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.