Preliminary site classification of free-field strong motion stations based on Wenchuan earthquake records

During the great Wenchuan earthquake, about 460 permanent free-field stations in National Strong Motion Observation Network System （NSMONS） of China captured the main shock acceleration records. These records can be applied to site effect analyses, and then the site classification of those permanent stations can be carried out firstly, which will served as the fundamental information for further research. In this paper, the site of near-fault stations is classified by horizontal-to-vertical spectral ratio （HVSR） method according to the site class description of Japan earthquake resistant design code and response spectral shapes （RSS） method following the site class description of the 1997 Uniform Building Code （UBC） provisions. Then based on the detailed borehole data of those free-field stations, the equivalent shear wave velocity and overburden thickness are calculated and the site classifications are given by Chinese code for seismic design of buildings. Furthermore, for the stations having successful microtremor test data, the site dominant periods are computed to verify the results of site classification. Finally, combined with all the above results, the recommended site classes of near-fault permanent free-field stations are given.

Response spectrum of seismic design code for zones lack of near-fault strong earthquake records

It was shown from the study on the recently near-fault earthquake ground motions that the near-fault effects were seldom considered in the existing Chinese seismic code. Referring to the UBC97 design concept for near-fault factors, based on the collected world-widely free-site records of near-fault earthquakes ground motions classified by earthquake magnitude and site condition, the attenuation relationship expressions of the acceleration spectrum demand at the key points within the long period and moderate period were established in term of the earthquake magnitude and the site condition. Furthermore, the near-fault factors＇ expressions about the earthquake magnitude and the fault distance were deduced for the area lack of near-fault strong earthquake records. Based on the current Chinese Building Seismic Design Code, the near-fault effect factors and the modified design spectral curves, which were valuable for the seismic design, were proposed to analyze the seismic response of structures.

Analysis of Digital Strong Motion Records of the M_S 8.0 Wenchuan Earthquake by the Network of Shaanxi Province

Records of the May 12, 2008 Ms8.0 Wenchuan, Sichuan, earthquake from 27 stations of Shaanxi Digital Strong Motion Network are processed and analyzed, including baseline adjustment of acceleration traces, filtering, and calculations of velocities, displacements and acceleration response spectra. The results show that direction energy radiation of the large earthquake and horizontal inhomogeneous medium along the wave traveling path might both have some effect on the scattering degree of the attenuation of PGAH, besides influence of local site conditions. For the same intensity, the ratios of PGA/PGV are commonly small, on average about 5, which indicates that long period components are rich in ground motion. Intensities for most sites of the stations are within V -VII. The larger one among PGAE-W and PGAN-S is quite close to their PGAH, which is less than 10 % in relative deviation. The dominated waves are surface waves. The thicker the soil overburden is, the stronger the surface wave will be and the longer the shaking will last. Local site conditions have strong effects on ground motion, especially those of thick sediment filled-in basins, as they can significantly amplify long-period components.

Strong motion observations and recordings from the great Wenchuan Earthquake

The National Strong Motion Observation Network System （NSMONS） of China is briefly introduced in this paper. The NSMONS consists of permanent free-field stations, special observation arrays, mobile observatories and a network management system. During the Wenchuan Earthquake, over 1,400 components of acceleration records were obtained from 460 permanent free-field stations and three arrays for topographical effect and structural response observation in the network system from the main shock, and over 20,000 components of acceleration records from strong aftershocks occurred before August 1, 2008 were also obtained by permanent free-field stations of the NSMONS and 59 mobile instruments quickly deployed after the main shock. The strong motion recordings from the main shock and strong aftershocks are summarized in this paper. In the ground motion recordings, there are over 560 components with peak ground acceleration （PGA） over 10 Gal, the largest being 957.7 Gal. The largest PGA recorded during the aftershock exceeds 300 Gal.

Analysis on the Characteristics of the Coseismic Response of Water Levels Recorded in Three Wells in Hainan,China to the Two Sumatra-Andaman Strong Earthquakes

The characteristics of seismic water level fluctuations of the two Sumatra-Andaman strong earthquakes with magnitude 8.7 and 8.5 on December 26,2004 and March 29,2005 recorded at Jiaji well,Qionghai,Hainan were analyzed,the features of the infrequent "step" changes of well water level after the two earthquakes were also analyzed and the mechanism of the "step change" of well water level was discussed.Then the high-sample-rate digital observation data of seismically-induced water level fluctuations of the Sumatra-Andaman strong earthquakes with magnitude 8.7 and 8.5 recorded at Nanbin well,Sanya and Tanniu well,Wenchang were analyzed.The results suggest that the dominant period of the seismic well water level fluctuation in all three wells was comparatively accordant,the amplitudes of seismic water level fluctuation of the same earthquake in different wells were clearly different,the time duration of seismic water level fluctuations of different earthquakes at the same well was also clearly different.

Estimation of the 2010 Mentawai tsunami earthquake rupture process from joint inversion of teleseismic and strong ground motion data

Joint inversion of teleseismic body-wave data and strong ground motion waveforms was applied to determine the rupture process of the 2010 Mentawai earthquake. To obtain stable solutions, smoothing and non-negative constraints were introduced. A total of 33 teleseismic stations and 5 strong ground motion stations supplied data. The teleseismic and strong ground motion data were separately windowed for 150 s and 250 s and bandpass filtered with frequencies of 0.001e1.0 Hz and 0.005e0.5 Hz, respectively. The finitefault model was established with length and width of 190 km and 70 km, and the initial seismic source parameters were set by referring to centroid moment tensor(CMT) solutions. Joint inversion results indicate that the focal mechanism of this earthquake is thrust fault type, and the strike, dip, and rake angles are generally in accordance with CMT results. The seismic moment was determined as 5.814 1020Nm(Mw7.8) and source duration was about 102 s, which is greater than those of other earthquakes of similar magnitude. The rupture nucleated near the hypocenter and then propagated along the strike direction to the northwest, with a maximum slip of 3.9 m. Large uncertainties regarding the amount of slip retrieved using different inversion methods still exist; however, the conclusion that the majority of slip occurred far from the islands at very shallow depths was found to be robust. The 2010 Mentawai earthquake was categorized as a tsunami earthquake because of the long rupture duration and the generation of a tsunami much larger than was expected for an earthquake of its magnitude.

Improved HVSR site classification method for free-field strong motion stations validated with Wenchuan aftershock recordings

Local site conditions play an important role in the effective application of strong motion recordings.In the China National Strong Motion Observation Network System（NSMONS）,some of the stations do not provide borehole information,and correspondingly,do not assign the site classes yet.In this paper,site classification methodologies for free-field strong motion stations are reviewed and the limitations and uncertainties of the horizontal-to-vertical spectral ratio（HVSR） methods are discussed.Then,a new method for site classification based on the entropy weight theory is proposed.The proposed method avoids the head or tail joggle phenomenon by providing the objective and subjective weights.The method was applied to aftershock recordings from the 2008 Wenchuan earthquake,and 54 free-field NSMONS stations were selected for site classification and the mean HVSRs were calculated.The results show that the improved HVSR method proposed in this paper has a higher success rate and could be adopted in NSMONS.

Ground motion attenuation of M_s8.0 Wenchuan earthquake

The great Ms8.0 Wenchuan earthquake has been the most destructive earthquake since 1949 in China. The earthquake occurred no more than half a year after the establishment of the National Strong Motion Observation Network System （NSMONS） of China; what is more, the epicenter was located in the area with dense strong motion observation sta- tions so that a large number of strong motion records of the main shock were obtained. In this paper, 501 strong motion re- cords from 167 observation stations are utilized to establish the ground motion attenuation relations in three directions in the range of fault distance less than 600 km. The result shows the difference of seismic motion attenuation in two horizontal di- rections is insignificant. It is the first time that strong-motion records are used to establish the ground motion attenuation relations of the Ms8.0 earthquake in China.

New characteristics of intensity assessment of Sichuan Lushan “4.20” M_s7.0 earthquake

The post-earthquake rapid accurate assessment of macro influence of seismic ground motion is of significance for earthquake emergency relief,post-earthquake reconstruction and scientific research. The seismic intensity distribution map released by the Lushan earthquake field team of the China Earthquake Administration(CEA) five days after the strong earthquake(M7.0) occurred in Lushan County of Sichuan Ya’an City at 8:02 on April 20,2013 provides a scientific basis for emergency relief,economic loss assessment and post-earthquake reconstruction. In this paper,the means for blind estimation of macroscopic intensity,field estimation of macro intensity,and review of intensity,as well as corresponding problems are discussed in detail,and the intensity distribution characteristics of the Lushan '4.20' M7.0 earthquake and its influential factors are analyzed,providing a reference for future seismic intensity assessments.

2023年2月6日土耳其M_(w)7.8和M_(w)7.6两次大地震的地震动特征及模拟

2023年2月6日土耳其东南部连续发生M_(w)7.8和M_(w)7.6大地震,强震动观测台网获取到了大量的强震动观测记录,利用两次地震的强震动记录,对比土耳其地区地震动预测方程,分析了地震动的衰减特征,探讨M_(w)7.8地震的震源破裂特征对近断层地震动空间分布、速度脉冲的产生及观测台站方位相关的地震动总残差的影响,采用随机有限断层三维地震动模拟方法给出了基于数值模拟记录的仪器地震烈度图,得到以下结论:(1)两次地震中地震动的震源效应明显强于土耳其地区浅地壳地震的平均震源效应,M_(w)7.8地震对中长周期地震动的震源效应比M_(w)7.6地震更强烈;(2)两次地震中远场地震动的非弹性衰减明显强于土耳其地区的平均水平;(3)M_(w)7.8地震中最强烈的地震动主要集中于卡赫拉曼马拉什—安塔基亚的狭长区域内,该区域观测记录普遍具有双向速度脉冲特征,地震动总残差与台站方位角有明显相关性,推断M_(w)7.8地震沿北东走向的东安纳托利亚断裂向北东向破裂传播,同时沿南南西走向的东安纳托利亚断裂的卡赫拉曼马拉什—安塔基亚段向南南西方向破裂传播;(4)模拟记录能大致反映近断层地震动强度及距离衰减特征,M_(w)7.8和M_(w)7.6地震中破裂面附近地震烈度分别达到Ⅸ和Ⅷ度,烈度分布主要受破裂断层和局部场地条件的影响.

基于HVSR方法的上海市典型场地特征研究

上海市强震台网获取了2020年5月3日日本九州6.0级地震等多次强震记录。通过对地震记录进行分析处理,分别计算了每个强震台站的H/V谱比曲线,结合上海市地质调查研究院的基岩埋深分布图,得到了每个台站的基阶共振频率与沉积层厚度。结果表明:H/V谱比曲线峰值与沉积层厚度存在明显相关性,并且通过数据拟合建立了上海市沉积层厚度与基阶共振频率的经验公式,绘制了上海市场地基本周期分布图,可以为上海市抗震设防提供一定的参考;厚沉积层场地上地震记录的谱比曲线会出现多个峰值,第二个峰值代表了沉积层的高阶共振频率;通过数据拟合得到上海地区剪切波速与沉积层厚度的经验公式。

2023年甘肃积石山M_(S)6.2地震随机有限断层三分向地震动模拟

2023年12月18日23时甘肃省临夏州积石山县发生M_(S)6.2地震,震中附近造成大量人员伤亡。基于不同数据和方法反演得到的震源破裂过程还有争议,为模拟此次地震的地面运动,分别采用SW倾向与NE倾向的震源破裂模型,利用随机有限断层法模拟三分向地震动。选取震中距约100 km内的57个触发强震台站,模拟其三分向加速度时程、速度时程及5%阻尼比拟加速度反应谱(PSA)。结果表明,模拟的峰值地面加速度(PGA)、峰值地面速度(PGV)能够体现地震动峰值的衰减规律,NE倾向的震源破裂模型会导致更大的地震动。模拟与观测记录的幅值接近、谱形相似,其中SW倾模型的PSA残差更小。基于三分向模拟地震动给出积石山M_(S)6.2地震的仪器烈度分布,SW倾向与NE倾向模型极震区烈度均达Ⅷ度,其中SW倾向模型与应急管理部发布的烈度图更加接近。积石山M_(S)6.2地震的模拟说明随机有限断层法可用于计算三分向地震动,并验证模拟输入震源、路径与场地参数的可靠性。

基于中国强震动数据的PhaseNet网络捡拾P波到时研究

快速和准确捡拾地震P波到时是地震预警技术的基础工作之一。PhaseNet等U形网络已在“谛听”测震数据集上取得良好的震相捡拾结果,旨在提升处理M L3.0以下的地震的能力。目前针对有限中国强震动数据的震相捡拾研究较少,地震预警主要是针对处理M L3.0以上的地震。该研究以在地震预警中快速和准确捡拾地震P波到时为目的,直接迁移和构建PhaseNet的衍生网络,探究利用有限中国强震动数据重训的网络模型是否具有良好的捡拾P波表现。研究结果表明:PhaseNet及其衍生网络模型的精确度、召回率、F 1分数、捡拾误差均值(μ)和标准差(δ)约为0.942、0.930、0.937、-20 ms和200 ms左右,具有良好的捡拾P波到时精度和泛化性能;此外,PhaseNet及其衍生网络在高信噪比条件下捡拾效果良好,但对于低信噪比数据的捡拾效果仍有待提升。

2024年1月23日乌什7.1级地震强震动记录特征分析

新疆地震台网中心共获取乌什7.1级地震813条仪器烈度信息、496组强震动记录,其中PH002台站峰值加速度280.68 cm·s^(-2)为本次获得的最大峰值加速度。对获取的219个台站的峰值加速度与第五代中国地震动参数区划图衰减关系对比发现两者在100 km范围内比较符合。计算分析5个峰值加速度超过100 cm·s^(-2)的台站加速度反应谱,并与地震动设计反应谱进行比对分析,同时计算出5个台站地震动有效持时,显示200 km内地震能量释放较迅速,体现出地震破坏力较大。

2023年12月18日甘肃积石山6.2级地震强震动记录特征分析

2023年12月18日,甘肃省临夏回族自治州积石山县发生6.2级地震。基于甘肃地震预警台网获取的此次地震强震动记录,完成基线校正、滤波等基本数据处理后计算出地震动相关参数,分析此次地震震中200 km内的幅值、反应谱和持时等重要特征,并绘制峰值加速度等相关参数空间分布图。结果表明:此次地震震中200 km内获取到的峰值加速度范围为1.2~1511.6 cm/s 2,峰值速度范围为0.1~85.9 cm/s,由于地形和场地影响,地震动峰值随着震中距的增大而衰减。将实际观测值与常用的五种衰减模型预测值进行对比,表明YU模型对甘肃积石山6.2级的预测结果要明显优于NGA-West2的四种模型,但五种衰减模型都明显低估了实际观测值的最大峰值。通过对比两个邻近震源台站的5%阻尼比加速度反应谱与设计谱,发现三分向高频成分加速度反应谱均远超过7度罕遇地震的设计谱,且反应谱峰值周期均在0.5 s以内。通过计算5%~75%和5%~95%两种重要持时,发现持时随着震中距的增大而增大,且与Afshari&Stewart 16预测方程均基本吻合。

基于随机相位差谱的全非平稳地震动建模

为合理表征地震动过程的全非平稳性和随机性,发展了一类基于相位差谱和功率谱结合的随机地震动模型。首先,阐明了相位差与地震动全非平稳性的关系,并利用实测强震记录,进行相位差谱模型参数的识别及统计分析。其次,采用确定性和随机性参数的功率谱模型,分别获得确定性参数的取值以及随机性参数的归一化最优概率分布。最后,通过选取基本随机参数的代表性点集,获得相应的地震动加速度代表性时程集合。研究结果表明:仅需2个或4个基本随机变量即可模拟具有自然变异性及丰富概率信息的地震动加速度过程,且模拟所得的加速度反应谱与实测强震记录拟合良好,为应用概率密度演化理论进行复杂工程结构的随机地震反应及抗震可靠性精细化分析奠定基础。

2022年四川泸定M_(S)6.8地震强震动记录特征

对地震监测台网获取的2022年四川泸定M_(S)6.8地震的1799组强震动记录进行处理后计算地震动参数,利用克里金插值方法得到峰值加速度PGA、峰值速度PGV和加速度反应谱的空间分布图。结果表明:基准站、基本站和一般站记录到的地震动峰值依次增大,反映出局部地形和场地条件对地震动的放大作用。高烈度区仪器地震烈度和调查地震烈度较为一致,Ⅵ度以下区域仪器地震烈度和调查地震烈度等值线方向差异较大,反映出在震害不显著情况下两种烈度的对应关系不太理想。初步震相分析发现,近断层台站垂直方向峰值较大,多个台站记录到水平向速度大脉冲现象。基于基岩地震动峰值统计了本次地震的衰减特性,PGV随距离衰减相比中国地震动参数区划图更慢,综合震相特征认为本次地震更符合浅源地震的特点。为进一步分析地震动与震害的关系,选取了2个震害较为严重乡镇的强震动记录计算5%阻尼比的拟加速度反应谱,在设计地震反应谱特征周期T g取值到1.5 s之间,台站记录反应谱值远超出设计地震反应谱,在部分周期区间还超出了罕遇地震设计反应谱值,体现出反应谱与震害较好的相关性。

辽宁普兰店M4.6地震加速度记录特征分析

2023年8月23日辽宁省普兰店区发生M4.6地震,辽宁省强震动台网共获取了34组强震记录,其中震中距为11.7km的普兰店台获取的EW分向最大加速度峰值为280.1cm/s²,是我省强震动台网迄今为止记录到的最大的加速度记录。本文针对强震动台站的记录峰值、频谱、PGA分布规律、V/H比值等进行了分析。

减隔震规则桥梁的模态分析方法研究

为改善模态分析方法应用于减隔震规则桥梁的计算精度,以高速铁路典型简支梁桥为研究背景,基于位移反应谱提出一种能够考虑高阶模态的分析方法。通过选取强震记录调整后进行非线性时程分析,与现有的基于加速度反应谱和基于位移反应谱的单模态简化分析方法进行计算精度的比较。分析结果表明,相比常规的单模态分析方法,考虑了高阶模态后的基于位移反应谱多模态分析方法更为准确。基于加速度谱的单模态分析方法严重低估了墩顶位移,误差超过60%;仅考虑基阶模态的基于位移反应谱的分析方法将低估墩底剪力,误差超过30%,造成设计偏于不安全;而基于位移反应谱的多模态分析方法能够更为准确地估算各项地震响应参数。

2021年漾濞地震序列破裂方向性研究

基于2021年云南漾濞M_(S)6.4地震序列的强震动记录,建立了地震动参数预测方程,采用破裂方向性效应拟合方法估计了强震动记录丰富且空间分布均匀的九次地震的震源破裂方向性特征.结果表明:其中的四次地震(1号、4号、8号、9号)表现出破裂方向性效应,且均为不均匀双侧破裂,但优势破裂方向不同(4号和8号为东南向,1号西北向,9号西南向),说明2021年漾濞地震序列的地震破裂较为复杂;由于主震(4号地震)的破裂速度较慢(约为2.2 km/s),其破裂方向性效应较弱且主要对峰值速度有影响,其它三次地震破裂方向性效应十分显著,破裂速度大于主震;此外,四次地震的破裂方向性效应还存在一定的周期相关性.