Systematic analysis of temporal changes in site response associated with strong ground motion in Japan Chunquan Wu Advisor: Prof. Zhigang Peng Collaborator: Prof. Dominic Assimaki (CEE) EAS Graduate Student Symposium 11/14/2008
Introduction Site 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
Region and Data • 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
Data Analysis Procedure 10s window 5s sliding 10s window 10s window An M5.2 earthquake on Nov 04, 2004 recorded by NIGH06
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
Investigating the Role of PGA • 3 M>6 events within 1 hour after the Niigata MS. • 9 Nearby stations with PGA larger than 200 gal during the MS.
Temporal Changes of Spectral Ratio NIGH06, 2004 Niigata earthquake sequence PGA: mainshock 409.8 gal, aftershock1 232.0 gal, aftershock2 202.8 gal, aftershock3 258.3 gal.
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. MYGH04, 4 aftershocks of 2003 Miyagi-Oki earthquake Assimaki, et al., BSSA, 2008a
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.