Hyogo-ken Nanbu Earthquake Research Articles

Seismic design and seismic performance retrofit study for the Akashi Kaikyo Bridge

This paper presents the original seismic design and seismic performance retrofit study against large-scale earthquakes of the Akashi Kaikyo Bridge. The Akashi Kaikyo Bridge is the world's longest suspension bridge and was completed in 1998. In the original seismic design, an inland near-field earthquake with large magnitude was not considered for the seismic design force. Although the bridge encountered the Hyogo-ken Nanbu Earthquake in 1995, it was not seriously damaged since the earthquake occurred during its construction. Therefore, a seismic performance retrofit study against large-scale earthquakes has been started. In the seismic performance retrofit study, site-specific large-scale earthquakes including inland near-field earthquakes were defined based on the latest seismological information. As a result, it was found that target seismic performance was ensured by taking some minor countermeasures.

Special Issue on Japan’s Advanced Technology for Building Seismic Protection

Modern buildings have more complex, important functions than ever before, and damage to these functions adversely impacted on socioeconomic activity during and after the 1995 Hyogo-Ken Nanbu Earthquake that leveled much of Kobe, Japan. Although many such buildings protected the lives of occupants, their impaired functioning required costly structural and nonstructural repair. Questions have been raised about conventional building structure performance enabling inelastic deformation or considerable damage during a major earthquake, as shown in Fig. 1a. Advanced technology such as building base isolation, shown in Fig. 1b, and passive control by dampers, shown in Fig. 1c, was developed prior to the Kobe disaster and became rapidly accepted after it, in line with a strong desire to better protect structural and nonstructural components. In base isolation, a building is placed on a flexible isolator that absorbs lateral ground movement, preventing vibration in the upper parts of the structure, as shown in Fig. 1b. In passive control, dampers connected to the structural frame dissipate seismic input energy, reducing kinetic energy and vibration of the building, as shown in Fig. 1c. Such advanced technology is currently used for all major buildings and even for small residences in Japan to better protect buildings and their contents. Japan has produced a large number of buildings with the technology, and is believed to have conducted the most extensive research in realizing base isolation and passive control schemes. This special issue of JDR addresses the present and future of Japan’s advanced technology with special reference to major activities related to design, construction, and research. Its purpose is to globally disseminate and share knowledge on promising schemes to help protect lives and social assets against catastrophic earthquakes. This issue covers the current status of base isolation and passive control schemes, unique projects promoting technology for structurally challenging cases, building requirements necessitating the use of advanced technology, the status of current codes and specifications, and new directions in technology. Papers in this issue were authored by leading structural designers and researchers in Japan, to whom we hereby express our deepest gratitude for their invaluable efforts.

Distribution of scatterers in the lower crust beneath the focal region of the 1995 Hyogo-ken Nanbu (Kobe) earthquake from explosion seismic observations

Recent studies indicate that the structure of the lower crust affects the occurrence of large intraplate earthquakes. Therefore, for understanding the generating processes of intraplate earthquakes, it is important to reveal the detailed structure of the lower crust. After the 1995 Hyogo-ken Nanbu (Kobe) earthquake ( M 7.3), the Research Group for Explosion Seismology (RGES) conducted a seismic refraction experiment in the northern part of Awaji Island in order to investigate the detailed structure around the focal region. In the surveyed area of 5 km × 10 km, 128 temporal observation stations were deployed two-dimensionally, and seven explosive shots with a 150-kg charge were detonated. The recorded waveforms contained many later phases with lapse times of 6–11 s from the shots. These later phases imply that strong inhomogeneity exists in the lower crust. Waveforms of the later phases were very complicated and appeared to be coherent only among 8–11 adjacent stations. In this study, we considered the later phases as P-to-P scattered waves and estimated the spatial distribution of scatterers by using the following procedure. We divided the stations into 18 groups, each consisting of 8–11 stations. For each group and each shot, the semblance coefficient is calculated using the seismograms in a frequency range of 4–8 Hz to pick up coherent later phases. The location of a scatterer was estimated from travel time, incoming azimuth, and incident angle of the phase. By applying this procedure, we obtained an image of the scatterer distribution. The scatterers were distributed in regions around 25 km depth in the central part and 30 km depth to the northwest of Awaji Island with a horizontal extent of about 10 km. These two regions were located on the edge of the low- V p, low- V s, and high- V p/ V s zones. The spatial relationship between the source fault, scatterer distribution, and velocity anomalies found in this study was similar to that obtained in other focal regions of intraplate earthquakes, the 2002 western Tottori earthquake ( M 7.3) and the 1962 northern Miyagi earthquake ( M 6.5).

Coseismic Change and Recovery of Scattering Environment in the Crust after the 1995 Hyogo-ken Nanbu Earthquake, Japan

We observe a unique temporal variation in crustal heterogeneity from the analysis of coda Q -1 for 14 years in the Tamba region, northeast to the rupture zone of the 1995 Hyogo-ken Nanbu earthquake ( M JMA 7.3) in southwest Japan. The values of ![Graphic][1] at lower frequencies (1.5–4.0 Hz) that increased coseismically due to the static stress change decreased back to the preevent values in about two years. No such variations are found at higher frequencies (5.0–24.0 Hz). We confirm that no tectonic events that cause a significant stress change occurred during the recovery period. The time required for the recovery of the scattering environment observed here, such as the number density of cracks and cracks opened by the stress change, is consistent with those of previous studies focused on the brittle shallower crust. This suggests a possibility that a similar mechanism of the recovery operates in both the brittle and the ductile parts of the crust. [1]: /embed/inline-graphic-1.gif

Estimation of seismic observation records in the vicinity of large earthquakes and its impact on geological disposal program

AbstractJapan is located at the converging plate boundaries and is one of the most earthquake-prone zones in the world. In order to ensure the safety of a geological repository against earthquakes, a site with a high possibility of direct destruction by active faults should be excluded, and the relationship between the characteristics of seismic ground motion, subsurface structures, geological disposal system and the propagation characteristics of seismic ground motion should be fully investigated. Earthquake-resistant design based on the latest technology is also very important for ensuring the safety of a geological repository.Following rapid improvement of seismic observation networks after the Hyogoken Nanbu earthquake in 1995, numerous seismic observation records have been obtained in the vicinity of large earthquakes. According to these seismic observations, some phenomena that might affect the safety of a geological disposal system have occurred. Some earthquakes occurred in the areas where active faults had not been identified, while some records showed that seismic motion in the deep underground environment was greater than that at the surface. We have identified the implications from the latest information concerning large earthquakes for the geological disposal program. This study made it clear that detailed investigation incorporating state-of-the-art technologies could reduce the likelihood of missing active faults to an extremely low level and a more practical analysis of seismic ground motion could be achieved by taking the latest information into account.

SHAKING TABLE TEST OF ROCKING-CONTROLLED WOODEN WALL WITH COLUMN-BASE ALLOWED UPLIFT BEHAVIOR

Many wooden structures suffered damage in Hyogo-ken Nanbu earthquake more than 10 years ago. Structural damage caused the termination of social and industrial activities, and severe economic loss. New technology which realizes not only seismic resistances but also protecting their functions will be needed for small residential buildings. In this study, wooden wall with column-base allowed uplift behavior is proposed. Rocking-controlled walls make conventional walls work following the 1st mode. Carrying out shaking table test of rocking-controlled wooden wall, non-degrading natural frequency of test specimen was achieved by pre-compression of rocking-controlled devices. And column-base allowed uplift behavior led to decrease in damage to wall.

C14 ウレタンゴム材料を用いた減振装置の基礎的研究(OS8 機械・機械構造物の耐震・免震・振動制御(2)1)

The detached house designed by old design index that was damaged at Hyogo-ken Nanbu earthquake is over 47% in total detached houses in Japan. The upgrading for seismic safety such detached houses is urgent duties because a possibility of occurrence of huge earthquake is expected in a high probability. This study tries to apply urethane material not only to seismic safety element but also to energy absorbing element for detached house as a vibration reduction material at the same time. A linearity of basic mechanical characteristics of urethane rubber is generally much stronger than viscoelastic material such as a natural rubber. Moreover, it is possible to arrange a proper mechanical characteristics that is satisfied the design demand to fit each application cases. The application of urethane material to a detached house contributes to expand the techniques for seismic safety of structures that is a social imperative duties, and also to realize ultra long life of detached house in Japan. As the result, it is confirmed that the test specimen using urethane rubber indicates high linearity in mechanical characteristics and deformation performance by loading tests in condition with the test parameters of shaking frequency and amplitude

地盤条件を考慮した地震による鉄道構造物の被害関数の構築

The damage survey data of railway structures in the 1978 Miyagi-ken Oki earthquake, the 1995 Hyogo-ken Nanbu earthquake, and the 2004 Niigata-ken Chuetsu earthquake are collected and compiled. The seismic vulnerability functions of railway structures for different geomorphologic conditions are constructed in the range of the JMA seismic intensity 4 to 7. The results indicate that the railway structure damage appears from the JMA seismic intensity 5+ and that the damage is higher at mountain, hill or valley plane.

ELASTO-PLASTIC RESPONSES AND PROCESS LEADING TO A COLLAPSE MECHANISM

Shaking table test of full-scale 4-story building was carried out at E-defense. The objectives of this experiment are to evaluate structural and functional performances of the steel building under design-level ground motions, and to determine the safety margin against collapse under exceedingly large ground motions. This paper presents detailed results concerning the nonlinear responses of the specimen by 0.4 and 0.6 times Takatori records of 1995 Hyogoken-Nanbu earthquake. The cause of difference between the mechanism assumed in design and the mechanism actually generated in experiment is investigated from elasto-plastic behaviors of structural members under three dimensional external forces.

防災科研 Hi-net の構築とその成果

After the 1995 Hyogoken-Nanbu earthquake, National Research Institute for Earth Science and Disaster Prevention (NIED) has constructed new nation-wide high sensitivity seismograph network. We call this network “NIED Hi-net” or “Hi-net”. Hi-net stations uniformly cover the Japanese Islands in addition to old high sensitivity seismographs. To reduce the seismic noise near the ground surface, a three-component velocity-type seismograph is installed at the bottom of a borehole. Recorded signal at each station is continuously transmitted to NIED data management center (DMC) at Tsukuba using IP-VPN technology. Hi-net waveform data are sent to the JMA and other related institutes for seismic activity monitoring and seismological research. NIED archives all waveform data generated by the Hi-net and other seismic networks. These waveform data and JMA hypocenter catalog are open to the public via the Hi-net website. Although less than 10 years has passed since the Hi-net data became available, this network has brought us a lot of contributions to geoscience. Stable and long-term observation by this seismograph network may play important roles to mitigate earthquake disasters and to predict earthquakes.

REINVESTIGATION ON PERIOD RANGE OF STRONG GROUND MOTIONS CORRESPONDING TO BUILDINGS DAMAGE

A period region of strong ground motions which have close relationship with actural damage to buildings was reinvertigated including the data by the 2007 Noto-hanto Earthquake and the 2007 Niigata-chuetsu-oki Earthquake, where 1.5-2 sec. and 2-2.5 sec. period dominated records were observed for the first time as the site with building damage data. I found that 1-1.5 sec. period region response had closer relationship with the actual damage to buildings, compare to the previously proposed 1-2 sec. region. The reason why 1-2 sec. in stead of 1-1.5sec. region had close relationship with actural damage to buildings in the previous study was investigated. I found the reason was that the JR Takatori record in the 1995 Hyogoken-Nanbu Earthquake which brought about severe damage around the observation site happened to have a large response around the 2 sec., in addition that 1.5-2 sec. and 2-2.5 sec. period dominated records which were observed in the 2007 Noto-hanto Earthquake and the 2007 Niigata-chuetsu-oki Earthquake were not included in the previous data set.

Revisiting the Basin-edge Effect at Kobe During the 1995 Hyogo-Ken Nanbu Earthquake

The basin edge effect, i.e., the interference of the direct S wave with the surface wave diffracted off the basin edge has been invoked by many authors to explain the damage distribution during the January 17, 1995 Hyogo-Ken Nanbu (Kobe) earthquake. Here we present the results of numerical experiments obtained with the spectral element method in 2-D geometry. Our results confirm that the amplification of horizontal motion close to the basin edge can be twice as large as the one measured in the center of the basin. This additional amplification is shown to depend strongly on the edge geometry and on frequency, due to physical dispersion of diffracted surface waves. In particular we obtain maximal amplification around 3 Hz, at frequencies critical for buildings.

Numerical analysis of deformation behaviour of quay walls under earthquake loading

In the last 50 years, there have been many incidences of failure of gravity quay walls. These failures are often associated with significant deformation of liquefiable soil deposits. Gravity quay wall failures have stimulated great progress in the development of deformation-based design methods for geotechnical structures. In this paper, the effective-stress analysis method has been used in conjunction with a generalised elasto-plasticity constitutive model implemented into a finite element procedure. Various monotonic and cyclic triaxial paths are simulated in order to demonstrate the capabilities of the constitutive model. The FEM is validated by back analysis of a typical Port Island PC1 caisson type quay wall, which was damaged during the 1995 Hyogoken-Nanbu earthquake. The numerical results are compared with the observed data obtained consisting of seaward displacement, settlement and tilting. In addition, both the influence of permeability, on the generation of pore water pressure and the influence of the relative density of the backfill and foundation layers, on the residual deformation of gravity quay walls are investigated.

Large-Scale Shake Table Experiment and Numerical Simulation on the Nonlinear Behavior of Pile-Groups Subjected to Large-Scale Earthquakes

ABSTRACT This paper describes the results of large-scale shake-table experiments involving a 3×3 pile-group. The pile-group was embedded in dry sand and subjected to sinusoidal waves and an earthquake motion recorded from the 1995 Hyogo-ken Nanbu (Kobe) earthquake. The load transfer between soil and pile was derived and the group effect was captured. Numerical simulations were also performed using a Beam-on-Nonlinear-Winkler-Foundation approach with a new hysteretic p-y curve. A comparison of the experimental and numerical results revealed that the numerical simulation is capable of accounting for the soil-pile interaction observed in the experiment.

Investigation of precursory phenomena in the ionosphere, atmosphere and groundwater before large earthquakes of M > 6.5

Investigation of precursory phenomena in the ionosphere, atmosphere and groundwater before large earthquakes of M > 6.5

Seismic performance of bond controlled RC columns

An experimental investigation was carried out to examine the enhancement of seismic performances of reinforced concrete (RC) columns such as shear strength and ductility by controlling the bond of longitudinal reinforcements. Four 300×300 mm square RC columns with ordinary bonded reinforcements and completely unbonded reinforcements were tested under reversed cyclic loading. The test results showed that this method is very effective in completely altering the failure mode at the ultimate state from an undesirable shear to a ductile flexural one. The hysteretic load–displacement behavior of the columns with unbonded reinforcements, however, greatly differs from those of the ordinary ones. A restoring force model capable of capturing the hysteretic behavior of the unbonded columns was therefore proposed. The proposed model was then experimentally verified by conducting pseudo-dynamic tests. The restoring force model was finally applied to conduct seismic response analysis by using input earthquake waves from Hyogoken Nanbu earthquake and El Centro earthquake.

DAMAGE PREDICTION MODELS FOR REINFORCED CONCRETE BUILDINGS BASED ON NON-LINEAR ANALYSIS WITH SOIL-STRUCTURE INTERACTION

It is very important to construct analytical models that can express the actual seismic strength of buildings in order to estimate the damage of building against the future earthquakes. Nagato and Kawase (2001) create such models by comparing the building damage ratios based on the damage statistics of RC (reinforced concrete) buildings during the Hyogo-ken Nanbu earthquake and those of the nonlinear response analyses of buildings models assumed from the current building code using simulated strong motions. In this paper, Nagato's models are improved by considering both the effect of amplification of the earthquake motion in the surface sediments and the effect of the soil-structure interaction. As the results, more appropriate models are proposed, especially for 9- and 12-storied RC buildings.

地震とラドン濃度異常（Ｉ）―従来の観測例―

The relationship between radon anomalies and earthquakes has been studied for more than 30 years. However, most of the studies dealt with radon in soil gas or in groundwater. Before the 1995 Hyogoken-Nanbu earthquake, an anomalous increase of atmospheric radon was observed at Kobe Pharmaceutical University. The increase was well fitted with a mathematical model related to earthquake fault dynamics. This paper reports the significance of this observation, reviewing previous studies on radon anomaly before earthquakes. Groundwater/soil radon measurements for earthquake prediction began in 1970's in Japan as well as foreign countries. One of the most famous studies in Japan is groundwater radon anomaly before the 1978 Izu-Oshima-kinkai earthquake. We have recognized the significance of radon in earthquake prediction research, but recently its limitation was also pointed out. Some researchers are looking for a better indicator for precursors; simultaneous measurements of radon and other gases are new trials in recent studies. Contrary to soil/groundwater radon, we have not paid much attention to atmospheric radon before earthquakes. However, it might be possible to detect precursors in atmospheric radon before a large earthquake. In the next issue, we will discuss the details of the anomalous atmospheric radon data observed before the Hyogoken-Nanbu earthquake.

地震とラドン濃度異常（ＩＩ）―兵庫県南部地震前に観測された大気中ラドン濃度異常―

Before the 1995 Hyogoken-Nanbu earthquake, various geochemical precursors were observed in the aftershock area: chloride ion concentration, groundwater discharge rate, groundwater radon concentration and so on. Kobe Pharmaceutical University (KPU) is located about 25km northeast from the epicenter and within the aftershock area. Atmospheric radon concentration had been continuously measured from 1984 at KPU, using a flow-type ionization chamber. The radon concentration data were analyzed using the smoothed residual values which represent the daily minimum of radon concentration with the exclusion of normalized seasonal variation. The radon concentration (smoothed residual values) demonstrated an upward trend about two months before the Hyogoken-Nanbu earthquake. The trend can be well fitted to a log-periodic model related to earthquake fault dynamics. As a result of model fitting, a critical point was calculated to be between 13 and 27 January 1995, which was in good agreement with the occurrence date of earthquake (17 January 1995). The mechanism of radon anomaly before earthquakes is not fully understood. However, it might be possible to detect atmospheric radon anomaly as a precursor before a large earthquake, if (1) the measurement is conducted near the earthquake fault, (2) the monitoring station is located on granite (radon-rich) areas, and (3) the measurement is conducted for more than several years before the earthquake to obtain background data.

DAMAGE RATIO CURVES OF BUILDING EQUIPMENTS BY USING THE DAMAGE DATABASE DUE TO 1995 HYOGOKEN NANBU EARTHQUAKE

The seismic performance of building equipments as well as building frames is one of critical factors to maintain the function of buildings after the occurrence of earthquakes. Therefore, the damage database of building equipments due to 1995 Hyogoken Nanbu earthquake is investigated to evaluate the seismic performance of building equipments from the statistical point. The damage occurrence is modeled by the binomial distribution, and the damage ratio curves, the shape of which is assumed to be the lognormal distribution, are calculated through the maximum likelihood method. It is evaluated that the differences of the building year, the structural type and the building story influence on the median of damage ratio curves. Moreover, the damage ratio curves are evaluated through the response acceleration and the response story deformation angle in order to compare the build code of equipments.