Converted phases from the mantle transition zone observed at European stations
Main Article Content
Converted phases from the mantle transition zone have been observed as precursors about 1 min before the main S-wave phases S, SKS and ScS in long-period records of the Grafenberg array (GRF) and a few European WWSSN stations, at epicentral distances from 70-90°. The 23 earthquakes used were located along the west coast of America from Alaska to Ecuador, with a concentration of events in Central America, and in East Asia from the Aleutians through Japan to Sumatra. Relatively strong converted energy was observed for the American earthquakes, except for two events in the South Mexico/Guatemala region. The East Asian earthquakes produced significantly less precursor energy. The interference of conversions from P to SV below the focus and from SV to P below the stations is studied with theoretical-seismogram calculations. Due to interference precursors are normally stronger on the horizontal-radial than on the vertical component; this is in agreement with the observations. In special cases with either maximum or minimum P radiation towards the station conversion takes place only on one side, and precursor observations can be related directly to structure either below the focus or the station. The data set includes such favorable cases. The interpretation of observed precursors in terms of the fine structure in the conversion zone is difficult, even in favorable cases, because of the low resolving power of long-period converted phases. Nevertheless, the following conclusions can be drawn from the observations presented. Most of the precursor observations for the American events are compatible with typical models of the transition zone between upper and lower mantle, having two discontinuities at depths of about 400 and 670 km. Such a structure applies for western Europe and for the Caribbean Sea/Gulf of Mexico region, in the latter case with a possible local interruption by a smoother transition zone. A relatively smooth transition zone below East Asia from about Korea to the Sea of Okhotsk can also explain the lack of precursor energy for a few earthquakes in and close to Japan. These results indicate large-scale lateral variations in the sharpness of the mantle transition zone.
Authors who publish with this journal as of Vol. 63 agree to the following terms:
a. Authors share the copyright with this journal in equal parts (50% to the journal, 50% to the lead author), and grant the journal right of first publication, with the work after publication simultaneously licensed under Creative Commons Attribution License CC BY-NC-ND 4.0 that allows others to share the work with an acknowledgment of the work's authorship and initial publication in this journal.
b. Authors may enter into separate, additional contractual arrangements for the non-exclusive distribution of the journal's published version of the work (e.g., post it to an institutional repository or publish it in a book), with an acknowledgment of its initial publication in this journal, and a reference to this copyright notice.
c. Authors are permitted and encouraged to post their work online (e.g., in institutional repositories or on their website) during the submission process, as this can lead to productive exchanges and earlier and greater citation of published work and better sales of the copyright.
Authors retain copyright and grant the Journal of Geophysics right of first publication, with the work three years after publication simultaneously licensed under the Creative Commons BY-NC-ND 4.0 License that allows others to share the work (with an acknowledgment of the work's authorship and initial publication in this journal), except for commercial purposes and for creating derivatives.
Authors can enter into separate, additional, but non-commercial contractual arrangements for the non-exclusive distribution of the journal's published version of the work (e.g., post it to an institutional repository, but not publish it in a book), with an acknowledgment of its initial publication in this journal.
Authors are permitted and encouraged to post their work online (e.g., in institutional repositories or on their website) before and during the submission process, as that can lead to productive exchanges, as well as earlier and greater citation of published work (See The Effect of Open Access).
This journal is one of a handful of scholarly journals that publish original scientific works under CC BY-NC-ND 4.0 - the only Creative Commons license affording the authors' intellectual property absolute worldwide protection.
Journal of Geophysics is published under the scholar-publishers model, meaning authors do not surrender their copyright to us. Instead, and unlike corporate publishers like Elsevier or Springer Nature that resell copyright to third-parties for up to $80,000 (per paper, per transaction!), the Journal of Geophysics authors share copyright equally with this journal.
Therefore, all the proceeds from reselling copyright to third parties get shared to equal parts (50% to the journal, 50% to the lead author). Under the Berne Convention, this protection is an inheritable right that lasts for as long as the rightsholder lives + 50 years.
By submitting to this journal, the lead author, on behalf of all co-authors, grants permission to this journal to represent all co-authors in negotiating copyright sales and collecting proceeds. The lead author should negotiate with his/her co-authors the modalities of distributing the lead author's portion of the proceeds. Usually, this is per pre-agreed percentage of each co-author's contribution to creating the copyrighted work. (more...)
Baumgardt, D.R. (1981) Seismic body-wave study of vertical and lateral heterogeneity in the Earth's interior, Ph.D. thesis, Pennsylvania State University, 532 p.
Bock, G., Ha, J. (1984) Short-period S to P conversion in the mantle at a depth near 700 km, Geoph. J. R. Astron. Soc. 77 (In press)
Burdick, L.J. (1978) t* for S waves with a continental ray path, BSSA 68:1013-1030
Burdick, L.J. (1981) A comparison of the upper mantle structure beneath North America and Europe, J. Geophys. Res. 86:5926-5936
Burdick, L.J., Helmberger, D.V. (1978) The upper mantle P velocity structure of the western United States, J. Geophys. Res. 83:1699-1712
Chael, E.P., Stewart, G.S. (1982) Recent large earthquakes along the Middle American trench and their implications for the subduction process, J. Geophys. Res. 87:329-338
Faber, S., Muller, G. (1980) Sp phases from the transition zone between the upper and lower mantle, Bull. Seism. Soc. Am. 70:487-508
Given, J.W., Helmberger, D.V. (1980) Upper mantle of Northwestern Eurasia, J. Geophys. Res. 85:7183-7194
Helmberger, D.V., Engen, G.R. (1974) Upper mantle shear structure, J. Geophys. Res. 79:4017-4028
King, D.W., Calcagnile, G. (1976) P-wave velocities in the upper mantle beneath Fennoscandia and Western Russia, Geophys. J. R. Astron. Soc. 46:407-432
Masters, G., Jordan, T.H., Silver, P.O., Gilbert, F. (1982) Aspherical Earth structure from fundamental spheroidal-mode data, Nature 298:609-613
Rademacher, H., Odom, R.I., Kind, R. (1983) The upper mantle structure under south-east Europe derived from ORF broadband records of Greek earthquakes, J. Geophys. 52:7-13
Stewart, G.S., Chael, E.P., McNally, K.C. (1981) The November 29, 1978, Oaxaca, Mexico, earthquake: a large simple event, J. Geophys. Res. 86:5053-5060
Vinnik, L.P. (1977) Detection of waves converted from P to SV in the mantle, Phys. Earth Planet. Inter. 15:39-45
Vinnik, L.P., Avetisjan, R.A., Mikhailova, N.G. (1983) Heterogeneities in the mantle transition zone from observations of P-to-SV converted waves, Phys. Earth Planet. Inter. 33:149-163