Abstract
AbstractTemporal 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 seriesτ(t) of the repeating earthquake sequences since the 2004Mw9.2 Sumatra and 2005Mw8.6 Nias earthquakes. TheScoda 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 strongδVLRreduction in late 2007.δVSrecovery is most likely driven by deep processes, whereas the temporal breaks inδVLRrecovery in 2007 mostly reflect surface damage and healing induced by the strong ground motions of the 2004Mw9.2, 2005Mw8.6, 2007Mw8.4, andMw7.9 Bengkulu and 2008Mw7.3 Simeulue earthquakes. The observed differences between the temporal variations inδVSandδVLRcan distinguish deep processes from healing of the surface damage zone.
Highlights
Long‐term monitoring of temporal seismic‐velocity variations in the crust is an important and long‐sought goal in geophysics because these temporal changes can be proxy measures for the mechanical processes and timescales of crustal responses to earthquake slip
The Rayleigh wave velocity changes 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
Numerous studies have suggested that these temporal velocity changes are often associated with the peak ground velocity (PGV) or acceleration (PGA) that is induced by the strong ground motion (GM) of an earthquake
Summary
Long‐term monitoring of temporal seismic‐velocity variations in the crust is an important and long‐sought goal in geophysics because these temporal changes can be proxy measures for the mechanical processes and timescales of crustal responses to earthquake slip. W. Yu et al (2013b, 2013a) detected repeating earthquakes (REs) near the Sumatra Subduction Zone and used these repeaters to probe the temporal changes in seismic velocity induced by the 2004 Mw 9.2 Sumatra and 2005 Mw 8.6 Nias earthquakes during 2005–2008 They found that the temporal velocity changes in the high‐frequency (HF, 0.5–2.0 Hz) S wave codas (δVS) and long‐period (LP, 0.03–0.1 Hz) Rayleigh waves (δVLR) followed similar logarithmic recovery timescales after the 2004 and 2005 earthquakes that were potentially linked to strong GM‐induced surface damage or/and postseismic afterslip. Through a forward modeling approach, we explore a series of perturbed velocity models and examine the sensitivity of these models against the observed attributes, to reconcile the first‐order pattern shown in the observed lag‐time time series τ(t) of the HF S coda and LP Rayleigh waves
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