Showing papers in "Earthquake Spectra in 1988"

The Mexico Earthquake of September 19, 1985—Relationships Between Soil Conditions and Earthquake Ground Motions

Abstract: Comparisons are presented between the characteristics of ground motions at five sites underlain by clay at which ground motions were recorded in Mexico City in the earthquake of September 16, 1985 and for which analyses of ground response have been made, based on the measured properties of soils and the motions recorded on hard formations at the National University of Mexico. It is shown that the ground response in areas of Mexico City underlain by clay is extremely sensitive to small changes in the shear wave velocity of the clay and it is suggested that a probabilistic approach which allows for uncertainties in shear wave velocity measurements and in the characteristics of the motions on the hard formations is desirable to assess these effects. Based on the results of such an approach it is concluded that simple ground response analyses can provide very useful data for engineering assessments of the effects of local soil conditions on the characteristics of ground motions likely to develop at s...

The Mexico Earthquake of September 19, 1985—A Study of Amplification of Seismic Waves in the Valley of Mexico with Respect to a Hill Zone Site:

Abstract: Since the installation of an extensive digital strong motion array by Fundacion Javier Barros Sierra in 1987 three moderate earthquakes have been recorded by the array and by the accelerographs operated by Instituto de Ingenieria, UNAM. Using this new data and results from the analysis of previous accelerograms we present spectral ratios at 40 sites in the valley of Mexico with respect to a hill zone site in Ciudad Universitaria (CU). Clear evidence for nonlinear behaviour of the clay is found at Central de Abastos Oficina (CDAO) site during the great Michoacan earthquake (Ms=8. 1). At four other lake bed sites this behaviour is not seen either because none occurred or because of poorer quality of data. The spectral ratio at a given site appears to be roughly independent of magnitude (except, perhaps, during great earthquakes when lake bed sites may behave nonlinearily), azimuth, and depth of earthquakes with epicenters ≥ 200 km from the city. On the lake bed sites of the valley the relative ampl...

The Mexico Earthquake of September 19, 1985—A Theoretical Investigation of Large- and Small-scale Amplification Effects in the Mexico City Valley

Abstract: The linear, large‐scale and small‐scale amplification effects in the Mexico City valley, related to both the surficial clay layer and the underlying thick sediments, are investigated with two‐dimensional (2D) models and compared with the results of simple one‐dimensional (1D) models. The deep sediments are shown to be responsible, on their own, for an amplification ranging between 3 and 7, a part of which is due to the 2D effects in case of low damping and velocity gradient. This result is consistent with the observed relative amplification around 0.5 Hz at CU stations with respect to TACY station. The amplification due to the clay layer is much larger (above 10), and the corresponding 2D effects have very peculiar characteristics. On the one hand, the local surface waves generated on any lateral heterogeneity exhibit a strong spatial decay, even in case of low damping (2%), and the motion at a given site is therefore affected only by lateral heterogeneities lying within a radius smaller than 1 k...

The Mexico Earthquake of September 19, 1985—On the Seismic Response of the Valley of Mexico

Abstract: In order to explain damage and observed ground motions in Mexico City during the 1985 Michoaca´n earthquake, simultaneous consideration must be given to source, path, and site conditions. This is clear from teleseismic records and local vertical displacements. Incident waves had an important part of energy in the frequency band of 0.3‐1 Hz. Damage distribution and observed motion in the lake bed zone cannot be satisfactorily explained using one‐dimensional theory. The effects of lateral irregularities are required. To assess its effects we describe the stratigraphic setting of the valley and discuss some features of damage distribution with results for one‐ and two‐dimensional wave propagation models. These are useful to establish on quantitative basis the importance of lateral heterogeneity.

The Mexico Earthquake of September 19, 1985—Natural Period of Sites in the Valley of Mexico from Microtremor Measurements and Strong Motion Data

Abstract: The period at which peak in the microtremor Fourier velocity spectra occurs in the transition and lake bed zones of the valley of Mexico is found to be the natural period of the site. These periods in the valley are compiled from the microtremor measurements carried out by Instituto de Ingenieri´a, UNAM and scientists from Japan (for a total of 181 sites). Using this data and the natural periods estimated from strong motion recordings (36 sites), an isoperiod contour map of the valley of Mexico is presented. This map may be useful in future design of important structures.

The Mexico Earthquake of September 19, 1985—Nonstationary Models of Seismic Ground Acceleration

Abstract: Characteristics of a nonstationary process obtained by modulating the amplitude and frequency of a stationary process differ from those of an oscillatory process. An accelerogram recorded in the soft soil of Mexico City during the 1985 earthquake serves to calibrate both nonstationary models. Responses of linear and nonlinear single‐degree‐of‐freedom systems indicate that the process with modulated amplitude and frequency is preferable for reliability studies. Coefficients of variation of ductility demands of systems excited with accelerograms generated by the model with modulated amplitude and frequency are close to those corresponding to actual accelerograms.

The Mexico Earthquake of September 19, 1985—Behavior of Building Foundations in Mexico City

Abstract: During the 1985 earthquake, a number of building foundations in the lacustrine soft soil area of Mexico City presented an inadequate performance. Previous high static contact stresses between the foundation and the supporting subsoil propitiated the appearance of plastic deformations of the soil under seismic cyclic stresses, which led to settlements and tilting of the buildings. Foundations of all kinds showed different degrees of vulnerability to the earthquake, but constructions on friction piles sustained the most severe damages. This can be attributed in some cases to non compliance with the accepted design criteria and current regulations. The main factor was however the pronounced dynamic magnification of seismic movements associated to the quasi coincidence between the natural period of some structures with moderate height on friction piles, and the long period of the subsoil motions, which led to large overturning moments and shear forces at the foundation level.

The Mexico Earthquake of September 19, 1985—The Incident Wavefield in Mexico City during the Great Michoacán Earthquake and Its Interaction with the Deep Basin:

Abstract: We used the vertical displacement records of the ground motion in Mexico City during the great 1985 Michoacan earthquake to identify the nature of the waves responsible for the heavy damage suffered by the town. The records at the different stations exhibit very similar waveshapes. A multichannel phase analysis shows the arrival, from the source zone, of a strong coherent wavetrain which represent the most of the energy incident at periods around 3 sec. We interpreted this arrival as the regional continental phase Lg. The study of other instrumental data in the period range relevant to the present investigation shows the efficiency of Lg propagation between the subduction zone and Mexico City. We have computed the response to Lg waves of a sedimentary basin whose characteristics correspond to the deep basin beneath Mexico City. The amplification reaches a value of about 5. The results obtained are in good agreement with the response of the basin to a single obliquely incident plane wave. The dura...

The mexico earthquake of september 19, 1985, effect of magnitude on the character of strong ground motion: an example from the guerrero mexico strong motion network

Abstract: The Guerrero digital accelerograph network has been operating, since spring of 1985, on rock sites along the coast of Mexico, above an active subduction zone. The accelerograms collected through June 1987 include examples from events with magnitudes from 3 to 8, all recorded at nearly the same hypocentral distance. Spectra from these accelerograms scale in a manner that is qualitatively consistent with earthquake source theory. Based on four selected events, peak accelerations attenuate more rapidly for small events than for large events.

The Mexico Earthquake of September 19, 1985—Consequences, Lessons, and Impact on Research and Practice

Abstract: Considerations generales sur l'inadequation des regles parasismiques, constatee apres le seisme de 1985 a Mexico City, ainsi que sur les difficultes de modelisation des sollicitations comme de la reponse des structures

The Whittier Narrows, California Earthquake of October 1, 1987—Damage to the I‐5/I‐605 Separator

Abstract: The I‐5/I‐605 separator, a major nine span bridge, was severely damaged in the Whittier earthquakes. All columns of Bent 6, which is supported on the median strip of the Santa Ana Freeway (I‐5) suffered shear failures, and the supported spans came close to collapse. Other minor damage included minor column hinging at more flexible bents, abutment pounding, and fracture of keeper plates at abutment rockers. Apparently much of this minor damage occurred during a major aftershock. Preliminary calculations indicate that the shear failures of Bent 6 resulted from disproportionate flexural and shear strengths of the columns, attributable to the working stress philosophy adopted in design, and the much greater stiffness of Bent 6 than adjacent bents. Movement joints with cable restrainers, at both ends of spans founded on Bent 6, make estimation of lateral load transfer from more flexible regions difficult.

The Mexico Earthquake of September 19, 1985—General Soil Conditions and Clay Properties in the Valley of Mexico

Abstract: We present and discuss the results of resonant column and cyclic triaxial tests on clay samples obtained from different sites within the Lake zone in the Valley of Mexico. Of particular interest are the nearly elastic behavior and low damping ratio even for shear strain amplitudes as high as 0.3 (%). A hyperbolic model reproduces adequately well the resulting shear modulus vs strain curves. Degradation of shear modulus caused by load repetition is negligible for strains lower than about 1 (%) but increases significantly for higher strains. A power‐type expression fits well the modulus degradation vs number of cycles curves. Results from static triaxial tests indicate that for compression stress paths the induced pore water pressure is uniquely related to axial strains. Analyses of ground motions show that one dimensional wave propagation models may be used to predict free field seismic motions in most parts of the Lake zone.

The Mexico Earthquake of September 19, 1985—Estimation of Response Spectra in the Lake Bed Zone of the Valley of Mexico

Abstract: We estimate pseudo acceleration response spectra with 5% damping (Sa) for the great Michoaca´n earthquake at 22 lake bed sites (in addition to five sites where the event was well recorded) in the valley of Mexico. The method used consists of (a) estimation of Fourier acceleration amplitude spectra (FAS) at the sites via observed FAS at Ciudad Universitaria (CU) and compiled spectral ratios of these sites with respect to CU obtained from recorded earthquakes, (b) estimation of the duration of strong ground motion during the Michoaca´n earthquake at the sites, and (c) application of random vibration theory to predict Sa. A constant duration of 71 sec at all sites is found to be satisfactory. The main uncertainties in estimated Sa come from (a) possible nonlinear behaviour of the clay during the great earthquake (the spectral ratios used in the computation are mostly for moderate earthquakes, when the clay behaves linearly), and (b) variations in site's spectral ratios due to randomness and/or its d...

The Whittier Narrows, California Earthquake of October 1, 1987—Performance of Buildings at California State University, Los Angeles

Abstract: The Whittier Narrows earthquake (Richter Magnitude 5.9), which struck Southern California in the morning of October 1, 1987, caused some $20.5 million damage to various engineered buildings at California State University, Los Angeles and the death of a student due to a falling precast concrete panel. Peak ground acceleration of 0.39 g and a building response of 0.48 g were recorded in one of the affected buildings. The damage was both structural (cracking of reinforced concrete beams, columns and shear walls) and non‐structural (cracked plaster and partition walls, dislodged and broken ceiling tiles, lights and other fixtures, damage to the science, computer and mechanical equipment and to utilities). A fire broke out in one chemistry laboratory and toxic combinations of chemicals were created by the spills in another chemistry laboratory. Shaking from the earthquake also aggravated existing friable asbestos contamination, forcing closure of one and portions of other buildings.

The Whittier Narrows, California Earthquake of October 1, 1987—CSMIP Strong Motion Data

Abstract: The Whittier Narrows earthquake of October 1, 1987 generated the largest set of strong‐motion records ever obtained from a single earthquake. The California Strong Motion Instrumentation Program (CSMIP) recovered 128 strong‐motion records from 101 stations. Of these 101 stations, 63 are ground‐response stations and 38 are extensively‐instrumented structures. The structures include 27 buildings, eight dams, a suspension bridge, an airport tower, and a power plant. This paper summarizes that data set and highlights records of particular interest. The duration of strong shaking was approximately 3 to 4 seconds at most stations. The maximum peak acceleration values in the CSMIP data set are 0.62 g on the ground and 0.54 g in a structure. The largest acceleration (0.62 g) was recorded at a station near Tarzana, approximately 45 km from the epicenter. Other records of particular interest discussed here include the record from a soft‐story building on the Los Angeles State University campus and the reco...

The Mexico Earthquake of September 19, 1985—Complementary Technical Norms for Earthquake Resistant Design, 1987 Edition

Abstract: Complement a la partie sismique du code mexicain du bâtiment (1976), tenant compte des lecons du seisme de 1985 a Mexico City

The Mexico Earthquake of September 19, 1985—Probability Distribution of Times Between Characteristic Subduction Earthquakes

Abstract: When earthquakes from a single source govern a structure's design, decisions are sensitive to the probability distribution of times between characteristic earthquakes and correlations between time elapsed and magnitude. Scarcity of data prevents a purely statistical study of these matters while absence of trustworthy theories prevents a purely analytical approach. Based on simulation using a physically founded mathematical model and on data from various parts of the world, we postulate a slip‐predictable model and four candidate distributions with random parameters for the distribution of interoccurrence times in subduction zones: lognormal, gamma, inverse gaussian and Weibull. Probabilities are updated using data from three groups of subduction earthquakes. In all cases the lognormal distribution minimizes the disutility due to choosing a single model. The same distribution with fixed parameters gives almost equally good results.

The Mexico Earthquake of September 19, 1985—Correlations Between Dynamic and Static Properties of Mexico City Clay

Abstract: It is the purpose of this paper to present and discuss several empirical correlations between dynamic shear modulus and void ratio, and static shear modulus; shear wave velocity and electric cone point resistance. Also a discussion and a comparison is made of the results of field and laboratory measurements of Mexico City clay dynamic properties.

The Mexico Earthquake of September 19, 1985—An Empirical Model to Predict Fourier Amplitude Spectra of Horizontal Ground Motion

Abstract: We analyze Fourier acceleration amplitude spectra of 14 coastal earthquakes (5.6 ≤ Ms ≤ 8.1) recorded at a hill zone site in Ciudad Universitaria (CU), Mexico City (282 ≤ R ≤ 466 km, R= closest distance from CU to the rupture area). The observed smoothed spectra are fitted to the following model log FS(T) = A(T) Ms + C(T) log(R) + B(T) where FS is the maximum spectral value of either of the two horizontal components at period T. The coefficients A, C, and B are obtained for 18 periods (2.5 ≥ T≥ 0.2 sec). The observed spectra at CU are not reliable for most earthquakes beyond this period range. The predicted FS for the September 19, 1985, Michoaca´n earthquake (Ms = 8.1) is less than the observed FS for T ≥ 0.6 sec suggesting an anomalously large radiation towards Mexico City during the earthquake at these periods. This anomalous radiation is in agreement with the teleseismic evidence. FS at several sites in Mexico City (many on the lake bed zone) can be obtained from the predicted FS at CU and th...

The Whittier Narrows, California Earthquake of October 1, 1987 ― simulation of recorded accelerations

Abstract: We have simulated accelerograms from many of the strong motion stations close to the mainshock of the 1987 Whittier Narrows earthquake using a semi‐empirical Green's function summation technique. This method allows gross aspects of the source rupture process to be treated deterministically using a kinematic model based on first motion studies, teleseismic modeling and the distribution of aftershocks. Stochastic aspects of the rupture process are then included to simulate irregularity in both rupture and slip velocity. Gross aspects of wave propagation are modeled using theoretical Green's functions calculated with generalized rays. Detailed aspects of the source radiation at high frequencies, as well as unmodeled propagational aspects such as scattering, are included empirically by using multiple recordings of a smaller Imperial Valley earthquake as empirical source functions. Our main objective is to see how well we can predict the peak ground accelerations, time histories and response spectra o...

The Whittier Narrows, California Earthquake of October 1, 1987 ― evaluation of strengthened and unstrengthened unreinforced masonry in Los Angeles City

Abstract: On November 15, 1987, the City of Los Angeles' Earthquake Division, launched a study of the performance of its strengthened unreinforced masonry buildings during the Whittier Narrows Earthquake. The objective of the study was twofold: (1) To analyze the damage to unstrengthened as well as to strengthened and tension‐anchored‐only buildings, and from that analysis (2) to determine the most effective ways of improving the design standards for strengthening unreinforced masonry buildings. The initial part of that objective has been completed, and the findings of that study are the primary basis for this article. Observed damage demonstrated a clear‐cut need to improve certain aspects of the Code design standards for strengthened buildings; more importantly, however, it sent out warning signals to owners of unstrengthened buildings and only to a slightly lesser extent to owners of tensioned‐anchors‐only buildings, of the very serious need to fully strengthen their buildings and to improve on the low ...

The Whittier Narrows, California Earthquake of October 1, 1987—Performance of Tilt‐up Buildings

Abstract: In the 1971 San Fernando earthquake the general performance of tilt‐up structures was poor, and a number of collapses occurred. After this event substantial changes were made to seismic code provisions governing the design and construction of these buildings. The October 1, 1987 Whittier Narrows earthquake produced strong shaking (up to Modified Mercalli Intensity VIII) in an area containing thousands of tilt‐up buildings of various ages. This recent earthquake thereby provided the first significant test of these new provisions.

The Whittier Narrows, California Earthquake of October 1, 1987—Response of Lifelines and Their Effect on Emergency Response:

Abstract: The response of lifeline systems was good, but there were component failures. Communication inadequacies did have a significant impact on the emergency response system. The dysfunction of lifelines, primarily power, communication, and natural gas systems put an added burden on the emergency response system and was the source of most fires. Damage and dysfunction included the frequently observed failure of ceramic elements on high voltage substation equipment, damage to large liquid storage tanks, and the saturation of the telephone system, including emergency response networks, with inappropriate calls. These hold the potential for compounding impacts after larger, more damaging earthquakes. The need to change operating procedures of some lifelines and to update emergency response public information is indicated.

The Whittier Narrows, California Earthquake of October 1, 1987—Social Aspects

Abstract: There were a relatively small number of fatalities and casualties in the Whittier Narrows, but social disruption was extensive. An estimated 10,000 households and thousands of businesses were affected. The earthquake also resulted in an increase in the need for mental health services. Due both to extensive mitigation and preparedness programs and to the moderate size of the event, the emergency response was, in general, well‐managed. Nevertheless, important insights were gained on potential weaknesses in areas such as communication co‐ordination. The response by local residents was generally positive and adaptive, but the need for improved public awareness and training was also evident. The earthquake demonstrated the importance of ethnicity as a factor that influences the public's response to the earthquake hazard. Although the long‐term impact of the earthquake on the Southern California will likely be imperceptable, some segments of the population, such as low‐income earthquake victims and bus...

The Whittier Narrows, California Earthquake of October 1, 1987—Note on Peak Accelerations During the 1 and 4 October Earthquakes

Abstract: Attenuation patterns of the recorded peak accelerations during two moderate earthquakes (ML = 5.9 and 5.3) in Los Angeles, California are described. It is shown that the recording of earthquake motions by dense arrays of accelerographs can yield a detailed and deterministic picture of the physical processes which are involved in shaping the observed variations of strong ground motion. For the two earthquakes the observed changes of peak amplitudes with respect to the azimuth and distance are slowly and continuously changing functions showing strong dependence of amplitudes on the radiation patterns of the two earthquakes and on the effects of wave propagation through irregular three‐dimensional geology of the Los Angeles basin.

The Mexico Earthquake of September 19, 1985—Model for Generation of Subduction Earthquakes:

Abstract: A mathematical model for rational decision‐making should be based not only on statistical data but also on the salient features of the physical phenomena at play. The model developed here simulates the magnitudes and occurrence times of subduction earthquakes but it may also apply to other tectonic seisms. It uses linear springs and dashpots to idealize the convection drag and ridge push on tectonic plates and the latter's behavior. It divides the rupture area into many faultlets whose frictional strength is lognormally distributed; this strength drops upon breakage and is then gradually recovered. Without introducing ad‐hoc features to fit the seismic data, the model produces essentially correct magnitude exceedance rates, quasiperiodic characteristic earthquakes, sequences of fore‐ and aftershocks, and background noise. The model can be expanded to include spatial correlation.

The Whittier Narrows, California Earthquake of October 1, 1987—Early Results of Isoseismal Studies and Damage Surveys

Abstract: Preliminary isoseismals for Modified Mercalli intensities are presented for the Whittier Narrows Earthquake. Isoseismals for intensities VI and lower are based on responses to a mail survey. Intensity VII and larger are based on a field survey of damage described in this paper. The maximum observed intensity of VIII was confined to Whittier. The shapes of the intensity contours compare favorably with the distribution of average peak acceleration data from the strong motion array in the greater Los Angeles area. The damage assessments appeared consistent with earthquake magnitude. However, the accelerations were higher than expected for the magnitude. The building classification and survey strategies developed were tested and found usable and adequate for describing damage. With further refinement this system can be used to describe damage within a limited geographical area and in a format useful for correlations with strong ground motions and the Modified Mercalli Intensity scale.

The Whittier Narrows, California Earthquake of October 1, 1987—Response of Elevators

Abstract: The Whittier Narrows earthquake has been the first earthquake to affect a significant number and many of the same elevator systems that now conform to the more stringent code provisions introduced after the 1971 San Fernando earthquake. While the relative damage potential of the two earthquakes is not now known, much less manchine room and rope damage was observed. The incidence of counterweights coming out of their guide rails was much less than it was from the San Fernando earthquake but still quite common. The performance and operation of elevator earthquake protection devices, seismic switches and counterweight derail detection devices, did not fully meet expectations but they did significantly reduce damage and the potential for injury. There was some inconsistency in the triggering of seismic switches. The seismic performance of elevators suggest that several changes to codes should be considered including a requirement to report earthquake damage and prevent seismic devices from being rese...

The Whittier Narrows, California Earthquake of October 1, 1987—Seismology

Abstract: The October 1, 1987 Whittier Narrows earthquake (ML = 5.9) was located at 34° 3.0′N, 118° 4.8′W, at the northwestern end of the Puente Hills. The sequence ruptured a small part, 4 km by 5 km, of a previously unidentified, buried, thrust fault that strikes east‐west and dips 25° down to the north. This fault may be part of a large system of thrust faults extending across the entire east‐west length of the northern margin of the Los Angeles basin. The focus of the mainshock is deep, at 14 ± 1 km. The largest aftershock (ML = 5.3) produced mostly strike‐slip movement on a steeply dipping, northwest plane, that bounds the mainshock rupture area to the west. Enhancement of the Los Angeles basin seismic network would facilitate investigation of the potential of these faults for moderate‐sized or large earthquakes.

The Whittier Narrows, California Earthquake of October 1, 1987—Preliminary Analysis of Peak Horizontal Acceleration

Abstract: The Ml=5.9 Whittier Narrows, California, earthquake triggered several hundred accelerographs in the greater Los Angeles area. One‐hundred and sixty‐eight of these were used to develop attenuation relationships for peak horizontal acceleration. The analysis indicates that the attenuation of peak acceleration during the earthquake was generally consistent with that predicted from the attenuation relationships of Campbell (in press). However, the acceleration amplitudes were about 65‐percent higher than predicted. An analysis of residuals clearly showed that the ground motions recorded during this earthquake were influenced by a complex interaction of source mechanism, building embedment, site geology, and geography. Source effects may have been responsible for the higher‐than‐expected accelerations as well as some of the observed azimuthal variation. The correlation of peak acceleration with geography may have been caused in part by the gross geologic structure of the region. Buildings with basemen...