Tomographic imaging of Lg and Sn propagation in the Middle East

Observations based on relatively limited data recorded by sparsely distribu ted stations have indicated that regional seismic phase propagation (Lg and Sn) is very complex in the Middle East. Accurate characterization of regio nal seismic wave propagation in this region necessitates the use of a large number of seismic stations. We have compiled a large data set of regional and local seismograms recorded in the Middle East. This data set comprises approximately four years of data from national short-period networks in Tur key and Syria, data from temporary broadband arrays in Saudi Arabia and the Caspian Sea region. and data from GSN, MEDNET, and GEOFON stations in the Middle East. We have used this data set to decipher the character and patte rn of regional seismic wave propagation. We have mapped zones of blockage a s well as inefficient and efficient propagation for Lg, Pg, and Sn througho ut the Middle East. Two tomographic techniques have been developed in order to objectively determine regions of lithospheric attenuation in the Middle East. We observe evidence of major increase in Lg attenuation, relative to Pg, ac ross the Bitlis suture and the Zagros fold and thrust belt, corresponding t o the boundary between the Arabian and Eurasian plates. We also observe a z one of inefficient Sn propagation along the Dead Sea fault system which coi ncides with low Pn velocities along most of the Dead Sea fault system and w ith previous observations of poor Sn propagation in western Jordan. Our obs ervations indicate that in the northern portion of the Arabian plate (south of the Bitlis suture) there is also a zone of inefficient Sri propagation that would not have been predicted from prior measurements of relatively lo w Pn velocities. Mapped high attenuation of Sn correlates well with regions of Cenozoic and Holocene basaltic volcanism. These regions of uppermost ma ntle shear-wave attenuation most probably have anomously hot and possibly t hin lithosphere.