Fig. 7

Temporal variations in the number of background earthquakes compared with the aseismic slip amount. a Temporal variations in the number of background earthquakes downdip of the large coseismic slip zone (red rectangle in Fig. 1). The blue curve is the best fit of the function \({{a + b} \mathord{\left/ {\vphantom {{a + b} {\left( {t + c} \right)}}} \right. \kern-0pt} {\left( {t + c} \right)}}\) to the observed number of background earthquakes per year (red points). The two black dashed lines provide a possible range for the number of background earthquakes per year before the 2011 Tohoku-Oki earthquake, which is deduced from Fig. 3 in Ogata (2011). b Relationship between the number of background earthquakes calculated using the estimated HIST-ETAS model and the aseismic slip amount, which was calculated using the similar earthquake catalog (Igarashi 2020) and the method outlined by Uchida et al. (2003). Black dashed lines are the same as in a. c Relationship between the number of background earthquakes calculated using the estimated HIST-ETAS model and the afterslip, which was estimated via a geodetic inversion analysis (Fukuda and Johnson 2021). Black dashed lines are the same as in a