Ground Motion Amplification at Natural Rock Arches in the Colorado Plateau

Abstract

Thousands of rock arches are situated within the central Colorado Plateau—a region experiencing small- to moderate-magnitude contemporary seismicity. Recent anthropogenic activity has substantially increased the seismicity rate in some areas, raising questions about the potential for vibration damage of natural arches, many of which have high cultural value. However, predictions of the vibration response and potential for damage at a given site are limited by a lack of data describing spectral amplification of ground motion on these landforms. We analyzed 13 sandstone arches in Utah, computing site-to-reference spectral amplitude ratios from continuous ambient seismic data, and compared these to spectral ratios during earthquakes and teleseismic activity. We found peak ground velocities on arches at their dominant natural modes (in the range of 2–20 Hz) are ∼20–180 times the velocity on adjacent bedrock, due to amplification arising from slender geometry and low modal damping (0.8%–2.7%). Ambient spectral ratios are generally 1.2–2.0 times the coseismic spectral ratios. Because arches experience highly amplified ground motion, the range of earthquakes considered potentially damaging may need to be revised to include lower-magnitude events. Our results have implications for conservation management of these and other culturally valuable landforms.