High-frequency earthquake ground motion scaling in mountainous regions: Examples from Zagros

Rates of ground motion dissipation during propagation and variation in source excitation as a function of magnitude are parameterized in the 1-12 Hz frequency range for earthquake ground motions recorded in the 5-500 km distance range at 10 seismic stations within northeastern Iraq. The availability of 761 small-to-moderate-sized earthquakes with local magnitudes of 1.64 - 5.9 allows for a parameterization of the regional scaling of high-frequency ground motions. This is achieved through regression modeling, relating the logarithm of measured ground motions to excitation, site, and propagation effects. Regression results for the Fourier velocity spectra and peak velocities at selected narrow bandpass-filtered frequencies are used to define a piecewise continuous geometrical spreading function, frequency-dependent Q(f), and distance-dependent duration that can be used with random vibrations to predict ground motion as a function of magnitude and distance. The duration results indicate significant variations of duration data with distance and with increasing frequency. The site terms agree well for all components of motion for mountainous sites and high values of site terms observed at sites where thick alluvial deposits are present and high anthropogenic noise is anticipated. Modeling indicates that the average regional attenuation of the peak ground motion is best fit by a geometrical spreading function of r^-1 for r ≥ 1 kkmm, r^-0.95for r > 40 kkmm, r⁰ for r > 120 kkmm, and r^-1.25 for rr > 195 kkmm. The frequency-dependent quality factor is Q(f) = 320 f^0.25.