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1.
Summary. Data for P and S beyond 85° are used for earthquakes in the four epicentral regions that travel times have been found for (Japan, Europe, Central and South Pacific). They seem to disagree seriously with suggestions of a considerable change in the times and dt/dΔ for S from the Jeffreys—Bullen tables of 1939–40. There are signs of a sharp drop in dt/dΔ for both Pand S in the range 93–95° except for the Southern Pacific.  相似文献   

2.
Summary. Two localized regions of velocity heterogeneity in the lower mantle with scale lengths of 1000–2000 km and 2 per cent velocity contrasts are detected and isolated through comparison of S, ScS, P and PcP travel times and amplitudes from deep earthquakes in Peru, Bolivia, Argentina and the Sea of Okhotsk. Comparison of the relative patterns of ScS-S differential travel times and S travel-time residuals across North American WWSSN and CSN stations for the different source regions provides baselines for interpreting which phases have anomalous times. A region of low S and P velocities is located beneath Northern Brazil and Venezuela at depths of 1700–2700 km. This region produces S -wave delays of up to 4 s for signals from deep Argentine events recorded at eastern North American stations. The localized nature of the anomaly is indicated by the narrow bounds in azimuth (15°) and take-off angle (13°) of the arrivals affected by it. The long period S -waves encountering this anomaly generally show 30–100 per cent amplitude enhancement, while the short-period amplitudes show no obvious effect. The second anomaly is a high-velocity region beneath the Caribbean originally detected by Jordan and Lynn, who used travel times from deep Peruvian events. The data from Argentine and Bolivian events presented here constrain the location of the anomaly quite well, and indicate a possible short- and long-period S -wave amplitude diminution associated with it. When the travel-time data are corrected for the estimated effects of these two anomalies, a systematic regional variation in ScS-S station residuals is apparent between stations east of and west of the Rocky Mountains. One possible explanation of this is a long wavelength lateral variation in the shear velocity structure of the lower mantle at depths greater than 2000 km beneath North America.  相似文献   

3.
Summary. The shear-wave velocity distribution in a spherically averaged Earth is estimated statistically from previously published short-period S travel-time measurements (Uhrhammer). An algorithm is defined for integral inversion techniques which allows estimation of the variance of the velocity distribution from the uncertainties in the S slowness model. Comparisons are made between the resulting S -velocity solution and other solutions in common use. There are significant differences (at the 95 per cent confidence level) between the 5-velocity model determined here and the Jeffreys-Bullen model over the depth ranges of 150–550 km and 2100–2350 km. The 95 per cent confidence level in the present velocity distribution ranges from ± 0.025 km/s at 625 km to ±0.32km/s at 2766 km and averages about ±0.063 or ±1 percent.
Correlations between azimuthally dependent source and station adjustments (which were previously determined (Uhrhammer)) indicate widespread lateral inhomogeneities (up to 3.4 per cent) to depths of approximately 700 km. Up to three-quarters of the source adjustments are due to lateral velocity variations in the source regions. Station adjustments for differential 5 minus P times are significantly correlated with elevation and crustal age, but not with station instrumental magnification.  相似文献   

4.
Upper mantle shear structure of North America   总被引：5，自引：0，他引：5  
Summary. The waveforms and travel times of S and SS phases in the range 10°–60° have been used to derive upper mantle shear velocity structures for two distinct tectonic provinces in North America. Data from earthquakes on the East Pacific Rise recorded at stations in western North America were used to derive a tectonic upper mantle model. Events on the north-west coast of North America and earthquakes off the coast of Greenland provided the data to investigate the upper mantle under the Canadian shield. All branches from the triplications due to velocity jumps near 400 and 660 km were observed in both areas. Using synthetic seismograms to model these observations placed tight constraints on heterogeneity in the upper mantle and on the details of its structure. SS–S travel-time differences of 30 s along with consistent differences in waveforms between the two data sets require substantial heterogeneity to at least 350 km depth. Velocities in the upper 170 km of the shield are about 10 per cent higher than in the tectonic area. At 250 km depth the shield velocities are still greater by about 4.5 per cent and they gradually merge near 400 km. Below 400 km no evidence for heterogeneity was found. The two models both have first-order discontinuities of 4.5 per cent at 405 km and 7.5 per cent at 695 km. Both models also have lids with lower velocities beneath. In the western model the lid is very thin and of relatively low velocity. In the shield the lid is 170 km thick with very high elocity (4.78 km s-1); below it the velocity decreases to about 4.65 km s-1. Aside from these features the models are relatively smooth, the major difference between them being a larger gradient in the tectonic region from 200 to 400 km.  相似文献   

5.
Summary. Four types of crustal and upper-mantle rocks have been used for the investigation of seismic P -wave velocities in three mutually perpendicular directions. Hydrostatic pressure, up to 6 kbar and temperatures up to 500°C were applied to the samples. Measurements of the ultrasonic P -wave travel times and velocities were carried out along two geotherms. All rock types show an anisotropic behaviour which is caused by the orientation of certain minerals. The anisotropy is not dependent on temperature and pressure. Gneiss and peridotite have 5–6 per cent anisotropy whereas granite and a metagabbro show values of only 2–3 per cent. The smallest velocity is always in the z direction, perpendicular to a schistocity or foliation. It is shown that the data agree with those of field observation. We conclude that anisotropy caused by preferred orientation of minerals must be expected in the whole lithosphere. Additional effects of layering, of cracks, and of nonhydrostatic stresses are estimated.  相似文献   

6.
summary . Approximately 1000 PKP observations in the range 110° < Δ < 170° have been statistically studied to examine the existence of Bolt's GH branch of the PKP travel-time curve. Data give firm evidence for the existence of this branch from 130 to 153° but the possibility of its extending further cannot be ruled out. Observations for this branch fit the form:
Where a =(1150.3254±0.5328)s, b = (16.4907±0.3803)s, c = (−0.1316±0.0630)s.
Times for this branch are always greater by 2 s than the corresponding Gogna times from Pacific earthquakes.  相似文献   

7.
A detailed and extensive record section constructed from recordings at the NORSAR array of presumed explosions in continental Russia exhibits two distinct ( T , Δ) triplications. The reliable identification of these upper mantle travel-time branches is possible because of the dense areal sampling of the NORSAR configuration. A simple upper mantle P- velocity model which can account adequately for the data involves velocity discontinuities at depths of 420 km and 690 km, and fairly uniform velocity gradients elsewhere. For this model, the first arrival branch for Δ≤ 21° extends as a second arrival to a distance of about 33°, at which distance it is terminated by the 420-km discontinuity. Rays bottoming between depths of 420 and 690 km span the distance range 16°≤Δ≤ 28°, and give first arrivals in the range 21°≤, Δ, 24°. Rays which penetrate the 690-km discontinuity give rise to secondary arrivals in the range 19°≤Δ≤ 25°, and first arrivals for distances Δ≤ 25°.  相似文献   

8.
Summary. A two-ship refraction profile was fired on the Australian continental shelf during the Banda Sea geophysical programme carried out by the Woods Hole Oceanographic Institution, the Scripps Institution of Oceanography and the Geological Survey of Indonesia. Some of the 55-kg shots fired during this profile were observed at an array of stations in northern Australia to a distance of 1150 km.
The first arrival P travel times at the land stations had apparent velocities of 6.52, 8.24 and 8.48 km/s. The observed travel times correspond closely with those for other stable continental platform or shield regions. The travel times in these regions are of the order of 6 s less than those given in the Jeffreys—Bullen tables at distances of 700 to 1150 km.
The observations are interpreted as implying an upper-mantle velocity of 8.4 km/s at a depth of about 75 km.  相似文献   

9.
b
A two ship refraction profile was undertaken on the Australian continental shelf during the Banda Sea geophysical program, carried out by the Woods Hole Oceanographic Institution, the Scripps Institution of Oceanography and the Geological Survey of Indonesia. S waves originating close to the sea bottom were observed to distances of up to 1150 km at an array of stations in northern Australia.
These observations are interpreted as implying S mantle velocities of 4.60 km s-1 from a depth of 45 km to a depth of 76 km and 4.72 km s-1 below a depth of 76 km.
Ratios of the P and S travel times (Vp/Vs) have been determined to be 1.74 in the crust rising to a value of greater than 1.79 below a velocity discontinuity at a depth of 200 km. It is inferred that this high value arises because the effect of temperature is greater for S than for P .
Using the data from this and other studies in the shield region of Northern Australia it has been found that the S travel times are significantly less than predicted by the Jeffreys—Bullen tables.  相似文献   

10.
Summary. A two-channel MESA or maximum entropy spectrum analysis (Morf et al. 1978) between Δ T °= ET – UT (Morrison 1973) and sunspot numbers spanning 1832–1975 yields the following results over a bandwidth 2–20yr: (1) The spectra of Δ T ° and sunspot numbers are both dominated by a narrow band signal at 11.0 yr; (2) On average the coherency over the continuum is 0.14 while at 11.0yr it peaks at 0.83; (3) The 11-yr sunspot cycle signal in length of day (lod) has an amplitude of 0.16 ms, and in time lags that in sunspot numbers by 3.4yr. Estimates obtained from segmenting the series yield extremal values which grossly bound the above estimates: The narrow band signal has period range (10.5–11.4yr) coherency (0.8–1.0), amplitude (0.06–0.31 ms), and time lag (3.0–3.Syr). In addition, a two channel analysis of sunspot numbers with a new Δ T n series from 1861–1978 (Morrison 1979) and an earlier segment of Δ T °, as well as a single-channel analysis of Δ T ° and Δ T n individually, further support the conclusion that the solar sunspot cycle in Earth rotation has been detected. These experimental results have implications in astronomy, solar physics, meteorology and climatology.  相似文献   

11.
Summary. Based on accurately located 23 very shallow earthquakes ( h = 1–14 km) in northern and central Greece by portable networks of seismic stations and by the joint epicentre method, the travel times of the Pn -waves from the foci of these earthquakes to the sites of 54 permanent stations in the Balkan region have been determined. The travel times of Pn -waves in the central and eastern part of the area (eastern Greece, south-eastern Yugoslavia, the Aegean Sea, Bulgaria, southern Romania, western Turkey) fit a straight line very well with the Pn velocity equal to 7.9 ± 0.1 km s-1. On the contrary, the travel times of Pn -waves to stations in the western part of the area (Albania, western Greece) do not fit this curve because the Pn -waves travelling to these stations are delayed by more than 1 s due to the thicker crust under the Dinarides–Hellenides mountain range. Time delays for Pn -waves have been calculated for each permanent station in the Balkan area with respect to the mean travel-time curve of these waves in the central and eastern part of the area. Corrections of the travel times for these delays contribute very much to the improvement of the accuracy in the location of the shallow earthquakes in the Aegean and surrounding area.  相似文献   

12.
Summary. Seismic travel times for extrema, zero-crossings, or entire body waves need to be determined precisely to one part in 103 or better in several varieties of seismic studies employing an impulsive artificial source. Examples are crosshole surveys which delineate rock crack distribution separating the holes and monitoring of crustal seismic travel times in earthquake precursor studies. A timing resolution of one part in 103 has been achieved previously using digitally recorded seismic data. These methods, however, do not use interpolation between digitized data points as a method to increase the timing resolution. We report travel-time determinations based on interpolation between digitized points which achieve a precision of two parts in 104, a five-fold improvement over the existing methods. In addition, the effects of seismic noise on travel-time measurement have been compared for the extremum location, the unnormalized correlation, and the normalized correlation method. The following conclusions are drawn from this comparison: (1) the normalized correlation method provides an 18–55 per cent improvement in the standard deviation of the mean over the extremum location method, and (2) results as accurate as those by the normalized correlation can be obtained by the unnormalized correlation if a complete up-and-down swing of the waveform is used as the master trace and if the master trace is close to being sinusoidal. The advantage of the unnormalized correlation over the normalized correlation is speed; the unnormalized correlation is faster by a factor of 28 in computing time.  相似文献   

13.
An Mw 5.9 earthquake occurred in the Lake Rukwa rift, Tanzania, on 1994 August 18, and was well recorded by 20 broad-band seismic stations at distances of 160 to 800 km and 21 broad-band stations at teleseismic distances. The regional and teleseismic waveforms have been used to investigate the source characteristics of the main shock, and also to locate aftershocks that occurred within three weeks of the main shock. Teleseismic body-wave modelling yields the following source parameters for the main shock: source depth of 25 ± 2 km, a normal fault orientation, with a horizontal tension axis striking NE-SW and an almost vertical pressure axis (Nodal Plane I: strike 126°–142°, dip 63°–66°, and rake 280°–290°; Nodal Plane II: strike 273°–289°, dip 28°–31°, and rake 235°–245°), a scalar moment of 4.1 times 1017 N m, and a 2 s impulsive source time function. Four of the largest aftershocks also nucleated at depths of 25 km, as deduced from regional sPmp–Pmp times. The nodal planes are broadly consistent with the orientation of both the Lupa and Ufipa faults, which bound the Rukwa rift to the northeast and southwest, respectively. The rupture radius of the main shock, assuming a circular fault, is estimated to be 4 km with a corresponding stress drop of 6.5 MPa. Published estimates of crustal thickness beneath the Rukwa rift indicate that the foci of the main shock and aftershocks lie at least 10 km above the Moho. The presence of lower-crustal seismicity beneath the Rukwa rift suggests that the pre-rift thermal structure of the rifted crust has not been strongly modified by the rifting, at least to depths of 25 km.  相似文献   

14.
Summary. One hundred and eight deep-focus earthquakes occurring in the Indian region (6–37° N, 70–100° E) were originally selected for the study of P residuals (Paper I). Of these, 61 earthquakes were found suitable for analysing S residuals.
S times for this region are generally greater than J—B S times. Analysis of residuals reveals that anomalous regions are present both in the upper mantle as well as the lower mantle.  相似文献   

15.
Summary. P -wave seismograms at ranges less than 10 km are synthesized by asymptotic ray theory and by summation of Gaussian beams for point sources located in a low-velocity wedge surrounding a fault. The computations are performed using models of the wedge inferred from the analysis of reflection and refraction experiments across the San Andreas and Hayward-Calaveras faults. Calculations in these models show that the 10–20Hz vertical displacements of earthquakes located at 3–10km depth are amplified by up to an order of magnitude in a 1–2km wide region centred on the fault trace compared to displacements predicted by laterally homogeneous models of the crust. This amplification is not cancelled by high attentuation in the fault zone and compensates for the reduction in amplitudes directly above the source predicted from the radiation pattern of a strike-slip earthquake. Depending on the source depth of the earthquake and the structure and velocity contrast of the wedge, multiple triplications in the travel-time curve of direct P - and S -waves will occur at stations in the fault zone. A wedge model successfully predicts the triplications observed in the P waveforms of aftershocks of the Coyote Lake earthquake recorded in the fault zone, showing that body waves from microearthquakes can be used to determine the three-dimensional velocity structure of the fault zone. The amplification, waveform complexity, and distortion of ray paths introduced by the low- velocity wedge suggest that its effects should be included in the interpretation of strong ground motions and travel times observed in the fault zone. For realistic models of the wedge, asymptotically approximate methods of calculating the body waveforms are strictly valid for frequencies greater than 20Hz. Numerical methods may be necessary to calculate accurately the wavefield at lower frequencies.  相似文献   

16.
Summary. Pacific earthquakes studied by Gogna, also three important explosions in the Tuamotu archipelago, are rediscussed. The results are very consistent, but those from Tuamotu are later than Gogna's by about 1 s in the times of P about 60°. Both sets of data give PKP residuals about -5 s about 140° - 142°, indicating that the observations there referred to the neighbourhood of the cusp of the travel-time curve but the ISS had compared them with the DEF branch. The corresponding difference in the 1940 tables is about 2 s.
Analysis at intervals of 1° indicated that the cusp of PKP is about 141° instead of 143° as in the 1940 tables and the difference between it and the DEF branch at these distances is about - 5 s.
Travel times of S under the Pacific were found but need more data, especially at distances under 10°.
Times of PcP reported by Kogan and Carder were compared with those calculated from P in Gogan's explosions, and indicated a radius of the core of 3479.8 ± 1.8 km.  相似文献   

17.
Summary. The asymptotic properties of spheroidal mode dispersion at high frequency for fixed phase velocity are related to the intercept times τβ( p ) for P and S waves. If the mode eigenfrequency and the ratio of horizontal to vertical displacement at the surface for the mode are known τα( p ) and τβ( p ) may be separately estimated. If discontinuities exist in the velocity model then 'solotone' effects occur, in frequency at fixed slowness, and in τα( p ), τβ( p ) estimated from the mode dispersion as a function of slowness. The coupling of P and S waves in the spheroidal modes means that the interaction of P waves with upper-mantle discontinuities affects also the estimates of the S wave τβ( p ) values for which the corresponding turning points lie in the lower mantle. The asymptotic formalism also shows that sharp pulses formed by superposition of spheroidal modes correspond to multiple PS reflections.
A study of τα( p ), τβ( p ) estimates derived from spheroidal modes with periods from 45–50s, calculated for model 1066B, shows that even in the presence of strong upper-mantle discontinuities the errors in intercept time are only about one-tenth of a period. The asymptotic properties may there-for provide a useful means of estimating intercept times from modes with a few seconds period as a supplement to travel-time methods.  相似文献   

18.
Summary. Teleseismic P and S arrival times to North American stations are obtained from the ISC bulletins for the 10-yr period 1964–73, and relative travel-time delays are calculated with respect to standard tables. Station anomalies as well as variations of the delays with azimuth and epicentral distance from station are analysed, and the location of the velocity anomalies responsible for them is discussed. Inversion of the P delays to infer upper mantle velocity structure down to a depth of 700 km is obtained using three-dimensional blocks, as proposed by Aki, Christofferson & Husebye. Three layers can be resolved in this depth range. It is found that the heterogeneities responsible for the travel-time delays are primarily located in the first 250 km of the upper mantle, and that they correlate with surface features. Significant heterogeneities subsist to depths of at least 700 km and their broad scale pattern also correlates with the surface features: in the third layer (500 to 700 km depth) there is an increase of velocity from the West to the East of the United States, while the second layer (250 to 450 km depth) exhibits a reversed pattern. A tentative interpretation of these deeper anomalies is made, as being due mainly to topography of the major upper mantle discontinuities, near 400 and 650 km depth.  相似文献   

19.
Summary. A first-order form of the Euler's equations for rays in an ellipsoidal model of the Earth is obtained. The conditions affecting the velocity law for a monotonic increase, with respect to the arc length, in the angular distance to the epicentre, and in the angle of incidence, are the same in the ellipsoidal and spherical models. It is therefore possible to trace rays and to compute travel times directly in an ellipsoidal earth as in the spherical model. Thus comparison with the rays of the same coordinates in a spherical earth provides an estimate of the various deviations of these rays due to the Earth's flattening, and the corresponding travel-time differences, for mantle P -waves and for shallow earthquakes. All these deviations are functions both of the latitude and of the epicentral distance. The difference in the distance to the Earth's centre at points with the same geocentric latitude on rays in the ellipsoidal and in the spherical model may reach several kilometres. Directly related to the deformation of the isovelocity surfaces, this difference is the only cause of significant perturbation in travel times. Other differences, such as that corresponding to the ray torsion, are of the first order in ellipticity, and may exceed 1 km. They induce only small differences in travel time, less than 0.01s. Thus, we show that the ellipticity correction obtained by Jeffreys (1935) and Bullen (1937) by a perturbational method can be recovered by a direct evaluation of the travel times in an ellipsoidal model of the Earth. Moreover, as stated by Dziewonski & Gilbert (1976), we verify the non-dependence of this correction on the choice of the velocity law.  相似文献   

20.
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   

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