CONTRIBUTION DU TRAITEMENT NUMERIQUE A L'ANALYSE ET A INTERPRETATION DES ENREGISTREMENTS REFRACTION*

The tools used in digital processsing have received little application, to date, to refraction seismic. It seems however that some problems encountered in the analysis of refraction records could be solved through techniques which are of common use in processing of reflection seismic data (correlation, stacking, f and k filtering). The experiments made with some of these techniques revealed the possibility of a more complete refraction record analysis, allowing the study of later arrivals. The interpretation of these records could also, to a fairly large extent, be automated by means of digital processing. While recording, some care ought to be taken to allow identification and interpretation by digital processing of as many events as possible. However, the results shown here have been obtained from fairly old analog records, for which no particular care had been taken, which makes it possible to complete and revise the interpretation of previous studies using the same processing techniques. Long line refraction shooting is justified for marine surveys in some areas, particularly where reflection is disturbed by singing; improvements in analysis and interpretation would therefore prove very useful for a faster and more thorough interpretation.     

河南平顶山平煤矿区天然地震、爆破、塌陷时频特征分析

选取2019年3月—8月河南平顶山市宝丰县平煤矿区发生的ML 2.0—2.9天然地震、爆破、塌陷等9次震动事件,在区域地质构造背景和波形特征分析基础上,采用短时傅里叶变换(STFT)方法开展时频波谱分析,提取不同类型事件的时频特征。结果显示:(1)天然地震频率成分丰富,且高、低频分布均匀,P波在约3 Hz和8 Hz处存在2个峰值,S波存在多个峰值;(2)爆破事件的时频谱相对集中,以低频为主,P波频率峰值约5Hz,信号主频随时间变化,大致呈线性降低至1—2 Hz;(3)塌陷事件频率成分以4 Hz以下的低频为主,P波无明显峰值且频率成分单一,主频出现在2 Hz左右的面波。本文结果可为今后该矿区震动事件类型判断提供参考依据。     

Occurrence of shallow earthquakes following periods of intense rainfall in Tenerife, Canary Islands

The question as to whether there is a seasonality in the occurrence of local seismic activity in the volcanic island of Tenerife, and if it could be associated to intense rainfall events is addressed. Analogue records from the TFMB seismic station and records on the daily precipitation at the Izaña Meteorological Observatory for the period December 1987–October 1992 were used to check this question. Statistical analyses show a non-random component in the temporal distribution of local microearthquakes at greater than 99% confidence, and a relatively strong contemporaneous correlation with intense rainfall periods. If the suggested correlation is confirmed by further investigations, it will allow better identification and discrimination of local seismic events that could be associated with volcanic activity, and thereby increase the performance of surveillance measures.     

Analysis of seismic patterns observed at Nevado del Ruiz volcano, Colombia during August – September 1985

The seismic activity of the Nevado del Ruiz volcano was monitored during August–September 1985 using a three-component portable seismograph station placed on the upper part of the volcano. The objective was to investigate the frequency content of the seismic signals and the possible sources of the volcanic tremor. The seismicity showed a wide spectrum of signals, especially at the beginning of September. Some relevant patterns from the collected records, which have been analyzed by spectrum analysis, are presented. For the purpose of analysis, the records have been divided into several categories such as long-period events, tremor, cyclic tremor episodes, and strong seismic activity on September 8, 1985.The origin of the seismic signals must be considered in relation to the dynamical and acoustical properties of fluids and the shape and dimensions of the volcano's conduits.The main results of the present experiment and analysis show that the sources of the seismic signals are within the volcanic edifice. The signal characteristics indicate that the sources lie in fluid-phase interactions rather than in brittle fracturing of solid components.     

Discrimination of earthquakes and explosions using multi-fractal singularity spectrums properties

A new method of discrimination of seismic records from earthquakes and explosions is proposed which is based on using properties of their multi-fractal singularity spectrums. The efficiency of the method is illustrated by analysing seismic records from the region of Aswan Dam in Egypt. Current pattern of seismicity in the Upper Egypt is composed by three types of records: tectonic earthquakes, reservoir-induced earthquakes and seismic events generated by quarry blasts. To discriminate quarry blasts from earthquakes of both natures, multi-fractal analysis of records were used. Singularity spectrum support width and multi-fractal generalised Hurst exponents were calculated for all seismic events in the selected data set from a given area. The linear Bayesian discriminator using these characteristics of seismic records provides correct classification for 93 % of earthquakes records and for 99 % of quarry blasts records.     

Correlation of JMA instrumental seismic intensity with strong motion parameters

The Japan Meteorological Agency (JMA) seismic intensity (I_JMA) has been used as a measure of strong shaking for many years in Japan, and it necessitates to know the correlation between the JMA seismic intensity and other strong motion indices, e.g. Peak Ground Acceleration (PGA), Peak Ground Velocity (PGV), and Spectrum Intensity (SI). In this study, two strong motion data sets were selected; in which, the first set consists of 879 three‐components non‐liquefied records selected from 13 major earthquake events that occurred in Japan, the United States, and Taiwan, and the second set consists of 17 liquefied records selected from 7 major earthquake events that occurred in Japan and the United States. The JMA seismic intensity and other ground motion indices were calculated using the selected data sets. The relationships between the JMA seismic intensity and PGA, PGV, and SI were then derived performing a two‐stage linear regression analysis. Results from the analysis show that the JMA instrumental seismic intensity shows higher correlation with SI than PGA or PGV, and it shows the highest correlation with the parameters such as the combination of PGA and SI or the product of PGA and SI. The obtained relationships are also compared with the ones obtained by other studies, which may be useful for the disaster management agencies in Japan and deployment of new SI‐sensors that monitor both PGA and SI. Copyright © 2002 John Wiley & Sons, Ltd.     

Spatial variation and stochastic modelling of seismic differential ground movement

Specially designed arrays of strong motion seismographs located near earthquake sources are required for engineering studies of near-source earthquake properties as well as spatial variation of seismic waves. The SMART-1 array in Tath provides good records for this type of study. Based on the SMART-1 array data, the analysis of the principal direction wave propagation and the space-time correlation of some events recorded by SMART-1 have been studied. A stoce model for predicting the differential ground movement was also developed. This stochastic model includes the effect of source characteristics, attenuation of wave passage and spatial correlation characteristics. The performance of this more discussed and compared with the ground movement recorded by the SMART-1 array. From the present study, it is that spatial correlations do exist as seismic waves propagate across the array site. Generally, the loss of coherence is direction of propagation can be explained by energy at the same frequency exhibiting a slightly different velocity with the measurement intervals. It is also concluded that the phase velocity of seismic waves and the corner frequency of the grep displacement spectrum are controlling factors in the prediction of the root mean square of differential grep displacement.     

台湾地区地震的空间关联维特征与构造环境研究

采用关联维方法对台湾地区地震活动的空间特征进行了研究。先利用 10 0a来台湾的地震目录计算各个地震区、带的关联维数 ,将地震空间分布的分形特征定量表达出来 ,然后综合分析地震空间分布的关联维数和孕震构造环境之间的关系 ,得出了以下结论 :1)台湾东、西部地震区由于地震属于不同的大地构造单元 ,因此关联维数有较大的差异 ;2 )在各地震区内部的各个地震带由于板块构造、地壳结构、活断层分布上的差异 ,而具有与其构造特征相对应的关联维数 ;3)各地震带内部的各个不同的部位又由于不同的构造应力场 ,而导致地震分布上出现不同的丛集性 ,表现为不同的关联维数。这些结论充分说明通过关联维分析所得到的地震活动的空间图像与地震活动所代表的不同地质构造背景有着良好的对应关系     

基于砌体结构破坏损伤的地震烈度物理标准研究

为了研究基于砌体结构破坏的地震烈度物理标准,将15个地震动参数按属性(峰值、频率、持时和能量)分成4类,将地震记录按地震动三要素(峰值、频率、持时)分成3组,求出每组记录作用下砌体结构的延性系数,计算出各个参数值和延性系数的相关系数,比较分析这些相关系数发现地震动峰值加速度、有效峰值加速度、地震动输入能量和滞回耗能都能表征地震动对砌体的破坏势,并且这4个参数都和烈度有很好的相关性,可以作为烈度的物理标准。     

A detailed analysis of two seismic sequences in Abruzzo, Central Apennines, Italy

A three-component digital seismic network has been installed along central Apennines since the end of 1991. Two seismic sequences having main shocks of magnitudes 3.9 and 3.7 were recorded in August 1992 and June 1994, respectively. A detailed analysis of these sequences, including multiplet relocation, fault-plane solutions and source parameter estimation, is performed in the present paper. A correlation analysis allowed us to recognize a number of correlated events in the two sequences which were used for relative locations using a master event technique. This analysis allowed to obtain a better alignment of epicentral data along two almost orthogonal directions, following an Apenninic and an anti-Apenninic trend. For the two sequences, fault-plane solutions were evaluated by using a first arrival technique, resulting in mechanisms with predominant normal faulting for the 1992 and 1994 swarms. S-wave polarization analysis allowed to check the stability of the previous solutions and to reduce their range of uncertainty. The same technique was also applied to derive the composite fault-plane solutions from the aftershocks, resulting in solutions which are in good agreement with those derived from the main shocks of both sequences. Source parameters were then derived from the three-component records of 28 well-recorded events with seismic moment in the range 8.5 × 10¹⁰–1.0 × 10¹⁴ Nm. Stress drops ranged in the interval 0.3–52.3 bar and source radii were of the order of 100 m. Their scaling relations are in good agreement with other results derived from the analysis of other Italian earthquakes that occurred in regions of predominantly normal faulting tectonics (Apennines and Calabrian arc).     

Initial results of bio-potential signal (Seismic Electric Signal) related to seismic activities

In recent year, there has been growing interest in the possible use of electromagnetic observations to study earthquakes and possible precursors prior to seismic activity, in response to the success in United States, Japan, Russia, China, and other countries using seismo-electromagnetic methods. We have established a new experimental setup (i.e., biopotential sensor) in Farah region (geographic coordinates: 27.17°N, 77.47°E), Mathura, India. The setup has started operating and analyzed the data since November 2011. The data have been tested by various methods and a good correlation with seismic events was found; thus, a real-time analysis from 21:00 p.m. through 8:00 a.m. every day was initiated. First, we recorded the amplitude enhancement in bio-potential and found positive correlation with seismic activities (near Delhi and Rajasthan) and analyzed the data with solar flares and magnetic storms during the same period, finding a negative correlation of these events. The studies of these events are in progress with statistical analysis of the data. We chose the observing site in Farah region because this region is well known for being a site of a conductive channel of seismic activity.     

乌兰浩特地震台典型地震事件震相特征分析

选取乌兰浩特地震台记录到的典型天然地震事件及爆破事件,根据断裂带分布及波形记录特征,分析地震及爆破事件的震相特征。结果表明,研究区域近震波形Pn、Pg、Sn、Sg震相及Sm面波较清晰,Pg、Sg波走时差一般不大于23 s,与中国地震台网中心发布的M_S震级间的偏差一般小于0.6级;远震P、S、pP、sP震相可较清晰识别,Pm、L面波记录较明显,P、S波走时差一般不小于25 s,震级偏差一般小于0.4级,且通过震相特征及实地考察发现,爆破多为霍林郭勒区域爆破事件。     

Geodetic evidence on segmentation of Cascadia subduction zone based on episodic tremors and slips using multivariate harmonic analysis of GPS time series

Episodic tremor and slip (ETS) events with different recurrence intervals have been observed in abundance all along the Cascadia subduction zone margin. Analysis of seismic records as well as Global Positioning System (GPS) time series of the Pacific Northwest Geodetic Array (PANGA) has suggested three distinct coherent zones for the occurrence of these events. In this paper multivariate harmonic estimation has been deployed for further analysis of the segmentation in this area. Raw time series of 43 permanent GPS stations have been used for this purpose. The GPS stations have been geographically divided into three distinct groups including those in the northern, middle and southern parts of the study area. After the reduction of time series for the linear trend as well as annual and semiannual effects, the data series of each group has been analyzed using the multivariate harmonic estimation technique. Subsequently, different combinations of GPS stations including the stations located in the southern, northern and middle zones have been analyzed. Furthermore, the northern and middle, southern and middle as well as the northern and southern zone pair combinations have also been analyzed. The statistical measure devised for identifying the significant frequencies suggests common periods that are consistent with the recurrence intervals of the ETS events already reported for each of the above three geographic zones. Moreover, the method can provide geodetic evidence, in addition to geophysical ones, on the segmentation of ETSs, provided that the adopted time series are of a sufficient length. The geodetic evidence obtained in this research is consistent with the recurrence intervals as well as the boundaries obtained by the analysis of seismic records. Contrary to univariate harmonic estimation, multivariate approach using spatio-temporal correlation of the GPS time series is capable to detect those ETSs whose impacts on the time series are weak.     

Observation and analysis of low-frequency crustal earthquakes in Lake Van and its vicinity,eastern Turkey

Lake Van is located in very intensely deformed tectonic settings in the eastern Turkey. The surrounding area of Lake Van has major tectonic structures; NE and NW trending active conjugate strike-slip faults, pull-apart basins along these faults, E-W trending active thrust faults, folds, compressional ramp basins, N-S trending folds, NE-SW aligned continental collision-related volcanoes and N-S trending tensional cracks functioned as magma conduits. Since the area is tectonically very active, the earthquake activity within the basin and the surrounding area of Lake Van is very high. In this study, we have analysed the records of VANB broadband station of Kandilli Observatory and Earthquake Research Institute's network for the seismic events (2000–2004) that occurred in the basin and in the vicinity of Lake Van. Based on the spectral analysis of earthquake data, we have mainly observed three types of seismic events; hybrid event, long period event and tremor. We have suggested that the observed hybrid, long period and tremor type of seismic events that occurred in the study area are related to the upward moving materials and stress loading in the mid and lower crust based on the geophysical data.     

多种表征强震动记录特性的参数对比分析

为研究强震动记录的破坏特性,对多条国内外常用的强震动记录的特性参数进行了对比分析.结果表明,强震动记录特性参数之间的相关件很小;PGA(PGV)和EPA(EPV)大小关系受震级和震中距影响较大;VSI较ASI能够更准确的评价强震动记录的破坏特性;与其它参数相比,强震动记录破坏强度参数(PD)同时考虑了地震加速度、时间和...     

乌兰浩特地震台典型地震事件震相特征分析

选取乌兰浩特地震台记录到的典型天然地震事件及爆破事件,根据断裂带分布及波形记录特征,分析地震及爆破事件的震相特征.结果表明,研究区域近震波形Pn、Pg、Sn、Sg震相及Sm面波较清晰,Pg、Sg波走时差一般不大于23 s,与中国地震台网中心发布的MS震级间的偏差一般小于0.6级;远震P、S、pP、sP震相可较清晰识别,Pm、L面波记录较明显,P、S波走时差一般不小于25 s,震级偏差一般小于0.4级,且通过震相特征及实地考察发现,爆破多为霍林郭勒区域爆破事件.     

Evaluation of dynamic eccentricities obtained using a probabilistic approach,response spectrum methods and modern building codes

In this paper a probabilistic approach has been adopted to study both the effects of uncertainty in earthquake frequency content and the correlation between earthquake frequency content and ground motion intensity on the response of a single-storey torsionally coupled elastic structure. The earthquake ground motion has been assumed to be a Gaussian, zero mean, stationary random process which is fully characterized by a power spectrum. The ground acceleration power spectrum is idealized as a probabilistic normalized power spectrum computed from actual earthquake records. The advantage of such an idealization is that it enables the effect of the natural frequency as a controlling structural parameter in torsional coupling to be assessed. Comparisons of the dynamic amplifications of eccentricity with those obtained from modern codes of practice and conventional response spectrum analyses have been made. The results of this study have shown that the variation in the frequency content has a significant effect on the response of low frequency structures, while the correlation between the frequency content and the intensity of seismic ground motion is insignificant for the wide range of structures considered. The structure natural frequency has been shown to be an important controlling parameter in the torsionally coupled response of structures subject to seismic loading. The frequency dependence of the dynamic amplification of eccentricity was found not to be reflected in the response spectrum analysis and the torsional provisions of modern building codes.     

Broadband seismic experiment at Merapi Volcano, Java, Indonesia: very-long-period pulses embedded in multiphase earthquakes

Multiphase (MP) and low frequency (LF) earthquakes with spectral peak amplitudes at 3–4 and 1 Hz, respectively, are two common types of shallow volcanic earthquakes previously recognized at Merapi Volcano. Their mechanisms are poorly understood but MPs have been temporally associated with lava dome growth. We conducted a seismic experiment in January–February 1998, using four broadband seismographs to investigate the nature of seismic activity associated with dome growth. During our experiment, Merapi experienced mild dome growth with low-level seismic activity. We compare our data to that recorded on a local short-period (SP) network, with the following preliminary results.MP and LF events as recorded and classified on the short-period network instruments were recognized on the broadband network. Frequency spectrograms revealed similar patterns in the near summit region at widely separated broadband stations. Higher frequency spectra than previously recognized were identified for both MP and LF events, and were strongly attenuated as a function of radial distance from the source. Thus the spectral characteristics of these events as recorded on far-field stations are not fully indicative of the source processes. In particular, many events classified as LF-type appear to have much high frequency energy near the source. This aspect of these so-called LF earthquakes, and their association with very-long-period (VLP) pulses, suggests that many events identified in the far-field as LF events are in actuality a variety of the MP event and involve similar source processes. Broadband records indicated that simple large-amplitude VLP pulses were embedded in MP and LF wavetrains. From event to event these pulses were similar in their waveforms and had periods of 4 s. VLP events embedded in LF and MP earthquakes were located using particle motions. The epicenters were clustered in a central region of the dome complex, and preliminary source depths were within about 100 m of the dome surface, suggesting a source region deep within the dome or the uppermost conduit. A similar source location was established by study of MP high-frequency onsets. Our broadband data suggests that we have recorded both elastic seismic waves and a simple embedded pulse that is interpreted to represent a surface tilt at the seismometer site. The inferred tilt indicates an inflation and subsequent deflation, possibly caused by a gas pressure pulse or episodic shallow magma transport with stick-slip movement of the conduit wall.     

Correlation of seismic activity and fumarole temperature at the Mt. Merapi volcano (Indonesia) in 2000

In this paper we present densely sampled fumarole temperature data, recorded continuously at a high-temperature fumarole of Mt. Merapi volcano (Indonesia). These temperature time series are correlated with continuous records of rainfall and seismic waveform data collected at the Indonesian–German multi-parameter monitoring network. The correlation analysis of fumarole temperature and precipitation data shows a clear influence of tropical rain events on fumarole temperature. In addition, there is some evidence that rainfall may influence seismicity rates, indicating interaction of meteoric water with the volcanic system. Knowledge about such interactions is important, as lava dome instabilities caused by heavy-precipitation events may result in pyroclastic flows. Apart from the strong external influences on fumarole temperature and seismicity rate, which may conceal smaller signals caused by volcanic degassing processes, the analysis of fumarole temperature and seismic data indicates a statistically significant correlation between a certain type of seismic activity and an increase in fumarole temperature. This certain type of seismic activity consists of a seismic cluster of several high-frequency transients and an ultra-long-period signal (<0.002 Hz), which are best observed using a broadband seismometer deployed at a distance of 600 m from the active lava dome. The corresponding change in fumarole temperature starts a few minutes after the ultra-long-period signal and simultaneously with the high-frequency seismic cluster. The change in fumarole temperature, an increase of 5 °C on average, resembles a smoothed step. Fifty-four occurrences of simultaneous high-frequency seismic cluster, ultra-long period signal and increase of fumarole temperature have been identified in the data set from August 2000 to January 2001. The observed signals appear to correspond to degassing processes in the summit region of Mt. Merapi.     

Identification of Local Seismic Events Using the Mikhnevo Seismic Array

A large number of events with sources in the immediate vicinity of an array are usually detected during seismological observations with seismic arrays. These events should be detected and correctly interpreted during processing of seismic array records in order to avoid clogging up the event catalog. This problem can be solved by classifying records of local events by genetic features and creating a databank with the most representative samples. The present paper considers local events recorded using a unique scientific setup, the Mikhnevo small aperture seismic array. Epicenters of local seismic events are located less than 5 km from the center of the array. Seismic responses of acoustic shock waves are also examined. Seismic events caused by anthropogenic sources are identified and classified using cluster, cross-correlation, and wavelet analysis. Events accompanied only by the arrival of surface waves, as well as events represented by body, surface, and acoustic waves, are identified. Shock wave events are classified as a separate category. A small group of supposedly natural weak events is also found. As a result, a databank of waveforms of local seismic events for the Mikhnevo seismic array is established. In the future, this will make it possible to automate their identification when investigating the seismicity of the East European Platform.