A procedure for simulating synthetic accelerograms compatible with correlated and conditional probabilistic response spectra

The modeling of seismic load is a major topic that has to be addressed thoroughly in the framework of performance based seismic analysis and design. In this paper, a simple procedure for simulating artificial earthquake accelerograms matching the statistical distribution of response spectra, as given by median ground motion prediction equations, the standard deviation and correlation coefficients, is proposed. The approach follows the general ideas of the (natural) ground motion selection algorithms proposed by Baker [4] and Wang [43] but using simulated (artificial) “spectrum-compatible” accelerograms. This allows to simulate spectrum-compatible accelerograms featuring variability similar to the one of recorded accelerograms when the match of median and ±1 standard-deviation response spectra is imposed by the regulator. The procedure is illustrated by an application to the NGA ground motion data and models.     

Delineation of the northern limit of the Congo Craton based on spectral analysis and 2.5D modeling of aeromagnetic data in the Akonolinga-Mbama area,Cameroon

Aeromagnetic data of the Akonolinga-Mbama region are analyzed in order to elucidate the subsurface geology of the area. The available data in the form of a residual aeromagnetic map is interpreted as a vast magnetically quiet region, and complex zones which do not correlate with the surface geology of the region.Within the magnetically quiet zone, a high negative circular elongated anomaly zone seems to represent an intrusion of a plutonic rock into the metamorphic formations of the region. Spectral analysis and two-and-a-half dimensional (2½-D) modeling are used to estimate the depth of the causative bodies and determine the source rocks along three profiles crossing the suspected areas. Models from various zones of granitic intrusions are obtained, thereby proposing some shallow fault lines along zones of contact. This permits us to mark out the northern margin of the Congo Craton, thus enabling us to distinguish the cratonic formations from the Pan African fold belt. Part of the belt has been thrust over the northern portion of the Congo Craton in Cameroon.     

小球藻和铜绿微囊藻的高浓度Chl-a高光谱定量模型

本研究将地物高光谱遥感技术应用于室内实验,从而得到小球藻和铜绿微囊藻的高光谱特征.通过多种半经验方法,如单波段、波段比值和微分法,建立了两藻种最优的Chl-a高光谱定量模型,并与室外情况进行了对比.结果表明:小球藻的最优定量模型为Chl-a=174.6 1138292(R703) 2.3(109[(R703)]2(p＜0.01),相应的方法适宜性为:一阶微分法＞单波段法＞波段比值法;铜绿微囊藻的最优定量模型为Chl-a=5299164(R757)1.9773(p＜0.01),相应的方法适宜性为:单波段法＞波段比值法＞一阶微分法;从高光谱特征来看,小球藻在540 nm和700 nm附近存在明显的特征波峰,其位置随Chl-a浓度增大而向长波方向偏离,铜绿微囊藻在530 nm、660 nm和700 nm附近存在3个较强的特征波峰,在610 nm和680 nm附近存在明显的波谷;与以往室外研究不同的是铜绿微囊藻的反射率在400-500 nm之间的R值并不低,是因为没有非藻类颗粒物的影响,总吸收明显降低.     

Influence of the quality factor on simulated seismic waves: A case study of the 1994 Northridge earthquake

In numerical simulations of ground motion, the constant quality factor Q of a viscoelastic medium can be determined using the time-domain constitutive approximation method of the generalized standard linear solid (GSLS) model. This study introduces a numerical seismic wavefield simulation method which combines the spectral element method with the constant-Q model. The method is used to simulate the seismic wavefield of the 1994 Northridge earthquake. The optimal attenuation coefficient for the simulated seismic waves in this study area is determined empirically based on a quantitative analysis of the deviation curve. Further, the effect of the quality factor on the simulated wavefield are analyzed according to the site characteristics of each seismic station. The quality factor shows a variable effect on the different frequency components of the simulated wavefield. The effect of the quality factor also varies with the characteristic parameters of each seismic station site, such as site velocity structure, fault distance, and azimuth angle.     

Characteristics and analysis of the geomagnetic variations in regions around the Qiongzhou Strait

Ｃｈａｒａｃｔｅｒｉｓｔｉｃｓ　ａｎｄ　ａｎａｌｙｓｉｓ　ｏｆ　ｔｈｅ　ｇｅｏｍａｇｎｅｔｉｃ　ｖａｒｉａｔｉｏｎｓ　ｉｎ　ｒｅｇｉｏｎｓ　ａｒｏｕｎｄ　ｔｈｅ　Ｑｉｏｎｇｚｈｏｕ　Ｓｔｒａｉｔ（范国华）（姚同起）（顾左文）（朱克佳）（陈伯舫）（冯戬...     

Site amplification in the Lijiang Valley, Yunnan Province

The site amplifications for three stations, Libin (LIBI), Baisha (BASH) and Yulong (YULG) situated respectively in the southern, middle and northern parts of Lijiang Valley, are obtained by analyzing the S-wave soil/bedrock and microtremor horizontal/vertical spectral ratios. The data are digital recordings for the aftershocks of the Lijiang MS=7.0 earthquake on February 3, 1996. In the frequency range of 1～4 Hz, the S-wave soil/bedrock spectral ratio of E-W component for LIBI is the largest and amounts to 4.5. The microtremor soil N-S/vertical (V) spectral ratio is approximate to 1, E-W/V is about 4.5 and the same with above soil/bedrock spectral ratio. It is shown that the vertical and N-S components of microtremor have not been amplified by the soil and the spectral ratios for BASH and YULG are further evaluated. They have similar characteristics with that of LIBI. In above frequency range, both N-S/V ratios are approximate to 1, while the E-W/V ratio is about 6 for BASH, 4.5 for YULG. Lijiang Valley is characterized by the trans-valley directional site response.     

岩石圈塑性流动与大陆板内构造变形研究进展评述

人们对大陆板内构造变形机制有两种不同认识,即“刚性”板块通过弹性实现应力远程传递和岩石圈通过下层塑性流动实现应力远程传递。通过对岩石圈各层变形属性和塑性流动的研究。认为通过处于塑性状态的下地壳和岩石圈地幔的塑性流动实现应力远程传递和控制板内构造变形更为合理。     

地球自由振荡的保结构模拟

大地震等诸多激励均能激发全球自由振荡现象,通常表现为驻波形式的全球整体振荡.现有的地震波数值模拟方法多为非保结构方法,无法压制长时程计算中的积累误差.本文采用优化的三阶辛格式谱元法,对地球自由振荡及全球尺度的地震波传播进行了长时程模拟.通过与传统的基于Newmark算法的谱元方法结果对比分析,明确验证了本文所得优化的三阶辛格式谱元法在模拟地球自由振荡等大规模长时程问题上的优越性和准确性.上述进展在方法论层面为今后探测、刻画全球尺度地球非均匀结构的驻波数值方法奠定了部分基础,并为相关研究领域提供了新的选择.     

施甸盆地三维地震动研究

本文利用三维施甸盆地模型以及川滇地区的地壳速度结构,采用谱元法和并行计算技术,考虑地形的影响,模拟了盆地外围中等设定地震作用下施甸盆地地震波的传播,分析了面波的产生和传播,研究了该盆地不同位置剖面地震动的差异和原因.研究表明,小型盆地在中等地震的作用下盆地内部地震动空间差异很大,盆地中较深的小型凹陷更容易汇聚面波,形成更强的振动,位于覆盖层厚度剧烈变化区域的地震动强度也会很大,甚至大于覆盖层厚的区域.较基岩地震动,盆地内地震动持续时间明显增加,地震动的放大作用明显,这也是盆地震害严重的主要原因之一.     

On the use of microtremor recordings in seismic microzonation

Experimental methods involving microtremor recordings are useful for determining site effects in regions of moderate seismic activity where ground motion records are few, and in urban or industrial contexts where the noise level is high. The aim of this study is to establish a microzonation by using the Nogoshi–Nakamura method,[1, 2] a simple experimental technique based on microtremor recordings. Since the physical phenomena underlying the method are only partially understood, the spectral responses obtained cannot be used alone. We, therefore, complete our experimental results by comparing them with the solutions of a one-dimensional numerical simulation (SHAKE91).[3, 4] The experimental programme was carried out on a plain near the Rhone Delta (South of France). H/V spectral ratios were calculated at 137 noise measurement points. In addition, we were able to compute the numerical transfer functions from soil columns defined by geotechnical characteristics inside the studied region. A comparison of the results obtained by the experimental and numerical methods showed that the fundamental frequencies are in good agreement, but that the amplitudes obtained by the two techniques are sometimes different. The analysis of H/V spectral ratios enabled us to establish maps to characterize the region: a resonance frequency map and maps of amplification levels as a function of frequency range, leading to a seismic microzonation for the whole of the region.