EVALUATION OF SITE AMPLIFICATION IN HAKUBA FROM OBSERVATIONS OF MICROTREMORS AND AFTERSHOCKS OF THE 2014 NORTHEN NAGANO EARTHQUAKE

The 2014 Northern Nagano Earthquake, occurred on 22 November 2014, caused the heavy damage to wooden houses especially in the Horinouchi and Mikkaichiba districts in Hakuba, Nagano prefecture, even though the maximum intensity was 6 lower observed during the event. However, we do not know the feature of the strong ground motion in the districts because of no availability of mainshock records. We therefore performed the aftershocks and the microtremor observations around the districts to evaluate the local site effect. The observed ground motions of aftershocks exhibit a variety within several km, such that the S-wave at periods of 0.5-1.0 s dominate in Horinouchi and Mikkaichiba, while the aftershock records are relatively small at K-NET Hakuba. The S-wave velocity structure was revealed with microremor exploration and we found that the Horinouchi district has the shallow soft soil with a depth of about 30 m, while the shallow soil is stiff at K-NET Hakuba. An average S-wave velocity in the top of 30 m depth shows a correlation with the seismic intensity and the amplification. The comparison of the spectral ratios of the aftershocks indicates that the ground motion at periods of 0.5-1.0 s is amplified due to the shallow soft soil in Horinouchi and Mikkaichiba. We observed the seismic intensity was large with an increase of about 1 in Horinouchi compared to the surrounding area. However, the effects of deep structure must be also considered in understanding of ground motion features since the observed amplification factor is larger than the estimation.