Strong Earthquake Events Research Articles

Precursory behavior of fractal characteristics of the ULF electromagnetic fields in seismic active zones before strong earthquakes

It is now recognized that the earthquake hazard systems evolve naturally to the self-organized critical (SOC) state, in which the system is very sensitive to any external perturbations, and when the large avalanches (strong earthquakes) are probable events. Since the principal feature of the SOC state is fractal organization of the output parameters both in space (scale-invariant structure) and in time (flicker noise or 1/ f noise) we can use fractal methods to investigate the evolutionary processes in the earthquake hazard system at different stages of the catastrophic event preparation. Here we apply fractal methods for extraction of earthquake precursory signatures from ULF ( f=0.001–1 Hz) geomagnetic data obtained in seismic active regions before strong earthquake events. We focus our attention on the massive Guam earthquake of August 8, 1993 ( M=8, depth=60 km), and on a swarm of Japanese earthquakes of June–August 2000 occurred near Izu Peninsula ( M>6, depth ≈ 10 km). We analyze scaling (fractal) characteristics of ULF geomagnetic fields registered in the area less than 100 km from the earthquake epicenters, study their dynamics as approaching the earthquake dates, and compare the results obtained in both regions. Three methods have been applied to calculate scaling parameters (spectral exponents and fractal dimensions of the ULF geomagnetic time series): FFT procedure, Burlaga–Klein approach and Higuchi method. It is found that fractal characteristics of the ULF emissions manifest specific precursory behavior with some common and individual peculiarities in both seismic active areas. As the common feature, we have revealed the same increase of the ULF time series fractal dimensions (and the corresponding decrease of spectral exponents) before the both events. As the distinctive peculiarity, we have found different character of such dynamics: gradual increase (decrease) in the case of the Guam earthquakes and relatively sharp alteration in the case of the Izu earthquake swarm. For the case of Japanese earthquakes, it turned out to be possible to reveal the most effective frequency range (around f=0.01 Hz), in which precursory behavior of fractal characteristics is more pronounced and manifested earlier than in the other frequency ranges. We give a physical interpretation of the peculiarities revealed taking into account possible specifics of the SOC processes at different depths of the earthquake focuses.

Seismic response of rail-counterweight systems in elevators

The rail-counterweight systems in building elevators are known to be susceptible to earthquake-induced ground motions. Besides avoiding costly repairs and economic disruptions, it is of special interest to maintain the functionality of the elevators in critical facilities such as hospitals during, and especially after, a strong earthquake event. This paper presents an approach to develop a realistic analytical model of these vulnerable systems to study their seismic response behaviour. The model includes the flexibilities of the guidance and supporting components of the counterweights in a systematic manner. Currently only the linear response behaviour is considered; however, the sources of non-linearities in the flexible components are clearly identified. The model considers the effect of the differential support motions from the building. Both the out-of-plane and in-plane responses of the rail-counterweight are studied and included in the stress calculations. Several sets of numerical results considering simultaneous action of the two orthogonal components of historic earthquakes are obtained to study the seismic response characteristics of the rail-counterweight system. Copyright © 2001 John Wiley & Sons, Ltd.

Seismic response of rail‐counterweight systems in elevators

AbstractThe rail‐counterweight systems in building elevators are known to be susceptible to earthquake‐induced ground motions. Besides avoiding costly repairs and economic disruptions, it is of special interest to maintain the functionality of the elevators in critical facilities such as hospitals during, and especially after, a strong earthquake event. This paper presents an approach to develop a realistic analytical model of these vulnerable systems to study their seismic response behaviour. The model includes the flexibilities of the guidance and supporting components of the counterweights in a systematic manner. Currently only the linear response behaviour is considered; however, the sources of non‐linearities in the flexible components are clearly identified. The model considers the effect of the differential support motions from the building. Both the out‐of‐plane and in‐plane responses of the rail‐counterweight are studied and included in the stress calculations. Several sets of numerical results considering simultaneous action of the two orthogonal components of historic earthquakes are obtained to study the seismic response characteristics of the rail‐counterweight system. Copyright © 2001 John Wiley & Sons, Ltd.

Integrated Use of Seismograph and Strong-Motion Data to Determine Soil Amplification: Response of the Fraser River Delta to the Duvall and Georgia Strait Earthquakes

We present a new method to estimate site amplification, in which re- gional seismographic data are used to constrain the input motions to the soil profile. The amplification is determined as the ratio of the Fourier spec- trum of the recorded motion on soil to the spectrum of the inferred input motion at the underlying bedrock interface. The method is demonstrated for sites on the Fraser River Delta, British Columbia (B.C.), that recorded the 1996 M 5.1 Duvall, Wash- ington, and 1997 M 4.3 Georgia Strait, B.C., earthquakes. Regional seismographic recordings on rock sites are used to determine the apparent source spectra of these events. These spectra are attenuated to sites in the Fraser River Delta, then input to a specified soil profile. We compare the empirical-inferred amplification to alternative estimates of the soil amplification based on (1) the quarter-wavelength approach and (2) the assumption that the amplification is approximately equal to the observed H/V ratio. The empirical-inferred amplification estimates are grossly consistent with the theoretical amplification estimates, but have larger peaks at specific frequencies, probably due to resonant effects not accounted for in the quarter-wavelength ap- proach. The H/V ratio appears to be a simple and useful first approximation to the site response. The Fraser River Delta amplifies weak motions by factors of three to six over a broad frequency range, from 0.2 to 4 Hz. High-frequency motions (f 10 Hz) are attenuated. The long-period amplifications pose a serious concern in the event of strong earthquake shaking from large earthquakes. A useful by-product of the analysis is the determination of apparent source spectra for the events. The apparent source spectrum of the Georgia Strait earthquake is in good agreement with the Brune point-source model, with a stress drop of approxi- mately 45 bars. The apparent source spectrum of the slightly larger Duvall earthquake is in reasonable agreement with the Brune model, with a stress drop of about 70 bars; however, for the Duvall earthquake there is evidence of a sag in spectral amplitudes, relative to the Brune model, at intermediate frequencies (0.3 to 1 Hz). This inter- mediate-frequency sag is an expected result of source complexity; it is well dem- onstrated by the broadband seismic data.

Earthquake events in Neoproterozoic and Early Paleozoic and its relationship with supercontinental Rodinia in North China

Carbonate liquefied veins in limestone and liquefied mud veins in mudstone are the result by earthquake, which is liquefied in sediments and a strong earthquake event remains clearly records in strata. 24 strata units with earthquake events (molar tooth), which are subdivided into 11 seismic periods, have been distinguished from Mid-Proterozoic to Early Paleozoic strata in the Sino-Korean platform. The distributions of them represent paleo-seismic tectonic zones in different periods, which is tallied with convergent continental block before Proterozoic supercontinental Rodinia and the evidence of the breakup or splitting developed in the inner Sino-Korean Platform in 700 Ma.

Disaster in relation to the Hanshin-Awaji earthquake: A comment on risk-informed design

Following the disastrous Hanshin-Awaji Great Earthquake, which occurred on 17 January 1995 and caused a loss of approximately 6000 lives and damage of more than US$100 billion, a concept of risk-informed design has gradually been discussed in Japan. This event, the so-called Kobe earthquake, brought unexpected structural damage to buildings and civil engineering structures. In Japan, the seismic design procedure of such structures had been developed to a high level, and various codes and guidelines had been established in approximately 40 fields including the Building Code. Even so, there was serious damage in a highly populated area of Kobe city, one of the most famous and beautiful port cities of Japan, stretching over several square kilometres. The author estimated that the probability of recurrence of such a serious event in the Kobe area will be less than one in 10 000 - 100 000 years, even if the cycle of occurrence of strong earthquake events is of the order of 1000 years. Accounting for the occurr...

An experimental study of a midbroken 2-bay, 6-storey reinforced concrete frame subject to earthquakes

A 2-bay, 6-storey model test reinforced concrete frame (scale l:5) subjected to sequential earthquakes of increasing magnitude is considered in this paper. The frame was designed with a weak storey, in which the columns are weakened by using thinner and weaker reinforcement bars. The aim of the work is to study the global response to a damaging strong motion earthquake event of such buildings. Special emphasis is put on examining to what extent damage in the weak storey can be identified from global response measurements during an earthquake where the structure survives, and what level of excitation is necessary in order to identify the weak storey. Furthermore, emphasis is put on examining how and where damage develops in the structure and especially how the weak storey accumulates damage. Besides the damage in each storey the structure is identified by a static load at the top storey while measuring the horizontal displacement of the stories and also visual inspection is performed. From the investigations it is found that the reason for failure in the weak storey is that the absolute value of the stiffness deteriorates to a critical value where large plastic deformations occur and the storey is not capable of transferring the shear forces from the storeys above so failure is unavoidable.