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1.
The method of coda waves was applied to two different sets of data for the evaluation of the relative site response.The first set of data consists of low magnitude earthquakes with closely spaced locations, recorded at a small aperture array of velocimeters located in the Abruzzo region, central Italy. The second set of data is composed of events with epicentral distance ranging from 20 to 300 km, recorded at a seismological network with an aperture of about 100 km located in the Puglia region, southern Italy.Results show that the coda wave method furnishes stable estimates of the site effect. An amplification, relative to an arbitrary site, of a factor of about 2 occurs in the 1.7–6 Hz frequency band for two stations of the Abruzzo network, while an amplification factor of about 0.5 occurs in the whole frequency band (1–24 Hz) for one of the stations of the Puglia network. This station is located in an area which is correlated with a low macroseismic intensity anomaly. 相似文献
2.
Bruce S. Gibson 《Pure and Applied Geophysics》1988,128(1-2):309-331
Three types of seismic data recorded near Coalinga, California were analyzed to study the behavior of scattered waves: 1) aftershocks of the May 2, 1983 earthquake, recorded on verticalcomponent seismometers deployed by the USGS; 2) regional refraction profiles using large explosive sources recorded on essentially the same arrays above; 3) three common-midpoint (CMP) reflection surveys recorded with vibrator sources over the same area. Records from each data set were bandpassed filtered into 5 Hz wide passbands (over the range of 1–25 Hz), corrected for geometric spreading, and fit with an exponential model of amplitude decay. Decay rates were expressed in terms of inverse codaQ (Q
c
–1
).Q
c
–1
values for earthquake and refraction data are generally comparable and show a slight decrease with increasing frequency. Decay rates for different source types recorded on proximate receivers show similar results, with one notable exception. One set of aftershocks shows an increase ofQ
c
–1
with frequency.Where the amplitude decay rates of surface and buried sources are similar, the coda decay results are consistent with other studies suggesting the importance of upper crustal scattering in the formation of coda. Differences in the variation ofQ
c
–1
with frequency can be correlated with differences in geologic structure near the source region, as revealed by CMP-stacked reflection data. A more detailed assessment of effects such as the depth dependence of scattered contributions to the coda and the role of intrinsic attenuation requires precise control of source-receiver field geometry and the study of synthetic seismic data calculated for velocity models developed from CMP reflection data. 相似文献
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通过对2013年1月至11月四川长宁地区M≥4.0地震震相的分析得出,在震中距大于260 km时,四川数字地震台网部分台站可记录到较为清晰的sPn震相。本文采用sPn和Pn震相确定震源深度的方法,分别对长宁地区2个M≥4.0地震的震源深度进行了重新计算,2次地震的震源深度较浅,均小于3 km,是较为精确的震源深度值。 相似文献
5.
选取2009—2018年江苏省射阳地震台200 km范围内M_L≥2.0地震波形资料,基于Sato单次散射模型,计算该区域S波尾波Q_c值,并探求其时间变化特征。采用相同方法计算盐城、灌云地震台地震波形资料尾波Q_c值,并将3个地震台的计算结果进行对比。结果表明:射阳地区是江苏省尾波Q_0低值区域,Q_c值随频率的变化关系为Q_c(f)=33.7±3.4f ~(1.16±0.04);在2016年10月20日射阳M_(L )4.8地震前,射阳地区尾波Q_c值有增大现象,Q_c值变化特征表现为上升—下降—上升—发震;盐城、射阳、灌云地震台所记录地震的尾波Q_c值在震前均出现明显的高值异常变化。 相似文献
6.
Phase and Correlation in `Random' Seismic Fields and the Reconstruction of the Green Function 总被引：3，自引：0，他引：3
Michel Campillo 《Pure and Applied Geophysics》2006,163(2-3):475-502
We first present a summary of recent results on coda interpretation. We emphasize the observation of the stabilization of
P to S energy ratio indicating the modal equipartition of the wavefield. This property clearly shows that the coda waves are
in the regime of multiple scattering. Numerical solutions of the elastic radiative transfer equation are used to illustrate
the evolution of the wave-field towards P-to-S energy stabilization, and asymptotically to complete isotropy. The energy properties
of the coda have been widely studied but the phase properties have often been neglected. The recently observed coherent backscattering
enhancement, an expression of the so-called `weak localization', demonstrates that interference effects still persist for
multiple diffracted waves. Another manifestation of the persistence of the phase is the possibility to reconstruct the Green
function between two stations by averaging the cross correlation of coda waves produced by distant earthquakes and recorded
at those two stations. This reconstruction is directly related to the properties of reciprocity and time reversal of any wavefield.
Using broadband seismic coda waves, we show that the dominant phases of the Green function in the band 2 s–10 s, namely fundamental
mode Rayleigh and Love waves, are reconstructed. We analyze the time symmetry of the cross correlation and show how the level
of symmetry evolves with the isotropization of the diffuse field with lapse time. Similarly we investigate the correlation
in continuous ambient noise records. Whereas the randomness of the coda results from multiple scattering by randomly distributed
scatterers, we assume that the seismic noise is random mostly because of the distribution of sources at the surface of the
Earth. Surface waves can be extracted from long time series. The dispersion curves of Rayleigh waves are deduced from the
correlations. On paths where measurements from earthquake data are also available, we show that they are in good agreement
with those deduced from noise correlation. The measurement of velocities from correlation of noise along paths crossing different
crustal structures opens the way for a `passive imaging' of the Earth's structure. 相似文献
7.
Comparison of crustal and upper mantle heterogeneity in different time periods: Indonesian subduction zone to northern Australia 
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下载免费PDF全文 Earthquake events from the Indonesian subduction zone recorded in northern Australia show a long and high-frequency coda associated with both P and S waves. Regional events recorded by Warramunga array in northern Australia can separate out wave propagation through the mantle by focusing on the coherent signal across the medium-aperture array. Most of the incoherent wave components result from structures in the vicinity of the array with small-scale lengths of 1–2 km or smaller. The coherent phases with relatively rapid changes in waveforms are associated with the scattering of seismic waves by crustal and mantle heterogeneity, but in some case can be related to structural effects near the source. As the depth of the source increases, the coherent portion of the seismic wavefield tends to become much simpler, which suggests that the heterogeneity tends to weaken at depth with larger-scale length. We compare the coherent signal features of earthquakes from the Indonesian subduction zone that have occurred in recent years with those in the early 1980s, first studied by Kennett (Phys Earth Planet Inter 47: 319–332, 1987). The general characteristics of the coherent signal variation with depth in recent years are the same as those observed in 1980s, but the variations are larger. This change suggests a stringer variation in heterogeneity with depth than before, which may bear important information about the dynamic processes and evolution of the crust and upper mantle. 相似文献
8.
Seismic stations, with automatic P-picking and satellite retransmission were set up on Mount Etna following the eruption started on March 1983. Positions of the stations were chosen in order to complement the permanent telemetered network of Catania University.Comparison between locations obtained by both networks were made for earthquakes recorded by at least 5 ARGOS DCP (Data Collection Platform) stations. We observed a satisfactory agreement for events inside both networks.By merging data of both networks, it has been possible to locate more than 50 earthquakes for which separate computation was not possible due to the low number of arrivals.On 3rd-4th June a swarm of deep seismic events was observed. Hypocenters of these earthquakes are clearly located in a NNW-SSE-trending vertical zone of 5 km width at a depth of 7–36 km.Changes in the distribution of shallow seismic activity, before and after this swarm, have been observed. 相似文献
9.
In this paper, regionalP-wave upper mantle structure is investigated using slant-stack velocity analysis of short-period earthquake data recorded at station MAJO (Matsushiro, Japan). Shallow earthquakes from 1980–1986 within 35° of MAJO are used to construct a common receiver gather. Processing of the wavefield data includes focal depth and static time corrections, as well as deterministic deconvolution, in order to equalize pulse shapes and align wavelets on the first arrivals. The processed wavefield data are slant stacked and interatively downward continued to obtain a regional upper mantle velocity model. The model includes a low velocity zone between 107 and 220 km. Beneath the LVZ, the velocity increases smoothly down to the discontinuity at 401 km. In the transition zone, the velocity model again increases linearly, although there is some suggestion of further complexity in the downward continued wavefield data. At the base of the transition zone, a second velocity discontinuity occurs at 660 km, with a linear velocity gradient below. In addition to slant-stack analysis, travel times and synthetic seismograms are computed and compared with the processed and unprocessed wavefield data. 相似文献
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S. Falsaperla G. Lanzafame V. Longo S. Spampinato 《Journal of Volcanology and Geothermal Research》1999,88(3):156
Crustal tectonic seismicity on Stromboli is here discussed along with the regional tectonic and geodynamic context of the Aeolian archipelago, Italy. The aim of this paper is to contribute to the reduction of the crucial gap in information concerning the tectonic activity originating at the volcano Stromboli in the last decade. It is indeed well known that Strombolian eruptions, related to the persistent state of activity on this volcano, allow a fast collection of seismic data which are mainly represented by explosion quakes and tremor. However, only some sporadic information concerns tectonic earthquakes due to their low occurrence rate. A catalog of 150 crustal tectonic earthquakes recorded at Stromboli from 1985 to 1996 is presented here. The analyses of the seismograms were carried out on the basis of the records of the station STR, located in the southwestern part of the island. The magnitude considered ranged from 1.5 to 3.7. Hypocentral location indicates foci at less than 12 km, although for the majority of the earthquakes, which are not recorded at the seismic stations deployed on the other islands of the Aeolian archipelago, a shallower (within a few kilometers) depth can be hypothesized. The strain release associated with the 150 tectonic earthquakes of our catalog was also evaluated. The low levels of occurrence and seismic energy release reached throughout the ca. twelve years analyzed here confirm the minor role played by processes of brittle failure of rocks. This aspect of seismic activity is nevertheless important as it can provide a contribution to the knowledge of the dynamics of this volcano. The focal mechanisms available, integrated with data from the literature, show the T-axes oriented about NE–SW and NW–SE. These findings are consistent with the deformative and tectonic characteristics of this volcanic edifice and of the Aeolian archipelago, which shows the same two directions of extension. This is in agreement with subduction, which is very old or which passively evolves towards the final stages, and is controlled by the transform activity of the Aeolian–Tindari–Giardini fault system. 相似文献
12.
This paper reports the results of two seismic experiments aimed at determining the wave field of explosion quakes at Stromboli Island (Mediterranean Sea, Southern Italy). The typical Strombolian activity mostly consists of explosive phenomena causing pyroclastic, materials to be emitted together with jets of volcanic gases from one or more craters. Stromboli is an active volcano characterized by persistent seismic activity consisting of explosion quakes that are seismic events associated with the explosive volcanic phenomena. Explosion quakes are short lived seismic events occurring intermittently whose amplitude tends to decrease with distance from the vent. A distinctive feature of explosion quakes is the presence on seismograms of two, often clearly distinct, seismic phases. The first, low-frequency seismic phase (<2 Hz) is in fact usually followed by a high-frequency seismic phase (>3–4 Hz) after one second or more. The first seismic phase of explosion quakes has been shown to be characterized by a nearly radial linear polarization and by an apparent propagation velocity estimated at 600–800 m/s. The second phase is characterized by a more chaotic motion and a lower apparent propagation velocity of 150–450 m/s. The wavefield associated with the first low-frequency seismic phase appears to be generated by a resonating P-wave seismic source accompanying gas explosion and emission of pyroclastic materials. The wavefield associated with the second high-frequency seismic phase of explosion quakes appears to be mainly composed of scattered and converted waves due to the critical topography of the volcano. 相似文献
13.
The Quality factor is the parameter that can be used to describe the energy attenuation on seismic wave. In theory, we can obtain the relationship between the change of the coda wave quality factor with time and the strong earthquake preparation process on the basis of the quality factor of a coda wave in a same ray path. However, in reality the coda wave quality factor measured by different seismic coda waves corresponds to different seismic wave ray paths. The change of the quality factor with time is related to non-elastic characteristics of the medium and the volume of scattering ellipsoid constrained by scattered wave phase fronts, besides the change of regional stress field. This paper discusses the relationship between quality factor, epicenter distance and different lapse time, and then discusses the relationship between quality factor and frequency. Furthermore the determination method of the coda wave quality factor is put forward. The improved determination method of the quality factor, which removes the influence of different earthquakes or propagation depth of scattered waves, may increase measurement precision, thus information pertaining to abnormal changes in quality factor and the relationship between the quality factor and earthquake preparation process can be acquired. 相似文献
14.
Statistical tests for regional seismic phase characterizations 总被引：1，自引：0，他引：1
L. Persson 《Journal of Seismology》2003,7(1):19-33
In seismic analysis some assumptions are often made aboutthe data, e.g. stationarity and Gaussianity. This is not obvious for all realseismic data. Here, we use statistical tests for characterization of regionalseismic data. We apply tests for stationarity, symmetry, linearity, andtime-reversibility. In the analysis we use twelve regional seismic events inFennoscandia recorded with the seismic small-aperture arrays NORESS,ARCESS and FINESA at hypocentral distances in the range from 160 to1580 km. For the tests we use the regional phases Pn, secondary P, Sn and Lg-phases and the preceding noise. Two of the eventsare explosions, two are rockbursts and eight are earthquakes. Theperformance and possibilities of using statistical tests based on bispectra, asa complementary tool for conventional analysis of seismic phases isdemonstrated. The preceding noise recorded before the first onset of theP-wave for the twelve events is tested to be spatially stationarybetween each channel within each array and temporal stationary in 21consecutive time windows of 3.2 sec each. Also, the preceding noise issymmetric and linear. The seismic phases defined by the symmetry test asnon-Gaussian (not symmetric) are all linear. This means a linear model canbe used to characterize both the noise and the phases. The first P-phase for the two explosions is characterized as non-Gaussian at thethree arrays. For all 36 possibly first P-phase arrivals at the three arraystations, 23 are non-Gaussian. The second P-phase is non-Gaussian at13 of 36 data records, the S-phase at eleven of 36 and the Lg-phase at nine of 36. For all the four phases more than 32 of possible36 are time-reversible. 相似文献
15.
R. K. Chadha 《Pure and Applied Geophysics》1994,142(2):319-328
T phases of three earthquakes from the Indian Ocean region, recorded by a short-period vertical-component seismic station network located in the vicinity of Kanyakumari on the southernmost tip of India, are studied. Two of these earthquakes are located west of 90°E ridge and one in the Nicobar Island region. However, seven other earthquakes which occurred 150–200 km south of Kanyakumari in the ocean did not produceT phases. An analysis ofT-waves (tertiary waves) travel time reveals the zone ofP-wave toT-wave conversion (i.e.,PT phase) region to coincide with the western continental slope of Srilanka. Further, it is observed that the disposition of the bathymetry between Srilanka and southern India strongly favours the downslope propagation mechanism ofT-wave travel to the southern coast of India through SOFAR channel. These observations are reported for the first time from India. 相似文献
16.
The detection and location of low magnitude earthquakes in northern Norway using multi-channel waveform correlation at regional distances 总被引：1，自引：0，他引：1
Steven J. Gibbons Mathilde Bttger Srensen David B. Harris Frode Ringdal 《Physics of the Earth and Planetary Interiors》2007,160(3-4):285-309
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. 相似文献
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Javier Almendros Jesús M. Ibez Enrique Carmona Daria Zandomeneghi 《Journal of Volcanology and Geothermal Research》2007,160(3-4):285-299
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. 相似文献
19.
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ML1.0—3.0重复地震,进而为重复地震的监测与研究提供依据。 相似文献
20.
单台Lg尾波Q_(Lg)~c的测量研究 总被引：7，自引：2，他引：5
将单台记录叠加频谱比法（简称SSR法）应用于震中距小于500km的区域地震Lg尾波为lHz下的值,n是与频率f有关的指数)的提取之中·为了检验方法的稳定性和可靠性,我们选用两组发生在不同地区的地震记录．一组是1989-10-18大同主震（ML＝5．9）的15个余震,北京遥测地震台网记录到的信噪比较高的Lg尾波序列。另一组是秦岭造山带数字流动地震观测台网记录到的1995年甘肃平凉两次地震信噪比较高的Lg尾波序列．每一组地震的震源参数基本相同,即它们到某一特定台站的路径几乎相同,得到的 可以认为是不受震源影响的地震一台站路径上的平均衰减效应·两区域 的测量结果表明：（1）尽管两个地震观测台网所用观测仪器不同,每个台站的观测条件不同,所记录的地震也不尽相同,但对Q0和,的拟合结果线性程度都比较高,说明SSR法是稳定的;（2）同一震群的不同地震到某一特定台站测得的结果基本一致,即Q0值及其平均值大都在其标准差范围之内,说明SSR法是可靠的。（3）对同一地震不同方位各台站的Lg尾波测量结果可以看出,穿过不同构造区的Q0值存在明显差... 相似文献