Near-surface velocity reduction and anisotropy for early aftershocks revealed by coda interferometry

Measuring stress change and damage-induced seismic velocity and anisotropy in near-surface structures and their temporal variations on elastic properties provide insights into the underlying mechanisms of healing processes and their evolution. In addition, it could provide information for site classification. We analyze three-component surface and borehole seismic data collected by NIED KiK-net to explore the near-surface temporal variations in S-wave velocity and S-wave azimuthal anisotropy through coda interferometry (Chen et al., 2017, EPSL). Coda cross-correlations suppress the effect of incident angle, which is more stable rather than S-wave deconvolution techniques. Our analysis mainly focuses on the Hokkaido area where experienced strong ground motions from the 2018 Mw 6.6 Hokkaido Eastern Iburi earthquake. We analyzed the KiK-net data in a frequency band of 2-10 Hz before and after the mainshock and evaluated seismic velocity and azimuthal anisotropy between surface and borehole sensors. The station IBUH03 shows a significant reduction of ~6.0% for S-wave velocity after the mainshock and quick logarithmic healing recovery; changes in the fast S-wave polarization direction and strength of anisotropy (~140 degrees and +0.6% at IBUH03) are also observable during the coseismic periods. Our analysis suggests that the polarization direction of fast S-wave was systematically changed from 30 degrees to 170 degrees (measured clockwise from the north) following the mainshock. We suggest that the stress changes after the events created water-saturated cracks, which are responsible for the reductions of seismic velocities and the changes of seismic azimuthal anisotropy. We calculated the Vs30-based site amplification and the increment of seismic intensity. Considering the near-surface velocity reduction, the seismic intensity for early aftershocks tends to be larger than expected, indicating the importance of time-dependent seismic hazard assessment.