Linear Amplification of Horizontal Strong Ground Motion in Zagreb (Croatia) for a Realistic Range of Scaled Point Sources

— The linear amplification of the larger horizontal component of strong ground motion along a selected profile in the city of Zagreb is estimated by examining the synthetic waveforms corresponding to a suite of 16 realistically chosen scaled point sources. The accelerograms, computed for the average bedrock model by modal summation, are propagated through local laterally heterogeneous anelastic models by the finite-difference algorithm. The ratio of peak ground acceleration (PGA) and of the response spectra (RS), obtained by using local and bedrock models, define the PGA and RS amplification AMP(PGA) and AMP(RS), respectively. Even variations of the order of commonly observed uncertainties of only dip and rake angles of the causative fault show that both AMP(PGA) and AMP(RS) vary at some sites by more than a factor of two. It follows that, especially for strongly laterally heterogeneous structures, local effects must be determined for each of the relevant sources considering all associated uncertainties as completely as possible. Such a conclusion certainly holds for the case of the microzonation of Zagreb, where the local geology is quite complex, and the seismicity is not confined to a single seismic source zone.     

Seismic microzoning from synthetic ground motion parameters: case study, Santiago de Cuba

Synthetic seismograms (P-SV and SH waves) have been calculated along six profiles in Santiago de Cuba basin, with a cutoff frequency of 5 Hz, by using a hybrid approach (modal summation for a regional 1D structure plus finite differences for a local 2D structure embedded in the first). They correspond to a scenario earthquake of M_S=7 that may occur in Oriente fault zone, directly south of the city. As initial data for a seismic microzoning, the characterisation of earthquake effects has been made considering several relative (2D/1D) quantities (PGDR, PGVR, PGAR, DGAR, I_AR—ratios of peak ground values of displacement, velocity and acceleration, and of design ground acceleration and Arias intensity-, etc.) and functions representative of the ground motion characteristics in soil (2D) with respect to bedrock (1D). The functions are the response spectra ratio RSR(f), already routinely used in this kind of work, and the elastic energy input ratio E_IR(f), defined, for the first time, in this paper. These data, sampled at 115 sites within all the profiles have been classified in two steps, using logical combinatory algorithms: connected components and compact sets. In the first step, from the original ground motion parameters or functions extracted from the synthetic seismograms, nine sets have been classified and the partial results show the spatial distribution of the soil behaviour as a function of the component of motion. In the second step, the results of the classification of the nine sets have been used as input for a further classification that shows a spatial distribution of sites with a quasi-homogeneous integral ground motion behaviour. By adding the available geological surface data, a microzoning scheme of Santiago de Cuba basin has been obtained.     

2D numerical simulations of earthquake ground motion: examples from the Marche Region,Italy

A detailed numerical simulation of the ground motion and a site response analysis for two towns in the Marche Region (Treia and Cagli) is carried out on the basis of structural models deduced from available geological and geophysical data. In both cases, the reference event is an M = 5.7 earthquake associated with a normal fault located beneath each town. The ground motion is computed using the 2D spectral element method (SPEM 2D). The method solves the propagation of the seismic field through complex geological structures and enables an estimate of the effects of deep crustal structure, superficial geology, and topography on ground motion. Numerical simulations of the seismic field are performed along 2D vertical planes containing the seismic source. Strong ground motion has not been yet recorded in the two towns; therefore, the numerical simulation of ground motion represents a way to overcome the lack of instrumental data. The simulations carried out for Treia show that ground motion is influenced by both source mechanism and effects due to propagation through the geological structure, while ground motion in Cagli features strong local effects, caused by the presence of alluvial deposits under a large area of the town.     

Site amplifications in the city of Benevento (Italy): comparison of observed and estimated ground motion from explosive sources

A hybrid approach is applied to construct 2-D synthetic seismograms for explosive sources. The computation of the signals has been performed with a hybrid technique, that couples the modal summation method, describing the P-SV propagation from the source to a target area, to the finite difference method, allowing the computation of the 2-D response of the target area. We use this technique to study the site response in the city of Benevento (Southern Italy). The results are compared both with those obtained for the same model, but considering an extended double-couple source and SH wave propagation and with those derived from a direct analysis of experimental data obtained during the Benevento Seismic Risk Project, funded by the Commission of European Communities. The type and depth of the source and the source-receiver distance can provide different excitations, but the response of the target area, in terms of ratios of response spectra, remains in general stable in the high frequency range (>2 Hz). The explosive source does not excite lower frequencies and is therefore unable to provide estimates of site effects in this frequency range. We also show that it is important to consider both P-SV and SH waves when assessing site effects. Finally, a very good definition of a structural model from geological data is fundamental in order to explain the observed data in the time domain. The observed data validate our synthetic modelling for evaluating the site effects and for a tentative seismic microzonation of the city of Benevento.     

Site effects of the 2002 Molise earthquake, Italy: analysis of strong motion, ambient noise, and synthetic data from 2D modelling in San Giuliano di Puglia

On 31 October and 1 November 2002, the Basso Molise area (Southern Italy) was struck by two earthquakes of moderate magnitude (M _L = 5.4 and 5.3). The epicentral area showed a high level of damage, attributable both to the high vulnerability of existing buildings and to site effects caused by the geological and geomorphological settings. Specifically, the intensity inside the town of San Giuliano di Puglia was two degrees higher than in neighbouring towns. Also, within San Giuliano di Puglia, the damage varied notably. The site response in the city was initially evaluated from horizontal-to-vertical spectral ratios (HVSR) from a limited number of strong motion recordings of the most severe aftershocks. Several microtremor measurements were also available. Both data sets indicated the simultaneous presence of two amplification peaks: one around 6 Hz, attributed in previous studies to the strong, shallow impedance contrast among landfill/clay and calcarenites, and one at 2 Hz related to the first S-wave arrivals and predominantly seen only on one receiver component. Further studies performed on weak-motion recordings also showed strong amplification on the vertical receiver component, thus indicating an underestimation of the amplification by the HVSR technique. Additionally, a 2D-model of the geology of the sub-surface was developed, reproducing the flower-shaped structure generated during the late orogenic transpressive regime. The numerical (finite-difference hybrid) simulation reproduced the two peaks of the observed data at slightly higher frequencies. The model also confirmed that the borders of the flower structure define a boundary between amplification levels, with higher amplification inside.     

自贡市西山公园地形对地震动的影响

不规则地形和土层对地震动的影响较大,建(构)筑物选址及其抗震设防必须考虑地形和土层场地的放大效应,以避免或减轻其震害.利用自贡地形台阵记录的汶川地震(Ms8.0)的主震加速度时程,基于传统谱比法分别研究了地形和土层场地对地震动的放大效应.结果表明:(1)地形场地在低频的放大效应不明显,最大仅为1.24;在高频的放大效应较显著,在1~10 Hz频带,山顶的放大效应最大,EW、NS和UD地震动的最大放大效应分别为4.15、3.61和2.41,对应频率分别为5.72 Hz、6.46 Hz和7.44 Hz;在10~20 Hz频带,靠近山顶的山脊上某个地震动分量的放大效应最大,7#台站EW、5#台站NS和7#台站UD地震动的最大放大效应分别为9.10、5.56和2.52,对应频率分别为16.97 Hz、16.91 Hz和17.91 Hz.(2)地形场地的最大放大效应随高度有增加的趋势,且在0.1~10 Hz频带随高度增加的趋势更加明显.(3)土层场地水平向地震动在2 Hz以上开始明显放大,竖向地震动在4 Hz以上开始明显放大;EW、NS和UD地震动的最大放大效应分别为13.4、12.168和6.0,对应频率分别为6.94 Hz、7.55 Hz和10.8 Hz.(4)土层场地与地形场地的最大放大效应相比较,前者显著大于后者,对于水平向地震动,前者至少是后者的3倍以上;对于竖向地震动,前者至少是后者的2.5倍以上.(5)无论是地形场地还是土层场地,地震动的最大放大效应均有水平向大于竖向的特征.     

渭河盆地中土层场地对地震动的放大作用

土层场地对地震动的影响较大,建(构)筑物的选址及其抗震设防必须考虑土层场地的放大作用,以避免或减轻其震害.汶川地震中,布设在渭河盆地中的数字强震动台网共有27个台站(包括2个基岩台站和25个土层台站)获得良好的主震加速度时程.利用这些加速度时程,选择汤峪台做为参考场地,基于考虑几何衰减的传统谱比法分析研究了25个土层场地的放大作用.结果表明:(1)汶川地震中渭河盆地土层场地上的加速度峰值(PGA)有随震中距增加而减小的趋势,且与土层的厚度无关.(2)根据汶川地震中渭河盆地土层场地的加速度反应谱特征及其对地震动的放大作用特征,渭河盆地的土层场地可分为3类:深厚、中厚和薄土层场地.而且,深厚、中厚和薄土层场地分别对地震动的低频、1 Hz附近和高频分量放大作用显著;值得注意的是,深厚土层对地震动的高频分量也有一定程度的放大,只不过相对于低频分量的放大作用较小.(3)汶川地震中渭河盆地的土层场地对不同方向地震动的放大作用不同,且有EW＞NS＞UD的关系.(4)汶川地震中从宝鸡到眉县沿着盆地边缘分布的Ⅶ度烈度异常区是盆地边缘效应和土层场地对地震动放大作用共同作用的结果.     

1975年海城MS7.3地震强地面运动模拟

地震强地面运动预测对工程的抗震设计,地震危害性分析以及减轻特定地区可能发生的大地震所造成的灾害具有重要的作用.本文根据辽宁省海城地区的地质资料和发生于1975年2月4日辽宁省海城市的M_S7.3地震资料,分别构造了海城地区的地下速度结构和海城地震的震源模型,并且使用可以准确描述地形起伏的曲线网格有限差分方法计算了海城地震的波场传播过程.通过对计算得到的波场快照、合成理论地震图以及地震烈度的分析表明：（1）震源模型、地下的速度结构和地形起伏对海城地震的波场传播模拟具有重要的影响,它们所产生的近断层效应、方向性效应和盆地效应明显;（2）通过计算得到的海城地震的理论烈度分布与通过震后调查得到的烈度分布大体符合,验证了本文所构造的震源模型和速度结构的合理性.

     

张北地震在北京市激发的二维强地面运动的模拟

张北县距离北京市约200多km,1998年1月的张北地震是近期北京周边地区发生的一次较强规模的地震,研究它在北京地区引起的强地面运动,有助于分析首都圈内的地震危害性,为地震减灾提供理论帮助.本文利用新的计算工具-局域离散波数法,模拟了张北地震激发的包括来自上地壳、moho面反射波、首播等在内的全波场在北京西部引起的2D强地面运动,分析了该区内第三纪、第四纪沉积地层对于地震的放大效应,得出的主要结论有,薄的第四纪沉积盖层的地点放大效应比厚的第三纪盖层的大很多,说明介质物性是决定地点放大效应的根本原因;不同物性的沉积盖层,在R、Z分量上的放大作用不一样,体现了地点放大效应的复杂性,值得在今后的研究中继续探讨.     

Preliminary results of strong ground motion simulation for the Lushan earthquake of 20 April 2013, China

The earthquake occurred in Lushan County on 20 April, 2013 caused heavy casualty and economic loss. In order to understand how the seismic energy propagates during this earthquake and how it causes the seismic haz- ard, we simulated the strong ground motions from a rep- resentative kinematic source model by Zhang et al. （Chin J Geophys 56（4）：1408-1411, 2013） for this earthquake. To include the topographic effects, we used the curved grids finite difference method by Zhang and Chen （Geophys J Int 167（1）：337-353, 2006）, Zhang et al. （Geophys J Int 190（1）：358-378, 2012） to implement the simulations. Our results indicated that the majority of seismic energy con- centrated in the epicentral area and the vicinal Sichuan Basin, causing the XI and VII degree intensity. Due to the strong topographic effects of the mountain, the seismic intensity in the border area across the northeastern of Boxing County to the Lushan County also reached IX degree. Moreover, the strong influence of topography caused the amplifications of ground shaking at the moun- tain ridge, which is easy to cause landslides. These results are quite similar to those observed in the Wenchuan earthquake of 2008 occurred also in a strong topographic mountain area.     

2008年汶川8.0级大地震近场强地面运动的模拟

运用经验格林函数法模拟了2008年5月12日汶川8.0级大地震的近场强地面运动.拟合过程中,首先参考其他学者反演结果给出的滑动量分布的特征,确定强震动生成区的大致范围;然后利用Somerville等(1999)提出的地震矩与凹凸体面积间的经验关系式确定强震动生成区(SMGA)细小划分的初值,继而利用遗传优化算法确定以上两者的最优值及其他震源参数.数值模拟波形同实际地震观测记录在时间域和频率域分别进行了比较,结果显示,在所选取的18个观测台中,多数台站的数值模拟结果同实际观测结果符合得很好,特别是大于1 Hz的高频部分.我们发现断层面上有5个强震动生成区,其中两个的位置与其他学者反演的滑动量集中分布区相一致,但强震动生成区规模和上升时间比Somerville等(1999)获得的定标率外延的估计值要小.     

C、D、Mf值综合异常交汇区在华东地区中强地震前的中期异常

从大震前中小地震活动有个增强的过程和大地震常常发生在异常区外围的现象出发,提出用描述地震分布时、空、强基本特点的空间集中度Ｃ、地震危险度Ｄ和地震强度因子Ｍｆ进行交汇预测未来震中位置。应用在华东地区的震例一般预测位置可达１°～１．５°半径,并能较好地避免单值预报的虚报、漏报。     

2013年四川芦山MS7.0地震近断层强地面运动模拟及烈度分布估计

2013年4月20日在我国四川省发生了芦山M_S7.0地震,地震给当地群众的生命财产安全带来了巨大的损失,其中最严重的破坏发生在震中附近的芦山、宝兴等地区.根据地震发生的快速反演结果,及发震断层面上滑动位移的分布情况,构建有限断层模型,对近断层区域的强地面运动进行初步计算,并基于强地面运动的模拟结果给出震区烈度分布的初步估计.模拟结果显示：模拟烈度图显示极震区的烈度在IX级左右,VI级烈度影响范围大致为16000 km²,该结果与中国地震局于4月27日给出的震区实测烈度图一致程度较高.强烈地震发生后,基于近断层强地面运动模拟计算的结果,可以给出相对合理的地震烈度分布情况估计,对震区震情判定及救灾工作具备较高的现实意义.     

应用动态复合震源模型模拟汶川Mw7.9地震强地面运动

2008年5月12日中国汶川地区发生Mw7.9地震,震中位置103.4°E,31.0°N.主要发震断层空间展布长达300多公里,由南西方向到北东方向呈现明显的分段性,汶川—映秀段逆冲为主兼有少量的右旋走滑分量;安县—北川段为逆冲-右旋走滑的断层错动;青川段以右旋走滑为主兼有少量逆冲分量.采用改进后的复合震源强地面运动预测模型,建立了长为320 km,宽为20 km的断层破裂运动学模型,实现了断层分段、空间倾角、滑动方向连续变化的动态设定.数值模拟结果给出了近断层两侧(上、下盘)的地面加速度的分布特征,并同卧龙、郫县走石山及绵竹清平强震观测记录进行了对比分析.模拟加速度时程曲线无论在波形、持续时间、频率分量、峰值大小同观测记录都具有较好的相似性.利用现有83个已知经纬度台站的强震实测数据及数值模拟的结果同Boore等的新一代衰减关系(NGA)进行比较,对比模拟与实际观测水平峰值加速度的一致程度.近断层峰值加速度分布特征则进一步显示了在汶川、北川和青川附近明显的高值分布区域,同野外地质调查相一致.进一步的分析结果也表明,汶川—映秀段逆冲为主的断层上盘的运动量远大于断层下盘,在距离断层地表出露位置5 km处,峰值加速度N-S、E-W及UP方向分量的比值分别为1.72∶1、2.5∶1及1.77∶1.本文中给出的动态复合震源模型和近断层区域强震模拟的计算方法,对大震强地面运动的预测及实现近实时强地面运动分布特征的圈定(Shaking Map)有着重要的实际意义.     

Duration of strong ground motion during Mexican earthquakes in terms of magnitude,distance to the rupture area and dominant site period

A study of the duration of strong ground motion using accelerometric data of subduction and normal‐faulting Mexican earthquakes is presented. Duration is obtained based on the time between 2.5 and 97.5 per cent of the Arias intensity. An expression to predict this duration in terms of the magnitude, distance to the rupture area and site period is proposed and compared with predictions available in the literature. The effect of large duration for very distant sites and the contribution of soft soils to the duration of strong ground motion are widely discussed. We have found that large magnitude not only yields long duration at the source, but also proportionally longer duration with distance and with dominant site period compared to small magnitude. The duration obtained from the regression is used as a parameter to obtain input and hysteretic energy and on the use of damage models available in the literature. Finally, duration is used together with the random vibration theory to predict response spectra. Copyright © 2001 John Wiley & Sons, Ltd.     

2010年墨西哥Baja Mw7.2地震与中国玉树Mw6.9地震强地震动特征的对比研究

2010年4月4日墨西哥Baja地区发生Mw7.2地震,2人遇难;同年4月14日中国青海省南部玉树地区发生Mw6.9地震,截至2010年4月25日,已造成2220人遇难.有报道指出,玉树地震矩震级小于Baja地震,人员伤亡却远大于后者,主要原因在于玉树地区抗震设防标准低、建筑物抗震性能差.地震造成破坏程度的大小并非仅仅取决于矩震级的大小,而同时与其释放的地震波辐射能及发震后造成的强地面运动的大小有关.玉树地震释放的地震波辐射能约相当于Baja地震的10倍,目前玉树地震尚无实测的强震记录.针对玉树地震和Baja地震建立动态复合震源模型,分别模拟基岩上及浅层速度结构(V30,地下30 m平均剪切波速)下近断层区域的强地面运动.结果表明,基岩上及V30下玉树地震近断层区域强地面运动整体约相当于Baja地震的2倍.因此,玉树地震造成发震区域内建筑物损毁程度及人员伤亡情况均严重于Baja地震,重要原因之一在于其地震波辐射能大,且强地面运动较强.本文中所应用的动态复合震源模型,在地震矩守恒和地震波辐射能守恒的条件约束下,可以作为地震发生后补充强地面运动数据的有效手段之一.     

2022年6月1日芦山MS6.1地震近场地震动特征及与西南地区地震动预测方程的对比分析

基于芦山M_S6.1地震的61组强震动记录, 分析近场强震动记录的幅值特征、水平向地震动强度的空间分布; 将近场台站加速度反应谱值与NGA-West2和ZYLW22模型的预测中值进行对比; 结合实际观测值与ZYLW22模型预测中值的对比、事件内残差的分布, 检验此次芦山M_S6.1地震是否符合西南构造活跃区主震的地震动衰减特性.研究结果表明, (1)近场台站记录的地震动强度均小于中国地震局划定地震烈度相对应的值; (2)空间分布显示断层破裂前方(震中北东向)的反应谱值大于相同距离处的破裂后方, 在长周期表现的更为明显.断层破裂前方的水平向SA在短周期大于NGA-West2和ZYLW22模型的预测中值, 而断层破裂后方的水平向SA在整个周期范围内与ZYLW22模型的预测中值相差不大, 地震动的方向性效应较为显著; (3)ZYLW22模型可以很好地预测芦山M_S6.1地震大多数水平向地震动.构造活跃区和稳定区的地震动衰减速率不同, 西南地区应按构造活跃区和稳定区分别进行地震动预测方程研究, 并需进一步考虑震源破裂方向性以及盆地效应的影响.