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1.
Using network averaged spectra Murphy (1989) demonstrates that if it is assumed that the source functions of explosions at Pahute Mesa, Nevada Test Site (NTS) are as predicted by the Mueller-Murphy (M-M) source model then the average t * at around 1 Hz for P waves radiated from the test site must be about 0.75 s. With this value of t * Murphy (1989) estimates the best fitting M-M spectrum for each explosion studied, by adjusting A, t o & c ; A & t o being the amplitude and delay time of pP relative to P and c the wave speed for the material in which the explosion was fired. The absolute amplitudes of the theoretical spectra are obtained using a calibration factor estimated from the data. Murphy (1993) extends the analysis to explosions in granite at the Nevada, French Sahara and E. Kazakh test sites. For the French Sahara explosion t * is assumed to be 0.75 s (Murphy's estimate for NTS explosions), and for the E. Kazakh explosion a t * of 0.55 s is used. For the French Sahara and E. Kazakh explosions Murphy (1993) shows that by using the same calibration factor as for the NTS it is possible by varying A & t o to fit the estimated average network spectra using the M-M granite source. Murphy (1993) states that the amplitudes and spectra for the largest NTS explosion in granite (PILE DRIVER) can also be predicted using the M-M model but these results are not shown.  相似文献   

2.
We present velocity constraints for the upper-mantle transition zones beneath Central Siberia based on observations of the 1982 RIFT Deep Seismic Sounding (DSS) profile. The data consist of seismic recordings of a nuclear explosion in north-western Siberia along a 2600 km long seismic profile extending from the Yamal Peninsula to Lake Baikal. We invert seismic data from the mantle transition zones using a non-linear inversion scheme using a genetic algorithm for optimization and the WKBJ method to compute the synthetic seismograms. A statistical error analysis using a graph-binning technique was performed to provide uncertainty values in the velocity models.
Our best model for the upper-mantle velocity discontinuity near 410 km depth has a two-stage velocity-gradient structure, with velocities increasing from 8.70–9.25 km s−1 over a depth range of 400–415 km, a gradient of 0.0433 s−1, and from 9.25–9.60 km s−1 over a depth range of 415–435 km, a gradient of 0.0175 s−1. This derived model is consistent with other seismological observations and mineral-physics models. The model for the velocity discontinuity near 660 km depth is simple, sharp and includes velocities increasing from 10.15 km s−1 at 655 km depth to 10.70 km s−1 at 660 km depth, a gradient of 0.055 s−1.  相似文献   

3.
Summary. NORSAR recordings of Rayleigh waves generated by presumed nuclear explosions on central and southern Novaya Zemlya and in northwestern Siberia have been studied. Using a frequency time analysing technique and correcting for presumed known dispersion effects across the Baltic Shield, dispersion curves for two different paths across the southern part of the Barents Sea were obtained. The curves are very unusual in that they give extremely low velocities even for periods up to 20 s. For the path to the middle part of the island, the inversion of the data gives a model with sediments and consolidated sediments down to 25 km, followed by a 15-km thick basaltic layer and an upper mantle with a P velocity as low as 7.9 km/s. For the path to the southern part of Novaya Zemlya the data inversion gives a somewhat different model with sediments and consolidated sediments down to 8 km, followed by a 17-km thick zone with velocities close to granitic and a 15-km thick layer with basaltic velocities. Again the upper-mantle P velocity is only 7.9 km/s. Other indications of lateral inhomogeneities in the Barents Sea are obtained by utilizing the array's capability to determine the angle of approach of seismic waves. It is demonstrated that reflections both from inhomogeneities in the Barents Sea and the continental margin off Norway can be detected. For waves from the southern end of the island, a reflection from a strong discontinuity close to the direct path to the middle part of the island is found, whereas signals from this area include a reflected wave possibly coming from the edge of the Svalbard platform.  相似文献   

4.
We infer the lithospheric structure in eastern Turkey using teleseismic and regional events recorded by 29 broad-band stations from the Eastern Turkey Seismic Experiment (ETSE). We combine the surface wave group velocities (Rayleigh and Love) with telesesimic receiver functions to jointly invert for the S -wave velocity structure, Moho depth and mantle-lid (lithospheric mantle) thickness. We also estimated the transverse anisotropy due to Love and Rayleigh velocity discrepancies. We found anomalously low shear wave velocities underneath the Anatolian Plateau. Average crustal thickness is 36 km in the Arabian Plate, 44 km in Anatolian Block and 48 km in the Anatolian Plateau. We observe very low shear wave velocities at the crustal portion (30–38 km) of the northeastern part of the Anatolian Plateau. The lithospheric mantle thickness is either not thick enough to resolve it or it is completely removed underneath the Anatolian Plateau. The shear velocities and anisotropy down to 100 km depth suggest that the average lithosphere–asthenosphere boundary in the Arabian Plate is about 90 and 70 km in Anatolian block. Adding the surface waves to the receiver functions is necessary to constrain the trade-off between velocity and the thickness. We find slower velocities than with the receiver function data alone. The study reveals three different lithospheric structures in eastern Turkey: the Anatolian plateau (east of Karliova Triple Junction), the Anatolian block and the northernmost portion of the Arabian plate. The boundary of lithospheric structure differences coincides with the major tectonic boundaries.  相似文献   

5.
Summary. Group velocities for first and second higher mode Rayleigh waves, in the frequency range 0.8–4.8 Hz, generated from a local earthquake of magnitude 3.7 M L in western Scotland, are measured at stations along the 1974 LISPB line. These provide detailed information about the crustal structure west of the line. The data divide the region into seven apparently homogeneous provinces. Averaged higher mode velocity dispersion curves for each province are analysed simultaneously using a linearized inversion technique, yielding regionalized shear velocity profiles down to a depth of 17 km into the upper crust. Shear wave velocity is between 3.0 and 3.4 km s−1 in the upper 2 km, with a slow increase to around 3.8 km s−1. P -wave models computed using these results agree with profiles from the LISPB and LUST refraction experiments.  相似文献   

6.
Summary. The paper gives the results of a study of the anisotropy of seismic wave velocities within the Ashkhabad test field in Central Asia. The anisotropy was studied by analysing variations in the values of apparent velocities of first arrivals for epicentral distances ranging from 30 to 130 km and by analysing the delays (Δ ts1-s2 ) between the arrival times of shear waves with different polarizations.
The velocities of P -waves vary with azimuth from 5.3 to 6.27 km s-1 and the velocities of S -waves vary from 3.15 to 3.5 km s-1.
The delay times Δ tS1 - S2 depend on the direction of the propagation. The character of the variation of the propagation velocity of the longitudinal wave, the presence of two differently polarized shear waves S 1 and S 2 propagating at different velocities, and the character of the distribution of Δ tS1 - S2 on the stereogram suggest that the symmetry of the anisotropic medium is close to hexagonal with a nearly horizontal symmetry axis coinciding with the direction of maximal velocity. The azimuth of the symmetry axis of the medium is 140° and coincides with the direction of geological faults.  相似文献   

7.
The crustal and upper mantle structure of the northwestern North Island of New Zealand is derived from the results of a seismic refraction experiment; shots were fired at the ends and middle of a 575 km-long line extending from Lake Taupo to Cape Reinga. The principal finding from the experiment is that the crust is 25 ± 2 km thick, and is underlain by what is interpreted to be an upper mantle of seismic velocity 7.6 ± 0.1 km s−1, that increases to 7.9 km s−1 at a depth of about 45 km. Crustal seismic velocities vary between 5.3 and 6.36 km s−1 with an average value of 6.04 km s−1. There are close geophysical and geological similarities between the north-western North Island of New Zealand and the Basin and Range province of the western United States. In particular, the conditions of low upper-mantle seismic velocities, thin crust with respect to surface elevation, and high heat-flow (70–100 mW m−2) observed in these two areas can be ascribed to their respective positions behind an active convergent margin for about the past 20 Myr.  相似文献   

8.
We present a regional surface waveform tomography of the Pacific upper mantle, obtained using an automated multimode surface waveform inversion technique on fundamental and higher mode Rayleigh waves, to constrain the   VSV   structure down to ∼400 km depth. We have improved on previous implementations of this technique by robustly accounting for the effects of uncertainties in earthquake source parameters in the tomographic inversion. We have furthermore improved path coverage in the South Pacific region by including Rayleigh wave observations from the French Polynesian Pacific Lithosphere and Upper Mantle Experiment deployment. This improvement has led to imaging of vertical low-velocity structures associated with hotspots within the South Pacific Super-Swell region. We have produced an age-dependent average cross-section for the Pacific Ocean lithosphere and found that the increase in   VSV   with age is broadly compatible with a half-space cooling model of oceanic lithosphere formation. We cannot confirm evidence for a Pacific-wide reheating event. Our synthetic tests show that detailed interpretation of average   VSV   trends across the Pacific Ocean may be misleading unless lateral resolution and amplitude recovery are uniform across the region, a condition that is difficult to achieve in such a large oceanic basin with current seismic stations.  相似文献   

9.
We have analysed the fundamental mode of Love and Rayleigh waves generated by 12 earthquakes located in the mid-Atlantic ridge and Jan Mayen fracture zone. Using the multiple filter analysis technique, we isolated the Rayleigh and Love wave group velocities for periods between 10 and 50  s. The surface wave propagation paths were divided into five groups, and average group velocities calculated for each group. The average group velocities were inverted and produced shear wave velocity models that correspond to a quasi-continental oceanic structure in the Greenland–Norwegian Sea region. Although resolution is poor at shallow depth, we obtained crustal thickness values of about 18  km in the Norwegian Sea area and 9  km in the region between Svalbard and Iceland. The abnormally thick crust in the Norwegian Sea area is ascribed to magmatic underplating and the thermal blanketing effect of sedimentary layers. Maximum crustal shear velocities vary between 3.5 and 3.9  km  s−1 for most paths. An average lithospheric thickness of 60  km was observed, which is lower than expected for oceanic-type structure of similar age. We also observed low shear wave velocities in the lower crust and upper mantle. We suggest that high heat flow extending to depths of about 30  km beneath the surface can account for the thin lithosphere and observed low velocities. Anisotropy coefficients of 1–5 per cent in the shallow layers and >7 per cent in the upper mantle point to the existence of polarization anisotropy in the region.  相似文献   

10.
It has long been known that S waves on seismograms of local explosions are often accompanied by strong low-frequency, low-velocity, Rg surface wave trains, often significantly diminished for earthquakes. We utilize this fact to construct a new formal discriminator between earthquakes and explosions by measuring the S -surface-wave group velocity. The method is based on analysing the velogram; that is, the display of the envelope of ground motion versus group velocity V = R/T , where R is the epicentral distance and T  the traveltime. We examine the distribution of seismic energy in time and space using envelopes of records from the Israel Seismic Network (ISN), from which we compute the velograms and observe differences in the velograms of quarry blasts and earthquakes. The data include 143 seismic events occurring in three areas (Galilee, Dead Sea, and Gilad) monitored by the ISN; the magnitude range is M L = 1.0–2.8 at distances of 15–310 km. From the velograms we measure the group velocity, V m s , within the 1–4 km s−1 range at which the velogram reaches its maximum for each available station. The resulting V m s (R) function is closely fitted by the empirical relationship a + b  ln  R , with a and b coefficients varying from event to event. A simple linear function c = b + 0.33a at a threshold C = 0.69 completely separates ( a,b ) pairs for the 67 Galilee events, and, for the 76 remaining events, one earthquake and four explosions are wrongly classified. After data validation and application of the Fisher linear discriminator, adapted to the events from Galilee, only two misidentified events remain for the whole data set.  相似文献   

11.
南极沿167°E子午线横贯南极山脉岩石圈速度结构   总被引：3，自引：0，他引：3       下载免费PDF全文
束沛镒  焦丞民 《极地研究》1999,11(3):221-227
依据沿大圆弧穿越南极点和斯科特站两地震台的地震瑞利面波波形资料,计算了两台之间的相速度频散,通过反演计算,获得了台间地下２００ｋｍ 岩石圈剪切波速度细结构。结果表明,横贯南极山脉地壳厚度约为４５ｋｍ ,５５～７５ｋｍ 之间存在明显低速带,它预示着这一深度有熔融的岩浆存在。  相似文献   

12.
Summary. A variety of near-regional (300 km) data, including spectral amplitudes of Pg , surface-wave forms, and close-in (5–10 km) accelerograms have been used to build an elastic seismic source model for a 1-Mton explosion in tuff at near-regional distances. The model consists of: (1) a pressure pulse which injects 3 × 1012 cm3 of volume into the medium, (2) a vertical, upward force impulse that imparts 1018 dyn-s of momentum to the medium, each source component having a time duration of 0.6 s and a depth of 1.3 km. The force impulse appears to be required by two considerations: (a) the striking similarity, apart from sign, of explosion surface waves with those of their cavity collapses, (b) the observation of considerable SV energy leaving the source of the 1-Mton explosions JORUM and HANDLEY . Scaling curves have been constructed which fit the proposed source model. These scaling curves employ: very slow decrease, as (yield)−0.10 of the primary corner frequency; decay as (frequency)4 or (frequency)3 to high frequency. While these scaling curves are unconventional, they appear to be the only ones which can satisfy the near-regional data. The slow scaling with yield of the spectral carner frequency suggests that it is caused by something other than the equivalent elastic radius, e.g. the time duration of motion at the source. The results, at odds with similar studies at teleseismic distances, suggest that significantly different equivalent elastic sources are required at near-regional (as compared with teleseismic) distances; therefore, the effect of the upward impulse might not be seen at teleseismic distances. Consequently, these results probably do not pertain to the seismic discrimination problem at teleseismic distances.  相似文献   

13.
The first detailed deep seismic refraction study in the Bransfield Strait, West Antarctica, using sensitive OBSs (ocean bottom seismographs) was carried out successfully during the Antarctic summer of 1990/1991. The experiment focused on the deep crustal structure beneath the axis of the Bransfield Rift. Seismic profile DSS-20 was located exactly in the Bransfield Trough, which is suspected to be a young rift system. Along the profile, five OBSs were deployed at spacings of 50-70 km. 51 shots were fired along the 310 km profile. This paper gives the first presentation of the results. A detailed model of the crustal structure was obtained by modelling the observed traveltimes and amplitudes using a 2-D ray-tracing technique. The uppermost (sedimentary?) cover, with velocities of 2.0-5.5 km s−1, reaches a depth of up to 8 km. Below this, a complex with velocities of 6.4-6.8 km s−1 is observed. The presence of a high-velocity body, with V p= 7.3-7.7 km s−1, was detected in the 14-32 km depth range in the central part of the profile. These inhomogeneities can be interpreted as a stage of back-arc spreading and stretching of the continental crust, coinciding with the Deception-Bridgeman volcanic line. Velocities of 8.1 km s−1, characteristic of the Moho, are observed along the profile at a depth of 30-32 km.  相似文献   

14.
A network of nine broad-band seismographs was operated from March to May 1994 to study the propagation of seismic waves across the Mexican Volcanic Belt (MVB) in the region of the Valley of Mexico. Analysis of the data from the network reveals an amplification of seismic waves in a wide period band al the stations situated in the southern part of the MVB.
The group velocities of the fundamental mode of the Rayleigh wave in the period range 2–13 s are found to be lower in the southern part of the MVB than in its northern part and in the region south of the MVB. The inversion of dispersion curves shows that the difference in group velocities is due to the presence of a superficial low-velocity layer (with an average S -wave velocity of 1.7 km s-1 and an average thickness of 2 km) beneath the southern part of the MVB. This low-velocity zone is associated with the region of active volcanism.
Numerical simulations show that this superficial low-velocity layer causes a regional amplification of 8–10 s period signals, which is of the same order as the amplification measured from the data. This layer also increases the signal duration significantly because of the dispersion of the surface waves. These results confirm the hypothesis of Singh et al. (1995), who suggested that the regional amplification observed in the Valley of Mexico is due to the anomalously low shear-wave velocity of the shallow volcanic rocks in the southern MVB  相似文献   

15.
Rayleigh wave phase velocity maps in southern Africa are obtained at periods from 6 to 40 s using seismic ambient noise tomography applied to data from the Southern Africa Seismic Experiment (SASE) deployed between 1997 and 1999. These phase velocity maps are combined with those from 45 to 143 s period which were determined previously using a two-plane-wave method by Li & Burke. In the period range of overlap (25–40 s), the ambient noise and two-plane-wave methods yield similar phase velocity maps. Dispersion curves from 6 to 143 s period were used to estimate the 3-D shear wave structure of the crust and uppermost mantle on an 1°× 1° grid beneath southern Africa to a depth of about 100 km. Average shear wave velocity in the crust is found to vary from 3.6 km s–1 at 0–10 km depths to 3.86 km s–1 from 20 to 40 km, and velocity anomalies in these layers correlate with known tectonic features. Shear wave velocity in the lower crust is on average low in the Kaapvaal and Zimbabwe cratons and higher in the surrounding Proterozoic terranes, such as the Limpopo and the Namaqua-Natal belts, which suggests that the lower crust underlying the Archean cratons is probably less mafic than beneath the Proterozoic terranes. Crustal thickness estimates agree well with a previous receiver function study of Nair et al. . Archean crust is relatively thin and light and underlain by a fast uppermost mantle, whereas the Proterozoic crust is thick and dense with a slower underlying mantle. These observations are consistent with the southern African Archean cratons having been formed by the accretion of island arcs with the convective removal of the dense lower crust, if the foundering process became less vigorous in arc environments during the Proterozoic.  相似文献   

16.
Summary. Normal mode theory, extended to the slightly laterally heterogeneous earth by the first-order Born approximation, is applied to the waveform inversion of mantle Love wave (200–500 s) for the Earth's lateral heterogeneity at l = 2 and a spherically symmétric anelasticity ( Q μ) structure. The data are from the Global Digital Seismograph Network (GDSN). The l =2 pattern is very similar to the results of other studies that used either different méthods, such as phase velocity measurements and multiplet location measurements, or a different data set, such as mantle Rayleigh waves from different instruments. The results are carefully analysed for variance reduction and are most naturally explained by heterogeneity in the upper 420 km. Because of the poor resolution of the data set for the deep interior, however, a fairly large heterogeneity in the transition zones, of the order of up to 3.5 per cent in shear wave velocity, is allowed. It is noteworthy that Love waves of this period range cannot constrain the structure below 420 km and thus any model presented by similar studies below this depth are likely to be constrained by Rayleigh waves (spheroidal modes) only.
The calculated modal Q values for the obtained Q μ model fall within the error bars of the observations. The result demonstrates the discrepancy of Rayleigh wave Q and Love wave Q and indicates that care must be taken when both Rayleigh and Love wave data, including amplitude information, are inverted simultaneously.
Anomalous amplitude inversions of G2 and G3, for example, are observed for some source-receiver pairs. This is due to multipathing effects. One example near the epicentral region, which is modelled by the obtained l = 2 heterogeneity, is shown.  相似文献   

17.
Summary. Rayleigh and Love wave group velocities were determined for 21 paths across the Barents shelf. Those group velocities exhibit regional variations of 1.0 km-1 or more at short periods, depending upon the location of the path within the shelf. Only two different crustal shear-velocity models beneath sedimentary layers are required, however, to explain all of the group velocity data. One model pertains to most of the shelf from a longitude near the eastern coast of Svalbard to Novaya Zemlya. The other pertains to a 200 or 300 km wide region at the western edge of the shelf. Shear velocities in the upper crust of the western region are significantly higher and the crust is much thinner than they are for the rest of the shelf. That region is known to have moved to its present prosition from a point several hundred kilometres to the north during the Caledonian orogeny.
Surface wave group velocities within each of the two regions are strongly influenced by sediments which have accumulated in basins within the Barents shelf. Some of these basins, in the southern portion of the shelf, may be 10km or more in thickness.  相似文献   

18.
Blockage of regional seismic waves by the Teisseyre-Tornquist zone   总被引：1，自引：0，他引：1  
During the Group of Scientific Experts Technical Test (GSETT, second experiment, 22 April-2 June 1991), several hundred seismic events were located in Europe. Associating these events with the detecting stations-altogether 28 European stations including seven arrays participated in the GSETT-2 experiment-clearly shows that the Teisseyre-Tornquist Zone (TTZ) influences the propagation of regional seismic phases. Large explosions in the Bay of Gdańsk, for example, were observed by the well-established Scandinavian arrays'NORSAR (Δ 830km) and ARCESS (Δ 1650km), but not by the Polish station KSP (Δ 470km) nor by the new highly sensitive GERESS array (Δ 750km), both situated south-west of the TTZ. For events in central Europe with comparable magnitudes, we observe a similar increase of the detection threshold at stations located north-east of the TTZ in Scandinavia. to explain these observations, the wave propagation of Pn and Pg perpendicular to the TTZ was modelled for a profile from the Estonian/Russian border region to GERESS with Gaussian-beam seismograms. Published crustal and uppermost mantle models for Poland and for Europe were used as a starting point for developing a model of the TTZ. the observations cannot be explained only by a graben-like crustal structure with a jump in Moho depth from 30km to 50km. to defocus the seismic energy, the TTZ as a structural anomaly between eastern and western Europe must reach down into the upper mantle to a depth of at least about 200 km. the proposed model has such a deep-reaching root of the TTZ.  相似文献   

19.
Seismic amplitude tomography for crustal attenuation beneath China   总被引：1，自引：0，他引：1  
Amplitude tomography reconstructs seismic attenuation directly from recorded wave amplitudes. We have applied the tomography to amplitude data reported in the 'Annual Bulletin of Chinese Earthquakes' and interpreted the regionally varying crustal attenuation in terms of tectonics. The seismic amplitudes were originally recorded for determining the M L and M S magnitudes. They generally correspond to the maximum amplitudes of the horizontal components of the short-period S waves and intermediate-period Rayleigh waves. Both sets of measurements are sensitive to crustal structure. The peak amplitudes from M L amplitudes spread spherically with significant dispersion and scattering. M S amplitudes show cylindrical spreading with little dispersion. Average crustal Q values for attenuation at 1 Hz are 737 and 505 for M L and M S, respectively, with substantial regional variations. Frequency dependence in the attenuation is also indicated. Regions with the lowest attenuation (high Q values) are beneath the south China Block, Sichuan Basin, Ordos Platform, the Daxinganling and the Korea Craton. These tend to be tectonically inactive regions, which are generally dominated by intrusive and cratonic rocks in the upper crust. Regions with the highest attenuation (low Q values) are beneath Bohai Basin, Yunnan, eastern Songpan-Ganzi Terrain, margins of the Ordos platform and the Qilian Shan. These are predominantly active basins, grabens and fold belts. The continental margin also highly attenuates both S and surface waves.  相似文献   

20.
A methodology is proposed for the quantification of volcanic explosions based on three parameters derived from broad-band seismic signals: the counter force of the eruption F , the power of the explosion P and the duration of the upward movement of the gas slug in the conduit to the free surface of magma, D . This methodology was applied to the 2004–2005 sequence of explosions at Volcán de Colima, Mexico. The broad-band records of more than 100 explosive events were obtained at a distance of 4 km from the crater. We determined the counter force of the eruption by modelling the low-frequency impulse of the seismic records of 66 volcanic explosions and estimated the power of 116 explosions from the spectra of the high-frequency impulse. The power of Colima explosions spans five orders of magnitude; the counter force spans four orders of magnitude. We show that the power of a volcanic explosion is proportional to the counter force of the eruption. These parameters may be used for the elaboration of a scale of volcanic explosions.  相似文献   

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