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

window

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

Stacked Spectral Ratio Traces

Drop & Recovery vs. PGA (peak ground acceleration)

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.