Time-varying damping ratios and velocities in a high-rise during earthquakes and ambient vibrations from coda wave interferometry

Coda wave interferometry is applied to data from Community Seismic Network MEMS accelerometers permanently installed on nearly every floor of a 52-story steel moment-and-brace frame building in downtown Los Angeles. Wavefield data from the 2019 M7.1 Ridgecrest, California earthquake sequence are used to obtain impulse response functions, and time-varying damping ratios and shear-wave velocities are computed from them. The coda waves are used because of their increased sensitivity to changes in the building's properties, and the approach is generalized to show that a building's nonlinear response can be monitored through time-varying measurements of representative pseudo-linear systems in the time domain. The building was not damaged, but temporary nonlinear behavior observed during the strong motions provides a unique opportunity to test this method's ability to map time-varying properties. Reference damping parameters and velocities are obtained from a month-long period during which no significant seismic activity had occurred. Damping ratios measured over narrow frequency bands increase by up to a factor of 4 over short time durations spanning the main shock, as well as M > 4.5 aftershocks and a foreshock. The largest damping ratio increases occur for the highest frequencies, and the increase is attributed to friction associated with structural and non-structural surface discontinuities which experience relative motion and impact during shaking, resulting in energy loss. Shear-wave velocities in the building's east-west and north-south directions are found by applying a waveform stretching method to the direct and coda waves. The broadband velocities are reduced by as much as 10% during building shaking, and their restoration to pre-earthquake levels is found to be a function of shaking amplitudes. Until recently, these techniques had been limited by temporal and spatial sparsity of measurements, but in this study, variations of the impulse response functions are resolved over time scales of tens of seconds and on a floor-by-floor spatial scale.