Fig. 2

Groundwater level changes in the six wells of the HSRI observational network for four earthquakes. a Responses to the M6.2 earthquake of November 22, 2014, in the northern Nagano prefecture, central Japan (event 9 in Table 2). The epicentral distance range is from 184 to 201 km. b Responses to the M6.5 earthquake of September 4, 2013, in the Izu Bonin region, southern Japan (event 4 in Table 2). The epicentral distance range is from 579 to 601 km. c Response to the M7.8 earthquake of April 25, 2015 (event 10 in Table 2). The epicentral distance range is from 5133 to 5148 km. The vertical line indicates the arrival time of Rayleigh waves identified on the seismograms. The propagation velocity is approximately 3.25 km/s, which is in agreement with the group velocities of Rayleigh waves with oscillation period of 30 s along the propagation path in and around China (Feng and An 2010). d Response to the M9.1 earthquake of March 11, 2011 near the east coast of Honshu, Japan (event 1 in Table 2). The epicentral distance range is from 433 to 453 km