Wave Propagation Characteristics of Short-Period Crustal Phases Near ARCESS and NORESS
Final rept. 1 May 1991-31 Oct 1992
PENNSYLVANIA STATE UNIV UNIVERSITY PARK DEPT OF GEOSCIENCES
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Wave propagation of crustal phases near the ARCESS array is examined and compared to characteristics near the NORESS array, with emphasis on Lg and its composition. f-k analysis is applied to the data, to identify arrivals in the records, then a composite of array beams is used to approximate each event. Record sections of composite-seismograms are constructed for different directions from ARCESS, to study propagational characteristics with distance and azimuth. Near surface velocity under NORESS and ARCESS is obtained through inversion of Rg wave dispersion curves. A composite ARCESS record section is compared to a NORESS record section of events located in the Caledonides. The differences that emerge are that Lg in the Caledonian region, north of NORESS, is dominated by discrete arrivals representing Moho reflections, with the order of reflection increasing with distance. In the Archean ARCESS region, however, Lg is dominated by turning waves, also with the order of reverberation increasing with distance, but with each confined to a small distance range. Rg wave propagation in the ARCESS region is much more efficient than in the Caledonian NORESS region, as Rg is observed to 400 km distance at ARCESS, but only to 200 km distance at NORESS. Synthetic record sections are constructed by wavenumber integration in order to model the observed character of Lg and Rg in the two regions. The dominating factor in the difference of Lg characteristics turns out to be the velocity gradient in the lower crust. A few earthquakes near ARCESS are also studied. Their depths can be constrained on the basis of phase velocities and depth phases. For reliable phase identification, however, the composition of Lg in the region needs to be accurately known.
- Geology, Geochemistry and Mineralogy