Recent California Earthquakes Research Articles

Which Earthquake Accounts Matter?

AbstractEarthquake observations contributed by human observers provide an invaluable source of information to investigate both historical and modern earthquakes. Commonly, the observers whose eyewitness accounts are available to scientists are a self-selected minority of those who experience a given earthquake. As such these may not be representative of the overall population that experienced shaking from the event. Eyewitness accounts can contribute to modern science only if they are recorded in the first place and archived in an accessible repository. In this study, we explore the extent to which geopolitics and socioeconomic disparities can limit the number of earthquake observers whose observations can contribute to science. We first revisit a late nineteenth-century earthquake in the central United States in 1882 that provides an illustrative example of an event that has been poorly characterized due to a reliance on English-language archival materials. For modern earthquakes, we analyze data collected for recent earthquakes in California and India via the online “Did You Feel It?” (DYFI) system. In California, online data-collection systems appear to be effective in gathering eyewitness accounts from a broad range of socioeconomic groups. In India, however, responses to the DYFI system reveal a strong bias toward responses from urban areas as opposed to rural settlements, as well a bias with literacy rate. The dissimilarity of our results from modern earthquakes in the United States and India provides a caution that, in some parts of the world, contributed felt reports can still potentially provide an unrepresentative view of earthquake effects, especially if online data collection systems are not designed to be broadly accessible. This limitation can in turn potentially shape our understanding of an earthquake’s impact and the characterization of seismic hazard.

Strain-Estimated Ground Motions Associated with Recent Earthquakes in California

ABSTRACTPeak ground velocity (PGV) is a commonly used parameter in earthquake ground-motion models (GMMs) and hazard analyses, because it is closely related to structural damage and felt ground shaking, and is typically measured on broadband seismometers. Here, we demonstrate that strainmeters, which directly measure in situ strain in the bulk rock, can easily be related to ground velocity by a factor of bulk shear-wave velocity and, thus, can be used to measure strain-estimated PGV. We demonstrate the parity of velocity to strain utilizing data from borehole strainmeters deployed along the plate boundaries of the west coast of the United States for nine recent M 4.4–7.1 earthquakes in California, including the largest two events of the July 2019 Ridgecrest earthquake sequence. PGVs derived from maximum horizontal shear strains fall within the range of seismic-estimated values recorded at the same distances. We compare the strain-estimated data with GMMs based on seismic PGVs and find consistency in residual polarity (positive vs. negative; the sign of the difference between observed and modeled data) for certain earthquake–station paths, where some paths indicate an overestimation and others indicate an underestimation of strain-derived PGVs, as compared with the GMMs. We surmise that this may be indicative of over or underestimation of shear-wave velocity along those paths, as compared with the average velocity used to derive PGV from strain measurements, or indicative of repeatable site and path effects that are not accounted for in our analyses. This direct comparison of strain with velocity can highlight physical path effects, as well as improve the density and capability of ground-motion recordings.

Methodological Considerations for the Evaluation of Seismic Risk on Road Network

The recent earthquakes in California and Japan have shown the fundamental role that the road infrastructure plays in emergencies. In fact, only the maintenance of a sufficient level of efficiency can help to quickly reach the affected areas and thus avoid further serious consequences. The necessity of guaranteeing the functionality of the transport network during seismic events therefore requires seismic risk planning extended also to the road infrastructures in order to support the management of post-earthquake emergencies. Analogously it is fundamentally important to have analysis instruments of the road system able to preventatively evaluate the effects of earthquakes in order to identify possible emergencies, therefore preparing a program of intervention to reduce seismic risk on road networks. This paper proposes a methodology for the evaluation of seismic risk of road infrastructures according to the following points: •Study of seismic hazard of the site for the definition of a seismic scenario using attenuation models in relation to historical seismology and the geological and tectonic characteristics of the territory; •Analysis of the direct exposure connected to the probability of the presence of road users on the different parts of the network directly exposed to the seismic event; •Analysis of the indirect exposure relative to the distribution of the population and the infrastructures for which post-earthquake accessibility must be guaranteed; •Evaluation of the functional vulnerability in relation to the potential replaceability of damaged stretches considering network configuration and geometrical characteristics; •Evaluation of structural vulnerability of the stretch correlated to the characteristics (structural, mechanical, technological, etc.) of the different components (bridges, embankments, trenches, tunnels) that make up the stretches obtained by the use of correctly elaborated tables for each component. The determination of global risk indexes of the single stretches and of the network, evaluated by means of a relationship between the ascertained parameters derived from the investigation of the previous points, provides the necessary information for the definition of mitigation measures to reduce the risk and for management planning before and after disaster. The proposed methodology, which has already been applied to a restricted area, is currently being applied to the province of Catania (Sicily, Italy), which is one of the geographical regions of highest seismic risk in Europe, and its future extension to all of eastern Sicily is foreseeable.

Simulation of digital earthquake accelerograms using the inverse discrete Fourier transform

AbstractAcceleration time histories of horizontal earthquake ground motion are obtained by inverting the discrete Fourier transform, which is defined by modelling the probability distribution of the Fourier phase differences conditional on the Fourier amplitude.The Fourier amplitude spectrum is modelled as a scaled, lognormal probability density function. Three parameters are necessary to define the Fourier amplitude spectrum. They are the total energy of the accelerogram, the central frequency, and the spectral bandwidth.The Fourier phase differences are simulated conditional on the Fourier amplitudes. The amplitudes are classified into three categories: small, intermediate and large. For each amplitude category, a beta distribution or a combination of a beta distribution and a uniform distribution are defined for the phase differences. Seven parameters are needed to completely define the phase difference distributions: two for each of the three beta distributions, and the weight of the uniform distribution for phase differences corresponding to small Fourier amplitudes.Approximately 300 uniformly processed horizontal ground motion records from recent California earthquakes are used to develop prediction formulas for the model parameters, as well as to validate the simulation model. The moment magnitude of the earthquakes ranges from 5.8 to 7.3. The source to site distance for all the records is less than 100 km. Copyright © 2002 John Wiley & Sons, Ltd.

Seismic Design and Experimental Verification of Concrete Multiple Column Bridge Bents

In the aftermath of widespread bridge damage in recent California earthquakes, a significant amount of research has been conducted to improve seismic performance of bridge structures. Compiling recent advances, 2 large-scale concrete multiple-column bridge bents were designed and subjected to simulated earthquake loading at the University of California-San Diego. In addition to serving as proof-test specimens, the bridge bents facilitated examination of improved cap beam-to-column joint reinforcement details to minimize construction problems. The design of the joints in the 1st test unit was simplified by using prestressing in the bent cap and examining the force transfer across the joint, which also enabled introduction of precast construction for concrete multiple column bents. In the 2nd test unit, joint reinforcement was minimized by utilizing efficient joint force transfer models and special reinforcement products. Design details, seismic performance, and selected test results are discussed.

Seismic evaluation and retrofit with steel jackets of reinforced concrete bridge piers detailed with lap-splices

Recent earthquakes around the world have confirmed the poor seismic behavior of reinforced concrete bridge piers incorporating typical pre-1971 reinforcement details. Since the 1971 San Fernando earthquake in California, procedures to evaluate accurately the flexural and shear behavior of reinforced concrete bridge piers, as well as retrofit techniques to address economically the most common deficiencies, have been elaborated. In eastern Canada, the majority of reinforced concrete bridge structures incorporate piers with similar reinforcement details as those that suffered severe damage, or collapse, during recent earthquakes in California and Japan. Very little research, however, has been conducted on the seismic behavior of these structures, which often exhibit complex cross-sectional geometries and lap-splices in the plastic hinge region. This paper presents a contribution towards a better understanding of the seismic behavior and retrofit of reinforced concrete bridge piers in eastern Canada through quasi-static tests performed on five 1/3.65-scale pier models of an existing bridge structure in the Montreal region. The first specimen was tested in its existing conditions, while the four others were retrofitted with steel jackets. The geometry of the jacket, the size of the gap at the base of the pier, and the properties of the fill material between the jacket and the original cross section were investigated in these last four tests. A numerical model, considering the bond-slip between the concrete and the longitudinal reinforcement, is proposed to simulate the experimental results. Key words: bond-slip, bridge piers, ductility, hysteresis loops, lap-splices, seismic retrofit, steel jackets.

Retrofit for Control and Repairability of Damage

Collapse or severe damage of a large number of bridges in recent earthquakes in California and Japan has emphasized the need to develop a suitable retrofitting technique for enhancing the flexural ductility and shear resistance of existing bridge columns. However, most retrofit techniques currently in vogue are essentially one-time fixes that do not address the issue of repairability following a seismic event. This paper reports the results of a research program investigating the performance of reinforced concrete bridge columns where the main objective is to control damage and ensure fast rehabilitation with minimum disruption to the traffic flow. Thus a retrofit for control and repairability of damage ( ReCARD) philosophy is developed and validated through experiments on near full-size bridge columns.

Dynamics of Seismic Pounding at Expansion Joints of Concrete Bridges

Motivated by a need to understand the effects of earthquake-induced pounding at thermal expansion joints of concrete bridges, the first part of this paper deals with collinear impact between concrete rods of the same cross section but different lengths. It is shown that (1) the coefficient of restitution between rods depends only on the length ratio and the damping ratio of the rod material and (2) the duration of impact is equal to the fundamental period of axial vibration of the shorter rod. These results are used in the second part of this paper to formulate a realistic yet simple analysis of seismic pounding in concrete bridges. In determining a suitable value of the coefficient of restitution, use is made of the strong-motion data recorded on a concrete bridge that experienced significant pounding during recent California earthquakes. It is shown that (1) seismic pounding generally reduces the column forces; (2) large impact forces generated in the superstructure are not transmitted to the bridge columns and foundations; and (3) pounding does not increase the longitudinal separation at the hinges.

Composite Materials Reinforcement of Existing Masonry Walls

Historically, masonry buildings have been designed with little or no regard for the effects of seismic loads. Recent earthquakes in California, Japan, and other parts of the world have demonstrated that these older masonry structures are extremely susceptible to the lateral forces imposed during such an event. The upgrading of such structures to meet current building code requirements has become a priority in the field of earthquake engineering; however, many of the reinforcing systems currently in use are either ineffective, economically unfeasible, or aesthetically inappropriate. Current research in the use of composite materials for the reinforcement of existing masonry walls has shown that such a system can substantially increase the lateral stability of the structure with minimal impact to the existing foundation. While outlining the preliminary results of the research, this paper focuses primarily on the architectural implications of reinforcing existing masonry walls with composite materials. The problems associated with the various masonry substrates, building code issues related to fire and smoke hazard, and compatible “architectural” finishes are evaluated.

Accidental torsion in buildings due to base rotational excitation

AbstractThis investigation is concerned with accidental torsion in buildings resulting from rotational excitation (about a vertical axis) of the building foundations as a result of spatially non‐uniform ground motions. Because of this accidental torsion, the displacements and deformations in the structural elements of the building are likely to increase. This increase in response is evaluated using actual base rotational excitations derived from ground motions recorded at the base of 30 buildings during recent California earthquakes. Accidental torsion has the effect of increasing the building displacements, in the mean, by less than 5 per cent for systems that are torsionally stiff or have lateral vibration periods longer than half a second. On the other hand, short period (less than half a second) and torsionally flexible systems may experience significant increases in response due to accidental torsion. Since the dependence between this increase in response and the system parameters is complex, two simplified methods are developed for conveniently estimating this effect of accidental torsion. They are the ‘accidental eccentricity’ and the ‘response spectrum’ method. The computed accidental eccentricities are much smaller than the typical code values, 0.05bb or 0.1b, except for buildings with very long plan dimensions (b ≥ 50 m). Alternatively, by using the response spectrum method the increase in response can be estimated by computing the peak response to each base motion independently and combining the peak values using the SRSS rule.

Evaluation of Code Accidental‐Torsion Provisions from Building Records

A procedure is presented for evaluating building‐code provisions for accidental torsion from analysis of earthquake‐induced motions of nominally symmetric‐plan buildings. This procedure is used to analyze the motions of three buildings recorded during recent California earthquakes. Two alternative approaches to evaluate the code accidental torsion provisions are developed. The first One considers the response histories of base shear and base torque in the building, and the second, the “actual” forces in the structural elements during the earthquake. The results show that base rotational motions cause between 25% and 45% of the total accidental torsion in the buildings. They also demonstrate that the accidental torsional moments specified by the Uniform Building Code are sufficient in representing the torsion in the recorded motions of the three buildings. Further, it is shown that accidental torsion need not be considered in the design of two of the three buildings studied. This observation, however, may ...

Medical Aspects of Urban Heavy Rescue

The concept of Urban Heavy Rescue is gaining increasing recognition within the emergency response community. Urban Heavy Rescue has come to denote the unique demands for special equipment and personnel as the result of structural collapse. Recent earthquakes in California, the Philippines, and Soviet Armenia as well as the building collapses in Brownsville, Texas, and New York City provided excellent demonstrations of the concept of specialized structural collapse teams. These events even have prompted the Federal Emergency Management Agency (FEMA) to establish a National Urban Search and Rescue System of special task forces trained in victim location and extrication from collapsed structures. This system will comprise an immediate federal response mechanism for assisting first responders in such activities. The National Search and Rescue System was based on post-event evaluations that pointed out a need not only for more applicable equipment and trained personnel, but also for the timely placement of the these resources.

Point-source inversion techniques

A variety of approaches for obtaining source parameters from waveform data using moment-tensor or dislocation point source models have been investigated and applied to long-period body and surface waves from several earthquakes. Generalized inversion techniques have been applied to data for long-period teleseismic body waves to obtain the orientation, time function and depth of the 1978 Thessaloniki, Greece, event, of the 1971 San Fernando event, and of several events associated with the 1963 induced seismicity sequence at Kariba, Africa. The generalized inversion technique and a systematic grid testing technique have also been used to place meaningful constraints on mechanisms determined from very sparse data sets; a single station with high-quality three-component waveform data is often sufficient to discriminate faulting type (e.g., strike-slip, etc.). Sparse data sets for several recent California earthquakes, for a small regional event associated with the Koyna, India, reservoir, and for several events at the Kariba reservoir have been investigated in this way. Although linearized inversion techniques using the moment-tensor model are often robust, even for sparse data sets, there are instances where the simplifying assumption of a single point source is inadequate to model the data successfully. Numerical experiments utilizing synthetic data and actual data for the 1971 San Fernando earthquake graphically demonstrate that severe problems may be encountered if source finiteness effects are ignored. These techniques are generally applicable to on-line processing of high-quality digital data, but source complexity and inadequacy of the assumed Green's functions are major problems which are yet to be fully addressed.

Seismic Amplitude Measurements Suggest Foreshocks Have Different Focal Mechanisms than Aftershocks

The ratio of the amplitudes of P and S waves from the foreshocks and aftershocks to three recent California earthquakes show a characteristic change at the time of the main events. As this ratio is extremely sensitive to small changes in the orientation of the fault plane, a small systematic change in stress or fault configuration in the source region may be inferred. These results suggest an approach to the recognition of foreshocks based on simple measurements of the amplitudes of seismic waves.

Proceedings

31 January–1 February 1974 Marine Studies Group & the Royal Society Meeting Submarine geology of the English Channel . The full proceedings of this meeting involving about 25 papers will be published as a special publication of the Royal Society. 19 February 1974 Engineering Group Meeting Aspects of hydrogeology overseas . 20 February 1974 Ordinary General Meeting The geology of Montserrat, West Indies . W. J. Rea. Interglacial coleoptera from Bobbitshole, Ipswich, Suffolk . G. R. Coope. These papers will appear in the July issue of the Journal 1974. The type Wenlock Series. M. G. Bassett, L. R. M. Cocks, G. H. Holland, R. B. Rickards & P. T. Warren. The discussion of this paper will accompany the paper itself in a future issue. 7 March 1974. Special General Meeting Discussion on revision of the Bye-laws . Discussion on the report of the Working Party on Professional Recognition . Both these discussions had been preceded by the circulation of documents to all Fellows. 12 March 1974 Engineering Group & British Geotechnical Society Meeting Discussion on the state of the art of engineering rock mechanics . E. Hoek & P. Londe 13 March 1974 Ordinary General Meeting held at Norwich Prediction of geological hazards . Earthquake magnitudes and recurrence intervals, San Andreas fault System, California—significant information for land-use planning . J. C. Cummings. Recent earthquakes in California (e.g. San Fernando 1971) show that existing building codes and development practices are inadequate safeguards against seismic hazards, and that future planning and construction must include greater consideration of the behaviour of

Unexpected geologic effects associated with recent California earthquakes

Summary not available.