S-Wave Velocity Structure beneath Southwest North America from Seismogram Comparisons of the Mexico Earthquake on 22 June 1997
DOI:
https://doi.org/10.5614/itbj.sci.2008.40.2.2Abstract
This research investigates earth structure beneath the Southwest North America landmass, especially between Mexico and California. Models based on S wave velocities for this area were obtained by carrying out seismogram fitting in time domain and three Cartesian components simultaneously. The data used is from an event, coded as C052297B that occurred in the state of Guerrero, Mexico and it was fitted to synthetic data computed with the GEMINI program at TS network stations. Earth model IASPEI91 and SPREM were used as input to create the synthetic data. Real and synthetic seismograms were subjected to a low-pass filter with a frequency corner of 20 mHz. Waveform analysis results show very unsystematic and strong deviations in the waveform, arrival times, amount of oscillation and the height of the wave amplitude. Discrepancies are met on S, Love, Rayleigh and ScS waves, where the stations epicentral distances are below 300. Deviation in analysis waveform because of the usage of model 1-D of SPREM and IASPEI91, because the 1-D was a kind of average value an elastic property at one particular depth of global earth. With the method of waveform analysis we can see how sensitive waveform is to structures within the layers of the Earth. To explain the discrepancies, a correction to the earth structure is essential. The corrections account for the thickness of the crust, speed gradient of h, the coefficient for the h and v in the upper mantle for surface wave fitting, a small variation of the S speed structure at a layer under the upper mantle above 771 km for S wave fitting, and a small variation at the base the mantle layers for ScS wave fitting. At some stations, a correction for S speed structure have yielded P wave fitting. Results of this research indicate that the 1-D earth model obtained through seismogram fitting at every hypocenter-observation station pair is unique. The Swave velocity on the upper mantle has strong negative anomalies. This paper criticized the previous earth models in the same area, which have been published by other seismologists, by analyzing the seismogram of C052297B earthquake in the TS seismological network station.References
Dziewonski, A.M. & Anderson, D.L., Preliminary Reference Earth Model, Phys. of the Earth and Plan. Int., 25, 297-356, 1981.
Kennett, B.L.N., IASPEI91 1991, Seismological Tables, Research School of Earths Sciences, Australian National University, 1991.
Kennett, B.L.N., Engdahl, E.R. & Buland, R., Constraints on Seismic Velocities in the Earth from Travel Times, Geophys. J. Int., 122, 108-124, 1995.
Gubbins, D., Seismology and Plate Tectonics, Cambridge University Press, Cambridge, 1990.
Keyser, M., Ritter, J. & Jordan, M., 3D Shear-Wave Velocity Structure of the Eifel Plume, Germany, Earth and Plan. Sci. Letters, 203, 59-82, 2002.
Tanaka, S., Very Low Shear Wave Velocity at the Base of the Mantle under the South Pacific Superswell, Earth and Plan. Sci. Letters, 203, 879-893, 2002.
Frederiksen, A.W., Bostock, M.G., Van Decar, J.C. & Cassidy, J.F., Seismic Structure of the Upper Mantle Beneath the Northern Canadian Cordillera from Teleseismic Travel-Time Inversion, Tectonophysics, 294, 43-55, 1998.
Souriau, A. & Poupinet, G., A Study of The Outermost Liquid Core using Differential Travel Times of the SKS, SKKS and S3KS Phases, Phys. of the Earth and Plan. Int., 68, Issue 1-2, 183-199, 1991.
Wysession, M.E., Valenzuela, R.W., Zhu, A. & Bartk, L., Investigating the Base of the Mantle using Differential Travel Times, Phys. of the Earth and Plan. Int., 92, Issue 1-2, 67-84, 1995.
Dziewonski, A., Block, S. & Landisman, M., A Technique for The Analysis of Transient Seismic Signals, Bulletin of the Seismological Society America, 59, 427-444, 1969.
Herrin, E. & Goforth, T., Phase-Matched Filters: Application to Study of Rayleigh Waves, Bulletin of the Seismological Society of America, 67, 1259-1275, 1977.
Feng, M., Assumpcao, M. & van der Lee, R.D., Group-Velocity Tomography and Lithospheric S-Velocity Structure of the South American Continent, Phys. of the Earth and Plan. Int., 147, 315-331, 2004.
van der Lee, R.D., James, A. & Silver, S., Upper Mantle S-Velocity Structure of Central and Western South America, J. of Geophys. Res., 106, No. 12, 30.821-30.885, 2001.
Okabe A., Kaneshima, S., Kanjo, K., Ohtaki, T. & Purwana, Surface Wave Tomography for Southeastern Asia using IRIS-FARM and JISNET Data, Phys. of the Earth and Plan. Int., 146, 101-112, 2003.
Zhao, D., Global Tomographic Imaging of Mantle Plumes and Subducting Slabs: Insight into Deep Earth Dynamics, Phys. of the Earth and Plan. Int., 146, 3-34, 2004.
Kennett, B.L.N., Gudmundsson, O. & Tong, C., The Upper-Mantle S And P Velocity Structure Beneath Northern Australia From Broad-Band Observations, Phys. of the Earth and Plan. Int., 86, 85 - 98, 1994.
Simons, F.J., Zielhuis, A. & van der Hilst, R.D., The Deep Structure of the Australian Continent from Surface Wave Tomography, Lithos, 48, 17-43, 1999.
Debayle, E. & Kennett, B.L.N., Anisotropy in The Australasian Upper Mantle From Love and Rayleigh Waveform Inversion, Earth and Plan. Sci. Letters, 184, 339-351, 2000.
Dalkolmo, J., Synthetische Seismogramme Fuer Eine Sphaerisch Symmetrische, Nichtrotierende Erde Durch Direkte Berechnung Der Greenschen Funktion, Diplomarbeit, Inst. fuer Geophys., Uni. Stuttgart, 1993.
Friederich, W. & Dalkolmo, J., Complete Synthetic Seismograms for a Spherically Symmetric Earth by a Numerical Computation of the Green's Function in the Frequency Domain, Geophys. J. Int., 122, 537-550, 1995.
Buland, R. & Chapman, C., Travel Time Calculation, Bulletin of the Seismological Society of America, 73, 1271-1302, 1983.
Friederich, W., Regionale, dreidimensionale Strukturmodelle des oberen Mantel aus der wellentheoritischen Inversion teleseismischer Oberflaechenwellen, Berichte des Instituts fuer Geophysik der Universitaet Stuttgart 9, 1997.
Garnero, E.J. & Lay, T., D" Shear Velocity Heterogeneity, Anisotropy and Discontinuity Structure Beneath the Caribbean and Central America, Phys. of the Earth and Plan. Int., 140, 219-242, 2003.
