Fig. 4

Time dependence of the peak frequency of the monitored microtremor H/V spectra normalized to the peak frequency at 14:15, December 6, 2016 (see black circle). Red circles indicate the results for the microtremor H/V spectra measured on April 15, April 16, April 17, April 23, May 3, May 5, May 14, May 19, May 22, June 4, August 20, September 13, October 9 and November 6, 2016, respectively; blue circle indicates the result measured on July 28, 2016, during the PS logging test by Yoshimi et al. (2016; 2017)