The ratio of the travel times for S and P at distances less than 10°

1980 ◽  
Vol 70 (3) ◽  
pp. 823-829
Author(s):  
A. L. Hales ◽  
K. J. Muirhead
Keyword(s):  
Upper Mantle ◽  
Depth Range ◽  
Travel Times ◽  
Laboratory Measurements ◽  
Mantle Minerals ◽  
Distance Range ◽  
Average Value ◽  
The Times

abstract The ratios of the S and P travel times, Ts/ Tp, observed at central Australian stations from Banda Sea earthquakes with depths between 100 and 200 km are less than the corresponding ratios for surface explosions in the same distance range. This result is surprising and inconsistent with the increase in the Vp/ Vs ratio with depth inferred from laboratory measurements on upper mantle minerals. Studies from several continental regions, including Japan, and oceanic regions, show similar low values. Possible reasons for the inconsistency are discussed. Studies of three well-observed earthquakes in the Hokkaido region lead to the conclusion that the depths of earthquakes in the 60 to 200 km depth range are systematically underestimated by ISC and that there is an associated error in the times of origin of about 3 sec. Using corrected origin times, the average value of the Ts/ Tp ratio over the distance range 0° to 8° is about 1.785.

An analysis of the travel times of S waves to North American stations in the distance range 28° to 82°

1967 ◽  
Vol 57 (4) ◽  
pp. 761-771 ◽  
Author(s):  
H. A. Doyle ◽  
A. L. Hales
Keyword(s):  
United States ◽  
Upper Mantle ◽  
Univariate Analysis ◽  
Western United States ◽  
Travel Times ◽  
Eastern United States ◽  
S Waves ◽  
Distance Range ◽  
Time Term ◽  
Mantle Velocity

abstract The travel times of S waves from 20 earthquakes to stations in North America in the distance range 28° to 82° have been studied. The deviations from J-B times were analyzed into station, source and distance components using the least-squares time-term approach of Cleary and Hales. Station anomalies had a range of about eight seconds, as compared to three seconds for the P anomalies, and are believed to be caused largely by variations in the upper mantle velocity distribution. S residuals, like the P residuals, were generally positive in the western United States, and negative in the central and eastern United States. P and S residuals at the same station correlated with a coefficient of 0.75, the slope of the regression of S anomaly on P anomaly being 3.72. Corrections to J-B times for S were of the order of the standard errors of the determinations. Within the distance range of 28° to 82° large changes of the S travel times, such as were required by the lower mantle velocities proposed by MacDonald and Ness (1961), are not permitted by the present data. The analysis was checked by carrying out a univariate analysis of variance of the same data.

Intensities and travel times of seismic rays in a sphere with a linear velocity function

1977 ◽  
Vol 67 (1) ◽  
pp. 33-42
Author(s):  
Mark E. Odegard ◽  
Gerard J. Fryer
Keyword(s):  
Velocity Profile ◽  
Travel Time ◽  
Upper Mantle ◽  
Intensity Ratio ◽  
Computation Time ◽  
Spherical Body ◽  
Travel Times ◽  
Velocity Function ◽  
Intensity Ratios ◽  
Distance Travel

Abstract Equations are presented which permit the calculation of distances, travel times and intensity ratios of seismic rays propagating through a spherical body with concentric layers having velocities which vary linearly with radius. In addition, a method is described which removes the infinite singularities in amplitude generated by second-order discontinuities in the velocity profile. Numerical calculations involving a reasonable upper mantle model show that the standard deviations of the errors for distance, travel time and intensity ratio are 0.0046°, 0.057 sec, and 0.04 dB, respectively. Computation time is short.

Shear velocity from differential travel times of short-period ScS-P in New Hebrides, Fiji-Tonga, and Banda Sea regions

1975 ◽  
Vol 65 (6) ◽  
pp. 1787-1796
Author(s):  
Mansur A. Choudhury ◽  
Georges Poupinet ◽  
Guy Perrier
Keyword(s):  
Upper Mantle ◽  
Focal Depth ◽  
Shear Velocity ◽  
Mean Value ◽  
Travel Times ◽  
Velocity Models ◽  
Depth Dependence ◽  
Short Period ◽  
New Hebrides ◽  
The Antarctic

abstract Behavior of P, S and ScS residuals as well as those of differential travel times of ScS-P from the Jeffreys-Bullen tables are analyzed. The phases have been read from short-period records of the Antarctic station, Dumont d'Urville (DRV); the earthquakes originating in New Hebrides, Fiji-Tonga, and Banda Sea regions. P residuals from all regions show a mean value of about −1 sec. On the contrary, S and ScS residuals, well correlated among themselves, show important regional as well as focal-depth dependence. ScS-P residuals from shallow and intermediate shocks are largely positive for New Hebrides and largely negative for Banda Sea; those from intermediate shocks are moderately positive for Fiji-Tonga. The anomalies disappear at depths greater than about 200 km. Upper mantle shear velocity models are presented for the three regions. The models are discussed in relation to a sinking lithosphere.

The travel times of the longitudinal waves of the Logan and Blanca atomic explosions and their velocities in the upper mantle

1962 ◽  
Vol 52 (3) ◽  
pp. 519-526 ◽  
Author(s):  
I. Lehmann
Keyword(s):  
Velocity Gradient ◽  
Upper Mantle ◽  
Discontinuity Surface ◽  
Travel Times ◽  
Longitudinal Waves

abstract The P travel times of the Logan and Blanca atomic explosions are found to be consistent with an upper mantile structure having a discontinuity surface at about 215 km depth at which the velocity and the velocity gradient increase abruptly while the velocity varies only slightly or is constant above this depth.

Upper-mantle travel times in Australia — A preliminary report

1975 ◽  
Vol 11 (2) ◽  
pp. 109-118 ◽  
Author(s):  
A.L. Hales ◽  
K.J. Muirhead ◽  
J.M. Rynn ◽  
J.F. Gettrust
Keyword(s):  
Upper Mantle ◽  
Preliminary Report ◽  
Travel Times

scholarly journals Seismic anisotropy: an original tool to understand the geodynamic evolution of the Italian peninsula

1997 ◽  
Vol 40 (3) ◽  
Author(s):  
L. Margheriti ◽  
C. Nostro ◽  
A. Amato ◽  
M. Cocco
Keyword(s):  
Upper Mantle ◽  
Common Property ◽  
Seismic Waves ◽  
Seismic Anisotropy ◽  
Structural Features ◽  
Relative Strength ◽  
Plate Motion ◽  
Mantle Minerals ◽  
Anisotropic Bodies ◽  
Shallow Crust

Anisotropy is a common property of the Earth's crust and the upper mantle; it is related to the strain field of the medium and therefore to geodynamics. In this paper we describe the different possible origins of anisotropic behavior of the seismic waves and the seismological techniques used to define anisotropic bodies. In general it is found that the fast polarization direction is parallel to the absolute plate motion in cratonic areas, to the spreading direction near rifts or extensional zones, and to the main structural features in transpressive regimes. The delay times between fast and slow waves reflect the relative strength and penetration at depth of the deformation field. The correspondence between surface structural trends and anisotropy in the upper mantle, found in many regions of the world, strongly suggest that orogenic processes involve not only the shallow crust but the entire lithosphere. Recently in Italy both shear wave splitting analysis and Pn inversion were applied to define the trend of seismic anisotropy. Along the Northern Appeninic arc fast directions follow the strike of the arc (i.e., parallel to the strike of the Miocene-Pleistocene compressional features), whereas in the Tyrrhenian zone fast directions are about E-W SW-NE; parallel to the post-Miocene extension that is thought to have reoriented the mantle minerals fabric in the astenosphere.

scholarly journals A Seismic Refraction Study of the Crust and Upper Mantle in the Vicinity of Bass Strait

1969 ◽  
Vol 22 (5) ◽  
pp. 573 ◽  
Author(s):  
R Underwood
Keyword(s):  
Crustal Structure ◽  
Upper Mantle ◽  
Seismic Refraction ◽  
Travel Times ◽  
Crust And Upper Mantle ◽  
Australian Institute ◽  
First Arrival Travel Times ◽  
First Arrival ◽  
Future Work

A reconnaissance seismic refraction study of the crust and upper mantle of Bass Strait and adjacent land was undertaken in 1966 under the sponsorship of the Geophysics Group of the Australian Institute of Physics. The shot locations and times, the station locations, distances, and first arrival travel times are presented. Analysis of these data is described; they indicate a P n velocity below 8 km sec-I. Time terms are less than expected and do not agree with previous work. Crustal thicknesses cannot be computed until studies of upper crustal structure are made. These, and several mantle refraction studies, are suggested for future work.

A study of the Earth's crust and upper mantle using travel times and spectrum characteristics of body waves

1970 ◽  
Vol 60 (6) ◽  
pp. 1921-1935
Author(s):  
B. M. Gurbuz
Keyword(s):  
Upper Mantle ◽  
Body Waves ◽  
Earth’S Crust ◽  
Travel Times ◽  
Time Data ◽  
Crust And Upper Mantle ◽  
Contour Maps ◽  
Spectrum Characteristics ◽  
Earth's Crust ◽  
Early Rise

Abstract The aim of this paper is to investigate the velocity distribution and structure of the Earth's crust and upper mantle from the close collaboration of theory and experimental results of travel times and spectrum characteristics of body waves. The interpretation was based on 38 seismic records which were obtained from the “Project Early Rise” experiment during July 1966. The results refer to the area bounded by latitudes 49°W and 51°30′ and longitudes 93°W and 98°W. A least-squares analysis of the travel-time data was made and the uncertainties of the slopes, intercept times, and corresponding velocities were determined. The observed wide-angle reflections were used to calculate the root mean square velocities applying the T2 - X2 method. Depth calculations for the velocity discontinuities and seismic depth contour maps were made. A model was constructed, and the validity of the proposed new model was tested by comparing the observed travel times, spectrum-amplitude ratios, and relative phase shifts of body waves with theoretically expected values. Evidence is given for three discontinuities in the Earth's crust with velocities of 6.11 ± 0.01 km/sec, 6.8 ± 0.08 km/sec, and 7.10 ± 0.04 km/sec at average depths 18 ± 2 km and 25.5 ± 0.9 km. Velocities in the uppermost part of the mantle were determined as 7.90 ± 0.05 km/sec and 8.48 ± 0.05 km/sec with interfaces at the average depths of 34 ± 1 km, and 47 ± 1 km, respectively.

P-wave travel times from shallow earthquakes recorded in India and inferred upper mantle structure

1968 ◽  
Vol 58 (6) ◽  
pp. 1879-1897
Author(s):  
K. L. Kaila ◽  
P. R. Reddy ◽  
Hari Narain
Keyword(s):  
Upper Mantle ◽  
P Wave ◽  
Mantle Structure ◽  
Travel Times ◽  
The North ◽  
Shallow Earthquakes ◽  
Upper Mantle Structure ◽  
Wave Travel ◽  
Excess Value ◽  
Velocity Changes

ABSTRACT P-wave travel times of 39 shallow earthquakes and three nuclear explosions with epicenters in the North in Himalayas, Tibet, China and USSR as recorded in Indian observatories have been analyzed statistically by the method of weighting observations. The travel times from Δ = 2° to 50° can be represented by four straight line segments indicating abrupt velocity changes around 19°, 22° and 33° respectively. The P-wave velocity at the top of the mantle has been found to be 8.31 ± 0.02 km/sec. Inferred upper mantle structure reveals three velocity discontinuities in the upper mantle at depths (below the crust) of 380 ± 20, 580 ± 50 and 1000 ± 120 km with velocities below the discontinuities as 9.47 ± 0.06, 10.15 ± 0.07 and 11.40 ± 0.08 km/sec respectively. The J-B residuals up to Δ = 19° are mostly negative varying from 1 to 10 seconds with a dependence on Δ values indicating a different upper mantle velocity in the Himalayan region as compared to that used by Jeffreys-Bullen in their tables (1940). Between 19° to 33° there is a reasonably good agreement between the J-B curve and the observation points. From Δ = 33° to 50° the J-B residuals are mostly positive with an average excess value of about 4 sec.

On locating local earthquakes using small networks

1969 ◽  
Vol 59 (3) ◽  
pp. 1201-1212
Author(s):  
David E. James ◽  
I. Selwyn Sacks ◽  
Eduardo Lazo L. ◽  
Pablo Aparicio G.
Keyword(s):  
Least Squares ◽  
Focal Depth ◽  
P Wave ◽  
Time Interval ◽  
Travel Times ◽  
Origin Time ◽  
Average Value ◽  
Fundamental Properties ◽  
P Wave Velocity ◽  
Wave Travel

abstract Mathematical instability in four-parameter least squares hypocenter solutions arises primarily from the fact that the four computed variables—origin time (T0), focal depth (h), latitude (θ), and longitude (λ)—are not strictly independent. Specifically, T0 exhibits a non-independent relationship with the geometric parameters. For small networks (< 10–15 stations), the lack of independence between T0 and the other variables results in unstable least-squares solutions. This instability is manifest most clearly by the fact that different station subsets of the observational network produce significantly different solutions for the same earthquake. The instability can be eliminated by computing T0 independently for each station using the formula ( T 0 ) i = ( T p ) i − V k ( T s − p ) i V p , where Tp = P-wave arrival time, Vk = S-P velocity, Vp = P-wave velocity, and Ts-p = time interval between P and S arrivals. An average value of T0 can be obtained from the individually calculated origin times and the P-wave travel times calculated. The variables ϕ, λ and z are then computed by the usual least-squares procedure using P-wave travel times only. The method is iterative and an average T0 is recalculated in the course of each iteration. Fundamental properties of travel times within the Earth impose definite limitations upon the accuracy of the locations. Low values of the derivative dTp/dh at epicentral distances of a few degrees introduce a large uncertainty in focal depth, particularly for shallow (0–60 km) earthquakes. There is normally little error in epicenter, however, even for solutions in which depth is poorly determined. The dimensions and geometric configuration of the network in relation to the epicenter and the proximity of the epicenter to any one station are controlling factors in predicting the minimum uncertainty for any given hypocenter solution.

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