How quickly does the subsurface damage heal after the 1999 Chi-Chi earthquake? 

Prof Kate Huihsuan Chen (Department of Earth Sciences, National Taiwan Normal University)


Abstract
========

We observe changes in the waveforms of repeating earthquakes in eastern Taiwan
following the 1999 Mw7.6 Chi-Chi earthquake, while their recurrence
intervals appear to be unaffected. There is a clear reduction in waveform
similarity and velocity changes indicated by delayed phases at the time of
the Chi-Chi event. These changes are limited to paths that cross the 70¡Ñ100
km region surrounding the Chi-Chi source area, the area where seismic
intensity and co-seismic surface displacements were largest. This suggests
that damage at the near-surface is responsible for the observed waveform
changes. Delays are largest in the late S-wave coda, reaching approximately
120 ms. This corresponds to a path averaged S wave velocity reduction of
approximately 1%.

To understand the behavior of velocity reduction induced by 1999 M7.6
Chi-Chi earthquake, we compute the postseismic changes in wavefield using a
2-D finite-difference method for seismic waves simulation. The simulation
model covers a 200x100 km2 area and is discritized by small grid of 50 m,
with a seismic source radiating seismic wave of frequency up to 8 Hz. The
effect of slightly different location and focal mechanism on waveform
cross-correlation coefficient (ccc) was first examined. The subtle change in
source location, dip angle, and rake produce the ccc drop over the whole
seismograms and in all frequencies, which is not consistent with the
observed predominance of ccc reduction in high-frequency energy and in
S-wave coda. The effects of near-surface damage and fault zone damage with
varying depth are next examined, to compare with the spatial extent and
magnitude of ccc reduction. The computed change in scattering properties
correlates with the spatial extent of such influence zone only if the
near-surface, low-velocity anomaly is placed in a ~50 km wide area, or if
the fault zone damage is set at 10-20 km depth. The correlation between the
observation and simulation explored here point to not only pervasive damage
near the surface but also the deep, along-fault damage at the time of the
Chi-Chi earthquake. The reduction in waveform similarity persists through
the most recent repeating event in our study (November 15, 2007), indicating
that the subsurface damage induced by the Chi-Chi earthquake did not fully
heal within the first 8 years following the Chi-Chi earthquake.