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1.
Analysis of microseismic oscillations in a period range of several minutes recorded before the Kronotsky (December 5, 1997), Simushir (November 15, 2006), Hokkaido (September 25, 2003), and Neftegorsk (May 27, 1995) earthquakes revealed series of asymmetric pulses (SAPs) arising a few days before the earthquakes. Records of IRIS stations located at various distances from epicenters of the earthquakes were analyzed. SAPs were recorded only by stations located in a seismoactive zone in which an earthquake occurred. The SAP onset times and amplitudes are uncorrelated with the microseismic noise level in a period range of several seconds caused by storm phenomena in the ocean and other meteorological factors. Some intervals between successive pulses had the same length, which indicated a periodicity of their occurrence. It is supposed that SAPs can be caused by elastoplastic displacements in a fault zone.  相似文献   

2.
Microseismic records from five broadband IRIS stations located at distances of 1000–2000 km from the earthquake source are studied. Unordinary programs are used to extract hidden periodicities, determine signal coherence at different stations, and reveal asymmetry in wave amplitudes. The records obtained at a few stations 60 h before the Sumatra earthquake include periodic oscillations in the range of periods from 20 to 60 min that arose after the McQuary earthquake and continued for about 24 h. Synchronization of waves recorded at all stations commenced 53 h before the Sumatra earthquake and continued up to the time of the earthquake, with the predominant period gradually increasing from a few minutes to tens of minutes.  相似文献   

3.
The dynamics of seismic noise before the catastrophic earthquake of March 11, 2011 in Japan is studied in a minute period range using the records of broadband IRIS stations. It is found that the dispersion of the noise and the number of asymmetric pulses recorded within 500 km from the epicenter of the earthquake have drastically increased 1.5 months before the event. This is the highest enhancement ever recorded by these stations during the same interval of the year for the past 15 years since the IRIS network started stably operating. No increase is revealed in the noise recorded beyond 1200 km from the epicenter. The pattern of decay in amplitudes of the anomalous noise with increasing distance from the epicenter to the corresponding stations likely indicates that the sources of the noise were located close to the source of the earthquake. The noise contains both regular and chaotic components.  相似文献   

4.
选取断层距小于200 km的64组强震记录数据,基于小波方法分析汶川地震近断层速度脉冲的地震动特性,并将此次地震中获取到的速度脉冲周期和幅值参数与Chi-Chi 地震和Northridge地震进行了比较,统计分析地震震级、距离对速度脉冲的周期和幅值参数的影响.研究表明:(1)汶川地震近断层速度脉冲具有周期长、幅值小的特点.速度脉冲周期主要分布在6~14 s之间,其中51MZQ台沿平行断层的分量脉冲周期最大为14.2 s,速度脉冲幅值与Chi-Chi 地震和Northridge地震相比明显偏小.(2) 速度脉冲记录出现在沿着地震断层破裂传播的方向上,且与地表断裂的距离都在30 km以内,这些长周期速度脉冲的形成可能主要由破裂传播的向前方向性效应引起.(3)速度脉冲的周期随矩震级呈对数线性增大,且随断层距增大有减小趋势.在矩震级小于Mw7.5时,观测到的地震动脉冲幅值为50~150 cm/s之间,与100 cm/s的典型断层滑动速率非常接近;而震级大于Mw7.5时,断层距10km范围内脉冲的幅值已经超过100 cm/s,个别记录的脉冲幅值甚至达到200 cm/s,远超过前人给出的饱和值,这可能与大的永久形变或该处土层介质条件有关.  相似文献   

5.
Maximum earthquake size varies considerably amongst the subduction zones. This has been interpreted as a variation in the seismic coupling, which is presumably related to the mechanical conditions of the fault zone. The rupture process of a great earthquake indicates the distribution of strong (asperities) and weak regions of the fault. The rupture process of three great earthquakes (1963 Kurile Islands, MW = 8.5; 1965 Rat Islands, MW = 8.7; 1964 Alaska, MW = 9.2) are studied by using WWSSN stations in the core shadow zone. Diffraction around the core attenuates the P-wave amplitudes such that on-scale long-period P-waves are recorded. There are striking differences between the seismograms of the great earthquakes; the Alaskan earthquake has the largest amplitude and a very long-period nature, while the Kurile Islands earthquake appears to be a sequence of magnitude 7.5 events.The source time functions are deconvolved from the observed records. The Kurile Islands rupture process is characterized by the breaking of asperities with a length scale of 40–60 km, and for the Alaskan earthquake the dominant length scale in the epicentral region is 140–200 km. The variation of length scale and MW suggests that larger asperities cause larger earthquakes. The source time function of the 1979 Colombia earthquake (MW = 8.3) is also deconvolved. This earthquake is characterized by a single asperity of length scale 100–120 km, which is consistent with the above pattern, as the Colombia subduction zone was previously ruptured by a great (MW = 8.8) earthquake in 1906.The main result is that maximum earthquake size is related to the asperity distribution on the fault. The subduction zones with the largest earthquakes have very large asperities (e.g. the Alaskan earthquake), while the zones with the smaller great earthquakes (e.g. Kurile Islands) have smaller scattered asperities.  相似文献   

6.
The low-frequency seismic noise recorded by the broadband IRIS stations in 1994–2012 is studied in the period range of 40 to 360 s. It is shown that for samples of a few months in length, the power spectra of noise at stations spaced apart a few thousand kilometers and operating in different meteorological and seismotectonic conditions are overall similar, which indicates that the sources of the noise are global. At the same time, the slope of the spectra changes with the increase in the period in the subintervals of 40–90, 120–200, and 200–360 s, which points to the difference of the sources generating the seismic noise. The amplitude of the noise at the stations located a few thousand km apart from the Sumatra earthquake of December 26, 2004, M = 9.2, and from the Tohoku earthquake of March 11, 2011, M = 9.0, increased after these events. This indicates the global character of the aftermath of these seismic catastrophes. After the Kronotskoe earthquake of December 5, 1997, which was weaker (M = 7.9), the noise grew only at the PET station located within 300 km of the epicenter. According to the records at the PET station, this earthquake was preceded by the increased noise level observed in 1994–1997. After 1999, the seismic noise declined and remained low up to the end of the studied interval with a duration of 14 years. Our results show that the low-frequency seismic noise generated by the sources in the atmosphere of the Earth is contributed by the processes taking place in the lithosphere.  相似文献   

7.
利用鄂西地区长时间段宽频地震台站的三分量背景噪声记录,采用波形互相关方法得到台站对间的互相关函数,并通过聚束分析获得瑞雷波和勒夫波的慢度谱,研究鄂西地区背景噪声源的时空分布特征。结果表明,5~10 s周期范围,背景噪声来源于南太平洋且没有季节变化;10~20 s周期范围,慢度谱上显示明显的能量环,表明噪声源来源于多个方向,且表现出强烈和急剧的季节变化;20~40 s周期范围,慢度谱上也存在明显的能量环,其产生机制可能与此周期下提出的次重力波机制相似。在不同的周期范围内,噪声源分布方位有所不同,但在周期10~40 s范围噪声源在各方向均有分布。因此,利用长时间段连续噪声数据计算的互相关函数在周期10~40 s范围内满足背景噪声面波层析成像的理论前提。  相似文献   

8.
In this paper the analysis of the ionospheric total electron content (TEC) variations obtained with using GPS measurements before the Hokkaido earthquake (M = 8.3) is presented. Anomalous behavior of TEC was detected within several days before the main event. Anomaly appeared as the local TEC enhancement situated in the vicinity of the forthcoming earthquake epicenter. These structures occurred during 5 days prior to the shock at the same interval of local time. At the process of the earthquake approach the amplitude of modification was increased, and it has reached the 85–90% level relative to the non-disturbed conditions 18 hours before the earthquake. The area of strong positive disturbance has extended over 1500 km in latitudes and 4000 km in longitudes. The analysis have shown that according to the series of characteristics (its locality, affinity with the epicenter, dome-shaped zone of manifestation, characteristic time of existence) the detected ionospheric anomaly may be associated to the precursors of seismic activity.  相似文献   

9.
唐山7.8级强震前震中周围形变电阻率的下降异常   总被引：16，自引：0，他引：16       下载免费PDF全文
1976年7月28日唐山7.8级强震前,北京、天津、唐山地区已布设14个形变电阻率观测台站,其中9个台,震前2-3年内记录到形变电阻率长趋势下降异常,这9个台围绕震中形成一个半长轴可达150公里的异常区.位于震中区的两个台还记录到临震前2-3个月内的加速下降异常.根据现场条件下地表浅部岩（土）层受压时,视电阻率下降的实验结果,认为在震中周围较大的区域内,震前浅部岩（土）层被压密,其压缩线应变的数量级约为3×10-5.这一认识得到唐山震前地面基线实测资料的支持.  相似文献   

10.
A magnitude MW7.0 earthquake struck north of Anchorage, Alaska, USA on 1 December 2018. This earthquake occurred in the Alaska-Aleutian subduction zone, on a fault within the subducting Pacific slab rather than on the shallower boundary between the Pacific and North American plates. In order to better understand the earthquake source characteristics and slip distribution of source rupture process as well as to explore the effect of tectonic environment on dynamic triggering of earthquake, the faulting geometry, slip distribution, seismic moment, source time function are estimated from broadband waveforms downloaded from IRIS Data Management Center. We use the regional broadband waveforms to infer the source parameters with ISOLA package and the teleseismic body wave recorded by stations of the Global Seismic Network is employed to conduct slip distribution inversion with iterative deconvolution method. The focal mechanism solution indicates that the Alaska earthquake occurred as the result of tensile-type normal faulting, the estimated centroid depth from waveform inversion shows that the earthquake occurred at the depth of 56.5km, and the centroid location is 10km far away in northeast direction relative to the location of initial epicenter. We use the aftershock distribution to constrain the fault-plane strike of a normal fault to set up the finite fault model, the finite fault inversion shows that the earthquake slip distribution is concentrated mainly on a rectangular area with 30km×20km, and the maximum slip is up to 3.6m. In addition, the slip distribution shows an asymmetrical distribution and the range of possible rupture direction, the direction of rupture extends to the northeast direction, which is same as that of aftershock distribution for a period of ten days after the mainshock. It is interesting to note that a seismic gap appears in the southwest of the seismogenic fault, we initially determined that the earthquake was a typical normal fault-type earthquake that occurred in the back-arc extensional environment of the subduction collision zone between the Pacific plate and the North American plate, this earthquake was not related to tectonic movement of faults near the Earth's surface. Due to the influence of high temperature and pressure during the subduction of the Pacific plate toward to the north, the subduction angle of the Pacific plate becomes steep, causing consequently the backward bending deformation, thus forming to a tensile environment at the trailing edge of the collision zone and generating the MW7.0 earthquake in Alaska.  相似文献   

11.
The records from 161 identical broadband seismic stations located in different regions of the world after the strong earthquakes off Sumatra Island on December 26, 2004 with magnitude M = 9.1, in Chile on February 27, 2010 with M = 8.8, and the Tohoku earthquake in Japan on March 11, 2011 with M = 9.0 are studied. Oscillations with a period of ~11 h are analyzed. They are observed as pulsations in the free radial oscillations of the Earth lasting more than one week. The stations located a few hundred kilometers apart from each other demonstrate identical records. As the distance between the stations becomes larger, the structure of the records becomes different. At interstation distances of about 3800 km, the records at the stations have opposite phases, and at distances of ~7600 km, the phases coincide. This is reflected in the spatial structure of the areas of the positive and negative phases of the oscillations on the Earth’s surface. This structure recurs at the same time instant after the three considered earthquakes, which indicates that this effect is independent of the properties of the sources. The spatial positions of the areas of positive and negative phases are also not correlated to the geological conditions in the vicinity of the stations which are located both in the subduction zone and within the platform. The structure of the pulsations and their spatial distribution differ from the variations of the Earth’s tides.  相似文献   

12.
The paper considers synchronous continuous records of microseismic background obtained within a month before the Kronotskii (Kamchatka) December 5, 1997, earthquake (M = 7.8) at six IRIS broadband stations that are located in a large region extending from central European Russia (the town of Obninsk) to the Far East (Kamchatka and Sakhalin). By averaging and downsampling, initial records were discretized at an interval of 30 s and the microseismic background was examined in the range of periods from 1 min to 2.4 h, after scale-dependent trends due to the effects of tides and temperature variations had been removed. Microseismic fluctuations were analyzed with the help of estimates of the evolution of their multifractal singularity spectra in a moving time window 12 h wide. As the criterion characterizing the background properties in a current time window, we took the values of the generalized Hurst exponent α* realizing the maximum of the singularity spectrum. Hidden synchronization effects of a microseismic field preceding a seismic event are identified by estimating the evolution of the spectral measure of coherent behavior of α* variations in a moving time window 5 days long for various combinations of jointly analyzed stations.  相似文献   

13.
本文搜集与整理了我国几十次大、中强地震,震前、震时的地倾斜异常图象,按其形态大致可分为五类:Ⅰ.震前倾斜记录曲线渐变——回复——发震(多见于远震);Ⅱ.震前倾斜记录曲线突变(阶跃)——发震(多见于近震);Ⅲ.震前倾斜记录曲线扰动或单向脉冲——正常——发震;Ⅳ.震前倾斜潮汐记录曲线畸变——发震;Ⅴ.震前倾斜记录曲线脉动(曲线加粗)——发震,其特征为:1.异常形态呈多样性;2.异常出现在震前几分钟到几十小时;3.异常幅值一般在几毫秒至十分之几角秒(10-8——10-6rad),个别达几角秒(10-5rad);4.异常倾斜方向多半与震中方位有关;5.异常有由远及近、从外到内(震中)的迁移现象。文章作者还结合模拟实验和地震模式,讨论了地震前倾斜异常的有关问题。   相似文献   

14.
大地震前近台背景噪声的频谱分析   总被引：1，自引：0，他引：1       下载免费PDF全文
选取了2008年汶川地震发生前12天四川测震台网震中附近三个台和2010年玉树地震前14天青海测震台网震中附近三个台的地震仪连续记录进行功率谱密度的分析。结果表明,近台背景噪声频谱与其它台站的记录存在差异,并且两次大地震的近台记录特征具有相似性。这种特征表达了震源区的岩层活动的物理过程与其后发生的大地震是有关联的。这些信息的提取和研究有助于对构造性地震的发震机理研究以及地震预报的探索。  相似文献   

15.
—The 12 November 1996 M w 7.7 Peru subduction zone earthquake occurred off the coast of southern Peru, near the intersection of the South American trench and the highest topographical point of the subducting Nazca Ridge. We model the broadband teleseismic P-waveforms from stations in the Global Seismic Network to constrain the source characteristics of this subduction zone earthquake. We have analyzed the vertical component P-waves for this earthquake to constrain the depth, source complexity, seismic moment and rupture characteristics. The seismic moment determined from the nondiffracted P-waves is 3–5 × 1020 N·m, corresponding to a moment magnitude M w of 7.6–7.7. The source time function for the 1996 Peru event has three pulses of seismic moment release with a total duration of approximately 45–50 seconds. The largest moment release occurs at approximately 35–40 seconds and is located ～90km southeast of the rupture initiation. Approximately 70% of the seismic moment was released in the third pulse.¶We find that the 1996 event reruptured part of the rupture area of the previous event in 1942. The location of the 1996 earthquake corresponds to a region along the Peru coast with the highest uplift rates of marine terraces. This suggests that the uplift may be due to repeated earthquakes such as the 1996 and 1942 events.  相似文献   

16.
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17.
The field of low-frequency microseisms is investigated with the use of data from 83 stations of the F-net broadband network in Japan over the period from the beginning of 1997 through June 2008. Vertical components with a sampling step of 1 s are used for analysis, as well as signals with a sampling step of 1 min obtained from the initial data by averaging and thinning. Long-period regularities of changes in the singularity spectrum support width Δα and the generalized Hurst exponent α* for the field of low-frequency microseisms were revealed by estimating multifractal singularity spectra in consecutive time windows 30 min long for 1-s data and 24 hour long for 1-min data. The average value of the parameter α* for 1-s data significantly decreased before the Hokkaido earthquake of September 25, 2003 (M = 8.3), and was not restored subsequently to its previous level. Prior to September 2003, 1-min data on α* variations experienced strong annual changes, which completely ceased afterwards. Both these effects are interpreted as an increase in the degree of synchronization of microseismic noise on Japan’s islands after the September 25, 2003, earthquake. This hypothesis is also supported by estimates of the measures of correlation and spectral coherence between variations in the average values of Δα and α* calculated for 1-min data inside five spatial clusters of stations from consecutive time fragments two months long. Based on the well-known statement of the theory of catastrophes that synchronization is one of the flags of an approaching catastrophe, it was suggested that the Hokkaido event could be a foreshock of an even stronger earthquake nucleating in the region of Japan’s islands.  相似文献   

18.
A study of the coseismic displacement and fling pulse recorded during the Mw 6.5 30 October 2016 Central Italy earthquake is presented. The near-field has been well documented, owing to the deployment of additional strong-motion stations following the earlier events of the 2016 Central Italy seismic sequence. As a result, there are numerous stations with evidence of coseismic displacement and fling pulse. In this study, 25 records with strike distance of less than 25 km and rupture distance under 28 km are considered. Approximate coseismic displacements have been recovered by a bilinear model to remove the low frequency noise in the records. The bilinear noise model uses two linear regression segments on the velocity trace to remove baseline offsets. After obtaining the coseismic displacement time series, the fling pulse period is examined. Existing methods of obtaining the fling pulse period are reviewed and a proposed algorithm is considered for automatic fling pulse detection. Both horizontal and vertical fling periods are obtained, unlike many studies which neglect the vertical fling. It is shown that the fling pulse period is highly variable (~?2–16 s) in the near-field region but exhibits some trends with various site-to-source distances.  相似文献   

19.
On 25th April, 2015 a hazardous earthquake of moment magnitude 7.9 occurred in Nepal. Accelerographs were used to record the Nepal earthquake which is installed in the Kumaon region in the Himalayan state of Uttrakhand. The distance of the recorded stations in the Kumaon region from the epicenter of the earthquake is about 420–515 km. Modified semi-empirical technique of modeling finite faults has been used in this paper to simulate strong earthquake at these stations. Source parameters of the Nepal aftershock have been also calculated using the Brune model in the present study which are used in the modeling of the Nepal main shock. The obtained value of the seismic moment and stress drop is 8.26 × 1025 dyn cm and 10.48 bar, respectively, for the aftershock from the Brune model .The simulated earthquake time series were compared with the observed records of the earthquake. The comparison of full waveform and its response spectra has been made to finalize the rupture parameters and its location. The rupture of the earthquake was propagated in the NE–SW direction from the hypocenter with the rupture velocity 3.0 km/s from a distance of 80 km from Kathmandu in NW direction at a depth of 12 km as per compared results.  相似文献   

20.
共搜集到１９８４￣１９９０年西南太平洋地区１２个板缘地震序列。多数地震序列的特征是：震中分布区域的长轴较长并且随主震震级和序列中强震次数而增加;震中分布区域的长、短轴长度的比值较高;地震序列的余震震源机制和主震的差异不大;震源深度下限超过地壳,可达７０ｋｍ以上。走滑型主震占的比例低,高倾角滑动面的走向既有与俯冲带走向平行的也有横切的,个别逆冲型地震的断层面走向横切俯冲带。它们显示出与板块俯冲带主体  相似文献   

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