Broad-band<i>Lg</i>attenuation modelling in the Middle East

Abstract

We present a broad-band tomographic model of Lg attenuation in the Middle East derived from source- and site-corrected amplitudes. Absolute amplitude measurements are made on hand-selected and carefully windowed seismograms for tens of stations and thousands of crustal earthquakes resulting in excellent coverage of the region. A conjugate gradient method is used to tomographically invert the amplitude data set of over 8000 paths over a 45°× 40° region of the Middle East. We solve for Q variation, as well as site and source terms, for a wide range of frequencies ranging from 0.5 to 10 Hz. We have modified the standard attenuation tomography technique to more explicitly define the earthquake source expression in terms of the seismic moment. This facilitates the use of the model to predict the expected amplitudes of new events, an important consideration for earthquake hazard or explosion monitoring applications. The attenuation results have a strong correlation to tectonics. Shields have low attenuation, whereas tectonic regions have high attenuation, with the highest attenuation at 1 Hz found in eastern Turkey. The results also compare favourably to other studies in the region made using Lg propagation efficiency, Lg/Pg amplitude ratios and two-station methods. We tomographically invert the amplitude measurements for each frequency independently. In doing so, it appears the frequency dependence of attenuation in all regions is not compatible with the power-law representation of Q( f ), an assumption that is often made.