Simulation of seismic response in an idealized city

We study the seismic response of an idealized 2D ‘city’, constituted by ten non equally-spaced, non equally-sized, homogenized blocks (i.e. buildings or groups of buildings) anchored in a soft soil layer overlying a hard halfspace. Our results display strong response inside the blocks and on the ground which qualitatively match the responses observed in some earthquake-prone cities.     

CODA Q estimates for Kumaun Himalaya

CodaQ (Q _c) estimates for the Kumaun Himalaya region have been obtained in high frequency range. Local earthquakes, recorded by a digital seismic network in the region, which fall in the epicentral distances range of 10 to 80 km and with a local magnitude range of 1.4 to 2.8, have been used. The coda waves of 30 sec window length, filtered at seven frequency bands centered at 1.5, 3, 6, 9, 12, 18 and 24Hz, have been analysed using the single backscattering model. The values ofQ _c estimates vary from 65 to 283 at 1.5 Hz to 2119 to 3279 at 24.0 Hz which showed thatQ _c is frequency dependent and its value increases as frequency increases. A frequency-dependentQ _c relationship,Q _c = (92 ± 4.73)f^(1.07±.023), is obtained for the region representing the average attenuation characteristics of seismic waves for Kumaun Himalaya region.     

长白山天池火山区的地震活动特征分析

2002年6月以来,长白山天池火山区的地震活动明显增强。利用长白山火山中心站的观测资料对长白山天池火山地震的活动特征进行了研究。火山地震的频谱分析和时频分析结果表明,自1999年以来发生的火山地震主要为构造型火山地震。对这些火山地震进行了尾波Q值分析并与四川地区的Q值进行了对比,结果表明长白山火山区尾波Q值偏低,说明这一地区构造活动水平相对较强     

云南地区尾波衰减特征研究

利用22个云南区域数字地震台网近震资料,研究了云南地区地震尾波在1~20 Hz内6个频段的衰减特征. 首先使用单次散射模型计算各台站的尾波衰减系数; 然后利用随机各向同性散射介质均匀分布的多次散射模型, 使用多流逝时间窗分析方法,计算各台站的平均自由程Le和地震反照率B0得到总衰减系数,并从中分离散射衰减系数 、吸收衰减系数 . 云南地区基本衰减特征是Le主要集中在10~30 km内, 在2~6 Hz之间有最大值;B0在1~2 Hz大致为0.5左右,在其它频段则小于0.5. 也即在1~2 Hz, 与 数值相当, 其它频段 逐渐大于,成为尾波总衰减的主要原因. 除1~2 Hz外,其它频段 值接近. 云南地区的Q-1尤其是, 在空间上有较大的差异,滇东略微高于滇西北, 滇西北高于滇西南地区. 与全球其它区域的研究结果比较发现,云南地区 略低于平均水平, 略高于平均水平, 而则处于其它区域结果的中间.      

地壳介质破裂的尺度理论在研究山丹-民乐 6.1级地震的地震学前兆特征中的应用

介绍了Allegre的地壳介质破裂的尺度理论和根据该理论建立一种预测地震发生时间方法的可能性. 作为震例研究了甘肃山丹——民乐6.1级地震的地震学前兆（地震空间相关长度和尾波Qc值）特征. 结果表明,地震空间相关长度和尾波Qc值在震前都呈增长趋势,用幂次率关系拟合了这两种参数的上升变化形态. 这为利用地震空间相关长度和尾波Q值的监测资料建立一种预测地震发生时间的方法和找到预测指标打下了基础.      

依舒断裂带北段尾波Q值特征研究

研究表明,地震波穿过构造活动相对稳定地区时,能量衰减不明显且具有高Q值;而穿过构造活动地区时,能量会发生强烈衰减且具有低Q值。本文利用Sato模型对依舒断裂带北段萝北-通河地区15个数字化地震台站记录到的266条M_L≥2.0地震尾波Q_c值进行了分析和研究,发现黑龙江萝北、通河附近地区Q_c值具有明显的复杂性和差异性,且萝北地区Q_c值远低于通河地区Q_c值,其原因一方面与区域构造活动有关,另一方面与区域地壳构造复杂、地下介质破碎、区域应力不断变化有关。本文还讨论了萝北、通河附近地区Q值随着频率变化的关系及Q_0值空间分布特征,有效地分析了该区域介质状态的变化过程,对萝北地区地震活动性的研究和预测有指导意义。     

Characterization of regional and local scattering effects from small-aperture seismic array recordings

In 1998, four small-aperture arrays separated by 20 km have been deployed in the southern French Alps in order to record the natural seismicity during two consecutive months. One of the main objectives of this experiment was to characterize the heterogeneities that influence the wavefield propagation, by analysis of the coda characteristics recorded by each of the seismic arrays.The time-azimuth-velocity characteristics of the coda phases have been obtained using a high-resolution wavenumber decomposition method.A statistical analysis, using the coda characteristics of the whole data set (20 regional earthquakes) recorded by the four arrays has been performed and lead to the calculation of the density of scattered energy within the medium. Three regional heterogeneous areas (> 10 km) have been characterized, and are located in the N–NE, W and S directions from the four arrays. Scattered energy is also located at local distances (< 10 km) from the four arrays.The comparison of the waveforms recorded (i) with one of the array, for different groups of earthquakes, and (ii) with the four arrays, for one group of earthquakes, show that the geometrical configuration of the source–scattering area–recording site system can strongly influence the energetic distributions related to the secondary phases of the seismograms.In particular cases, we experimentally show that the interactions of the direct wavefield with the heterogeneous structures of the medium can be sufficiently energetic to induce secondary scattered phases that dominate the seismic motions recorded at a given site. In such case, these effects should be taken into account for the quantification of the expected ground motion recorded during an earthquake.     

新疆及周边地区Lg尾波的Qem值分布

首先运用叠加谱比法， 对新疆地区22个数字化台网台站、乌鲁木齐台站以及周边的7个台站1999～2003年上半年所记录的1 156个宽带垂直记录，进行了处理， 得到了与各射线相对应的Q0值（1 Hz处的Q值）和频率相关因子eta;. 在此基础上， 应用反投影技术， 得到了新疆地区1deg;times;1deg;网格的Q0值和eta;值的分布图象. 结果表明: 在塔里木地台及西伯利亚地台边缘，Q0值较高，为300～450； 而在南部的西昆仑褶皱系、松潘甘孜褶皱系，Q0呈现出低值，在150～250的范围内变化；在北部的准葛尔褶皱系和天山褶皱系，Q0现出较低的值，在250～300的范围内变化，eta;值的变化范围为0.5～0.9. 