Effect of near-fault earthquake on bridges:lessons learned from Chi-Chi earthquake

The objective of this paper is to describe the lessons learned and actions that have been taken related to the seismic design of bridge structures after the Chi-Chi,Taiwan earthquake.Much variable near-fault ground motion data was collected from the rupture of Chelungpu fault during the Chi-Chi earthquake,allowing the seismic response of bridge structures subjected to these near-fault ground motions to be carefully examined.To study the near-fault ground motion effect on bridge seismic design codes,a two-level seismic design of bridge structures was developed and implemented.This design code reflects the near-fault factors in the seismic design forces.Finally,a risk assessment methodology,based on bridge vulnerability,is also developed to assist in decisions for reducing seismic risk due to failure of bridges.

Effects of the hanging wall and footwall on peak acceleration during the Jiji (Chi-Chi), Taiwan Province, earthquake

The M=7.6 Jiji (Chi-Chi) earthquake, Taiwan Province, on September 21, 1999 (local time) is a thrust fault style earthquake. The empirical attenuation relations of the horizontal and vertical peak ground accelerations (PGA) for the Jiji (Chi-Chi) earthquake are developed by regression method. By examining the residuals from the Jiji (Chi-Chi) earthquake-specific peak acceleration attenuation relations, it is found that there are systematic differences between PGA on the hanging-wall and footwall. The recorded peak accelerations are higher on the hanging-wall and lower on the footwall. The clear asymmetry of PGA distribution to the surface rupture trace can also be seen from the PGA contour map. These evidences indicate that the PGA attenuates faster on the hanging-wall than on the footwall. In the study of near-source strong motion, seismic hazard assessment, scenario earthquake and seismic disaster prediction, the style-of-faulting must be considered in order that the attenuation model can reflect the characteristic of ground motion in various seismic environmental regions.

Bi-normalized response spectral characteristics of the 1999 Chi-Chi earthquake

To develop uniform and seismic environment-dependent design spectrum,common acceleration response spectral characteristics need to be identified.In this paper,a bi-normalized response spectrum (BNRS) is proposed,which is defined as a spectrum of peak response acceleration normalized with respect to peak acceleration of the excitation plotted vs.the natural period of the system normalized with respect to the spectrum predominant period,Tp.Based on a statistical analysis of records from the 1999 Chi-Chi earthquake,the conventionally normalized response spectrum(NRS) and the BNRS are examined to account for the effects of soil conditions,epicentral distance,hanging wall and damping.It is found that compared to the NRS the BNRS is much less dependent on these factors.Finally,some simple relationships between the BNRS for a specified damping ratio and that for a damping ratio of 5%,and between the spectra predominant period and epicentral distance for different soil types are provided.

Dynamic rupture process of the 1999 Chi-Chi,Taiwan,earthquake

In this study, we preliminarily investigated the dynamic rupture process of the 1999 Chi-Chi, Taiwan, earthquake by using an extended boundary integral equation method, in which the effect of ground surface can be exactly included. Parameters for numerical modeling were carefully assigned based on previous studies. Numerical results indicated that, although many simplifications are assumed, such as the fault plane is planar and all heterogeneities are neglected, distribution of slip is still consistent roughly with the results of kinematic inversion, implying that for earthquakes in which ruptures run up directly to the ground surface, the dynamic processes are controlled by geometry of the fault to a great extent. By taking the common feature inferred by various kinematic inversion studies as a restriction, we found that the critical slip-weakening distance Dc should locate in a narrow region [60 cm, 70 cm], and supershear rupture might occur during this earthquake, if the initial shear stress before the mainshock is close to the local shear strength.

Characteristics of response spectra for long-periods of main-shock recordings of the Chi-Chi earthquake

Current practice uses predictive models to extrapolate long-period response spectra based on far-field recordings in moderate and weak earthquakes. However, the spectra are not long enough and the data are often not reliable, which means that the seismic design code cannot accurately define seismic design requirements for long-period structures. The near-field recordings in the main-shock of the Chi-Chi earthquake have a large signal-to-noise ratio （SNR）, which makes them suitable for studying the long-period acceleration response spectrum up to 20 sec. The acceleration response spectra from 246 stations within 120 km of the causative fault are statistically analyzed in this paper. The influence of distance and site conditions on long-period response spectrum is discussed, and the shapes of the amplification spectra are compared with the standard spectra specified in the seismic design code of China. Finally, suggestions for future revisions to the code are proposed.

Detecting changes in long-period site responses after the M_w 7.6 Chi-Chi earthquake, Taiwan, using strong motion records

Temporal changes in site effects are obtained using the HVSR(horizontal-to-vertical spectral ratio) method and strong motion records after the M w 7.6 Chi-Chi earthquake, Taiwan. Seismic data recorded between 1995 and 2010 are used, comprising 3,708 data from 15 stations adjacent to the Chelungpu fault. Temporal fl uctuations are determined by analyzing the site effect variation using a time–frequency variation(TFV) diagram based on these seismic data. Stations adjacent to the fault show signifi cant disturbances in the resonance frequency at 16–26 Hz. Station TCU129 shows a 40% drop in fundamental frequency after the main shock, and a gradual return to the original state over nine years. For stations located farther from the fault zone, sudden changes in tectonic stress play a dominant role in temporal changes to the HVSR. An impact analysis of the directional factor confi rms our fi nding that the proximity of the fault to seismic stations has the most infl uence on data.

Seismicity gap and seismic quiescence before 1999 Jiji(Chi-Chi) M_W7.6 earthquake

The September 21, 1999, Jiji （Chi-Chi） Mw7.6 earthquake is the strongest event occurred since 1900 in Taiwan of China. It is located in the middle segment of the western seismic zone of Taiwan. Based on several versions of China earthquake catalogue this study found that a seismic gap of M≥5 earthquakes appeared, in and around the epicenter region, 24 years before and lasted up to the mainshock occurrence. This study also noticed that there existed a lager seismically quiet region of M≥4 earthquakes, which lasted for about 2.5 years before the mainshock occurrence, The spatial variation pattern of regional seismicity before the mainshock seems to match with its coseismic source rupture process. The mentioned seismicity gap and seismic quiescence might be an indication of the preparation process of the Jiji strong earthquake.

Source time functions of the 1999, Jiji (Chi-Chi) earthquake from GDSN long period waveform data using aftershocks as empirical Green's functions

A large earthquake (Mw=7.6) occurred in Jiji (Chi-Chi), Taiwan, China on September 20, 1999, and was followed by many moderate-size shocks in the following days. Two of the largest aftershocks with the magnitudes of Mw=6.1 and Mw=6.2, respectively, were used as empirical Green's functions (EGFs) to obtain the source time functions (STFs) of the main shock from long-period waveform data of the Global Digital Seismograph Network (GDSN) including IRIS, GEOSCOPE and CDSN. For the Mw=6.1 aftershock of September 22, there were 97 pairs of phases clear enough from 78 recordings of 26 stations; for the Mw=6.2 aftershock of September 25, there were 81 pairs of phases clear enough from 72 recordings of 24 stations. For each station, 2 types of STFs were retrieved, which are called P-STF and S-STF due to being from P and S phases, respectively. Totally, 178 STF individuals were obtained for source-process analysis of the main shock. It was noticed that, in general, STFs from most of the stations had similarities except that those in special azimuths looked different or odd due to the mechanism difference between the main shock and the aftershocks; and in detail, the shapes of the STFs varied with azimuth. Both of them reflected the stability and reliability of the retrieved STFs. The comprehensive analysis of those STFs suggested that this event consisted of two sub-events, the total duration time was about 26 s, and on the average, the second event was about 7 s later than the first one, and the moment-rate amplitude of the first event was about 15% larger than that of the second one.

Characteristics of frequency content of near-fault ground motions during the Chi-Chi earthquake

The frequency content of earthquake ground motion is very important because it affects the dynamic response of structural systems. In the paper, we use five scalar parameters (the response spectral predominant period Tp, smoothed spectral predominant period To, Fourier amplitude spectral mean period Tm, equivalent pulse period Tv, and the pseudo-velocity spectral predominant period Tpv) reflecting the characteristics of frequency content of strong ground motion to examine the near-fault three-component motions during the 1999 Chi-Chi earthquake. The result indicates that the frequency content of near-fault motions at the Hanging wall is less than that at the foot wall; Tp shows a smaller value than that of To and Tm and it emerges a reverse relation of three-component motions as compared with that of To and Tm; Tv and Tpv of the near-fault motions at the north end of the rupture display a similar trend to that generated by the rupture directivity effect of strike-slip faulting. We therefore conclude that these observations are useful in the formulation of near-fault design spectra for seismic codes and in zoning studies in seismic risk.

Effects of hanging wall and forward directivity in the 1999 Chi-Chi earthquake on inelastic displacement response of structures

The characteristics of the inelastic response of structures affected by hanging wall and forward directivity in the 1999 Chi-Chi earthquake are investigated. Inelastic displacement ratios （IDRs） for ground motions impacted by these nearfield effects are evaluated and comprehensively compared to far-field ground motions. In addition, the inelastic displacement responses to hanging wall and footwall ground motions are compared. It is concluded that the inelastic displacement response is significantly affected in the short period range by hanging wall and in the long period range by footwall. Although high peak ground acceleration was observed at hanging wall stations, the IDRs for structures on hanging wall sites are only larger than footwall sites in the very long period range. Forward directivity effects result in larger IDRs for periods longer than about 0.5s. Adopting statistical relationships for IDRs established using far-field ground motions may lead to either overestimation or underestimation in the seismic evaluation of existing structures located in near-field regions, depending on their fundamental vibration periods.

Intrinsic and attenuative dispersion characteristics of direct P-waves in and near the source area of the 1999 MW7.6 Chi-Chi,Taiwan,earth-quake before and after the mainshock

Based on the measurement of the arrival time of maxima magnitude from band-pass filtering signals which were determined using a new Morlet wavelet multiple-filter method, we develop a method for measuring intrinsic and attenuative dispersion of the first cycle direct P-wave. We determine relative group delays of spectral components of direct P-waves for 984 ray paths from SML and ALS stations of the Taiwan Central Weather Bureau Seismic Network （CWBSN）. Using continuous relaxation model, we deduce a new transfer function that relates intrinsic dispersion to attenuation. Based on the genetic algorithm （GA）, we put forward a new inversion procedure for determining which is defined the flat part of quality factor Q（ω） spectrum, τ1 and τ2 parameters. The results indicate that ① The distribution of Om values versus epicentral distance and depth show that Qm values linearly increase with increasing of epicentral distance and depth, and Qm values is clearly independent of earthquakes magnitude; ② In the different depth ranges, Qm residual show no correlation with variations in epicentral distance. Some significant changes of Qm residual with time is likely caused by pre-seismic stress accumulation, and associated with fluid-filled higher density fractures rock volume in the source area of 1999 Chi-Chi Taiwan earthquake. We confirm that Qm residual with time anomaly appears about 2.5 years before the Chi-Chi earthquake; ③ A comparison of Qm residual for different depth range between SML and ALS stations show that the level of stress has vertical and lateral difference; ④ The area near observation station with both anomalously increasing and decreasing averaged Qm residual is likely an unstable environment for future strong earthquake occurrence. This study demonstrates the capability of direct P-waves dispersion for monitoring attenuation characteristics and its state changes of anelastic medium of the Earth at short propagation distance using seismograms recorded from very small events.

Long-period ground motion characteristic of the 1999 Jiji (Chi-Chi),Taiwan,main-shock and aftershocks

This paper investigates long-period ground motion characteristic of the 1999 Jiji （Chi-Chi）, Taiwan, mainshock and aftershocks on the basis of lots of high quality digital strong motion records. The study attaches the importance to the variation of strength of the long-period ground motion with the magnitude, distance, and site condition. In the meantime, the near-fault long-period ground motion characteristic is analyzed. The result shows that the shape of the long-period response spectrum is mainly controlled by site condition and magnitude （the spectrum of class D＋E is wider than that of class B＋C, and the spectrum of larger magnitude is wider than that of smaller magni- tude）, and the effect of fault distance on the shape is not evident. And near-fault long-period ground motion characteristic depends on fault activity apparently, that is to say, the long-term ground motion in the hanger is stronger than that in the footwall, and the long-term ground motion in the north is stronger than that in the south.

Dynamic mechanisms of the post-seismic deformation following large events:Case study of the 1999 Chi-Chi earthquake in Taiwan of China

The mechanism of postseismic deformation related to strong earthquakes is important in geodynamics, and presumably afterslip or viscoelastic relaxation is responsible for the postsesimic deformation. The 1999 Chi-Chi, Taiwan of China, earthquake occurred in the region where GPS observation station is most densely deployed in the world. The unprecedented GPS data provides a unique opportunity to study the physical processes of postseismic deformation. Here we assume that the interactions of viscoelastic relaxation, afterslip, fault zone collapse, poroelastic rebound, flow of underground fluids, and all these combined contribute to the surface displacements following the main shock. In order to know the essence of the postseismic deformation after the strong event, fault zone collapse, poroelastic rebound, flow of underground fluids, and so on, are represented equivalently by the variations of the focal medium properties. Therefore, the viscoelastic relaxation, afterslip, and the variations of the equivalent focal medium properties are inverted by applying the GPS temporal series measurement data with viscoelastic finite element method. Both the afterslip rate distribution along the fault and the afterslip evolution with time are obtained by means of inversion. Also, the preliminary result suggests that viscosities of the lower crust and the upper mantle in Taiwan region is 2.7×1018 and 4.2×1020 Pa·s, respectively. Moreover, the inversion results indicate that the afterslip contributing to postseismic deformation of 44.6% in 450 days after the Chi-Chi earthquake, with 34.7% caused by the viscous relaxation and 20.7% by other factors such as fault zone collapse, poroelastic rebound, and the flow of liquids.

Taiwan’ Chi-Chi Earthquake Precursor Detection Using Nonlinear Principal Component Analysis to Multi-Channel Total Electron Content Records

This research uses eigenvalue characteristics of nonlinear principal component analysis （NLPCA） and principal component analysis （PCA） to investigate total electron content （TEC） anomalies associated with Taiwan＇s Chi-Chi earthquake of 21 September 1999 （LT） （M_w=7.6）. The transforms are used for ionospheric TEC from 01 August to 20 September 1999 （local time） using data from 13 GPS receivers. The data were collected at 22°N-26°N Lat. and 120°E-122°E Long.. Applying the NLPCA to the multi-channel total electron content records of GPS receivers, the earthquake-associated TEC anomalies were represented by large principal eigenvalues of NLPCA （〉0.5 in a normalized set） on 14 August and 17, 18, and 20 September, with allowance given for the Dst index, which was quiet for the study period. Comparisons were then made with other researchers who also found TEC anomalies on September 17, 18, and 19 associated with the Chi-Chi earthquake, which cannot be detected by PCA.Consideration is also given for reported ground level geomagnetic field activity that occurred between mid-August and late October, leading up to and including the Chi-Chi and Chia-Yi earthquakes, which are associated with the same series of faults. It is possible that Aug. 14 is representative of an earthquake-associated TEC anomaly. This is an interesting result given how much earlier than the earthquake it occurred.

地震荷载作用下新三义隧道地震反应分析

为探究隧道衬砌在地震荷载作用下的破坏机理,根据新三义隧道某区段建立Abaqus数值计算模型,分析地震波施加方法对衬砌动力响应的影响,研究不同地震波类型对衬砌地震动破坏的差异,总结地震灾害对隧道衬砌的破坏机理。研究结果表明:地震波的振动输入法会导致数值模拟结果明显失真,波动输入法计算精度较高。由于结构特殊性,拱顶与侧壁避车洞会出现应力集中现象,为衬砌破坏多发处。由于隧道围岩的物理特性,岩体隧道衬砌受周围岩体的影响在一定范围内随地震动而产生整体性震动,使得隧道衬砌没有产生大范围结构性破坏。集集地震灾害对隧道衬砌造成的破坏主要由S波引起;地震对衬砌破坏过程为地震波自下而上传递到衬砌,使衬砌发生整体性震动;避车洞周围由于结构特殊性而产生超过混凝土极限强度的应力,造成衬砌发生开裂、剥落等现象。

1999年中国台湾集集M_(W)7.6地震跨断层场地相对运动

为了获得近断层地震动准确的残余位移信息,提出了一种变步长网格搜索地震动基线校正方法。首先基于1999年我国台湾M_(W)7.6集集地震强震动数据,探讨了近断层地震动的位移特征,然后通过地震动时程初始时刻的校正获得了集集地震多组上、下盘台站之间的相对运动时程数据,分析了分别靠近断层上、下盘场地之间相对运动的位移时程特征。结果显示:近断层地震动的滑冲效应显著,断层的平均滑冲速度可高达1.4 m/s;断层两盘间的相对运动类似于近断层地震动的运动特征,且幅值更大;近断层的残余位移约是断层两盘间最大相对位移的80%.

Inversion of near-field waveform data for earthquake source rupture process(Ⅱ):Inversion of Chi Chi earthquake source,20 September,1999,Taiwan

The new inversion algorithm developed based on the recent progress in thenonlinear programming study by us is used to invert the earthquake source process of Chi Chiearthquake M_w7.6, 20 Semptember,1999, Taiwan. A curve fault model is constructed in our inversionto make the fault model close to the real rupturing fault to reduce the influence from thediscrepancy between the constructed fault model and the real rupturing fault. The results show that(1) the rupture process of the Chi Chi earthquake source lasted about 32 seconds and the mainfaulting occurred between 6th to 21st second after the start of the ruptures and the high slip areawere mainly located at the northern segment of the fault. (2) The slip was dominated by thrustfaulting. The average rake angle was 64.5°, which was very consistent with those inverted by USGS,Harvard and CWB (Central Weather Bureau of Taiwan). The amount of the moment inverted in this paperwas 7.76x10^(20) NM, which was a slightly bigger than those inverted by USGS and Harvard. (3) Aclear nucleation step existed in the source faulting process and it lasted about 6 seconds. Themoment release rate accelerated obviously at the end of the nucleation step. The faulting startedfrom the southern segment and mainly occurred at the northern segment after 10 seconds. At the endof this paper, we analyzed the reliability of the inversion result via comparing with the GPSobservations and discussed its scientific signification.

珍贵的史料--陶然士的《1933年8月叠溪地震及震后情况》

英国传教士陶然士(T.Torrance)曾长期在中国四川居住,亲身经历过1933年四川叠溪地震。1934年7月,他又到地震现场进行考察,事后写成The Tieh-chi earthquake of August 1933 and afterwards(1933年8月叠溪地震及震后情况)一文,同年11月发表在The West China Missionary News(华西教会新闻)上。本文介绍了陶然士的生平和他在叠溪地震时的经历,披露了《1933年8月叠溪地震及震后情况》一文的主要内容,并进行了评述。陶然士在他的文章中对叠溪地震灾害及其次生水灾的记述基本客观真实,不少细节可以与地质学家常隆庆的《四川叠溪地震调查记》以及后人的研究成果相互印证,具有一定的科学价值,是又一份珍贵的地震史料。并对进一步发掘有关地震史料等有关问题提出了建议。

汶川M_W 7.9和集集M_W 7.6地震前应力场转换现象及其可能的前兆意义

2008年四川汶川M_W 7.9地震和1999年台湾集集M_W7.6地震均为挤压推覆构造环境下发生的板内逆断层型地震.通过对比分析2次地震前的CMT解、震源区附近的中小地震震源机制解及其反演的应力场可知,集集地震主震震源机制解与用台湾内陆中西部的CMT解反演得到的逆断层类型构造应力场吻合,而在主震前震源区附近中小地震震源机制解反演得到与构造应力场不一致的走滑类型局部应力场,但当局部应力场变化到与构造应力场一致时,数月后发生主震;同样,用青藏高原东部的CMT解震源机制反演得到走滑类型的构造应力场,逆断层类型的汶川主震与构造应力场的压应力轴吻合,震区附近中小地震反演得到了与构造应力场一致的区域应力场,但震前局部应力场变化与逆断层类型应力场一致时,随即发生主震.说明逆断层型主震区附近随着震源区应力积累,在震前会出现相似的局部应力场转换现象,当最终转换到与发生主震的应力状态一致时,表明震源区附近应力已达到相当高的应力水平,可能是发生大地震的征兆,应引起进一步的关注.

地震仪器烈度标准初步研究

地震仪器烈度是指根据仪器观测记录计算得到的地震动强度,并能直接反映地震的影响程度.地震仪器烈度具有直观、简便等特点,能够在震后快速获取.随着我国地震观测台站密度的不断加大及各种技术手段的不断成熟完善,观测数据的数量和质量都得到了很大的提高,因而在破坏性地震发生后根据观测台站的记录直接计算仪器地震烈度成为可能.通过对比分析美国、日本、以及我国大陆地区的地震烈度算法,本文提出一个借鉴日本气象厅测定烈度算法的仪器地震烈度表,并根据我国国情对其中所采用的幅值滤波器进行重新设计,同时对有效峰值持续时间重新选定.利用汶川地震和集集地震的强震记录对本文仪器地震烈度算法的验证结果表明,采用本文仪器地震烈度方法评定结果与现场宏观烈度调查结果符合良好,具有较高的准确率.