Temporal changes of subsurface velocities during strongshaking as seen from seismic interferometry

摘要

We apply a deconvolution method to a strong motion data set recorded at the surfaceand in boreholes in northeast Honshu, Japan. We try to characterize the nonlinear effectsof the subsurface soil during strong shaking and show the change of the subsurfacevelocity structure during the shaking. The deconvolved waveforms reflect the subsurfacevelocity structure, and their horizontal and vertical components correspond to S and Pwave, respectively, traveling from the borehole to the ground surface. The strong motionrecords with smaller values of peak acceleration do not include significant nonlineareffects, so the deconvolved waveforms of the observed accelerations can be well simulatedby the program SHAKE91. For high acceleration motions during the shaking of twoseparate earthquakes, large reductions of near-surface velocities are seen. In results for the2008 Iwate-Miyagi Nairiku earthquake, the large high-frequency ground motions over 4gat one near-source station caused a nonlinear response of the soil, and the reduction ofthe average shear wave velocity reached 24%. This corresponds to a stiffness change ofover 75%. The soil properties and the stiffness coefficient which changed during theshaking did not fully recover after the shaking, leaving a static change.