Fig. 2

(a) Example of stacked strain change. During this period, an S-SSE of \(M_w 5.9\) (Japan Meteorological Agency 2019) was detected. The gray shade indicates the period shown in (c) and (d). (b) Time series of correlation coefficients between the stacked strain shown in (a) and a ramp function of 1 h rise time (CC\(_{\textrm{sr}}\)). The gray shade is the same as that in (a). (c) Detrended stacked strain shown in (a) during a 5-day interval that is the middle part of (a) and (b). (d) Detrended accumulated number of tectonic tremors in the same time period as that of (c). The periods shown in (c) and (d) with gray shades show durations of strain steps with CC\(_{\textrm{sr}}> 0.88\)