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1.
—?In this paper we describe a technique for mapping the lateral variation of Lg characteristics such as Lg blockage, efficient Lg propagation, and regions of very high attenuation in the Middle East, North Africa, Europe and the Mediterranean regions. Lg is used in a variety of seismological applications from magnitude estimation to identification of nuclear explosions for monitoring compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT). These applications can give significantly biased results if the Lg phase is reduced or blocked by discontinuous structure or thin crust. Mapping these structures using quantitative techniques for determining Lg amplitude attenuation can break down when the phase is below background noise. In such cases Lg blockage and inefficient propagation zones are often mapped out by hand. With our approach, we attempt to visually simplify this information by imaging crustal structure anomalies that significantly diminish the amplitude of Lg. The visualization of such anomalies is achieved by defining a grid of cells that covers the entire region of interest. We trace Lg rays for each event/station pair, which is simply the great circle path, and attribute to each cell a value equal to the maximum value of the Lg/P-coda amplitude ratio for all paths traversing that particular cell. The resulting map, from this empirical approach, is easily interpreted in terms of crustal structure and can successfully image small blockage features often missed by analysis of raypaths alone. This map can then be used to screen out events with blocked Lg prior to performing Q tomography, and to avoid using Lg-based methods of event identification for the CTBT in regions where they cannot work.¶For this study we applied our technique to one of the most tectonically complex regions on the earth. Nearly 9000 earthquake/station raypaths, traversing the vast region comprised of the Middle East, Mediterranean, Southern Europe and Northern Africa, have been analyzed. We measured the amplitude of Lg relative to the P-coda and mapped the lateral variation of Lg propagation efficiency. With the relatively dense coverage provided by the numerous crossing paths we are able to map out the pattern of crustal heterogeneity that gives rise to the observed character of Lg propagation. We observe that the propagation characteristics of Lg within the region of interest are very complicated but are readily correlated with the different tectonic environments within the region. For example, clear strong Lg arrivals are observed for paths crossing the stable continental interiors of Northern Africa and the Arabian Shield. In contrast, weakened to absent Lg is observed for paths crossing much of the Middle East, and Lg is absent for paths traversing the Mediterranean. Regions that block Lg transmission within the Middle East are very localized and include the Caspian Sea, the Iranian Plateau and the Red Sea. Resolution is variable throughout the region and strongly depends on the distribution of seismicity and recording stations. Lg propagation is best resolved within the Middle East where regions of crustal heterogeneity on the order of 100?km are imaged (e.g., South Caspian Sea and Red Sea). Crustal heterogeneity is resolvable but is poorest in seismically quiescent Northern Africa. 相似文献
2.
Yu. F. Kopnichev I. N. Sokolova K. N. Sokolov 《Izvestiya Physics of the Solid Earth》2013,49(6):786-795
The characteristics of the attenuation field of short-period shear waves in the region of Nevada nuclear test site (NNTS) are studied. The seismograms of underground nuclear explosions (UNEs) and earthquakes recorded by three seismic stations in 1975–2012 at the epicentral distances of up to 1000 km are processed by the methods based on the analysis of the amplitude ratios of Sn to Pn and Lg to Pg waves, as well as the S-coda envelopes for close events. It is shown that the structure of the attenuation field in the Earth’s crust and upper mantle in the NNTS region experienced significant temporal variations during the interval of nuclear operations. The strongest variations were associated with UNEs conducted in the Pahute Mesa area, which held about two-thirds of the most intense explosions. Our data indicate that temporal variations in the structure of the attenuation field are related to the migration of deep fluids. A comparison of the general characteristics of the attenuation field in the regions of the three large nuclear test sites is presented. 相似文献
3.
v--vS/P amplitude ratios have proven to be a valuable discriminant in support of monitoring a Comprehensive Nuclear Test Ban Treaty. Regional S and P phases attenuate at different rates and the attenuation can vary geographically. Therefore, calibration is needed to apply the S/P discriminant in new regions. Calibration includes application of frequency-dependent source and distance corrections for regional Pn, Pg, Sn, and Lg phases.¶Jenkins et al. (1998) developed Pn, Pg, Sn, and Lg amplitude models for nine geographic regions and two global composite models, stable and tectonic. They determined frequency-dependent source and attenuation corrections from a large data set obtained from the Prototype International Data Center (PIDC). We use their corrections to evaluate calibrated S/P discriminants.¶Our discrimination data set includes >1000 amplitude ratios from earthquakes, industrial explosions, chemical explosions, and nuclear explosions from Lop Nor, India and Pakistan. We find that the calibrated S/P ratio is largest for earthquakes and smallest for the nuclear explosions, as expected. However, the discriminant is not universally valid. In particular, the S/P ratio for the Pakistan nuclear explosion fell within the normal range for the earthquakes. This event was recorded by only a few stations at far-regional distances and appears to have an anomalously high Sn amplitude. The industrial explosions overlap with the earthquake population, however the buried chemical explosions generally register lower S/P ratio than earthquakes. 相似文献
4.
N.N. Belyashova V.I. Shacilov N.N. Mikhailova I.I. Komarov Z.I. Sinyova A.V. Belyashov M.N. Malakhova 《Pure and Applied Geophysics》2001,158(1-2):193-209
—?Two chemical calibration explosions, conducted at the former Semipalatinsk nuclear test site in 1998 with charges of 25 tons and 100 tons TNT, have been used for developing travel-time curves and generalized one-dimensional velocity models of the crust and upper mantle of the platform region of Kazakhstan. The explosions were recorded by a number of digital seismic stations, located in Kazakhstan at distances ranging from 0 to 720?km. The travel-time tables developed in this paper cover the phases P, Pn, Pg, S, Sn, Lg in a range of 0–740?km and the velocity models apply to the crust down to 44?km depth and to the mantle down to 120?km. A comparison of the compiled travel-time tables with existing travel-time tables of CSE and IASPEI91 is presented. 相似文献
5.
D. R. Baumgardt 《Pure and Applied Geophysics》2001,158(7):1207-1250
v--vThe phenomenon of "Lg blockage," where Lg is strongly attenuated by crustal heterogeneities, poses a serious problem to CTBT monitoring because Lg is an important seismic phase for discrimination. This paper examines blockage in three continental regions where the Lg blockages may be caused by large, enclosed sedimentary basins along the propagation path. The Barents Sea Basin blocks Lg propagation across the Barents Sea from the Russian nuclear test sites at Novaya Zemlya to Scandinavian stations. Also, "early Lg" waves are observed in Sn codas on NORSAR, NORESS, and ARCESS recordings of Novaya Zemlya explosions where direct Lg is blocked. Early Lg waves may have resulted from Sn-to-Lg mode conversion at the contact between the Barents Basin and the Kola Peninsula. The Northern and Southern Caspian Sea Basins also block Lg waves from PNEs and earthquakes, perhaps due to thick, low-velocity, low-Q sediments replacing the granitic layer rocks in the crust. Lg blockage has also been observed in the Western Mediterranean/Levantine Basin due to low-Q sediments and crustal thinning. A "basin capture" model is proposed to explain Lg blockage in sedimentary basins. In this model, shear waves that reverberate in the crust and constitute the Lg wave train are captured, delayed, and attenuated by thick, low-velocity sediments that replace the "granitic" layer rocks of the upper crust along part of the propagation path. Sn waves, which propagate below the basin, would not be blocked and in fact, the blocked Lg waves may be diverted downward into Sn waves by the low velocity sediments in the basin. 相似文献
6.
Using seismic data from regional earthquakes in Tibet recorded by the Hi-CLIMB experiment, Pn attributes are used to constrain the velocity gradient and attenuation structure of the Tibetan lithosphere under the Hi-CLIMB array. Numerical modeling is performed using the spectral-element method (SEM) for laterally varying upper-mantle velocity and attenuation, and the seismic attributes considered include the Pn travel-time, envelope amplitude, and pulse frequency. The results from the SEM modeling provide two alternative models for the upper-mantle beneath the Hi-CLIMB array in Tibet. The first model is derived from the 3D velocity model of Griffin et al. (Bull Seism Soc Am 101:1938–1947, 2011) with a constant upper-mantle velocity gradient, and laterally varying upper mantle attenuation. The second model has a laterally varying upper-mantle velocity gradient, and constant upper-mantle attenuation. In both cases, the Qiangtang terrane is distinguished from the Lhasa terrane by a change in Moho depth and upper-mantle velocities. The lower upper-mantle velocities, as well as higher Pn attenuation, suggest hotter temperatures beneath the Qiangtang terrane as compared to the Lhasa terrane. Although the fits to the Pn amplitude and pulse frequency data are comparable between the two models, the first model with the constant upper-mantle velocity gradient fits the travel times somewhat better in relation to the data errors. 相似文献
7.
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
8.
The Korean Peninsula is located in the far-eastern Eurasian plate margin where crustal structures vary laterally, causing significant raypath-dependent modulations of crustal phases. The discriminative variations of crustal phases hinder application of conventional local magnitude scales in the continental margin. The mantle-lid phase is less affected by the crustal structures than the crustal phases, providing a better constraint to magnitude estimation. A regional body-wave magnitude scale based on the mantle-lid P wave (Pn), m b(Pn),?is developed for regional events around the Korean Peninsula. The m b(Pn) scale is determined to be m b(Pn)?=?0.380 (±0.299)?+?log A?+?2.012 (±0.122) log d, where A is the peak-to-peak Pn amplitude in?μm and d is the epicentral distance in km. The m b(Pn) estimates of regional events around the Korean Peninsula are determined. The m b(Pn) estimates are compared with other available magnitude estimates (m b(Lg),?M L). The influence of structures beneath stations on Pn amplification is investigated from inter-station magnitude residuals. A characteristic spatial variation of inter-station magnitude residuals with strengths mostly between ?6 and 6 %, but with maximum strengths of?±10 %, is observed. The inter-station magnitude residuals appears to be correlated well with geological and seismic structures in the crust. 相似文献
9.
—?We present a hybrid boundary-element (BE) and generalized screen propagator (GSP) method for the 2-D SH problem to model the combined effects of arbitrarily irregular topography, large-scale crustal variation, and the associated small-scale heterogeneities on regional wave propagation. We develop a boundary connection technique to couple the wave fields calculated by the BE method with those of the GSP method. Its validity is tested by numerical experiments. For a long crustal waveguide, the relatively short sections with severe surface topography can be modeled by the time-consuming BE method to high frequencies, and the exterior field in the relatively weak heterogeneous media of large volume can be calculated by the GSP method. For the waveguide with severe topography, the BE method can be used section by section via the boundary connection technique to model the combined effects of rough topography and large-scale structural variation on Lg wave propagation at extended regional distances.¶Numerical comparisons with independent methods showed that the hybrid method is relatively accurate for Lg simulation. We apply the hybrid method to Lg wave propagation in two real crustal waveguides in the Tibet region; one with Lg blockage and another without blockage. We found that the most characteristic effect from the irregular topography is the strong scattering by the topographic structures. The scattering by local irregular topographies leads to anomalous near-receive effects and tends to remove energy from the guided waves, which causes decay of amplitude and waveform distortion. It can be expected that rough surface topography and random heterogeneities with scale length close to the dominant wavelength will be very efficient in attenuating regional waves. The dramatic lateral variation of the topography-Moho large-scale structure combined with the small-scale rough topography and random heterogeneities could be the cause of Lg anomalous attenuation and blockage observed in this region. More quantitative assessment of the topographic effects must be conducted in the future. 相似文献
10.
To investigate the nature of Pn propagation, we have implemented the spectral-element method (SEM) for vertically and laterally varying media with and without attenuation. As a practical measure, essential features of the Pn waves are distilled into seismic attributes including arrival times, amplitudes and pulse frequencies. To validate the SEM simulations, we first compare the SEM results with reflectivity calculations of Braile and Smith (Geophys. J.R. Astr. Soc. 40, 145–176, 1975) and then to the asymptotic results of ?erveny and Ravindra (Theory of Seismic Headwaves, University of Toronto Press, pp. 235–250, 1971). Models with random, laterally varying Moho structures are then simulated, where the amplitude and pulse frequency characteristics are found to be stable to small Moho interface perturbations. SEM calculations for models with different upper-mantle velocity gradients are next performed where it is found that interference effects can strongly influence the Pn amplitudes and pulses frequencies. For larger-scale, laterally varying structures, SEM models similar to that found along the Hi-CLIMB array in Tibet are then performed. It is observed that large-scale structures, along with small-scale structures, upper-mantle velocity gradients and attenuation, can all significantly affect the Pn attributes. Ambiguities between upper-mantle velocity gradients and attenuation are also found when using Pn amplitudes and pulse frequency attributes. These ambiguities may be resolved, to some degree, by using the curvature of the travel times at longer regional distance, however, this would also be complicated by lateral variability. 相似文献
11.
W. A. Woodward S. R. Sain H. L. Gray B. Zhao M. D. Fisk 《Pure and Applied Geophysics》2002,159(4):889-903
—?We consider the problem of multivariate outlier testing for purposes of distinguishing seismic signals of underground nuclear events from training samples based on non-nuclear seismic events when certain data are missing. We consider the case in which the training data follow a multivariate normal distribution. Assume a potential outlier is observed on which k features of interest are measured. Assume further that the available training set of n observations on these k features is available but that some of the observations in the training data have missing features. The approach currently used in practice is to perform the outlier testing using a generalized likelihood ratio test procedure based only on the data vectors in the training data with complete data. When there is a substantial amount of missing data within the training set, use of this strategy may lead to a loss of valuable information. An alternative procedure is to incorporate all n of the data vectors in the training data using the EM algorithm to appropriately handle the missing data in the training set. Resampling methods are used to find appropriate critical regions. We use simulation results and analysis of models fit to Pg/Lg ratios for the WMQ station in China to compare these two strategies for dealing with missing data. 相似文献
12.
We propose a novel Lg attenuation tomography model (QLg tomography) for the state of Gujarat, Western India, using earthquake data recorded by the Gujarat Seismic Network, operated by the Institute of Seismological Research in Gandhinagar. The waveform dataset consist of 400 3-component recordings, produced by 60 earthquakes with magnitude (ML) spanning from 3.6 to 5.1, recorded at 60 seismic stations having epicentral distances spanning between 200 and 500 km. Spectral amplitude decays for Lg wave displacement were obtained by generalized inversion at 17 frequencies spanning between 0.9 and 9 Hz. Lg wave propagation efficiency was measured by Lg/Pn spectral ratio categorizing as efficient ratio ≥6 for 86%, intermediate ratio of 3–6 for 10% and inefficient ratio <3 for 4% paths of total 400 ray paths. The earthquake size and quality of waveform recorded at dense network found sufficient to resolve lateral variation of QLg in Gujarat.Average power-law attenuation relationship obtained for Gujarat as QLg(f) = 234f0.64, which corresponds to high attenuation in comparison to peninsular India shield region and other several regions around the world. QLg tomography resolves the highly attenuating crust of extremely fractured Saurashtra region and tectonically active Kachchh region. The Gujarat average attenuation is also lying in between them. The low attenuation in Cambay and Narmada rift basins and extremely low attenuation in patch of Surendranagar area is identified. This study is the first attempt and can be utilized as pivotal criteria for scenario hazard assessment, as maximum hazard has been reported in highly attenuating tectonically active Kachchh region and in low attenuating Cambay, Narmada and Surendranagar regions. The site and source terms are also obtained along with the QLg inversion. The estimated site responses are comparable with observed local geological condition and agree with the previously reported site amplifications at the same sites. The source terms are comparable with local magnitude estimated from Network. The Mw (Lg) is nearly equivalent to ML (GSN) and the slight differences are noted for larger magnitude events. 相似文献
13.
Markus Båth 《Pure and Applied Geophysics》1966,64(1):19-30
Summary
Sn andPn waves propagated to teleseismic distances are investigated by means of short-period seismograph records of the Swedish network.Sn is found in the distance range of 2400 to 4600 km andPn in the range 3500 to 3900 km, but only provided the path is exceptionally homogeneous. Almost all paths are restricted to the Russian platform. There are probably very few areas in the world offering similar propagation paths. The velocities just under the Mohorovii discontinuity are found to be 4.72 km/sec and 8.26 km/sec for transverse and longitudinal waves respectively. In addition, other properties of the teleseismicSn andPn are investigated, such as periods, dispersion, amplitudes, particle motions, propagation mechanisms, and comparisons are made withPa, Sa, withLi, Lg1, Lg2 and withP. The fact that teleseismicPn occurs much more seldom than teleseismicSn could be explained by different velocity profiles just under the crust. 相似文献
14.
H. J. Patton 《Pure and Applied Geophysics》2001,158(11):1951-2015
—?Data sets of m b (Pn) and m b (Lg) measurements are presented for three continental regions in order to investigate scaling relationships with moment magnitude M w and event discrimination at small magnitudes. Compilations of published measurements are provided for eastern North American and central Asian earthquakes, and new measurements are reported for earthquakes located in western United States. Statistical tests on M w :m b relationships show that the m b (Lg) scale of Nuttli (1973) is transportable between tectonic regions, and a single, unified M w :m b (Lg) relationship satisfies observations for M w ～4.2–6.5 in all regions. A unified relationship is also developed for nuclear explosions detonated at the Nevada Test Site and test sites of the former Soviet Union. Regional m b for explosions scale at higher rates than for earthquakes, and of significance is the finding that m b (Pn) for explosions scales at a higher rate than m b (Lg). A model is proposed where differences in scaling rates are related to effects of spectral overshoot and near-field Rg scattering on the generation of Pn and Lg waves by explosions. For earthquakes, m b (Pn) and m b (Lg) scale similarly, showing rates near 1.0 or 2/3?·?log10 M o (seismic moment).¶M w :m b (Lg) scaling results are converted to unified M s :m b (Lg) relationships using scaling laws between log M o and M s . For earthquakes with M s greater than 3.0, the scaling rate is 0.69?·?M s , which is the same as it is for nuclear explosions if M s is proportional to 1.12?·?log M o, as determined by NTS observations. Thus, earthquake and explosion populations are parallel and separated by 0.68 m b units for large events. For small events (M s ?M s :m b (Lg) plots for stable and tectonic regions, respectively. While the scaling rate for explosions is ～0.69, this value is uncertain due to paucity of M o observations at small yields. Measurements of [m b (P)???m b (Lg)] for earthquakes in the western United States have an average value of ?0.33?±?.03 m b units, in good agreement with Nuttli's estimate of m b bias for NTS. This result suggests that Nuttli's method for estimating test site bias can be extended to earthquakes to make estimates of bias on regional scales. In addition, a new approach for quick assessments of regional bias is proposed where M s :m b (P) observations are compared with M s :m b (Lg) relationships. Catalog M s :m b (P) data suggest that m b bias is significant for tectonic regions of southern Asia, averaging about ?0.4 m b units. 相似文献
15.
Igor B. Morozov 《Journal of Seismology》2010,14(4):803-822
Analysis of the frequency dependence of the attenuation coefficient leads to significant changes in interpretation of seismic attenuation data. Here, several published surface-wave attenuation studies are revisited from a uniform viewpoint of the temporal attenuation coefficient, denoted by χ. Theoretically, χ( f) is expected to be linear in frequency, with a generally non-zero intercept γ?=?χ(0) related to the variations of geometrical spreading, and slope dχ/df = π/Q e caused by the effective attenuation of the medium. This phenomenological model allows a simple classification of χ( f) dependences as combinations of linear segments within several frequency bands. Such linear patterns are indeed observed for Rayleigh waves at 500–100-s and 100–10-s periods, and also for Lg from ~2 s to ~1.5 Hz. The Lg χ( f) branch overlaps with similar linear branches of body, Pn, and coda waves, which were described earlier and extend to ~100 Hz. For surface waves shorter than ~100 s, γ values recorded in areas of stable and active tectonics are separated by the levels of \(\gamma _{D} \approx 0.2 \times 10^{-3}\) s???1 (for Rayleigh waves) and 8 ×10???3 s???1 (for Lg). The recently recognized discrepancy between the values of Q measured from long-period surface waves and normal-mode oscillations could also be explained by a slight positive bias in the geometrical spreading of surface waves. Similarly to the apparent χ, the corresponding linear variation with frequency is inferred for the intrinsic attenuation coefficient, χ i , which combines the effects of geometrical spreading and dissipation within the medium. Frequency-dependent rheological or scattering Q is not required for explaining any of the attenuation observations considered in this study. The often-interpreted increase of Q with frequency may be apparent and caused by using the Q-based model of attenuation and following preferred Q( f) dependences while ignoring the true χ( f) trends within the individual frequency bands. 相似文献
16.
《Journal of Geodynamics》1999,27(4-5):451-468
The seismic activity of the Sinai subplate region on the basis of both historical (2200B.C.–1900 A.D.) and recent (1900–1995) earthquake catalogs have been evaluated.Moderateand large earthquakes occurred mainly at the subplate boundaries, Dead Sea Fault (DSF) systemin the east, Cyprean arc in the north, and Suez rift in the southwest.Along the Dead Sea Fault system the activity concentrated at the southern andcentralsegments. The earthquake distribution appears to have a tendency to cluster in time andspace.The swarms (February, 1983; April, 1990; August, 1993 and November, 1995) in the GulfofAqaba indicate that the southern segment of the Dead Sea Fault system is the mostseismogenicthrough the last two decades. North of the Dead Sea depression the seismic activitytends to haveoccurred with NW trend to extend under the Levantine Sea. Although the northernsegment ofthe Dead Sea Fault system is well defined from geological, geophysical and historicalearthquakeactivity recent seismic activity is practically absent especially north of Latitude 34°N.In the eastern Mediterranean the seismicity is much higher in the area of the Hellenicarcthan in the Cyprean arc. Moreover, the activity occurs in a wide belt suggesting that theplateboundary is a deformation zone instead of a single line.The seismic activity in the Gulf of Suez is scattered and does not have any distincttrend.However, three active zones are delineated. At the mouth of the gulf most of activityisconcentrated where the Sinai triple junction (Africa, Arabia, Sinai) is situated. The centralpartand the northern part of the gulf include the adjacent area as far as the river Nile. Actually,theactivity is markedly decreased from south to north.Although there is no seismological evidence that the Suez rift continues into theeasternMediterranean, the activity in the Gulf of Suez region cannot be ignored.The parameters of magnitude-frequency relation (a, b) indicate thatthelevel of earthquake activity in the Sinai subplate region is generally moderate. Moreover,theenergy release curve shows a regular trend and reflects occasional high activity. © 1999ElsevierScience Ltd. All rights reserved. 相似文献
17.
We use the recorded seismicity, confined to the Dead Sea basin and its boundaries, by the Dead Sea Integrated Research (DESIRE) portable seismic network and the Israel and Jordan permanent seismic networks for studying the mechanisms of earthquakes in the Dead Sea basin. The observed seismicity in the Dead Sea basin is divided into nine regions according to the spatial distribution of the earthquakes and the known tectonic features. The large number of recording stations and the adequate station distribution allowed the reliable determinations of 494 earthquake focal mechanisms. For each region, based on the inversion of the observed polarities of the earthquakes, we determine the focal mechanisms and the associated stress tensor. For 159 earthquakes, out of the 494 focal mechanisms, we could determine compatible fault planes. On the eastern side, the focal mechanisms are mainly strike-slip mechanism with nodal planes in the N-S and E-W directions. The azimuths of the stress axes are well constrained presenting minimal variability in the inversion of the data, which is in agreement with the Eastern Boundary fault on the east side of the Dead Sea basin and what we had expected from the regional geodynamics. However, larger variabilities of the azimuthal and dip angles are observed on the western side of the basin. Due to the wider range of azimuths of the fault planes, we observe the switching of σ1 and σ2 or the switching of σ2 and σ3 as major horizontal stress directions. This observed switching of stress axes allows having dip-slip and normal mechanisms in a region that is dominated by strike-slip motion. 相似文献
18.
Russian Federation/ United States Calibration Working Group 《Pure and Applied Geophysics》2001,158(1-2):59-77
—?Joint Research Program of Seismic Calibration of the International Monitoring System (IMS) in Northern Eurasia and North America has been signed by the Nuclear Treaty Programs Office (NTPO), Department of Defense USA, and the Special Monitoring Service (SMS) of the Ministry of Defense, Russian Federation (RF). Under the Program historical data from nuclear and large chemical explosions of known location and shot time, together with appropriate geological and geophysical data, has been used to derive regional Pn/P travel-time tables for seismic event location within the lower 48 States of the USA and the European part of the RF. These travel-time tables are up to 5?seconds faster in shields than the IASPEI91 tables, and up to 5?seconds slower in the Western USA. Relocation experiments using the regional Pn travel-time curves and surrogate networks for the IMS network generally improved locations for regional seismic events. The distance between true and estimated location (mislocation) was decreased from an average of 18.8?km for the IASPEI91 tables to 10.1?km for the regional Pn travel-time tables. However, the regional travel-time table approach has limitations caused by travel-time variations inside major tectonic provinces and paths crossing several tectonic provinces with substantially different crustal and upper mantle velocity structure.¶The RF members of the Calibration Working Group (WG): Colonel Vyacheslav Gordon (chairman); Dr. Prof. Marat Mamsurov, and Dr. Nikolai Vasiliev. The US members of the WG: Dr. Anton Dainty (chairman), Dr. Douglas Baumgardt, Mr. John Murphy, Dr. Robert North, and Dr. Vladislav Ryaboy. 相似文献
19.
K. Koch 《Pure and Applied Geophysics》2002,159(4):759-778
20.
Sakir Sahin Xueyang Bao Niyazi Turkelli Eric Sandvol Ugur Teoman Metin Kahraman 《Pure and Applied Geophysics》2013,170(3):337-351
We estimate Lg wave attenuation using local and regional seismic phases in the Isparta Angle and the Anatolian Plateau (Turkey). The Isparta Angle (IA) is a tectonically active zone forming the boundary between the African Plate and the Anatolian Plateau, and is currently undergoing N–S extensional deformation. The Anatolian Plateau contains many intra-continental faults including the North Anatolian Fault Zone and the East Anatolian Fault Zone as well as the Menderes Massif. A large waveform data set was compiled from a variety of local and regional seismic networks including 121 digital seismic stations (broad-band and short period) between 1999 and 2008 spanning the IA, the Anatolian Plateau and Azerbaijan. The data set was used to determine the nature of Lg wave propagation and characterize the nature of seismic attenuation within the crust of these regions. Lg waveforms were used to calculate the frequency-dependent Lg-Q o and Lg- $ \eta $ . A wide range of Lg-Q o values was obtained between ~52 ± 6 and 524 ± 227. Low Lg-Q o values (~90–155) are calculated towards the north of IA, Iskenderun Gulf and its vicinity, Bingöl-Karl?ova, Izmit and its vicinity. Lg-Q o values are especially low (<90) along the Menderes Massif and the Aksehir-Simav Fault Zones. This may be due to intrinsic attenuation of Lg associated with the partially molten crust and young volcanism. The high Lg-Q o values (~350) are probably caused by the crust not being subject to large amounts of extensional deformation like the Antalya Gulf and apparently being thick enough to support Lg propagation. Relatively higher values along the border of this subduction zone and plate boundary might be related to the Taurus Mountain belts and Bitlis-Zagros Suture Zone. The lateral frequency dependency Lg- $ \eta $ is also consistent with high tectonic activity in this region. 相似文献