High-Frequency Earthquake Ground-Motion Scaling in Utah and Yellowstone

Velocity seismograms from the University of Utah Seismograph stations are used to quantify high-frequency ground motion scaling for the Yellowstone region and the Wasatch front in Utah. This study uses a data set consisting of over 3000 waveforms from 93 stations and 506 regional earthquakes and mining-associated seismic events that occurred in the years 1995-2001. All events lie within the upper crust with magnitudes less than 5.5. The signals were processed to examine the peak ground velocity and Fourier velocity spectra in the frequency range of 1.0-16.0 Hz. Regression results for Utah are characterized by rapid decreases of amplitude at short distances, which requires low Q and a rapid geometrical spreading g(r) for the forward modeling. Q(f) = 160f0.75, (kappa)eff = 0.05 sec and (delta)(sigma)= 300 bars are selected to predict the excitation terms when combined with the derived geometrical spreading. The frequency-dependent excitation of mining-associated events differs significantly from that of earthquakes because of either an additional attenuation due to shallow source depth or apparent low stress drops associated with the slow collapse of long tunnels. A Q (f) = 140f0.70 and geometrical spreading less rapid than r-1.0 at short distance ranges are used to describe the wave propagation distance dependence in the Yellowstone region. The Yellowstone attenuation functional D(r, f) is very different from that of Utah, perhaps because a shallow wave guide in the caldera traps more energy. The Yellowstone excitation spectra were modeled using (delta)(sigma) = 30 bars and (kappa)eff = 0.04 sec.