Seismic Investigations of the Zagros-Bitlis Thrust Zone
We present results of crustal studies obtained with seismic data from the Northern Iraq Seismic Network (NISN). NISN has operated 10 broadband stations in north-eastern Iraq since late 2005. At present, over 800 GB of seismic waveform data have been analyzed. The aim of the present study is to derive models of the local and regional crustal structure of north and north-eastern Iraq, including the northern extension of the Zagros collision zone. This goal is, in part, achieved by estimating local and regional seismic velocity models using receiver function- and surface wave dispersion analyses and to use these velocity models to obtain accurate hypocenter locations and event focal mechanisms. Our analysis of hypocenter locations produces a clear picture of the seismicity associated with the tectonics of the region. The largest seismicity rate is confined to the active northern section of the Zagros thrust zone, while it decreases towards the southern end, before the intensity increases in the Bandar Abbas region again. Additionally, the rift zones in the Read Sea and the Gulf of Aden are clearly demarked by high seismicity rates. Our analysis of waveform data indicates clear propagation paths from the west or south-west across the Arabian shield as well as from the north and east into NISN. Phases including Pn, Pg, Sn, Lg, as well as LR are clearly observed on these seismograms. In contrast, blockage or attenuation of Pg and Sg-wave energy is observed for propagation paths across the Zagros-Bitlis zone from the south, while Pn and Sn phases are not affected. These findings are in support of earlier tectonic models that suggested the existence of multiple parallel listric faults splitting off the main Zagros fault zone in east-west direction. These faults appear to attenuate the crustal phases while the refracted phases, propagating across the mantle lid, remain unaffected. We will present surface wave analysis in support of these findings, indicating multi-pathing for surface waves from events located to the south-east of NISN, indicating the complex structure of the Zagros fault zone. In combination with receiver function, our first structural model of the crust beneath north-eastern Iraq indicates crustal depth of 40-50 km for the foothills, which increases to 45-55 km below the core of the Zagros-Bitlis zone.
Seismic Imaging of Microblocks and Channel Flow in Crust Beneath the Southeastern Margin of Tibetan Plateau
We derived three-dimensional Vp, Vs and Vp/Vs models of the crust for the southeastern margin of the Tibetan Plateau using the 2001-2004 earthquake data recorded on Seismological Bureau of Sichuan Province network stations. The Spherical-Earth Finite-Difference (SEFD) travel time calculation method was used in the tomographic system. The basic concept is the extension of a standard Cartesian FD travel time algorithm to the spherical case by developing a mesh in radius, co-latitude, and longitude, expressing the FD derivatives in a form appropriate to the spherical mesh, and constructing "stencil" to calculate extrapolated travel times. In shallow depths, the Sichuan Basin is imaged as a low velocity region with high Vp/Vs, caused by sedimentary materials filled in the basin. Outside the basin, the upper crust shows strong velocity heterogeneity with high velocity bodies surrounded by low velocities. The velocity transition zones are generally consistent with block boundaries derived from modeling the GPS data. We found widespread low velocity layers at various depths in crust, which are generally associated with relatively high Vp/Vs values. These low velocity layers are likely caused by partial melting, thus are weak and may act as a flow channel for crustal materials to move toward east from the Tibetan Plateau. The 2008 Wenchuan Ms8.0 earthquake was very likely caused by strain accumulation around the high velocity region when the channel flow was obstructed.