Jeffrey R. Moore

Continuous ambient seismic monitoring is becoming common for structural health assessment of geologic features. However, the ability to detect or predict permanent mechanical change associated with damage requires detailed understanding of reversible drifts in resonance attributes associated with changing meteorological conditions. Here, we analyze the response of 17 sandstone rock arches to changing meteorological conditions during extended vibration‐based monitoring, with a focus on 1 arch in Utah which was continuously monitored for 15 months. Our results show that variations in resonance are correlated with temperature on daily and yearly time scales, but that the temperature sensitivity of frequency changes are variable at different sites and resonance modes, generally ranging from 0.5% to 6% per 10°C. Numerical modeling suggests the primary mechanism governing these frequency drifts is stress stiffening, where confined thermal dilation induces bulk stress changes in the low‐stress, nonlinear elastic regime via fracture closure. Secondary mechanisms affecting resonance attributes are driven by moisture, including formation of shallow pore ice, which can generate sharp frequency changes of up to 17% per −10°C, and moisture‐induced softening. Daily lag times of several hours between temperature and frequency extrema provide constraints on the rock volumes affected by these mechanisms, indicating modal attributes are sensitive to thermally driven changes occurring in the outermost centimeters of the structure. Through improved understanding of the reversible variations of rock mass resonance, our results aid future assessment of irreversible frequency changes associated with damage, and thus structural health monitoring of fragile geologic features.