Systematic analysis of temporal changes in site response associated with strong ground motion in
Japan
Chunquan WuAdvisor: Prof. Zhigang Peng
Collaborator: Prof. Dominic Assimaki (CEE)
EAS Graduate Student Symposium11/14/2008
IntroductionSite Effects: Ground motion can be significantly affected by the near-surface geological configuration.
From http://www-tamaris.cea.fr, http://eqseis.geosc.psu.edu, http://www.ce.washington.edu
Nonlinear Site Effects
dynamic properties are strain-dependent
widely observed during major EQs
significant influence on ground shaking
important for realistic ground-motion predictions
the controlling parameters are still under discussion
Kokusho, Current Science, 2004
KiK-net: 659 stations with a surface/downhole pair of strong-motion seismographs
~70 station-event pairs with PGA above 300 gal
download records of ~17000 events
focus on the records of several large earthquakes
Region and Data
Data Analysis ProcedureAn M5.2 earthquake on Nov 04, 2004 recorded by NIGH0610s
window
10s window10s
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5s sliding
Temporal Changes of Spectral Ratio
MYGH04, 2003 Miyagi-Oki earthquake, PGA: 664.7 gal
MYGH03, 2003 Miyagi-Oki earthquake, PGA: 809.1 gal
TTRH02, 2000 Western Tottori earthquake, PGA: 927.2 gal
Surface-to-downhole horizontal spectral ratio Peak spectral ratio decrease: 20-80% Peak frequency drop: 15-85% Recovery time scale: tens of seconds to several years
Station NI GH01 NI GH06 NI GH09 NI GH10 NI GH11 NI GH12 NI GH13 NI GH14 NI GH15
PGA (gal) 818.2 409.8 390.1 214.2 587.9 545.2 264.1 316.1 242.8
3 M>6 events within 1 hour after the Niigata MS.
9 Nearby stations with PGA larger than 200 gal during the MS.
Investigating the Role of PGA
NIGH06, 2004 Niigata earthquake sequence
PGA: mainshock 409.8 gal, aftershock1 232.0 gal, aftershock2 202.8 gal, aftershock3 258.3 gal.
Temporal Changes of Spectral Ratio
The role of site condition: inversion (ongoing work)
Hybrid two-step optimization scheme
Successfully applied in Japan and California (Assimaki and Steidl, 2007; Assimaki et al., 2008a,b).
Currently applying to the Niigata mainshock and aftershock sequence.
Assimaki, et al., BSSA, 2008a
MYGH04, 4 aftershocks of 2003 Miyagi-Oki earthquake
Conclusions and Future Plan
Surface-to-downhole peak spectral ratio decreases 20-80%, peak frequency drops 15-85%
Recovery time scale: tens of seconds to several years
The co-seismic peak frequency drop and the post-seismic recovery time increase with the PGA at a given site.
The co-seismic peak frequency drop and the post-seismic recovery time can vary significantly at a certain PGA level under various site conditions.
The controlling mechanisms for the time scale of recovery is still not clear.
The ongoing work of seismogram inversion would shed more light on the role of soil properties & layer structures.