High-Frequency Attenuation in the Lake Van Region, Eastern Turkey

Abstract

We provide a complete description of the characteristics of excitation and attenuation of the ground motion in the Lake Van region (eastern Turkey) using a data set that includes three‐component seismograms from the 23 October 2011 M w 7.1 Van earthquake, as well as its aftershocks. Regional attenuation and source scaling are parameterized to describe the observed ground motions as a function of distance, frequency, and magnitude. Peak ground velocities are measured in selected narrow frequency bands from 0.25 to 12.5 Hz; observed peaks are regressed to define a piecewise linear regional attenuation function, a set of excitation terms, and a set of site response terms. Results are modeled through random vibration theory (see Cartwright and Longuet‐Higgins, 1956). In the log–log space, the regional crustal attenuation is modeled with a bilinear geometrical spreading ![Graphic][1] characterized by a crossover distance at 40 km: ![Graphic][2] fits our results at short distances ( r is better at larger distances (40< r <200 km). A frequency‐dependent quality factor, Q ( f )=100( f / f ref)0.43 (in which f ref=1.0 Hz), is coupled to the geometrical spreading. Because of the inherent trade‐off of the excitation/attenuation parameters (Δ σ and κ ), their specific values strongly depend on the choice made for the stress drop of the smaller earthquakes. After choosing a Brune stress drop Δ σ Brune=4 MPa at M w=3.5, we were able to define (1) an effective high frequency, distance‐ and magnitude‐independent roll‐off spectral parameter, κ eff=0.03 s and (2) a size‐dependent stress‐drop parameter, which increases with moment magnitude, from Δ σ Brune=4 MPa at M w 3.5 to Δ σ Brune=20 MPa at M w 7.1. The set of parameters mentioned here may be used in order to predict the earthquake‐induced ground motions expected from future earthquakes in the region surrounding Lake Van. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif [3]: /embed/inline-graphic-3.gif