Crustal and upper-mantle structure in the central United States of America from body-wave spectra,surface-wave dispersion,travel-time residuals and synthetic seismograms |
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| Authors: | Tuneto Kurita |
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| Institution: | Institute of Meteorology and Geophysics, Johann Wolfgang Goethe-University, Frankfurt am Main Federal Republic of Germany |
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| Abstract: | Regional variations of the velocity structure down to a depth of about two hundred kilometers in the central United States are investigated by a combined use of body-wave spectra, surface-wave dispersion, travel-time residuals and synthetic seismograms. With minor adjustments to models obtained in a preceding study of body-wave transfer ratios the revised models FLO74, OXF74 and SHA74 are proposed which reasonably satisfy all data in the above four kinds of methods. These models retain the same characteristic feature of the deepening of the low-velocity zone from the Gulf of Mexico to the Interior Plain nearly along 89°N longitude while decreasing its thickness and increasing its velocities as in the preceding models. The low-velocity zone is about 50 km thick ranging in depth from about 150 to 200 km around a junction of the Interior Plain with the Interior Highlands, about 80 km thick from about 120 to 200 km in the Coastal Plain, and about 90 km thick from about 90 to 180 km in the continental shelf of the Gulf of Mexico. Although it is not clear what relation, if any, exists between the above characteristic feature of the low-velocity zone and the recurrent relief of intra-plate stress along the central Mississippi Valley, an intricate crustal structure found along the valley northwest of the confluence of the Mississippi and Ohio rivers is apparently related to the occurrence of historical earthquakes. Comparisons of synthetic seismograms of the above models with teleseismic records of deep earthquakes reveal that the undulations in a time interval of over 100 sec between the onsets of the P and pP phases on long-period records are good surface expression of the underlying layered structure below the station and are not associated with the source. Relevant future high-quality data promise a comprehensive elucidation of the fine configuration of structure in the crust and upper mantle. |
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