Validation of the Deep Velocity Structure of the Tokachi Basin Based on 3-D Simulation of Long-Period Ground Motions

The construction of 3-D basin velocity structures is ongoing in many regions of Japan. The structure models are constructed mainly for the prediction of long-period ground motions from future large earthquakes. In this paper, we validate the 3-D velocity structure model of the Tokachi basin, a deep sedimentary basin located in eastern Hokkaido, Japan, based on 3-D simulation of long-period (2–20 s) ground motions from three nearby intermediate-depth earthquakes; this model was constructed by the National Research Institute for Earth Science and Disaster Prevention (NIED). We make comparisons between the observed and synthetic long-period ground motions for the basin-induced surface waves as well as the direct S-wave. We also try to revise the 3-D velocity structure in the western part of the Tokachi basin based on 1-D velocity structures estimated using long-period S-wave modeling and the microtremor survey method. We then perform the 3-D simulation again to validate the revised model. Based on quantitative comparisons of the long-period ground motions from these simulations with those observed, we conclude that the NIED and revised velocity structure models are generally good at the central basin sites, but that both models require modification at the basin edges to explain the details of the observed basin-induced surface waves.     

直下型断层的破裂速度对盆地地震效应的影响

首先基于有限断层破裂下的运动学震源模型,对比验证了三维谱元法对于近场地震动的模拟精度。 进而通过含盆地模型与不含盆地的一维水平成层模型的地震动强度之间和放大系数分布特征之间的对比,详细研究了直下型断层的破裂速度对盆地地震效应的影响。结果表明,盆地的存在会显著改变近断层地震动的分布特征,同时盆地内不同分量强地震动的分布特征变化较大。断层破裂速度对盆地地震效应影响显著,随破裂速度的增大盆地地震动强度逐渐增加,但不同分量上地震动强度的增加速率显著不同,受盆地效应的影响,放大系数表现出与强地震动不同的分布特征。盆地放大系数整体表现出随破裂速度的增加而减小的趋势,但不同分量放大系数所受影响程度差异明显。同时,盆地内地震动强烈放大区域的位置也受破裂速度的显著影响,但其总体上集中在断层两侧区域及垂直于破裂方向的盆地边缘附近。      

Validation of 3-D basin structure models for long-period ground motion simulation in the Osaka basin, western Japan

We studied the applicability of two types of existing three-dimensional (3-D) basin velocity structure models of the Osaka basin, western Japan for long-period ground motion simulations. We synthesized long-period (3–20 s) ground motions in the Osaka basin during a M6.5 earthquake that occurred near the hypothetical Tonankai earthquake source area, approximately 200 km from Osaka. The simulations were performed using a 3-D finite-difference method with nonuniform staggered grids using the two basin velocity structure models. To study the ground motion characteristics inside the basin, we evaluated the wave field inside the basin using the transfer functions derived from the synthetics at the basin and a reference rock site outside the basin. The synthetic waveforms at the basin site were obtained by a convolution of the calculated transfer function and the observed waveform at the reference rock site. First, we estimated the appropriate Q values for the sediment layers. Assuming that the Q value depends on the S wave velocity V _S and period T, it was set to Q = (1/3V _S)(T ₀/T) where V _S is in m/s and the reference period T ₀ is 3.0 s. Second, we compared the synthetics and the observations using waveforms and pseudovelocity response spectra, together with a comparison of the velocity structures of the two basin models. We also introduced a goodness-of-fit factor to the pseudovelocity response spectra as an objective index. The synthetics of both the models reproduced the observations reasonably well at most of the stations in the central part the basin. At some stations, however, especially where the bedrock depth varies sharply, there were noticeable discrepancies in the simulation results of the models, and the synthetics did not accurately reproduce the observation. Our results indicate that the superiority of one model over the other cannot be determined and that an improvement in the basin velocity structure models based on simulation studies is required, especially along the basin edges. We also conclude that our transfer function method can be used to examine the applicability of the basin velocity structure models for long-period ground motion simulations.     

中国东部及其邻区上地幔顶部Pn波速度结构及各向异性

中国东部地处欧亚板块东南部,紧邻西太平洋板块俯冲带,有着复杂的地质构造和深部结构,是国内外学者研究的重点区域.本文采用Pn波层析成像方法反演得到了中国东部及其邻区上地幔顶部Pn波速度结构及各向异性.研究中应用的Pn到时数据主要来自多种地震观测报告,并特别补充了东北流动台阵、华北流动台阵以及区域地震台网记录的地震事件,拾取了大量高精度的Pn到时数据,最终挑选出2049个台站记录的24072次地震的240814条Pn波到时数据.结果显示,中国东部地区上地幔顶部平均速度为8.06 km·s^-1,速度变化范围为7.81~8.32 km·s^-1.东北地区东部表现为显著的低速异常,华北克拉通中、东部Pn呈现低速,而西部地区则表现为高速异常,华南地块主体表现为高速.反演结果还揭示江汉盆地、下辽河盆地、二连盆地及海拉尔盆地都显示出高速,而四川盆地和松辽盆地内部则呈现出不均匀的结构特征.四川盆地的高速异常显示出明显的分块现象,这可能是该盆地在沉积前具有不同的基底物质;松辽盆地的北部呈现为高速,而南部却表现为低速异常,这一特征与松辽盆地南、北部分别为高、低热流相对应,暗示盆地南部的岩石圈已经历了改造.研究进一步揭示Pn波低速区和高、低速过渡带的各向异性也较为强烈,而大部分强震都发生在这些区域之上的地壳内,说明这些部位容易发生变形而应力集中或产生应力差.     

Basin and crustal velocity structure models for the simulation of strong ground motions in the Kinki area,Japan

We constructed a prototype of the basin and crustal structure model for the Kinki area, southwest of Japan, for the simulation of strong ground motions of hypothetical crustal and subduction earthquakes. We collected results of the deep seismic velocity profiles obtained by the reflection experiments and seismic imaging results, which were conducted in the Kinki area. The obtained profiles give underground velocity structures of the crust, from the surface to the subducting slab. We also gather the basin velocity structure information of the Osaka, Kyoto, Nara, and Ohmi basins. To examine the applicability of the constructed velocity structure model to the ground motion simulation, we simulated waveforms of an intermediate size event occurred near the source area of the hypothetical subduction earthquakes. Simulated ground motions using the basin and crustal velocity structure model are fairly well reproducing the observations at most of stations, and the constructed basin and crustal velocity structure model is applicable for the long-period ground motion simulations.     

3D S-wave velocity structure of the Ningdu basin in Jiangxi province inferred from ambient noise tomography with dense array

The Ningdu basin, located in southern Jiangxi province of southwest China, is one of the Mesozoic basin groups which has exploration prospects for geothermal energy. A study on the detailed velocity structure of the Ningdu basin can provide important information for geothermal resource exploration. In this study, we deployed a dense seismic array in the Ningdu basin to investigate the 3D velocity structure and discuss implications for geothermal exploration and geological evolution. Based on the dense seismic array including 35 short-period (5 s-100 ?Hz) seismometers with an average interstation distance of ～5 ?km, Rayleigh surface wave dispersion curves were extracted from the continuous ambient noise data for surface wave tomographic inversion. Group velocity tomography was conducted and the 3D S-wave velocity structure was inverted by the neighborhood algorithm. The results revealed obvious low-velocity anomalies in the center of the basin, consistent with the low-velocity Cretaceous sedimentary rocks. The basement and basin-controlling fault can also be depicted by the S-wave velocity anomalies. The obvious seismic interface is about 2 ?km depth in the basin center and decreases to 700 ?m depth near the basin boundary, suggesting spatial thickness variations of the Cretaceous sediment. The fault features of the S-wave velocity profile coincide with the geological cognition of the western boundary basin-controlling fault, which may provide possible upwelling channels for geothermal fluid. This study suggests that seismic tomography with a dense array is an effective method and can play an important role in the detailed investigations of sedimentary basins.     

三峡地区上地壳结构的远震虚震源反射地震成像

远震虚震源反射成像方法利用远震初至波在台网之下地表与地下界面间形成的反射波(PPdp震相)波形资料进行台网地区地震反射结构研究.此方法先用台网各台站的平均初至波形求取震源信号,再用该震源信号与各道地震记录作反褶积从而取得反射剖面.本文介绍了远震虚震源反射成像的基本原理和实现步骤,并以三峡地区的观测资料为例,得到三峡库首区上地壳的反射地震剖面.成像剖面中解释出的四川盆地和秭归盆地的底界面位置和形态与地表地质观测和大地构造背景吻合.为验证虚震源成像的能力,本文使用弹性波正演模拟合成地震数据,经过处理实际资料一样的步骤获得虚震源成像结果.正演模拟表明,叠加多个远震的反射地震剖面可以有效地改善成像的信噪比和连续性;虚震源成像需要选择特定的震源频率范围,以减轻高频噪音以及低频造成的成像问题.在研究上地壳结构时,建议在不适合主动源采集和缺乏低频信号的地区尝试远震虚震源方法.     

A finite difference study on the basement structure beneath the Tianshan Orogen

We use the Pg seismic phase along the Korla-Jimsar profile across the Tianshan orogen and the 3D finite difference method to inverse the velocity structure of the upper crust beneath the basement of this mountain. Based on the velocity structure, the Korla-Jimsar profile can be divided into three parts, i.e. the north edge of the Tarim basin, the Tianshan orogen, and the south margin of the Junggar basin. Within the Tianshan there is a pattern of four convexities and three concavities, which correspond to the southern Tianshan, the Yanqi basin, the middle Tianshan, the Turpan basin, and the Bogda Mountains. In the north edge of the Tarim basin, the basement is about 10km deep with small lateral variations of velocity. In the Tianshan the velocity varies greatly laterally. The basement depth of the Yanqi basin is 6 km, which becomes shallow rapidly northward, and almost to the surface at the middle Tianshan. South to Kumux there is a small intermountain basin, where the maximum basement depth is 3 km, and also turns very shallow near Kumux. The Luntai fault, which bounds the Tarim basin and Tianshan, has vertical dislocation of about 5 km. The Turpan basin is covered with so thick a sediment that its basement is 7 km deep. The boundary fault between the Tianshan and Turpan is the Bolohoro fault which is characterized by quick deepening basement and 7 km vertical dislocation. In the Junggar basin the basement is 8 km deep. On the Korla-Jimsar profile, the velocity distribution of the upper crust and the structure are featured by NS symmetry on both sides of the axis of the Middle Tianshan, consistent with the deep structure revealed by this profile. It means that the Tarim basin and the Junggar basin underthrust toward the Tianshan from south and north, respectively. Such a structural style is different from that of another profile, i.e. the Xayar-Burjing profile, suggesting that there may be an important tectonic boundary between these two profiles.     

松辽盆地沉积层结构的短周期地震背景噪声成像研究

使用位于松辽盆地内部的NECESSArray台阵连续两年背景噪声数据,通过波形互相关和多重滤波方法提取到2~14 s较短周期的Rayleigh波群速度和相速度频散曲线,基于快速行进（FMM）面波成像方法得到群速度和相速度成像结果,并采用最小二乘迭代线性方法反演获得了松辽盆地深至12 km的三维S波速度结构.本文成像结果显示：松辽盆地内部S波速度分布的横向不均匀性与该区域的构造单元呈现出良好的空间对应关系.从地表至下方的6 km深度,盆地北部比南部表现出更加强烈的低速异常,这一特征可能与盆地南北的沉积构造差异有关.中央坳陷区低速异常的边界与嫩江断裂走向相互平行,表明盆地基底断裂对盆地形成演化具有一定的控制作用.在垂直速度结构剖面中,2.9 km·s^-1的S波速度等值线与地震反射剖面显示的盆地基底深度大致对应.基于S波速度模型和盆地基底速度（2.9 km·s^-1）,我们获得精细的松辽盆地沉积层厚度模型,结果表明松辽盆地的沉积层厚度分布呈现出中间厚、四周薄的特征,中央坳陷区的沉积层厚度范围大约在3~6 km.     

晋冀蒙交界地区地壳三维P波速度结构及地震活动性研究

2014年9月晋冀蒙临时地震台网正式运行,为晋冀蒙交界地区的地震研究提供了丰富的资料。结合临时台与固定台记录的P波走时资料,采用双差层析成像方法对该区进行地震重定位和三维P波速度结构联合反演,分析该区3个典型盆地的速度扰动结果,结果表明,地震重定位残差显著降低,震源深度分布更加合理;P波速度浅层基本符合平原地区低速、山区高速的特点,深层则有相反表现,典型盆地P波速度在浅层较为复杂,高低速相间,深层表现为低速;研究时段内忻定盆地和延怀盆地地震活动密集,大同盆地地震活动较弱,低速体及边缘地震活动强度高于高速体及边缘。     

Seismic response of multi-layered basins with velocity gradients upon incidence of plane shear waves

A boundary integral scheme based on boundary-integral discrete-wave-number approach has been developed to compute the seismic response of two-dimensional irregular-shaped basins with horizontal soil layers. Each layer exhibits a linear gradient of shear wave velocity with depth. The approach combines the boundary-integral representation of the seismic wave field outside the basin with plane wave representation of the seismic wave field inside the basin. The propagation throughout the layers is performed by matrix propagators in which the effect of the vertical variation of the velocity is incorporated by using confluent hyper-geometric functions of the Whittaker type. Our method is tested against otherwell-accepted solutions for the case of a circular basin with excellent agreement. Test of the ground response for a semi-circular basin with radius a shows that stable solutions are obtained if the chosen model parameters satisfy following conditions: (1) the distance from the sources to the interface is greater than 0·1a; (2) the distance between the sources is smaller than a quarter of the incident wavelength; and (3) the discrete wave-number step is smaller than 2π/4a. The computation of ground response of basins with a sharp interface and several horizontal deposits leads to the following main results: (1) the amplification of a basin with velocity gradients is larger than that of a basin with homogeneous layers; (2) the frequencies of the second- and third-order harmonics for a basin with velocity gradients are lower than those of a basin with homogeneous layers; and (3) the response amplitude of the basin with velocity gradients attenuates more slowly in time domain than when layers are homogeneous. Since these results have been obtained for realistic values of basin geometrical and mechanical consideration, they should find some interest in earthquake engineering or seismic microzonation studies. © 1998 John Wiley & Sons, Ltd.     

合肥市地壳浅部三维速度结构及城市沉积环境初探

合肥市位于合肥盆地东南缘,东侧紧邻郯庐断裂带,多条大型隐伏断裂穿过市区.为进一步认识合肥城市下方隐伏断裂的空间展布、性质,以及城市复杂的沉积环境,本文利用布设在合肥市区的58套三分量短周期地震仪组成的台阵,获得了37天的三分量连续波形数据,通过基于射线追踪的面波走时直接成像方法反演得到了合肥市地壳浅部0.6~3.6 km的三维剪切波速度结构,速度结构图像展现了地壳浅部的横向不均匀性和纵向成层性,揭示出NNE、NWW和近EW三组不同走向的隐伏断裂在城市地下浅部的构造特征.取得以下认识：（1）合肥市南、北方向在浅地表（2 km以内）存在显著速度差异,速度分界线位置与已知的近EW向的蜀山断裂一致,断裂南侧呈现低速凹陷,北侧则为高速隆起.低速中心深度达2~3 km,速度异常与该断裂在合肥盆地东部演化过程中的构造反转沉积了不同地层有关;（2）合肥市区存在明显的高速异常带,其走向、位置与穿过该区域的郯庐断裂带西支主干断裂相符,其中五河—合肥断裂在市区北部以东呈现低速凹陷特征,低速区范围与肥东凹陷晚白垩纪以来的沉积构造边界一致,认为肥东凹陷的最大沉积厚度可达2 km以上;（3）合肥市中心跨郯庐断裂带西支主干断裂之间呈现明显的凹、隆相间的复杂构造,推测其是在多组断裂的共同拉伸作用下形成的小型沉积盆地,沉积中心位于郯庐断裂带内部,最大厚度可达3~4 km.由于其展布方向在不同深度与该区域断裂的走向具有明显的相关性,推测不同深度的沉积形态与郯庐断裂带在不同时期的构造演化过程有关.     

辽宁地区波速比分布特征及构造意义

利用辽宁数字台网2001年以来的观测报告,采用单台和达法计算辽宁测震台网34个台站的平均波速比,重点分析辽宁地区波速比的空间分布特征.结果显示,辽宁地区的波速比空间分布呈横向不均匀性,下辽河盆地的波速比平均值低于整个辽宁地区波速比的均值,而辽西隆起和辽东隆起区的波速比却明显高于下辽河盆地的波速比和辽宁地区的均值.研究也...     

逆断层地震作用下三维沉积盆地地震动谱元法模拟

近断层地震动对近场区域会造成严重的地震灾害,尤其区域内存在沉积地形时会进一步加重灾害。目前,针对释放能量更大、破坏力更强的逆冲断层地震作用下沉积盆地的地震动响应研究还未见报道。本文即采用谱元法,研究了动力学逆断层地震作用下的三维沉积盆地的动力响应。文中以椭球形沉积盆地为例,对其逆断层地震动响应特性进行了分析,并探讨了改变内外介质波速比和沉积厚度时沉积盆地内部观测点地震动时程和峰值变化规律。研究表明:①沉积内外介质波速比对沉积盆地的地震动影响显著,当沉积内部介质波速比降低时,盆地内部地表的峰值响应增大,地震动持续时间明显延长,尤其是位于盆地中心加速度峰值放大2.08倍,持时延长1.97倍;②沉积盆地厚度同样对其地震动响应产生影响,当沉积厚度增加时盆地中心位置地震动响应减小,加速度峰值缩小约0.64倍,而盆地边缘区域的地震动响应明显增大,峰值放大约1.35倍。      

利用初至P震相测定石嘴山ML4.4地震序列的震源深度

基于初至P震相较易辨识、读取误差小、有效震相丰富等优点,提出联合Pg与Pn震相到时数据测定震源深度的方法,并将其应用于石嘴山M_L4.4地震序列(11次)的震源深度计算,结果显示该地震序列的震源较浅,这也是此次有感地震震感强烈的原因之一．然后利用初至P震相到时方法分别计算了双层和4层地壳速度模型下的震源深度,并与双差定位所得深度进行对比,结果显示: 利用初至P震相到时方法较双差定位方法得到的震源深度整体上一致性较好,但前者得到的震源深度较之后者略深;对于双层与4层地壳速度模型,二者的震源深度计算结果具有较好的一致性．因此,基于可靠区域地壳速度模型下的初至P震相到时方法可以应用于震源深度的计算,并能够取得较好的结果．另外,石嘴山M_L4.4地震序列的主震深度为7—8 km,最大余震深度为6 km,根据银川盆地的速度成像结果可知,主震和最大余震发生于银川盆地基底下部;其余地震的震级偏小,多集中于3—5 km深度,主要发生于银川盆地基底顶部或基底覆盖层内．      

华北地区上部地壳结构的三维有限差分层析成像

采用正方形网格对华北地区速度模型进行参数化、网格维数为649×90×16,网格间距为1km. 通过对华北地区30条深地震测深剖面的Pg波三维有限差分速度层析成像反演,获得了该地区上部地壳的三维速度结构. 反演结果显示,低速的"棱形"华北盆地被高速的隆起所包围;而华北盆地又由两个大的断陷带中间夹一个隆起所组成.两个大的断陷带又被分割为多个小的断陷盆地.研究区内的断层多为铲式正断层,结晶基底深度变化较大. 所有这些表明华北地区具有复杂的地质构造和发生强震的深部背景.     

利用地震折射和反射波资料研究银川盆地浅部结构和隐伏断裂

银川盆地是华北克拉通西部构造活动较为强烈的一个新生代断陷盆地.为了研究银川盆地的地壳浅部结构和活动断裂特征,我们利用2014年在银川盆地完成的深地震反射剖面数据,采用初至波层析成像方法得到了银川盆地高精度的基底P波速度结构和构造形态;考虑到仅根据速度结构剖面还难以确定断裂的准确位置、断层上断点埋深、断层的近地表构造组合样式等特征,研究中还采用浅层地震反射波勘探方法对银川盆地内的隐伏断裂和1739年平罗8.0级地震的地表破裂带浅部结构进行了高分辨率成像.研究结果表明:银川盆地与两侧地块的浅层P波速度结构和沉积盖层厚度差异较大,银川盆地总体呈现出明显的低速结构特征,盆地基底面起伏变化较大,基底最深处位于芦花台断裂和银川断裂之间的银川市下方,其深度约为7000~7200 m;贺兰山隆起区显示为明显的高速特征,地表出露中-古生代基岩地层,缺失新生代地层;鄂尔多斯地块西缘的浅层P波速度明显高于银川盆地,基底埋深相对较浅,推测其新生界地层厚度小于2500 m.浅层地震反射剖面揭示的地层反射界面形态和断裂的浅部构造特征非常清楚,黄河断裂、贺兰山东麓断裂、银川断裂和芦花台断裂不仅是错断盆地基底的断裂,而且还是第四纪以来的隐伏活动断裂,这些断裂的交替活动形成了"堑中堑"的盆地结构,并对银川盆地的形成、盆地内的新生代地层厚度和第四纪沉降中心具有重要的控制作用;在近地表这些断裂表现为由2~3条断层组成的"Y字形"断裂构造,且主断裂的最新活动可追踪至晚更新世末期或全新世,是构造继承性活动的结果.本文的研究结果不仅可为进一步分析银川盆地的基底结构、隐伏断裂特征和活动构造研究等提供新的地震学证据,而且还可为该区城市规划中避让活动断层提供科学依据.     

鄂尔多斯及周边区域噪声层析成像研究

基于中国地震科学探测台阵在鄂尔多斯及周边地区布设的461个地震台为期2年的地震观测资料,采用背景噪声层析成像方法,研究获得了鄂尔多斯及周边地区5—46 s周期、分辨率高达0.3°×0.3°的瑞雷面波相速度分布图像.与基于程函方程的地震面波成像结果对比看出,噪声层析成像在较短周期具有明显的优势(5—16 s),可以获得更...     

应用远震有限频率层析成像反演首都圈上地幔速度结构

本文选用首都圈数字地震台网2003年9月~2005年12月记录的300多个远震事件的波形资料,采用分频带多道互相关方法得到三个不同频段的P波相对走时数据共18499个,计算了每个频段的走时灵敏度核,应用有限频率层析成像反演得到首都圈地区的上地幔三维P波速度结构模型.利用检测板估计了反演结果的分辨率,并与射线层析成像方法的结果进行了比较,说明了反演结果的可靠性.研究结果表明,各构造单元具有明显不同的速度结构特征,其差异可到150 km深:燕山隆起区表现高速;太行山隆起区整体以低速为主并存在小范围高速块体;华北盆地、渤海湾下浅层上地幔中存在大范围的强低速异常,其顶面在50~70 km,可视为软流圈顶面的埋深,这一结果说明华北盆地、渤海湾下岩石圈明显减薄;张家口-蓬莱断裂带是上地幔浅部速度结构的变异带,也是岩石圈减薄的边界带,区内大部分强震都发生在该构造带上,由此看来该带上强震的发生不仅与地壳结构的不均匀性有关,还可能有较深的构造背景.     

Subsoil structure and site effects: A comparison between results from SPAC and HVSR in sites of complex geology

We present the results of the analysis of array recorded microtremors at 14 sites, close to the edges of the Mygdonian basin in northern Greece (Euroseistest). These measurements were made in order to better constrain the geometry and velocity structure of the basin as the soil layers taper out close to rock outcrop, where geology is complex and we may expect significant changes of the subsoil structure over short distances. The data were analysed using the SPAC method and HVSR. The first interprets the measurements as Rayleigh waves (for the vertical component we analysed) and allows to invert a phase velocity dispersion curve from computed correlation coefficients. The second estimates a local transfer function directly, from ratios of Fourier amplitude spectra. A phase velocity dispersion curve could be derived for 12 of the 14 measurement sites, and at three of the sites no resonant frequency was observed in the HVSR. It is encouraging that we obtained good results at most of our sites, in spite of the lateral heterogeneity expected close to the edges of Euroseistest. Our results allow us to obtain shear wave velocity models at most of the measurement sites (12 out of 14). They are also useful to explore the relation between size of the array and wavelength range for which a dispersion curve may be estimated, which in our case has strong limitations. We identify the frequency of resonance of the sediments as a small loss of coherency in SPAC’s correlation coefficients. Finally, we also consider the applicability of the joint inversion of the resonance frequency determined using HVSR and the phase velocity dispersion curve obtained from SPAC.