S-coda and Rayleigh waves from a decade of repeating earthquakes reveal discordant temporal velocity changes since the 2004 Sumatra Earthquake
[post]
Wen-Che Yu, Jiun-Ting Lin, Jun Su, Teh-Ru Alex Song, Chu-Chun Kang
2020
unpublished
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
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... ies 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 motions of the 2004 M w 9.2, 2005 M w 8.6, 2007 M w 8.4, and M w 7.9 Bengkulu and 2008 M w 7.3 Simeulue earthquakes. The observed differences between the temporal variations in δV S and δV LR can distinguish deep processes from healing of the surface damage zone. Plain Language Summary Earthquake slip leads to stress relaxation in the crust, whereas healing of the damage induced by strong ground motion predominantly occurs in the near surface. Temporal changes in the seismic velocity structure after large earthquakes can be driven by diverse mechanisms, such as aseismic slip or fault zone healing, but the timescales governing these processes are very similar, making them difficult to distinguish. We detect temporal velocity changes in the crust since the great 2004 Sumatra and 2005 Nias earthquakes using the high-frequency late-arriving scattered waves after the S phase and long-period Rayleigh waves of repeating earthquakes. We find that the temporal velocity changes in the scattered waves exhibit steady logarithmic recovery from 2005 to 2015, whereas the Rayleigh wave velocity recovery was interrupted by several large earthquakes after late 2007. The difference between these two temporal trends in velocity change is the key to distinguishing between a damage/healing/redamage cycle near the surface and slow deformation (e.g., afterslip and postseismic relaxation) at depth. Rayleigh waves are highly sensitive to the near-surface damage and healing after the 2004/2005 events and also the repeated damage induced by the 2007 and 2008 earthquakes. Steady velocity recovery of the scattered waves primarily corresponds to slow deformation at depth.
doi:10.1002/essoar.10502533.1
fatcat:okyw6mcjdbhqnad75qfm647rga