Resolving minute temporal seismic velocity changes induced by earthquake damage: the more stations, the merrier?

Ground shaking induced by earthquakes often introduces transient changes in seismic velocity monitored with ambient noise. These changes are usually attributed to relaxation behaviour following the coseismic damage in the subsurface and are of relevance for post-seismic hazard mitigation. However, the velocity evolution associated with this phenomenon can occur at very small timescales and amplitudes that are not resolved with seismic interferometry and are therefore challenging to link to laboratory experiments. A way to improve the temporal resolution of the velocity time-series is to test whether the estimation of the relative seismic velocity changes dv/v obeys the ergodic hypothesis in which the joint use of colocated stations would lead to better resolved measurements. In this study, we present results from a dense seismic array that was deployed for 2 weeks at the remarkable Patache site in Chile. Thanks to high temporal averaging capabilities, we are able to resolve seismic velocity changes in the 3–6 Hz frequency band at a 10-min resolution around the occurrence of a moderate earthquake (PGV ∼1 cm s ^–1 ). We report a velocity drop of ∼0.4 per cent in the first 10 min after ground shaking. Half of this initial drop was recovered within the 2 following days. The shape of the recovery follows a log-linear shape over the whole observed recovery phase, analogous to slow dynamics experiments. When normalized by the total amount of processed data, we show that the ergodic hypothesis almost perfectly holds in our network: the dv/v signal-to-noise ratio (SNR) obtained when averaging a few observation with large stacking durations for the correlation functions is almost equal to the SNR when using a large number of observations with small stacking durations. To understand if the ergodicity is linked to a particular site property, we use the array capabilities to identify the surf at the shoreline as the source of the noise and to derive a 1-D shear velocity profile with the focal spot imaging technique and a transdimensional Bayesian inversion framework. The inversion shows that hard rocks lie close to the surface indicating that this material hosts the observed shallow velocity changes. We discuss our high-resolution measurements and attribute them to a stable noise source excited by the shore, the ergodicity property and an ideal subsurface structure. Finally, we discuss the effect of moderate earthquakes on subsurface damage and the potential relaxation processes in hard rocks.