Detided tilt records from March 8, 2011 prior to the mainshock. (a) Records at stations in group A. (b) Records at stations in group B. The station groups are shown in Fig. 1. The sampling rate of these records is one minute. Atmospheric pressure changes and hourly precipitation at Morioka and Fukushima meteorological observatories of JMA are also plotted. The origin times of (1) the March 9 earthquake, and (2) the mainshock are indicated as solid arrows on the top of each frame.