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 Mw 9.2 Sumatra and 2005 Mw 8.6 Nias earthquakes. The S coda velocity changes (δVS, equivalent to −τS) recover steadily during the 2005–2015 period. The Rayleigh wave velocity changes (δVLR, or −τLR) undergo transient recovery, followed by a strong δVLR reduction in late 2007. δVS recovery is most likely driven by deep processes, whereas the temporal breaks in δVLR recovery in 2007 mostly reflect surface damage and healing induced by the strong ground motions of the 2004 Mw 9.2, 2005 Mw 8.6, 2007 Mw 8.4, and Mw 7.9 Bengkulu and 2008 Mw 7.3 Simeulue earthquakes. The observed differences between the temporal variations in δVS and δVLR can distinguish deep processes from healing of the surface damage zone.