抗震设计中多遇地震的问题和讨论

指出作为截面抗震验算标准的“小震”混淆了两种明显不同的概念:其一是对应于50年超越概率63％的地震动参数(以下简称“多遇小震”);其二是50年超越概率10％的地震动参数经过折算得到的参数(以下简称“折算小震”).上述混淆导致抗震设计中实际上执行着不同的标准.文中讨论了中、小地震动参数之间关系的复杂性,包括从地震危险性分...     

Stress drop in the sources of intermediate-magnitude earthquakes in northern Tien Shan

The paper is devoted to estimating the dynamical parameters of 14 earthquakes with intermediate magnitudes (energy class 11 to 14), which occurred in the Northern Tien Shan. For obtaining the estimates of these parameters, including the stress drop, which could be then applied in crustal stress reconstruction by the technique suggested by Yu.L. Rebetsky (Schmidt Institute of Physics of the Earth, Russian Academy of Sciences), we have improved the algorithms and programs for calculating the spectra of the seismograms. The updated products allow for the site responses and spectral transformations during the propagation of seismic waves through the medium (the effect of finite Q-factor). By applying the new approach to the analysis of seismograms recorded by the seismic KNET network, we calculated the radii of the sources (Brune radius), scalar seismic moment, and stress drop (release) for the studied 14 earthquakes. The analysis revealed a scatter in the source radii and stress drop even among the earthquakes that have almost identical energy classes. The stress drop by different earthquakes ranges from one to 75 bar. We have also determined the focal mechanisms and stress regime of the Earth’s crust. It is worth noting that during the considered period, strong seismic events with energy class above 14 were absent within the segment covered by the KNET stations.     

The study of the radiation characteristics and propagation of seismic waves in the North Caucasus by modeling the accelerograms of the recorded earthquakes

For evaluating the parameters of the vibrations of the Earth’s surface in the case of strong earthquakes, which are possible in the future, the regular patterns of the emission and propagation of seismic waves in the North Caucasus regions are investigated. The regional parameters of emission and propagation of seismic waves are evaluated by solution of the inverse problems of stochastic modeling of the accelerograms of the earthquakes, recorded by the seismic station in Sochi. The horizontal components of the strongest earthquakes (M _w ～ 3.9?5.6), that occurred in 2002–2006 within a radius of ～300 km from the seismic station, with source depths up to 60 km are modeled. For calculations of accelerograms, estimates of the quality are used, obtained earlier for this region in the form: Q(f) ～ 80 ～ f ^0.9. The parameter settings are carried out, which determine the shapes of the source spectra, the amplification of the seismic waves in the Earth’s crust, the weakening of the waves at high frequencies (κ), the parameters that determine the shape and duration of accelerograms, etc. Sufficiently good agreement of the calculated and recorded accelerograms is obtained, the regional characteristics of emission and propagation of seismic waves, which can be used for prediction of the parameters of strong motions in the North Caucasus, are evaluated; however, in the future these characteristics should be studied in more detail.     

Tsunami and mechanism of great earthquakes

The relation between tsunamis and sea-bottom deformations associated with the Kurile Islands earthquake of 1969 and the Tokachi-Oki earthquake of 1968 is studied on the basis of a fairly complete set of seismological and tsunami data. The seismic results are included in the calculation of static crustal deformations. The calculated deformations are compared with the tsunami source area as obtained by the inverse refraction diagram, the first motion of tsunami waves, and the height of the sea-level disturbance at the source. It is found that such deformations as predicted by the seismic results can quantitatively explain the source parameters of tsunamis. These findings strongly favor the idea that tsunamis are generated by tectonic deformations rather than by large submarine landslides and slumps. This conclusion is supported by additional analyses for the 1964 Niigata, 1944 Tonankai, 1933 Sanriku earthquakes. For the 1946 Nankaido earthquake, the source deformation responsible for the tsunami generation is of much greater magnitude than that for seismic waves.     

Source parameters of some significant earthquakes near Koyna dam,India

Seismic moment, stress drop, source radius, and fault dislocation have been determined for nineteen significant earthquakes in the Konya area, India using displacement spectra of shear waves computed from strong-motion accelerograph records. Though the stress-drop shows a definite increasing trend with the seismic moment, its correlations with source radius and corner frequency indicate that a constant stress drop of 170 bars represents a good mean value to describe the source mechanism of the Koyna dam earthquakes. An empirical relationship also has been established between magnitude and seismic moment.     

基于谱元法数值模拟研究地震判定准则

本文以宁夏区域地震台网为例,分析了波形互相关法在判定重复地震中可能存在的问题并讨论了相应的处理方法,通过构建三维非均匀体模型并利用谱元法数值模拟地震波的传播,统计了不同台站观测到的地震对波形互相关系数的分布,进一步研究了互相关系数与非均匀体性质及震源机制解之间的关系。结果表明:针对宁夏区域地震台网,利用波形互相关法判定重复地震比地震定位方法更有效;互相关系数在不同台站的取值与震源附近三维非均匀体强度和直达波与尾波的振幅比有关,对于相同的震源间距,较弱震源、较弱非均匀体或者较强振幅的直达波均会导致波形互相关系数变高,因此应选取更高的互相关系数阀值来判定重复地震。宁夏区域地震台网平均台间距为30—50 km,通过选取直达波较弱的台站或只截取尾波窗口计算互相关系数并设定较高的阀值,利用波形互相关法可有效地判定M_L1.0—3.0重复地震,进而为重复地震的监测与研究提供依据。      

Source parameters of the 2004 Kaliningrad earthquakes

An analysis of source parameters of the two unexpected earthquakes from the Kaliningrad (Russia) area is presented. The earthquakes occurred on 21 September 2004 at 11:05:01 and 13:32:31 UT, respectively. The first event was located at the latitude φ = 54.924°N and the longitude λ = 20.120°E, with a depth h = 16 km, and the second event at φ = 54.876°N, λ = 20.120°E and h = 20 km. Magnitudes Mw of the two events were very similar: 5.1 and 5.2. The magnitude values reported by various international data centers have been meaningfully different. The reason is the presence of high-frequency components in Z velocity component of the S wavefield. They were observed along the direction defined by two stations, BLEU in Sweden and SUW in Poland, located in opposite sides of the source. Along the direction perpendicular to it, the effects are relatively very small. The high-frequency waves are understood to mean components in the 6–8 Hz band for event 1 and 2-4 Hz for event 2. The effects in question are also clearly visible on displacement spectrograms. The magnitude values calculated at such stations from S-wave amplitudes or from seismic spectra are clearly overestimated and are close to 6. Therefore, we made a careful selection of channels in order to determine the spectral parameters and, on this basis, the source parameters. The size of the source is relatively small, of about 2 km. The closest seismic station is at 100 source radii from the source. One can clearly see the effect of the TT zone which markedly reduces the seismic moment value for seismic stations laying on the opposite sides of the source. Both events have very similar spatial distributions of the source parameters: magnitude, seismic moment and radius.     

Source parameters of the earthquakes of the Baikal rift system

The dynamic parameters of the earthquake source—the seismic moment, the moment magnitude, the source radius, the stress drop, and the amplitude of displacement—are determined by the amplitude Fourier spectra of the body shear waves (S-waves) for 62 earthquakes of the Baikal rift system with the energy class of K _P = 9.1–15.7. In the calculations I used the classical Brune model. The seismic moment of the earthquakes being investigated changes from 3.65 × 10¹¹ N m to 1.35 × 10¹⁸ N m, and the radii of earthquake sources vary from 390 m to 1.84 km. The values of the drop in stress Δσ grow with an increase in the seismic moment up to 1.7 × 10⁸ Pa. For the group of weak earthquakes (M _w = 1.7–3.3), extremely low values of the drop in stress 10³–10⁴ Pa are observed. The maximum amplitude of displacement in the source amounts to 5.95 m. The empirical equations between the seismic moment and the other dynamic parameters of the source are determined. The regional dependence of the seismic moment and energy class is obtained: log M ₀ ± 0.60 = 1.03K _P + 3.17. The character of the relationship between the seismic moment and the corner frequency indicates that the classical scaling law of the seismic spectrum for the earthquakes in question is not fulfilled. The obtained estimates of the dynamic parameters are in satisfactory agreement with the published data concerning the analogous parameters of the other rift zones, which reflects the general regular patterns of the destruction of the lithosphere and the seismicity in the extension zones of the lithosphere.     

Simulation of Ground Motion from Strong Earthquakes of Kamchatka Region (1992–1993) at Rock and Soil Sites

To estimate the parameters of ground motion in future strong earthquakes, characteristics of radiation and propagation of seismic waves in the Kamchatka region were studied. Regional parameters of radiation and propagation of seismic waves were estimated by comparing simulations of earthquake records with data recorded by stations of the Kamchatka Strong Motion Network. Acceleration time histories of strong earthquakes (M _w = 6.8–7.5, depths 45–55 km) that occurred near the eastern coast of Kamchatka in 1992–1993 were simulated at rock and soil stations located at epicentral distances of 67–195 km. In these calculations, the source spectra and the estimates of frequency-dependent attenuation and geometrical spreading obtained earlier for Kamchatka were used. The local seismic-wave amplification was estimated based on shallow geophysical site investigations and deep crustal seismic explorations, and parameters defining the shapes of the waveforms, the duration, etc. were selected, showing the best-fit to the observations. The estimated parameters of radiation and propagation of seismic waves describe all the studied earthquakes well. Based on the waveforms of the acceleration time histories, models of slip distribution over the fault planes were constructed for the studied earthquakes. Station PET can be considered as a reference rock station having the minimum site effects. The intensity of ground motion at the other studied stations was higher than at PET due to the soil response or other effects, primarily topographic ones. At soil stations INS, AER, and DCH the parameters of soil profiles (homogeneous pyroclastic deposits) were estimated, and nonlinear models of their behavior in the strong motion were constructed. The obtained parameters of radiation and propagation of seismic waves and models of soil behavior can be used for forecasting ground motion in future strong earthquakes in Kamchatka.     

火山地震信号和火山颤动震源模型

本文主要介绍了火山地震的类型以及在火山区特有的地震信号,如谐频信号、家族事件和地震震群.并简要介绍了与岩浆活动有着密切关系的火山颤动震源模型.     

浙江珊溪水库地震震源参数特征研究

利用2014年1月-2015年12月浙江省区域数字地震台网记录的1 360次珊溪水库M_L0.5-4.4地震资料,采用Brune模式,将台站获取的速度记录进行几何扩散校正、介质衰减校正及仪器校正后得到速度谱,对速度谱在频率域进行傅里叶积分得到震源位移谱,之后使用遗传算法计算得到拐角频率和震源位移谱零频极限值,进而计算出该区的地震矩为1010-1014 N·m,震源破裂半径为33-550 m,拐角频率为2.4-39.7 Hz,地震应力降为0.04-6.74 MPa,视应力为0-2.75 MPa.在此基础上,对各参数特征及其关系进行系统性分析,结果表明:各震源参数之间表现出一定的对数线性或半对数线性关系;空间上,应力降和拐角频率在库区西北段较高,东南段较低;应力降与地震矩在空间上呈反向关系,拐角频率与破裂半径在空间上呈负相关.      

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.     

Seismic events with non-shear component: I. Shallow earthquakes with a possible tensile source component

The concept of seismicity of fast tensile fracturing is introduced and supported by the results of shear and of combined shear and tensile displacements along a loaded stress concentrator. The seismicity of tensile fracturing is demonstrated by means of acoustic (elastic) signals obtained during tensile fracturing in physical models under load; the basic physical relations between the parameters characterizing the loaded medium, load conditions, shear and tensile displacements, and release of acoustic energy are presented. For determining the tensile-source component in earthquakes a procedure based on the construction of radiation patterns is suggested and submitted for discussion. The criteria for selecting earthquakes with possible shear-tensile source mechanisms are listed and discussed. The existence of such a combined seismic source is sought in two shallow earthquakes which occurred in southern Iran in March 1977. In general, the paper should be regarded as a proposal to utilize the radiation characteristics of a seismic source—with all their insufficiencies—as a quick and simple tool for seeking combined shear and tensile mechanisms of seismic energy release.     

Ground-motion prediction equations for the intermediate depth Vrancea (Romania) earthquakes

We present the regional ground-motion prediction equations for peak ground acceleration (PGA), peak ground velocity (PGV), pseudo-spectral acceleration (PSA), and seismic intensity (MSK scale) for the Vrancea intermediate depth earthquakes (SE-Carpathians) and territory of Romania. The prediction equations were constructed using the stochastic technique on the basis of the regional Fourier amplitude spectrum (FAS) source scaling and attenuation models and the generalised site amplification functions. Values of considered ground motion parameters are given as the functions of earthquake magnitude, depth and epicentral distance. The developed ground-motion models were tested and calibrated using the available data from the large Vrancea earthquakes. We suggest to use the presented equations for the rapid estimation of seismic effect after strong earthquakes (Shakemap generation) and seismic hazard assessment, both deterministic and probabilistic approaches.     

Array analyses of volcanic earthquakes and tremor recorded at Las Cañadas caldera (Tenerife Island, Spain) during the 2004 seismic activation of Teide volcano

We analyze data from three seismic antennas deployed in Las Cañadas caldera (Tenerife) during May–July 2004. The period selected for the analysis (May 12–31, 2004) constitutes one of the most active seismic episodes reported in the area, except for the precursory seismicity accompanying historical eruptions. Most seismic signals recorded by the antennas were volcano-tectonic (VT) earthquakes. They usually exhibited low magnitudes, although some of them were large enough to be felt at nearby villages. A few long-period (LP) events, generally associated with the presence of volcanic fluids in the medium, were also detected. Furthermore, we detected the appearance of a continuous tremor that started on May 18 and lasted for several weeks, at least until the end of the recording period. It is the first time that volcanic tremor has been reported at Teide volcano. This tremor was a small-amplitude, narrow-band signal with central frequency in the range 1–6 Hz. It was detected at the three antennas located in Las Cañadas caldera. We applied the zero-lag cross-correlation (ZLCC) method to estimate the propagation parameters (back-azimuth and apparent slowness) of the recorded signals. For VT earthquakes, we also determined the S–P times and source locations. Our results indicate that at the beginning of the analyzed period most earthquakes clustered in a deep volume below the northwest flank of Teide volcano. The similarity of the propagation parameters obtained for LP events and these early VT earthquakes suggests that LP events might also originate within the source volume of the VT cluster. During the last two weeks of May, VT earthquakes were generally shallower, and spread all over Las Cañadas caldera. Finally, the analysis of the tremor wavefield points to the presence of multiple, low-energy sources acting simultaneously. We propose a model to explain the pattern of seismicity observed at Teide volcano. The process started in early April with a deep magma injection under the northwest flank of Teide volcano, related to a basaltic magma chamber inferred by geological and geophysical studies. The stress changes associated with the injection produced the deep VT cluster. In turn, the occurrence of earthquakes permitted an enhanced supply of fresh magmatic gases toward the surface. This gas flow induced the generation of LP events. The gases permeated the volcanic edifice, producing lubrication of pre-existing fractures and thus favoring the occurrence of VT earthquakes. On May 18, the flow front reached the shallow aquifer located under Las Cañadas caldera. The induced instability constituted the driving mechanism of the observed tremor.     

中强地震能量震级测定

本文根据地震波衰减特性,开展了利用宽频带地震波形数据测定地震波能量E_S和能量震级M_e的方法研究。利用震中距处于20°—98°范围内的宽频带远震P波波形数据,测定了4次国外和4次国内中强震的能量震级M_e,并对其面波震级M_S、矩震级M_W及能量震级M_e进行了分析对比。结果表明:面波震级M_S表示的是地震在某一固定频率所辐射的地震波能量大小;矩震级M_W与地震所产生的断层长度、断层宽度、震源破裂的平均位错量等静态构造效应密切相关;而能量震级M_e反映的是震源动态特征,与地震震源的动力学特性密切相关。由于地震是以地震波形式辐射,能量主要集中在震源谱的拐角频率附近,因此能量震级M_e更适合描述地震的破坏性。由此可见,联合测定面波震级M_S,矩震级M_W和能量震级M_e对于地震定量研究以及地震灾害与风险评估具有重要作用。      

Prediction of ground motion spectra on solidly frozen bedrock during large earthquakes

Two methods are discussed for determining changes in ground displacement spectra and the velocity and acceleration spectra derived from these when recorded on solidly frozen bedrock (T < ?3°C) versus epicentral distance and energy class. The first of these methods characterizes the entire epicenter field of earthquakes, while the second aims at studying possible differences between spectra in different source zones. We have found the maximum spectral level as a function of epicentral distance and energy class. The calculated spectra are compared with the available records of large earthquakes. The manner in which near and comparatively small earthquakes can be used to find ground motion spectra is shown for solidly frozen bedrock for earthquakes as large as the 15–17 energy class, as well, determination of the differences between the spectra of seismic signals due to earthquakes occurring in different source zones is performed. The results can be used both directly and for the zonation and prediction of seismic hazard within the zone of solidly frozen rock, and also for the case where the temperature regime of the frozen rock has been changed or disturbed.     

宁夏地区中小地震震源参数的研究

利用宁夏及邻区的数字测震资料,选取2009年5月至2010年10月间的31个M2.7-ML4.9地震事件的波形记录,以Brune提出的震源谱为平方模型基础,通过消除观测谱中的几何扩散、非弹性衰减、场地响应、仪器响应等影响因素,得到这些地震的震源谱,根据Brune的圆盘震源模型计算并研究地震矩、矩震级、地震应力降、震源尺...     

甘肃地区震源参数的相关性研究

通过对甘肃数字地震台网2008年1月-2009年6月记录的ML 2.0-ML 6.0中小地震,S波观测谱中噪音、仪器、路径、场地等影响的逐一消除,利用遗传算法反演得到甘肃地区273条地震的震源谱,进而根据Brune的圆盘震源模型,计算地震矩、应力降、拐角频率、震源半径等震源参数,并研究甘肃地区震源参数的特征及相似关系....     

Spatial changes in volcanic and seismic activity prior to great earthquakes

Spatial relationship between volcanism and seismicity prior to the occurrence of several great interplate earthquakes in the circum Pacific area has been examined in order to understand the process of underthrusting in detail. The locations of the epicenters of the great earthquakes have been examined in terms of locations of concentrated volcanic and seismic activity in a short time period in order to determine if there is any relationship between these activities, and whether such relationships can be used to refine the underthrusting model. The study shows that in most cases, (1) there is no volcanic activity in the vicinity of the epicenter of the great earthquake and (2) maximum volcanic activity is localized, i.e., volcanoes adjacent to one another exhibit considerable activity in a short time period. Very little systematic spatial relationship between these three parameters is observed although in most cases, there is no volcanic activity at the time of maximum earthquake activity. Locations of active volcanoes and earthquake activity, during the five years prior to the occurrence of the great earthquake, do not appear to be a guide to the epicenter of the great earthquake. This study therefore suggests that although there is temporal relationship between the occurrence of maximum volcanic and seismic activity and the occurrence of great earthquake, there appears to be no systematic spatial relationship between these three parameters.