Estimation of Nonlinear Time-dependent Soil Behavior in Strong Ground Motion Based on Vertical Array Data

To improve our understanding of nonlinear elastic properties of soils, a method is proposed of estimation of stress-strain relations of soils in situ in strong ground motion based on vertical array data. Strong motion records provided by seismic vertical arrays allow estimation of nonlinear stress-strain relations in soil layers at different depths, from the surface down to the location of the deepest device. As an example, records obtained during the main shock of the 1995 Kobe earthquake at Port-Island, SGK, and TKS sites were used to estimate the stress-strain relations in the soil profiles. For different layers, different types of nonlinear stress-strain relations were selected, according to the profiling data. To account for temporal changes in the soil behavior, consecutive parts of records were examined, and for successive time intervals, the relations were found showing the best-fit approximation to the observed data. At Port Island and SGK sites, where the strongest accelerations were recorded, the obtained stress-strain relations showed systematic changes in the upper layers (0–14 m), such as, a progressive reduction of the slopes of the stress-strain curves due to liquefaction at Port Island and reduction and recovery of the slopes at SGK and TKS sites. At the three sites, the stress-strain relations remained stable in layers below 11–14 m. Thus, the proposed approach gives us a representation of the soil behavior in layers at different depths in strong ground motion; it allows calculation of the propagation of arbitrary seismic signals in the studied profiles and estimation of nonlinear components in the ground response by the nonlinear system identification technique. The method can also be applied to evaluate the ground response at sites where profiling data are available and an imposed motion can be estimated.     

A parametric study of borehole influence in resolving seismic velocity in well-to-well tomograms

The data collected in a well-to-well tomography experiment is inherently incomplete even when augmented by VSP data. The nature of the experiment suggests a geometric limitation to the resolution of any central structure.A parametric model has now been developed which examines the effect on reconstructed velocity due to the borehole size, transducer standoff, altered annulus, and the degree of velocity attenuation of the altered annulus, using a range of host velocities and hole separations.It was found that there are significant velocity variations caused by perturbations in borehole size. These errors are particularly significant for boreholes with large diameters and small transducer offsets. For an altered annulus, errors in both the dimensions and the degree of velocity alteration gave significant velocity variation in reconstructed velocity, particularly with large boreholes. In all cases it is observed that the variation and alteration in reconstructed velocities increased as hole separation is decreased.     

Matrix Green's functions for array-type sources and receivers in multiwave layered media

Summary. A previous formulation (Lu, Felsen & Kamel) of source-excited wave propagation in a multiwave layer is here extended to multiple layers, each of which may propagate different multiple wave species, and to simultaneous excitation and detection at arbitrarily specified multiple levels. Field variables are arranged so as to reveal 'interesting'layers requiring access (for example, those containing a source and/or receiver) but to hide in collective form all other 'uninteresting'layers. An ordering of wave constituents into array vectors provides not only a physically appealing view of the wave phenomena pertaining to array-type source and receiver arrangements but may also furnish numerical advantages. The variety of alternative representations in Lu et al. can be brought to bear directly on the present formulation which is thereby endowed with substantial versatility, especially that embodied within the hybrid ray-mode format.     

线阵探测器(LDA)的现状及发展趋势

本文概述了当前的线阵探测器(LDA)技术及未来发展，同时详细说明了LDA的重要性能特征，并对不同应用下不同的阵列设计进行了讨论。目前芬兰探测技术公司已将该项技术成功应用于大型集装箱检测系统，工业CT，医用CT，L型双能探测器等工业医学应用中．     

Seismic attenuation values obtained from instantaneous-frequency matching and spectral ratios

In this paper, attenuation values are obtained from seismic data using instantaneous-frequency matching and spectral ratios. to obtain differential t * values using instantaneous-frequency matching, a near offset reference pulse is attenuated until the resulting instantaneous frequency matches the observed value at the receiver. Prior to matching, filtering can be applied to each trace in order to reduce the effects of noise on the calculated instantaneous frequencies. In the second method, the spectral ratio between a receiver pulse and a reference pulse is used to obtain differential t * values. to obtain an unbiased estimate, a variable spectral bandwidth is used depending on the noise level of the data. the two methods are tested using synthetic traces and then applied to crustal refraction data from the 1986 PASSCAL Ouachita experiment. Results show that the differential t * values obtained using filtered, instantaneous-frequency matching are consistent with and have less scatter than those obtained from spectral ratios with a variable bandwidth.     

Local seismic array observations at north Evoikos, central Greece, delineate crustal deformation between the North Aegean Trough and Corinthiakos Rift

In order to better constrain and define the microseismic activity at the north Evoikos Gulf and its surrounding area we deployed an onshore/offshore seismic array consisting of 31 three-component seismic digital stations. The array was active from 30 June to 24 October 2003, and covered an area of 2500 km². We located more than 2000 seismic events ranging from 0.7 to 4.5 M_L by using six stations as a minimum in order to define the foci parameters. Recorded seismicity delineated three major zones of deformation: from south to north, the Eretria–Parnis–eastern Corinthiakos zone, the Psachna–Viotia zone, and the Northern Sporades–North Evia–Bralos zone. Alignments of the recorded seismicity follow the tectonic trends and their orientation in the above zones. The whole area accommodates the stress field between the North Aegean Trough and the Corinthiakos Gulf. Rate of deformation intensifies from north to south, as revealed also by historical and instrumental seismicity. The successive change of orientation between the two stress fields fragments the crust in relatively small units and the fault systems developed do not permit the generation of major earthquakes in the north Evoikos area and its immediate vicinity. This is also supported by the instrumental seismicity of the last century. Larger events reported in historical times are probably overestimated.Most seismic activity is crustal. Subcrustal events were recorded mainly below the Lichades area and are interpreted as the consequence of the subduction of the Ionian oceanic lithosphere below the Hellenides. The Lichades volcano is the most northern end of the Hellenic volcanic arc.At present the highest seismic activity is associated with the Psachna region of north Evia that has been continuously active since 2001. Considering, however, the development of the seismic activity during the last decade, there has been a sequence of large events, i.e., Parnis in 1999, Skyros in 2001 and Psachna in 2001–2003. This demonstrates the fact that the tectonic deformation in all this area is intense and important for the accommodation of the stress field of the North Aegean Trough to that of the Corinthiakos Rift.     

The detection and location of low magnitude earthquakes in northern Norway using multi-channel waveform correlation at regional distances

A fortuitous sequence of closely spaced earthquakes in the Rana region of northern Norway, during 2005, has provided an ideal natural laboratory for investigating event detectability using waveform correlation over networks and arrays at regional distances. A small number of events between magnitude 2.0 and 3.5 were recorded with a high SNR by the Fennoscandian IMS seismic arrays at distances over 600 km and three of these events, including the largest on 24 June, displayed remarkable waveform similarity even at relatively high frequencies. In an effort to detect occurrences of smaller earthquakes in the immediate geographical vicinity of the 24 June event, a multi-channel correlation detector for the NORSAR array was run for the whole calender year 2005 using the signal from the master event as a template. A total of 32 detections were made and all but 2 of these coincided with independent correlation detections using the other Nordic IMS array stations; very few correspond to signals detectable using traditional energy detectors. Permanent and temporary stations of the Norwegian National Seismic Network (NNSN) at far closer epicentral distances have confirmed that all but one of the correlation detections at NORSAR in fact correspond to real events. The closest stations at distances of approximately 10 km can confirm that the smallest of these events have magnitudes down to 0.5 which represents a detection threshold reduction of over 1.5 for the large-aperture NORSAR array and over 1.0 for the almost equidistant regional ARCES array. The incompleteness of the local network recordings precludes a comprehensive double-difference location for the full set of events. However, stable double-difference relative locations can be obtained for eight of the events using only the Lg phase recorded at the array stations. All events appear to be separated by less than 0.5 km. Clear peaks were observed in the NORSAR correlation coefficient traces during the coda of some of the larger events; the local stations confirm that these are in fact aftershocks exhibiting very similar waveforms to the main events. Many of the more marginal correlation detections are not made when the calculations are repeated using shorter signal segments, fewer sensors or more distant stations. We demonstrate in addition how these almost repeating seismic sources have been exploited to detect and measure timing anomalies at individual sites within the arrays and network.     

Rational approximation of stress and strain based on downhole acceleration measurements

Recordings from downhole accelerometer arrays offer unique insight into soil behavior and ground response during earthquakes. In this paper we present a scheme for interpolating displacement and acceleration measurements to provide approximations for subsurface shear strain and stress as continuous functions of time. Our suggested interpolating functions are constructed in such a way that the free surface boundary condition will always be satisfied and the interpolated displacement and acceleration remain finite for all depths. We also show how the functions can be adapted to represent layered soil profiles. Depending on the number of instruments in the downhole array, a truncated series of functions can be derived so that each represents a modal shape for the layered soil profile. The resulting approximations for strain and stress are considered more accurate and robust than previous approximations. © 1998 John Wiley & Sons, Ltd.     

基于中远场小震结构台阵观测数据的地震响应分析

近年来我国布设的强震动观测结构台阵越来越多,并在数次地震中捕获了一批宝贵的观测记录,这些记录既是对应用日益广泛的结构数值模拟结果的检验和参数标定的依据,也是结构健康监测的重要数据。本文选取昆明防灾减灾中心大楼观测台阵捕获的3次中远场小震记录作为研究对象,对比ANSYS结构数值模拟和观测值之间在时程以及谱特性等方面的响应差别,证明了在弹性范围下二者结果具有较好的一致性。最后,基于观测记录,采用高效简单的经验谱比法进行了结构模态参数快速识别,3次地震下的自振频率计算结果相对稳定,与数值建模计算结果的差值可控制在0.17Hz以内。     

Array analysis using circular-wave-front geometry:an application to locate the nearby seismo-volcanic source

The zero-lag cross-correlation technique, used for array analysis in the hypothesis of plane waves, has been modified to allow the wave front to be circular. Synthetic tests have been performed to check the capability of the method, which returns the input test data when the source–array distances are not greater than two or three times the array aperture. For this distance range the method furnishes an estimate of the apparent velocity and the epicentral coordinates of the source. For more distant sources the method becomes equivalent to that based on the planar-wave approximation, which gives an estimate of the backazimuth to the source and the apparent velocity. The method has been applied to seismic data recorded at the active volcano located at Deception Island, Antarctica. 35 volcanic long-period events occurring in a small swarm were selected. Results show that the epicentres are close to the array (between 0.4 and 2 km) and aligned in a SW direction, in agreement with one of the main directions of the fracture system of Deception volcano.