Shear-wave Velocity Structure around Teide Volcano: Results Using Microtremors with the SPAC Method and Implications for Interpretation of Geodetic Results

Deformation analysis and simulation of volcanic edifices require the construction of models of elastic properties of those structures. In this paper we present an analysis of microtremor measurements recorded during the performance tests of two temporary seismic arrays installed in the eastern portion of the Teide caldera in 1994. We take advantage of recent developments of the SPAC method and use spatial cross-correlation computations to estimate phase velocity dispersion of Rayleigh waves at the location of the arrays. We show that the extension of the standard SPAC method is valid in the case of our data, justifying its use and supporting the generalization of the SPAC method to single station pairs. The phase velocity dispersion curve obtained was inverted to recover the shear-wave profile at the site of the arrays. Our results indicate that the subsoil structure of the caldera is laterally homogeneous at the scale of a few km about the location of the arrays. We obtained about 315 m of volcanic sediments overlying rocks with a shear-wave velocity of 2 km/s. These results are robust and are a starting point to further modelling of deformation, permanent or transient, at this volcanic edifice, which can be useful in the interpretation of different observed fields. In fact, the computation of deformations and gravity changes due to possible volcanic intrusions in two models; one considering the volcanic sediments and the other without considering them, provided different results in the near field.     

Estimating site-specific strong earthquake motions

The Campus Earthquake Program (CEP) of the University of California (UC) started in March 1996, and involved a partnership among seven campuses of the UC—Berkeley, Davis, Los Angeles, Riverside, San Diego, Santa Barbara, Santa Cruz—and the Lawrence Livermore National Laboratory (LLNL). The aim of the CEP was to provide University campuses with site-specific assessments of their earthquake strong motion exposure, to complement estimates they obtain from consultants according to the state-of-the-practice (SOP), i.e. Building Codes (UBC 97, IBC 2000), and Probabilistic Seismic Hazard Analysis (PSHA). The Building Codes are highly simplified tools, while the more sophisticated PSHA is still somewhat generic in its approach because it usually draws from many earthquakes not necessarily related to the faults threatening the site under study.Between 1996 and 2001, the site-specific studies focused on three campuses: Riverside, San Diego, and Santa Barbara. Each campus selected 1–3 sites to demonstrate the methods and procedures used by the CEP: Rivera Library and Parking Lots (PL) 13 and 16 at UCR, Thornton Hospital, the Cancer Center, and PL 601 at UCSD, and Engineering I building at UCSB. The project provided an estimate of strong ground motions at each selected site, for selected earthquake scenarios. These estimates were obtained by using an integrated geological, seismological, geophysical, and geotechnical approach, that brings together the capabilities of campus and laboratory personnel. Most of the site-specific results are also applicable to risk evaluation of other sites on the respective campuses.The CEP studies have provided a critical assessment of whether existing campus seismic design bases are appropriate. Generally speaking, the current assumptions are not acknowledging the severity of the majority of expected motions. Eventually, both the results from the SOP and from the CEP should be analyzed, to arrive at decisions concerning the design-basis for buildings on UC campuses.     

Seismic microzonation of the greater Bangkok area using microtremor observations

Bangkok, the capital city of Thailand, is located at a remote distance from seismic sources. However, it has a substantial risk from these distant earthquakes due to the ability of the underlying soft clay to amplify ground motions. It is therefore imperative to conduct a detailed seismic hazard assessment of the area. Seismic microzonation of big cities, like Bangkok, provides a basis for site‐specific hazard analysis, which can assist in systematic earthquake mitigation programs. In this study, a seismic microzonation map for the greater Bangkok area is constructed using microtremor observations. Microtremor observations were carried out at more than 150 sites in the greater Bangkok area. The predominant periods of the ground were determined from the horizontal‐to‐vertical (H/V) spectral ratio technique. A microzonation map was then developed for the greater Bangkok area based on the observations. Moreover, the transfer functions were calculated for the soil profile at eight sites, using the computer program SHAKE91, to validate the results from the microtremor analysis. The areas near the Gulf of Thailand, underlaid by a thick soft clay layer, were found to have long natural periods ranging from 0.8s to 1.2s. However, the areas outside the lower central plain have shorter predominant periods of less than 0.4s. The study shows that there is a great possibility of long‐period ground vibration in Bangkok, especially in the areas near the Gulf of Thailand. This may have severe effects on long‐period structures, such as high‐rise buildings and long‐span bridges. Copyright © 2003 John Wiley & Sons, Ltd.     

一种以等差分级的DEM填洼算法

DEM的填洼是水系提取中最耗时的过程。真实的地表低高程点通常在空间上相邻,在对低高程栅格进行填洼时,少数栅格的高程值更新会导致级内大量栅格的循环迭代,从而消耗大量运算时间。为了提高DEM的填洼效率,在分级填洼的基础上,提出一种等差分级填洼算法,该算法顾及DEM各级填洼时间呈幂函数逐级递减这一地形规律。算法首先创建一系列数组,数组大小随等差数列递增;然后依次将排序后的DEM数据存放至数组中,由于定义的数组大小逐渐增大,因此,低高程区域被“分割”得很细;最后,当填洼运算从级内转至数组内时,低高程区域细致的划分极大缩短了栅格循环迭代时间,从而使得算法获得了较高的效率。新算法既能有效缩短填洼过程中的迭代时间,又能保证所提取水系的完整性与连贯性。为验证算法的有效性,选用四川省不同地区的SRTM 90 m分辨率DEM作为实验数据,并与已有的DEM分级填洼算法进行对比分析。实验结果显示：当研究区域栅格总数达到两千万个时,本文算法填洼效率提升了50%左右,且随着DEM数据量的增大,算法效率的提升更为明显,与此同时,利用新方法进行填洼后,DEM水文线连续性较强,表明了新方法的有效性。     

体波地脉动单点谱比法研究

采用弹性成层介质模型,应用概率分析方法,推导地下体波斜入射成层介质,在自由表面形成体波地脉动的水平分量与竖向分量谱比表达式。然后,通过大量模型计算,论证体波斜入射单层覆盖半空间形成地脉动水平分量与竖向分量谱比特性与场地传递函数间的关系。结果表明,利用地脉动单点谱比法分析体波为主要成分地脉动时,与场地水平向传递函数相比,可以得到较准确阻抗比大于3的场地卓越频率和偏小的场地放大因子。     

CORRELATION PATTERNS IN CHARACTERISTICS OF SPATIALLY VARIABLE SEISMIC GROUND MOTIONS

A methodology for the investigation of the spatial variation of seismic ground motions is presented; data recorded at the SMART-1 dense instrument array in Lotung, Taiwan, during Events 5 and 39 are used in the analysis. The seismic motions are modeled as superpositions of sinusoidal functions, described by their amplitude, frequency, wavenumber and phase. For each event and direction (horizontal or vertical) analysed, the approach identifies a coherent, common component in the seismic motions at all recording stations, and variabilities in amplitudes and phases around the common component sinusoidal characteristics, that are particular for each recording station. It is shown that the variations in both the amplitudes and the phases of the motions at the station locations around the common component characteristics contribute significantly to the spatially variable nature of the motions, and, furthermore, they are correlated: increase in the variability of the amplitudes of the motions recorded at individual stations around the common amplitude implies increase in the variability of the phases around the common phase. The dispersion range of the amplitude and phase variability around their corresponding common components appear also to be associated with physical parameters. The spatially variable arrival time delays of the waveforms at the stations due to their upward travelling through the site topography, in addition to the wave passage delays identified from signal processing techniques, constitute another important cause for the spatial variation of the motions; their consideration in the approach facilitates also the identification of the correlation patterns in the amplitudes and phases. © 1997 by John Wiley & Sons, Ltd.     

基岩表面地脉动谱比研究

目前,场地影响评价方法可以分为理论法和经验法。地脉动水平分量与竖向分量谱比法属经验法。这种方法将地脉动水平分量与竖向分量谱比的最大值对应的频率和幅值分别看作是观测场地的卓越频率和放大因子。虽然该法在国内外很多工程实例中得到了理想的结果。但由于这种方法建立过程中应用了许多假设,所以很多地震工程学家对这种方法持怀疑态度。本文讨论了基岩地脉动水平分量与竖向分量谱比为1这一假定的合理性。并采用泊松弹性半空间模型,应用概率分析方法考虑地脉动的随机性,推导了地下体波斜入射弹性半空间自由表面形成地脉动的水平分量与竖向分量谱比表达式。通过计算论证体波斜入射泊松弹性半空间形成的地脉动水平分量与竖向分量谱比特性。结果表明：当地脉动主要成分为体波时,基岩表面地脉动水平分量与竖向分量谱比接近于1;当地脉动主要成分为面波时,基岩表面地脉动水平分量与竖向分量谱比介于0.54～0.79之间。对泊松基岩,该比值是0.68。     

地表/井下反应谱比值非线性统计特征与影响因素研究

浅地表软弱覆盖层对地震动的影响具有强非线性,但因观测记录样本量过小,难以通过参考谱比法等直接方法予以可靠分析.本文基于日本KiK-net台网136个竖向钻井台阵获取的141881组加速度记录的统计分析,研究了地表/井下反应谱比值随地震动强度变化的非线性变化规律与主要影响因素.利用变窗口尺度的滑动窗口平均法加强数据线性度...     

微振动台阵记录中空间自相关法的时间延迟校正

本文给出了微振动台阵的波长与台站对间距离的半理论-半经验关系;根据前人的理论和实验,指出微振动波场存在优势方向,即在有效波长范围内,微振动的各道记录存在相位差;提出了投影到某方位的相位校正公式和校正方法,由此实现空间自相关法的相关函数和相速度曲线的时间延迟校正,通过实例验证了该方法的有效性。     

The future of large-format InSb arrays for astronomy

Introduced to astronomy less than a decade ago, infrared detector arrays have radically and forever changed astronomer's observational techniques, instruments, and telescope designs. This paper first examines the array technology development that caused this change, with emphasis on the 1- to 5-m band. Technology trends and projections for further development within the infrared industry are analyzed to assist astronomers in planning future instruments. This technology development is predicted to result in arrays with lower noise, greater well depth, response to a wider band of wavelengths, and, above all, much larger formats than arrays currently in use.