S-coda and Rayleigh waves from a decade of repeating earthquakes reveal discordant temporal velocity changes since the 2004 Sumatra Earthquake

Abstract

Temporal changes in the subsurface seismic velocity structure reflect the physical processes that modulate the properties of the media through which seismic waves propagate. These processes, such as healing of the surface damage zone and deep crustal deformation, are described by similar functional forms and operate on similar timescales, making it difficult to determine which process drives the observed changes. We examine earthquake-induced velocity changes using the measured lag-time time series τ(t) of the repeating earthquake sequences since the 2004 M w 9.2 Sumatra and 2005 M w 8.6 Nias earthquakes. The S coda velocity changes (δV S , equivalent to −τ S ) recover steadily during the 2005-2015 period. The Rayleigh wave velocity changes (δV LR , or −τ LR ) undergo transient recovery, followed by a strong δV LR reduction in late 2007. δV S recovery is most likely driven by deep processes, whereas the temporal breaks in δV LR recovery in 2007 mostly reflect surface damage and healing induced by the strong ground