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Lawrence Livermore National Laboratory, Earth Sciences Division, L-202, Livermore, California 94551
Department of Physics New Mexico State University, Las Cruces, New Mexico 88003
Abstract
We present propagation characteristics of the short-period regional seismic phases Sn and Lg for the Middle East. Digital waveform data from the Iranian Long Period Array (ILPA) and the Global Seismic Network (GSN) stations ABKT (Alibek, Turkmenistan), ANTO (Ankara, Turkey), KEG (Kottamya, Egypt), and KIV (Kislovodsk, Georgia) were used in this study. Three-component rotated seismograms were bandpass filtered (0.5 to 5.0 Hz) and examined for the presence and strength (amplitude) of Sn and Lg relative to the Pn and Pg phases. The short-period Sn and Lg waveforms show great variability. Sn propagates efficiently for paths crossing the Mediterranean, Black, and Caspian Seas as well as for paths passing north of the Greater Caucasus, Alborz, and Kopet-Dag Mountains. Generally, Sn is not observed or propagates inefficiently for paths with segments beneath the Turkish plateau and northern Iranian plateau, consistent with a previous study by Kadinsky-Cade et al. (1981). This study finds that the zone of inefficient Sn propagation coincides with a zone of low Pn velocities (<7.9 km/sec) determined by Hearn and Ni (1994) from Pn tomography. Lg is consistently not observed for paths crossing the Mediterranean, Black, and Caspian Seas. Lg propagates efficiently for paths crossing the northern Arabian shield. Generally, Lg propagates efficiently for paths contained within the Turkish-Iranian plateau. Paths in the Middle East greater than about 2000 km consistently show no or little Lg energy.
The correspondence of zones of inefficient Sn propagation and low Pn velocities suggests that high temperatures and partial melt lie at shallow depths in the upper mantle beneath the Turkish plateau and northern Iranian plateau. The presence of water, presumably due to subduction of hydrated oceanic crust, lowers the solidus temperature and enhances partial melt. Lg is blocked by oceanic crustal structures as well as by structural and topographic variations associated with tectonic boundaries.
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