Summary
To the residual data ofEnayatollah [11] are fitted sine curves with back-azimuth as the independent variable. The result is a clear regional trend towards apparently earlyP-wave arrivals from the north relative to those from the south at any one station for δ≥55o. Explanations are plausible, but not entirely satisfactory, on the basis of systematic epicentral mislocation, or of lateral inhomogeneity inP-velocity structure of the Fennoscandian crust and/or uppermost mantle. A zone of lateral inhomogeneity deeper in the mantle, for example a dipping 600-km discontinuity, consistent with the residual observations, is more likely than a shallower emplacement considering the physical implications of extending such a structure to underlie all or most of Fennoscandia. More fundamental inhomogeneous velocity structure throughout most or all of the mantle may have to be considered to enable epicentral determination and travel-time prediction within the accuracy that present instruments would theoretically permit.
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References
Y. Beaufils, P. Mechler andY. Rocard,Variationd'une couche de discontinuité de propagation sismique vers 600 km de profondeur, Comptes Rendus Acad. Sci. Paris270, Série B (1970), 1070.
C. Blamey andP. G. Gibbs,The epicentres and origin times of some large explosions, Geophys. J. R. Astr. Soc.16 (1968), 1.
B. A. Bolt andO. W. Nuttli,P wave residuals as a function of azimuth 1. Observations, J. Geophys. Res.71 (1966), 5977.
R. J. Brown,Lateral inhomogeneity in the crust and upper mantle from P-wave amplitudes, Pure and Appl. Geophys. (in press, 1972).
R. J. Brown, H. Borg andM. Båth,Strike and dip of crustal boundaries — a method and its application, Pure and Appl. Geophys.88 (1971), 60.
D. S. Carder,Travel times from central Pacific nuclear explosions and inferred mantle structure, Bull. Seism. Soc. Am.54 (1964), 2271.
J. Cleary andA. L. Hales,Azimuthal variation of U.S. station residuals, Nature210 (1966), 619.
J. R. Cleary andK. J. Muirhead,Comparison of the 1968 P tables with times from nuclear explosions, 1, Longshot and Greeley, Earth Planet. Sci. Lett7 (1969), 119.
A. Douglas,Joint epicentre determination, Nature215 (1967), 47.
A. Douglas andR. C. Lilwall,Does epicentre source bias exist? Nature220 (1968), 469.
M. A. Enayatollah,Travel times of P-waves for the Swedish-Finnish seismograph network, Pure and Appl. Geophys.94 (1972), 101.
E. R. Engdahl andE. A. Flinn,Seismic waves reflected from discontinuities within the upper mantle, Science163 (1969), 177.
R. W. E. Green andA. L. Hales,The travel times of P waves to 30 o in the central United States and upper mantle structure, Bull. Seism. Soc. Am.58 (1968) 267.
R. J. Greenfield andR. M. Sheppard,The Mohorovičié depth variations under the LASA and their effect on dT/dδ measurements, Bull. Seism. Soc. Am.59 (1969), 409.
E. Herrin, E. P. Arnold, B. A. Bolt, G. E. Clawson, E. R. Engdahl, H. W. Freedman, D. W. Gordon, A. L. Hales, J. L. Lobdell, O. Nuttli, C. Romney, J. Taggart andW. Tucker,1968 seismological tables for P phases, Bull. Seism. Soc. Am58 (1968), 1193.
E. Herrin andJ. Taggart,Regional variations in Pn velocity and their effect on the location of epicenters, Bull. Seism. Soc. Am.52 (1962), 1037.
E. Herrin andJ. Taggart,Epicenter determination for the SALMON event, J. Geophys. Res.71 (1966), 3503.
E. Herrin andJ. Taggart,Regional variations in P travel times, Bull. Seism. Soc. Am.58 (1968), 1325.
E. Herrin andJ. Taggart,Source bias in epicenter determinations, Bull. Seism. Soc. Am.58 (1968), 1791.
E. Husebye,Correction analysis of Jeffreys-Bullen travel time tables, Bull. Seism. Soc. Am.55 (1965), 1023.
E. S. Husebye, R. Kanestrøm andR. Rud,Observations of vertical and lateral P-velocity anomalies in the Earth's mantle using the Fennoscandian continental array, Geophys. J. R. Astr. Soc.25 (1971), 3.
K. H. Jacob,Global tectonic implications of anomalous seismic P travel times from the nuclear explosion Longshot, J. Geophys. Res.77 (1972) 2556.
H. Jeffreys,The Earth, 4th ed. (The University Press, Cambridge (1962).
H. Jeffreys andK. E. Bullen,Seismological Tables, Brit. Ass., Gray-Milne Trust (1967).
L. R. Johnson,Array measurements of P velocities in the upper mantle, J. Geophys. Res.72 (1967). 6309.
K. L. Kaila, P. R. Reddy andH. Narain,P-wave travel times from shallow earthquakes recorded in India and inferred upper mantle structure, Bull. Seism. Soc. Am.58 (1968), 1879.
R. Kanestrøm,Upper mantle structure of northern Europe from apparent velocities of P waves, Geophysica (Helsinki)11 (1971), 241.
K. Kogeus,Teleseismic Relative Location of Closely Spaced Epicenters, FOA 4 rapport C4370-26, Defence Res. Inst. Sweden, Stockholm (1968).
R. C. Lilwall andA. Douglas,Estimation of P-wave travel times using the joint epicentre method, Geophys. J. R. Astr. Soc.19 (1970), 165.
C. Lomnitz,Travel-time errors in the laterally inhomogeneous Earth, Bull. Seism. Soc. Am.61 (1971), 1639.
A. A. Lukk andI. L. Nersesov,Structure of the upper mantle as shown by observations of earthquakes of intermediate focal depth, Dokl. Akad. Nauk. USSR162 (1965), 559.
D. Mayer-Rosa,Die Geschwindigkeitsverteilung seismischer Wellen im oberen Erdmantel Europas, Doctoral thesis, Univ. of Stuttgart (1969).
P. Mechler, R. Mseddi andJ-L. Plantet,Carte générale du bas dell'asthénosphère sous la France, Comptes Rendus Acad. Sci. Paris,273, Série D (1971), 298.
M. Niazi andD. L. Anderson,Upper mantle structure of western North America from apparent velocities of P waves, J. Geophys. Res.70 (1965), 4633.
O. W. Nuttli,Table of angles of incidence of P waves at focus, calculated from 1968 P tables, Earthq. NotesXL, 3 (1969), 21.
O. W. Nuttli andB. A. Bolt,P wave residuals as a function of azimuth 2. Undulation of tthe mantle low-velocity layer as an explanation. J. Geophys. Res.74 (1969), 6594.
A. Ritsema,Remarks on some seismic signal velocities, inProceedings of the VESIAC Special Study Conf. on Seismic Signal Anomalies, Travel times, Amplitudes, and Pulse Shapes, VESIAC Report 4410-99-X, Univ. of Mich., Ann Arbor (1966), 57.
D. L. Springer andR. L. Kinnaman,Seismic source summary for U.S. underground nuclear explosions, 1961–1970, Bull. Seism. Soc. Am.61 (1971), 1073.
M. N. Toksöz, J. Arkani-Hamed andC. A. Knight,Geophysical data and long-wave heterogeneities of the Earth's mantle, J. Geophys. Res.74 (1969), 3751.
J. H. Whitcomb andD. L. Anderson,Reflection of P'P' seismic waves from discontinuities in the mantle, J. Geophys. Res.75 (1970), 5713.
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Brown, R.J. Azimuthally varyingP-wave travel-time residuals in Fennoscandia and lateral inhomogeneity. PAGEOPH 105, 741–758 (1973). https://doi.org/10.1007/BF00875824
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DOI: https://doi.org/10.1007/BF00875824