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1.
A new 3D velocity model of the crust and upper mantle in the southeastern (SE) margin of the Tibetan plateau was obtained by joint inversion of body- and surface-wave data. For the body-wave data, we used 7190 events recorded by 102 stations in the SE margin of the Tibetan plateau. The surface-wave data consist of Rayleigh wave phase velocity dispersion curves obtained from ambient noise cross-correlation analysis recorded by a dense array in the SE margin of the Tibetan plateau. The joint inversion clearly improves the v S model because it is constrained by both data types. The results show that at around 10 km depth there are two low-velocity anomalies embedded within three high-velocity bodies along the Longmenshan fault system. These high-velocity bodies correspond well with the Precambrian massifs, and the two located to the northeast of 2013 M S 7.0 Lushan earthquake are associated with high fault slip areas during the 2008 Wenchuan earthquake. The aftershock gap between 2013 Lushan earthquake and 2008 Wenchuan earthquake is associated with low-velocity anomalies, which also acts as a barrier zone for ruptures of two earthquakes. Generally large earthquakes (M ≥ 5) in the region occurring from 2008 to 2015 are located around the high-velocity zones, indicating that they may act as asperities for these large earthquakes. Joint inversion results also clearly show that there exist low-velocity or weak zones in the mid-lower crust, which are not evenly distributed beneath the SE margin of Tibetan plateau.  相似文献   

2.
The tsunami caused by the 2007 Peru earthquake (Mw 8.0) provoked less damage than by the seismic shaking itself (numerous casualties due to the earthquake in the vicinity of Pisco). However, it propagated across the Pacific Ocean and small waves were observed on one tide gauge in Taiohae Bay (Nuku Hiva, Marquesas, French Polynesia). We invert seismological data to recover the rupture pattern in two steps. The first step uses surface waves to find a solution for the moment tensor, and the second step uses body waves to compute the slip distribution in the source area. We find the slip distribution to consist of two main slip patches in the source area. The inversion of surface waves yields a scalar moment of 8.9 1020 Nm, and body-wave inversion gives 1.4 1021 Nm. The inversion of tsunami data recorded on a single deep ocean sensor also can be used to compute a fault slip pattern (yielding a scalar moment of 1.1 1021 Nm). We then use these different sources to model the tsunami propagation across the Pacific Ocean, especially towards Nuku Hiva. While the source model taken from the body-wave inversion yields computed tsunami waves systematically too low with respect to observations (on the central Pacific Ocean DART buoy as on the Polynesian tide gauge), the source model established from the surface-wave inversion is more efficient to fit the observations, confirming that the tsunami is sensitive to the low frequency component of the source. Finally we also discuss the modeling of the late tsunami arrivals in Taiohae Bay using several friction coefficients for the sea bottom.  相似文献   

3.
The dispersion that must accompany absorption is taken into account in many recent body-wave investigations but has been largely ignored in surface-wave and free-oscillation studies. In order to compare body-wave and free-oscillation data a correction must be made to travel times or periods to account for absorption-related physical dispersion. The correction depends on the frequency and Q of the data and can be as high as 1% which is much larger than the uncertainty of the raw data. Corrected toroidal mode data is inverted to obtain shear velocity and density versus depth. The average shear velocity in the upper 600 km is ～2% greater than obtained from the uncorrected data. The resulting shear-wave travel times oscillate about the Jeffreys-Bullen values with an average baseline of only +0.5 second. Thus, the discrepancy between body-wave and free-oscillation studies is eliminated.  相似文献   

4.
地震各向异性是反映地球内部介质特性的重要指针之一。常用的横波分裂法和二维面波方位各向异性层析成像方法很难准确反映各向异性随深度的变化。将与周期相关的区域化面波方位各向异性转换成与深度相关的一维横波速度方位各向异性可以弥补深度信息不足的缺陷。现有三维横波速度各向异性研究多是通过两步方法来实现的,即逐个周期二维面波方位各向异性层析成像以及逐个格点一维横波速度方位各向异性反演。这种分步反演的方式既不利于三维先验约束的引入,也不利于利用原始观测拟合误差对三维模型进行直接评估。因此本文开发了基于面波频散曲线的三维横波速度方位各向异性层析成像方法,并编制了相关正演和反演程序。为了检测方法和程序的有效性,我们对规律分布的三维检测板模型进行了模拟测试。测试结果显示:该方法可以很好地恢复各向同性波速异常、各向异性相对强度和快波方向等三维结构信息;而且反演模型相对于参考模型明显改善了对观测数据的拟合,降低了对观测数据的均方根误差。但对各向同性理论模型进行各向异性反演时,在波速均匀区可产生小于0.5%的假各向异性幅值,在波速非均匀区该假的各向异性幅值会更大,浅部可达3.5%。因此在实际应用中需要谨慎解释（浅部）非均匀区的各向异性结果。   相似文献   

5.
The particle size distributions of fault gouge from the San Andreas, the San Gabriel, and the Lopez Canyon faults in Southern California were measured using sieving and Coulter-Counter techniques over a range of particle sizes from 2 m to 16 mm. The distributions were found to be power law (fractal) for the smaller fragments and log-normal by mass for sizes near and above the peak size. The apparent fractal dimensionD of the smaller particles in gouge samples from the San Andreas fault, the San Gabriel fault and the Lopez Canyon gouge were 2.4–3.6, 2.6–2.9 and 2.4–3.0, respectively. The averageD for the Lopez Canyon gouge was 2.7±0.2, which is in agreement with earlier studies of this gouge using planar 2-D sections. The fractal dimension of the finer fragments from all three faults is observed to be correlated with the peak fragment size, with finer gouges tending to have a largerD. A computer automaton is used to show that this observation may be explained as resulting from a fragmentation process which has a grinding limit at which particle reduction stops.  相似文献   

6.
Phase-equilibrium studies of the nepheline normative portion of the basalt tetrahedron nepheline-forsterite-silica-diopside ofYoder andTilley have shown that during the course of crystallization the composition of the liquid phase leaves this tetrahedron. When the tetrahedron is expanded to nepheline-forsterite-silica-Ca2SiO4 the courses of crystallization and composition of the liquid can be and have been followed by studying a series of joins within this expanded tetrahedron. These studies show that the ultimate goal of crystallization is the quaternary invariant point diopsidic pyroxene + nepheline solid solution + sodic plagioclase + wollastonite solid solution + liquid at 950 ± 5°"C. Attention is called to this low melting point in a dry system. With perfect equilibrium between the solid phases and liquid all melilite disappears by reaction with liquid below 1065 ± 5°C, the temperature of the quaternary reaction point with diopsidic pyroxene + melilite + nepheline solid solution + wollastonite solid solution + liquid. A flow sheet showing the reactions between all quaternary invariant points in the geologically interesting portion of the expanded tetrahedron is presented. This shows the interrelations (daughter-parent relations) between a large number of rock types found in intrusive and extrusive nepheline normative compositions as well as in the quartz normative tholeiitic basalts.  相似文献   

7.
Source inversion of small-magnitude events such as aftershocks or mine collapses requires use of relatively high frequency seismic waveforms which are strongly affected by small-scale heterogeneities in the crust. In this study, we developed a new inversion method called gCAP3D for determining general moment tensor of a seismic source using Green's functions of 3D models. It inherits the advantageous features of the “Cut-and-Paste” (CAP) method to break a full seismogram into the Pnl and surface-wave segments and to allow time shift between observed and predicted waveforms. It uses grid search for 5 source parameters (relative strengths of the isotropic and compensated-linear-vector-dipole components and the strike, dip, and rake of the double-couple component) that minimize the waveform misfit. The scalar moment is estimated using the ratio of L2 norms of the data and synthetics. Focal depth can also be determined by repeating the inversion at different depths. We applied gCAP3D to the 2013 Ms 7.0 Lushan earthquake and its aftershocks using a 3D crustal-upper mantle velocity model derived from ambient noise tomography in the region. We first relocated the events using the double-difference method. We then used the finite-differences method and reciprocity principle to calculate Green's functions of the 3D model for 20 permanent broadband seismic stations within 200 km from the source region. We obtained moment tensors of the mainshock and 74 aftershocks ranging from Mw 5.2 to 3.4. The results show that the Lushan earthquake is a reverse faulting at a depth of 13–15 km on a plane dipping 40–47° to N46° W. Most of the aftershocks occurred off the main rupture plane and have similar focal mechanisms to the mainshock's, except in the proximity of the mainshock where the aftershocks' focal mechanisms display some variations.  相似文献   

8.
This paper presents uniformly calculated surface-wave magnitudes of over 400 earthquakes in the European area and adjacent regions associated with strong-motion recordings. Seismic moments and body-wave magnitudes are estimated also, so that they may be used in the derivation of attenuation laws. Using a more complete set of data the attenuation law derived for western North American earthquakes is modified for surface-wave magnitudes.  相似文献   

9.
— A pair of papers in 1976 lead-authored by Kei Aki heralded the beginning of the field of seismic tomography of the lithosphere. The 1976 paper by Aki, Christoffersson, and Husebye introduced a simple and approximate yet elegant technique for using body-wave arrival times from teleseismic earthquakes to infer the three-dimensional (3-D) seismic velocity heterogeneities beneath a seismic array or network (teleseismic tomography). Similarly, a 1976 paper by Aki and Lee presented a method for inferring 3-D structure beneath a seismic network using body-wave arrival times from local earthquakes (local earthquake tomography). Following these landmark papers, many dozens of papers and numerous books have been published presenting exciting applications of and/or innovative improvements to the methods of teleseismic and local earthquake tomography, many by Aki's students.¶This paper presents a brief review of these two types of tomography methods, discussing some of the underlying assumptions and limitations. Thereafter some of the significant methodological developments are traced over the past two and a half decades, and some of the applications of tomography that have reaped the benefits of these developments are highlighted. One focus is on the steady improvement in structural resolution and inference power brought about by the increased number and quality of seismic stations, and in particular the value of utilizing shear waves. The paper concludes by discussing exciting new scientific projects in which seismic tomography will play a major role — the San Andreas Fault Observatory at Depth (SAFOD) and USArray, the initial components of Earthscope.  相似文献   

10.
The Stone Canyon well penetrates 600 m of highly fractured and severely altered quartz diorite intimately associated with the creeping segment of the San Andreas fault of central California. Geophysical logs reveal a complex hydrology dominated by major fractures. Fluid pressure in some fractures is sufficient to prevent invasion of the formation by heavy drilling mud, implying pore pressures at least 10% higher than hydrostatic ones. At least three chemically distinct waters are encountered, including a chloride brine clearly segregated from the shallow, potable groundwater. Chemical alteration of the quartz diorite persists throughout the well, far below the depth where the water-rock reactions responsible for the ubiquitous chlorite and mixed-layer clays can be considered weathering. Whole-rock 18O analyses indicate significant interaction of the rocks with a low 18O fluid within some of the fractured and altered zones, whereas a deeper sample shows18O enrichment. High pore pressures encountered in Stone Canyon may be due to tectonic compression. Measurements of temporal variations in the pore pressure at the well may provide a means of predicting earthquakes along this segment of the San Andreas fault.  相似文献   

11.
In recent years, surface-wave analysis method has been developed rapidly in many fields. Multichannel analysis of surface waves can provide near-surface one-dimensional shear-wave velocity profiles. Because linearized inversion of surface-wave dispersion curves relies heavily on the choice of the initial model, setting an inappropriate initial model can lead to poor inversion results, or even failure of inversion. However, it is difficult to establish a reasonable initial model without a priori information, which is unavailable in most cases. To cope with this problem, a multiscale linearized inversion method is proposed for surface-wave dispersion curves inversion. In contrast with the traditional single-scale linearized inversion, the key idea of the proposed multiscale surface-wave inversion method is the introduction of a merging and splitting process of layers. After every scale inversion, the merging and splitting operations automatically optimize the inversion model, making it gradually approach to a reasonable subsurface stratification. Multiscale surface-wave inversion method reduces the difficulty of establishing the initial model and has high computational efficiency. In addition, it has strong ability to identify high-velocity or low-velocity interlayers and thin layers, especially suited for the geological conditions with obvious stratification. In synthetic tests, the proposed method was compared with the single-scale surface-wave inversion and particle swarm optimization algorithm to demonstrate the effectiveness and practicability of multiscale surface-wave inversion method. We also applied the multiscale surface-wave inversion method to field seismic data acquired in Guizhou, China and Texas, USA. Borehole and crosshole test data were compared with the inversion results of field data to prove the reliability of the proposed method.  相似文献   

12.
Most paleomagnetic applications require a precise, rationally organized and up-todate catalogue or database of paleomagnetic results worldwide. These include reconstructions of continents, calculations of the Apparent Polar Wander Paths (APWPs) or paleolatitude drift curves, testing the Geocentric Axial Dipole (GAD) model, studies of geomagnetic paleosecular variation or reversal asymmetries, comparison of coeval results obtained from different types of rocks, estimation of inclination shallowing in sedimentary rocks and understanding the delay in remanence acquisition caused by slow cooling in large intrusions. For this purpose, various databases, such as the Global Paleomagnetic Database (GPMDB), and the Magnetics Information Consortium Database (MagIC) have been generated. This paper presents a new relational database (PALEOMAGIA) where 3278 entries of Precambrian data have been split geographically, sorted according to age and rock types and ranked using a revised version of the Van der Voo grading scheme. The latest geochronologic information is included wherever available. Significant effort has been put to the retrieval and archiving of data published in the last decade, which are virtually nonexistent in GPMDB. Here we present the database and its browser-based user interface from a scientific and a technical point of view.  相似文献   

13.
This paper presents the results of an investigation of the magnitude–intensity and intensity–attenuation relationships for earthquakes in the Atlas block and Algeria using macroseismic data. This work is based on a selected sample of isoseismal maps from 32 events which were recently revised. Surface-wave magnitudes, Ms, are recalculated using the Prague formula and range from 4·2 to 7·45. Because the Atlas mountains block is in a collision zone, earthquakes occur in general within a layer 15 km deep. Expressions of general form for the magnitude–intensity and intensity–attenuation correlations are adopted and are, respectively, and where R2 = d2 + h2, d the source distance in km, h the focal depth in km, Ms the revised surface-wave magnitude, Msc the predicted surface-wave magnitude, Ii the intensity at isoseismal i, I the predicted intensity, σ the standard deviation and P is zero for 50-percentile values and one for 84-percentile, and the coefficients A's and B's are determined by regression analysis. The results of this study show that the intensity–attenuation models are adequate to predict quite well the die-out of intensity with distance in the Atlas zone and coastal Algeria; it is also found that magnitude can be predicted accurately by calibrating isoseismal radii against revised instrumental surface-wave magnitude. Such magnitude–intensity relationships may be used to evaluate the magnitude of historical earthquakes in the region under survey, with no instrumental data, for which isoseismal radii and intensities are available.  相似文献   

14.
We apply a newly developed numerical method to improve the Moho geometry by the implementation of gravity data. This method utilizes expressions for the gravimetric forward and inverse modeling derived in a frequency domain. Methods for a spectral analysis and synthesis of the gravity field and crust density structures are applied in the gravimetric forward modeling of the consolidated crust-stripped gravity disturbances, which have a maximum correlation with the (a priori) Moho model. These gravity disturbances are obtained from the Earth’s gravity disturbances after applying the topographic and stripping gravity corrections of major known anomalous crust density structures; in the absence of a global mantle model, mantle density heterogeneities are disregarded. The isostatic scheme applied is based on a complete compensation of the crust relative to the upper mantle density. The functional relation is established between the (unknown) Moho depths and the complete crust-stripped isostatic gravity disturbances, which according to the adopted isostatic scheme have (theoretically) a minimum correlation with the Moho geometry. The system of observation equations, which describes the relation between spherical functions of the isostatic gravity field and the Moho geometry, is defined by means of a linearized Fredholm integral equation of the first kind. The Moho depths are determined based on solving the gravimetric inverse problem. The regularization is applied to stabilize the ill-posed solution. This numerical procedure is utilized to determine the Moho depths globally. The gravimetric result is presented and compared with the seismic Moho model. Our gravimetric result has a relatively good agreement with the CRUST2.0 Moho model by means of the RMS of differences (of 3.5 km). However, the gravimetric solution has a systematic bias. We explain this bias between the gravimetric and seismic Moho models by the unmodelled mantle heterogeneities and uncertainties in the CRUST2.0 global crustal model.  相似文献   

15.
Approximately one thousand microearthquakes with body-wave magnitude mb have been located in northern Venezuela and the southern Caribbean region (9–12° N; 64–70° W) since the installation in 1980 of the Venezuelan Seismological Array, together with forty events of mb 4, one of them with surface-wave magnitude Ms 6. Focal depths are in the range of 0 to <15 km. This geologically complex region is part of the boundary between the Caribbean and the South American Plates. Epicentral locations indicate that this E–W oriented portion of the boundary is formed by two 400 km long subparallel fault zones: San Sebastián fault zone (SSF), 20 km north of Caracas along the coast; and La Victoria fault zone (LVF), 25 km south of the city. They are clearly delineated by the microseismicity. New composite focal mechanism solutions (CFMS) along these faults show right-lateral strike-slip (RLSS) motion on nearly E–W oriented fault planes. NW-striking subsidiary active faults occur in the region and intercept the two main E–W fault zones. These interceptions show high levels of microearthquake activity and seismic moment release when compared to other portions of both, the main and subsidiary faults. New CFMS at those fault crossing sites show NW-striking RLSS motion and normal faulting, in an en-echelon-like structural behavior. Geological data and quantitative comparisons with other transcurrent plate boundaries in the world suggest that the rate of plate motion in this area is on the order of 20 mm/y. Several moderate and large shocks have occurred along the SSF and LVF since 1640, including an Ms 7.6 event in 1900 on SSF. Although the region may be relatively far from a repeat of this earthquake, seismicity data indicate that strong shocks could take place along segments of the seismically active faults identified in this study.  相似文献   

16.
Most large earthquakes of magnitude 6.0 in California during 1852–1987 appear to show a southeast-to-northwest tendency of epicenter migration. This finding is consistent with earlier findings ofSavage (1971) for a relatively few large earthquakes along the west coast of North America, and ofWood andAllen (1973) for smaller events along the San Andreas fault in central California. The average speed of migration is approximately 130 km/yr, which is within the range of speeds observed for other major seismic zones in the world. The epicenter migration in California may be the result of some small but broad-scaled episodic strain changes associated with creep waves induced by magma injections at the East Pacific Rise and propagating northwestwardly along a broad transform boundary between the Pacific and North American plates at subseismogenic depths as proposed bySavage (1971).  相似文献   

17.
The 2.1 b.y. old Rooiberg Felsite roofs and is intruded by the mafic layered rocks and granites of the Bushveld Complex. The felsite unit, which locally exceeds 5 km in thickness and may represent an originally erupted volume of more than 300,000 km3, is dominated by rhyolitic to dacitic lavas with minor pyroclastic and sedimentary rock types. Volcanic rocks of more mafic composition occur towards the base of the sequence. The Rooiberg episode essentially terminated the volcanic activity in the Transvaal basin and heralded the emplacement of the Bushveld Complex. Despite the close spatial and temporal relationships between the Rooiberg Felsite and the Bushveld Complex, the precise nature of the petrogenetic link is obscure. Chemical analyses of felsite have been variously interpreted to suggest cyclic differentiation along a comagmatic trend or to demonstrate anomalous enrichment in SiO2. Several characteristics delineate the Rooiberg Felsite as a possibly unique occurrence of rhyolitic magmatism, notably the immense volume of the unit, the marked preponderance of lavas over pyroclastic types, and the unusually great thickness and lateral extent of the flows. The thesis that the Rooiberg Felsite represents a shock-produced, meteorite-impact melt cannot be supported on the available evidence.  相似文献   

18.
Natural hot spring waters ascending rapidly to the surface become supersaturated with respect to quartz because of rapid cooling, separation of steam and sluggish deposition of quartz and other crystallineSiO 2 phases. Large amounts of silica are likely to be deposited in hot spring systems only after the solubility of amorphous silica has been exceeded. Cristobalite and chaleedony probably form in hot spring systems only by the crystallization of previously deposited silica gel rather than by direct deposition from solution. Experimental data indicate that the solubilit of quartz in water rises with increasing temperature along the vapor pressure curve to a maximum value of 725 ppm at 330°C. However, the maximum amount of silica likely, to occur in hot spring systems where quartz precipitates at depth is appreciably greater. Steam formation during adiabatic cooling of a water quickly brought to the surface from 330°C at depth might leave the silica in the remaining liquid concentrated to about 1150 to 1400 ppm. Under such conditions, amorphous silica might precipitate (probably as a colloidal suspension) after the water cooled below about 200°C to 250°C. Waters initially in equilibrium with quartz at a temperature less than 210°C probably will precipitate amorphous silica in channelways underground only when and where large quantities of steam separate from the waters as a result of sudden decreases in pressure or hydrostatic head. Above 150° to 200°C amorphous silica and volcanic glass can contribute very large quantities of silica to the solution. However, at these temperatures in natural systems they are eventually converted to crystalline phases. Thus, control of dissolved silica at depth is likely to be relatively short lived in respect to the ages of most hot spring systems. The dissolved-silica content of 90 hot spring waters from Yellowstone National Park was measured colorimetrically in the field immediately after collection. Comparison with laboratory studies on the solubility of amorphous silica indicates that many waters in «alkaline» springs are markedly undersaturated with silica with respect to amorphous silica at the temperatures of the pools. Thus, the dissolved silica content of these waters cannot be accounted for by equilibria with amorphous silica. Rather, silica appears to be controlled by the dissolution, deposition, or alteration of other silica-bearing phases at depth. Furthermore, many springs now have compositions essentially identical (with respect to all components) to those determined in 1888, indicating that either equilibrium or steady state conditions have prevailed at depth for a long time. Veins of fine-grained quartz were found in drill core from the Upper Basin, and it is reasonable to assume that quartz controlled the quantity of silica in solution in those places of deposition. Possibly the silica content of the surface waters might allow an estimate of the temperatures at which these waters were last in equilibrium with quartz at depth. Assuming adiabatic cooling along the vapor pressure curve and correcting for steam formation, quartz solubility data compared with natural water analyses suggests that underground temperatures approach 205°C in the Upper Geyser Basin of Yellowstone. In the Norris Geyser Basin, underground temperatures of 245°C are suggested.  相似文献   

19.
—We report the analysis of over 16 years of fault creep and seismicity data from part of the creeping section of the San Andreas fault to examine and assess the temporal association between creep events and subsequent earthquakes. The goal is to make a long-term evaluation of creep events as a potential earthquake precursor. We constructed a catalog of creep events from available digital creepmeter data and compared it to a declustered seismicity catalog for the area between San Juan Bautista and San Benito, California, for 1980 to 1996. For magnitude thresholds of 3.8 and above and time windows of 5 to 10 days, we find relatively high success rates (40% to 55% 'hits') but also very high false alarm rates (generally above 90%). These success rates are statistically significant (0.0007 < P < 0.04). We also tested the actual creep event catalog against two different types of synthetic seismicity catalogs, and found that creep events are followed closely in time by earthquakes from the real catalog far more frequently than the average for the synthetic catalogs, generally by more than two standard deviations. We find no identifiable spatial pattern between the creep events and earthquakes that are hit or missed. We conclude that there is a significant temporal correlation between creep events and subsequent small to moderate earthquakes, however that additional information (such as from other potential precursory phenomena) is required to reduce the false alarm rate to an acceptable level.  相似文献   

20.
— Surface-wave amplitudes from explosion sources show less variation for a given event han body wave amplitudes, so it is natural to expect that yield estimates derived from surface waves will be more accurate than yield estimates derived from body waves. However, yield estimation from surface waves is complicated by the presence of tectonic strain release, which acts like one or more earthquake sources superimposed on top of the explosion. Moment-tensor inversion can be used to remove the tectonic component of the surface waves, however moment-tensor inversion for shallow sources is inherently non-unique so the explosion isotropic moment cannot be determined with the necessary accuracy by this means. Explosions on an island or near a mountain slope can exhibit anomalous surface waves similar to those caused by tectonic strain release. These complications cause yield estimates derived from surface waves to be less accurate than yield estimates from body waves recorded on a well-calibrated network with good coverage. Surface-wave amplitudes can be expressed as a surface-wave magnitude M s , which is defined as the logarithm of the amplitude plus a distance correction, or as a path corrected spectral magnitude, log $M^{\prime}_0$ , which is derived from the surface-wave spectrum. We derive relations for M s vs. yield and log $M^{\prime}_0$ vs. yield for a large data set and estimate the accuracy of these estimates.  相似文献   

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