PROPAGATION CHARACTERISTICS OF SHORT-PERIOD SR AND LG IN THE MIDDLE-EAST

We present propagation characteristics of the short-period regional se ismic 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 stu dy, 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 wa veforms show great variability, Sn propagates efficiently for paths cr ossing the Mediterranean, Black, and Caspian Seas as well as for paths passing north of the Greater Caucasus, Alborz, and Kopet-Dag Mountain s. Generally, Sn is not observed or propagates inefficiently for paths with segments beneath the Turkish plateau and northern Iranian platea u, consistent with a previous study by Kadinsky-Cade et al. (1981). Th is study finds that the zone of inefficient Sn propagation coincides w ith 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 path s crossing the Mediterranean, Black, and Caspian Seas, Lg propagates e fficiently for paths crossing the northern Arabian shield. Generally, Lg propagates efficiently for paths contained within the Turkish-Irani an plateau. Paths in the Middle East greater than about 2000 km consis tently show no or little Lg energy. The correspondence of zones of ine fficient Sn propagation and low Pn velocities suggests that high tempe ratures and partial melt lie at shallow depths in the upper mantle ben eath 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 o ceanic crustal structures as well as by structural and topographic var iations associated with tectonic boundaries.