Relaxation timescales after small and large earthquakes: similarity and controls from seismic velocity changes estimated in Patache, Chile.

Ground shaking induced by earthquakes often introduces transient changes in subsurface
rock's physical properties. Evidences for these changes come from estimated
seismic velocity changes that show co-seismic velocity drops, which are succeeded by a
phase of recovery (the so-called relaxation process). Because this transient behaviour may
influence hydraulic properties, friction properties in fault zones, material strength or landslide rates,
understanding the duration of the relaxation is important for post-earthquake hazard
mitigation. However, there is poor constraint on the recovery timescale, especially after
small seismic events. In this study, we present seismic interferometry results obtained
from a  seismic array at the Patache field site in Chile. Thanks to high 
averaging capabilities with this dense deployment of 13 stations, we are able to resolve relative seismic velocity
changes (3-6 Hz) at a 10-minutes resolution following a moderate seismic event (PGV ~
5 mm/s). After inferring the 1D shear velocity profile of our field site, we report a velocity
drop of ~0.4 % in the first 10 minutes after ground shaking, that precedes a recovery to
~50% of the initial pre-event value during the 48 hours following the event. We compare
this high resolution velocity change observation with a longer term, multi-annual velocity time-series that we obtained at
the same site and which exhibits the recovery induced by 2 large earthquakes (the 2007 Mw 7.7
Tocopilla and the 2014 Mw 8.2 Iquique). This combination of short and long observations allows us to
discuss the effect of ground shaking levels and earthquake sequences on the observed
relaxation timescales and highlights its key controls to possibly derive meaningful
predictive relationship for transient mechanical changes following earthquakes.