High Resolution Models of Regional Phase Attenuation and Velocity Structure of the Turkish-Iranian Plateau and Zagros

Abstract:

We present an approach for understanding the origin and nature of seismic anomalies in the continental crust of the Northern Middle East. We have constructed detailed models of crustal attenuation and velocity structure for the Northern Middle East based on the analysis of waveforms of the regional seismic phases Lg and Pg from regional earthquakes recorded at more than 550 stations in Turkish and Iranian Plateaus and the surrounding regions. The attenuation and velocity models are assumed to serve as proxies for the bulk average effective crustal P-wave and S-wave attenuation (Q_a and Q_b) and velocities (V_p and V_s). About 30000 reliable spectra were collected for both Lg and Pg phases and used to measure the Two-Station Method (TSM) and Reverse Two-station/event Method (RTM) Lg and Pg Q at 1 Hz (Q_Lg0 and Q_Pg0) and their frequency dependence factor (η). The Q_Lg0 and Q_Pg0 and η values measured over the individual TSM and RTM paths are then used to perform an LSQR tomographic inversion for lateral variations in Q₀ and η. We observe a strong correlation between the effective Q and velocity models. Our models show lateral variations that coincide with the major tectonic boundaries in the region. The tomographic models as well as the individual TSM and RTM measurements show lower values of Q_Lg0 and Q_Pg0 over the Turkish-Anatolian Plateau (Q_Lg0<150 and Q_Pg0<200) than those observed over the Iranian Plateau (150< Q_Lg0<300 and 150< Q_Pg0<400). Furthermore, we obtained the Lg and Pg group velocity models (V_Lg and V_Pg) by inverting the time of the first arrival of the Lg and Pg envelopes. Our Q_Lg0 and Q_Pg0 models are strongly correlated with the V_Lg and V_Pg models suggesting that the source of many of the low Q and velocity anomalies is likely the same. Our Q models have implication for any hazard assessment in different regions of the northern Middle-East and can also be used for the magnitude determination of the local and regional seismic events. A combined knowledge about the attenuation of Pg and Lg waves has also application in developing reliable seismic discriminants.