Lateral variations of high‐frequency seismic wave propagation at regional distances across the Turkish and Iranian plateaus
Authors: Kadinsky‐Cade Katharine , Barazangi Muawia , Oliver Jack , Isacks Bryan 
Published: February 2008

In this study we investigate crustal and uppermost mantle physical properties that characterize some of the continental plateaus of the Middle East. This is done as part of a larger effort to map and compare high‐frequency wave propagation at regional distances across the earth's continental plateaus. Thousands of short‐period WWSSN seismograms recorded at stations located in the Middle East and produced by earthquakes with epicentral distances less than about 20° were examined visually in an effort to study lateral variations of high‐frequency (0.5–2 Hz) seismic wave propagation across this area, particularly to the north of the zone of continental collision between the Africa‐Arabian and Eurasian plates. Variations of frequencies and amplitudes of Sn and Lg relative to P are examined and mapped throughout the region, and this work is supplemented by a study of velocities of Pn, Sn, and Lg. Sn amplitude variations are very striking in this area. An important observation of this study is that Sn propagates efficiently beneath a major part of the Turkish and Iranian plateaus. Sn is strongly attenuated, however, in the northernmost portion of the plateaus south of the Black and Caspian seas and in an area between the two seas. These regions are characterized, in general, by active tectonism, including volcanism, faulting, and folding. However, this active tectonism is not restricted to the areas of high Sn attenuation but appears to extend beneath other parts of the Iranian and Turkish plateaus. Patterns of lateral variations in the propagation of Lg are not as consistent as those for Sn. Lg propagates efficiently across Turkey, Iran, and adjacent regions, but the Lg waves that cross the Turkish and Iranian plateaus are weak and have relatively long predominant periods of about 2–5 s. The Lg phase is not observed when the path of propagation crosses the southern Caspian and the Black seas, consistent with the evidence of oceanic‐type crustal structure beneath these seas. Lg is also not observed from subcrustal events located in the Hindu Kush region. The velocity of Pn beneath most of the plateau areas, and particularly in the regions of Sn attenuation, ranges between 8.0 and 8.2 km/s. Velocities for Sn and Lg throughout the Middle East are about 4.5 and 3.4 km/s, respectively. It is possible to interpret the efficient Sn propagation and the relatively normal uppermost mantle P and S velocities beneath a major part of the Turkish‐Iranian plateau as indicating a partial underthrusting of the Arabian continental plate beneath the Iranian and Turkish continental blocks. Alternatively, the mapped regions of efficient Sn propagation and high Sn attenuation may result from a differential cooling of a past thermal anomaly. In this case the isostatic compensation of the plateaus could be due to an anomalously hot uppermost mantle that developed behind the mid‐to‐upper Tertiary subduction zone(s) in this region and to a subsequent crustal shortening after the collision of the Arabian and Eurasian plates. Our results cannot uniquely confirm the validity of the above proposed models.