal,India

We deliver a next-generation Probabilistic Seismic Hazard model of West Bengal based on improved seismogenic source characterization considering both the Layered Polygonal sources & Tectonic sources in the hypocentral depth range of 0–25 km & 25–70 km, seismic local-specific site condition, and adaption of appropriate region specific ground motion prediction equations in a logic tree framework. The surface consistent Probabilistic seismic hazard distribution in terms of Peak Ground Acceleration (PGA) & 5% damped Pseudo Spectral Acceleration (PSA) at different time periods for 10% probability of exceedance in 50 years have been generated and the design response spectra computed.     

Analytical Seismic Assessment of a Tall Long-Span Curved Reinforced-Concrete Bridge. Part I: Numerical Modeling and Input Excitation

The seismic response of bridges is affected by a number of modeling considerations, such as pier embedment, buried pile caps, seat-type abutments, pounding, bond slip and architecturally flared part of piers, and loading considerations, such as non-uniform ground excitations and orientation of ground motion components, which are not readily addressed by design codes. This article addresses a methodology for the nonlinear static and dynamic analysis of a tall, long-span, curved, reinforced-concrete bridge, the Mogollon Rim Viaduct. Various modeling scenarios are considered for the bridge components, soil-structure interaction system, and materials, i.e., concrete and reinforcing steel, covering all its geotechnical and structural aspects based on recent advances in bridge engineering. Various analysis methodologies (nonlinear static pushover, time history response to uniform and spatially variable seismic excitations, and incremental dynamic analyses) are performed. For the dynamic analyses, a suite of nine earthquake accelerograms are selected and their characteristics are investigated using seismic intensity parameters. A recently developed approach for the generation of non-uniform seismic excitations, i.e., spatially variable simulations conditioned on the recorded time series, is used. Methods for the evaluation of structural performance are discussed and their limitations addressed. The numerical results of the seismic assessment of the Mogollon Rim Viaduct are presented in the companion article (Part II). The sensitivity of the bridge response to the adopted modeling, loading and analyzing strategies, as well as the correlation between structural damage and seismic intensity parameters are examined in detail.     

Reconnaissance Report on Damage of Bridges in 2008 Wenchuan,China, Earthquake

This is a reconnaissance report on the damage to bridges during the 2008 Wenchuan, China, earthquake. Site investigation was conducted by the authors on August 10–14, 2008. Presented is a detailed discussion of the damage to 12 bridges as well as possible damage mechanisms. Characteristics of two near-field ground accelerations and Chinese seismic bridge design practices are also presented. An investigation of the damage finds insufficient intensity of seismic design force, inadequate structural detailing for enhancing the ductility capacity, and an absence of unseating prevention devices.     

Seismic Refraction at Ancestral Puebloan Sites on the Pajarito Plateau,New Mexico

Abstract

Geophysical techniques can be used for non-invasive surveys at archaeological sites. One under-utilized technique, seismic refraction, has many potential applications. It is an inexpensive, efficient way to characterize subsurface deposits, especially at sites with shallow accumulations over bedrock. Archaeologists and geophysicists participating in the Summer of Applied Geophysics Experience from Los Alamos National Laboratory and Bandelier National Monument on the Pajarito Plateau in northern New Mexico characterized Ancestral Puebloan sites (A.D. 1200–1450) using this technique. We provide an overview of how seismic refraction works and demonstrate the applicability of seismic refraction for identifying buried archaeological features such as communal structures and walls.     

Seismic Damage to Structures in the 2015 Nepal Earthquake Sequences

The 7.8 Mw Gorkha earthquake struck the east of Lamjung in Nepal, followed by a sequence of powerful aftershocks. Chinese Team Six including the authors inspected the seismic damage to civil structures along 10 paths in densely populated areas with a seismic intensity of VII to IX, 40 days after the main shock. The damage was categorized according to structure types and described in detail. Several conclusions are made: powerful aftershocks can significantly affect the failure patterns; geological conditions, structure types, and height have great influence on the level of damage; and the local risky retrofitting technique needs improvement badly.     

The Strength Reduction Factors for Seismic-Isolated Bridges Characterized by SDOF Bilinear Systems in Far-Fault Areas

This article presents a statistical study on strength reduction factors for seismic-isolated bridges in far-fault areas. 1410 ground motions are selected and modified to be compatible with the recommended response spectra. Then, they are divided into 60 groups to investigate the effects of PGA/PGV ratios, soil conditions and post-to-pre-yield stiffness ratio. Results show that reduction factors are significantly affected by the PGA/PGV ratio, while the latter two items are not as important as the first one. Finally, an improved equation to estimate the reduction factor is proposed, and the accuracy of the equation is verified by additional records.     

Seismic Hazard Analysis Using Discrete Faults in Northwestern Pakistan: Part II – Results of Seismic Hazard Analysis

This article is the second of two companion articles that evaluate seismic hazard in northwestern (NW) Pakistan. Using the properties and characteristics of discrete faults in NW Pakistan described in the first article, probabilistic and deterministic seismic hazard analyses for 11 major cities in NW Pakistan were conducted. The results from both probabilistic and deterministic seismic hazard analyses exhibit good agreement. Median deterministic spectra compare favorably with uniform hazard spectra (UHS) for 475- or 975-year return periods, while the 84^th-percentile deterministic spectra compare favorably with the UHS for a 2475-year return period. Peak ground accelerations (PGAs) for 2475-year return periods exceed 1.0 g for the cities of Kaghan and Muzaffarabad, which are surrounded by major faults. The PGAs for a 475-year return period for these cities are approximately 0.6g — 3 to 4 times greater than estimates by previous studies using diffuse areal source zones. The PGAs for some cities located farther from faults (including Astor, Malakand, Mangla, Peshawar, and Talagang) are similar to those predicted using diffuse areal source zones. Seismic hazard maps for PGA and spectral accelerations at periods of 0.2 s and 1.0 s corresponding to three return period (2475, 975, and 475 years) were produced. Based on deaggregation results, a discussion of the conditional mean spectra for engineering applications is presented.     

Simplification and Assessment of Modal-Based Story Damage Index for Reinforced Concrete Frames Subjected to Seismic Excitations

Different relations have been represented for the local damage index of structures to date, while the same application is defined for them as can be an indicator of relative sustained damage by the components or stories. Since different force-resisting systems subjected to the ground motions can behave differently, some well-known story damage indices are evaluated for the reinforced concrete frames with regards to their operation during nonlinear time history analysis. Two general concepts of story damage determination are selected for this purpose. SDI is a modal-based story damage index, which is calculated by the modal frequency and mode shapes. The behavior of this local index is evaluated during the seismic excitations. The results were compared with Park-Ang and modal flexibility story damage indices. Based on analytical study on seismic responses of some RC frames subjected to a suit of earthquake records a new story damage index has been developed. It has been derived from a simple global damage equation (softening index) using a normalized ratio of inelastic story shear to its drift. A procedure is recommended to use the proposed equation without any requirement to perform nonlinear dynamic analysis, which can significantly reduce the computational efforts. Distribution of the new represented SDI along the structural height shows a good agreement with damaged state of the RC frames after seismic excitations.     

Seismic Fragility Study of Displacement Demand on Fire Sprinkler Piping Systems

Seismic damage to fire sprinkler piping systems is not only caused by inertial forces or interstory drifts, but also by impact with surrounding objects. The collision of constituents of piping systems with nearby objects increases the chance of damage to the piping itself and to adjacent objects. In this study, the probability of seismic damage to fire sprinkler systems due to impact is quantified by obtaining seismic fragility parameters for large diameter pipes passing through walls and floors, as well as small diameter pipes that typically interact with suspended ceilings. The results of two shaking table experiments conducted at the University of Nevada, Reno and E-Defense test facility, and a high-fidelity numerical model of a hospital piping system are used to evaluate the displacement demands. Piping interaction fragility curves are generated based on clearances between adjacent objects and pipes. The probability of piping interactions and damage to piping systems subjected to different levels of peak floor acceleration is compared for different clearances. It is found that the probability of damage due to impact is comparable with the probability of exceeding other limit states, like the leakage in fittings, when a 1 in or 2 in gap is provided around large and small diameter pipes, respectively.     

Seismic Site Response Modeling for Three Missouri River Highway Bridges

Three highway bridges spanning the Missouri River flood plain were selected for evaluation of seismic site response for moderate size earthquakes emanating from the New Madrid Seismic Zone (NMSZ) in the Midwestern United States. The NMSZ is known to be capable spawning earthquakes larger than magnitude (M) 7.0, four of which occurred in a three-month period between 1811 and 1812, and the M_w 6.0 earthquake of October 1895 centered near Charleston, Missouri. This study evaluated the likely impacts of long period motion of these historic earthquakes on three long-span highway bridges using geotechnical data obtained from recent investigations. Our results suggest site amplification between 6× and 9×, depending on the magnitude and epicentral distance. We believe that threshold magnitude for serious foundation failure and damage to these bridges is between M_w 6.5 and 6.6. Above these magnitudes widespread liquefaction is predicted, which would effect the peak horizontal acceleration and spectral accelerations, causing the ground motions to be different than predicted. Increase in amplification of the response spectra also should be expected where the periods are higher than 1.0 sec. Therefore, M_w 6.5+ earthquakes at ranges 210–260 km could be expected to engender resonant frequency problems for multiple span bridges and tall buildings (10 to 25 stories) in channel corridors containing 20 to 46 m of unconsolidated sediment.