Fig. 1

Epicenter distributions of the detected earthquakes (01/10/1997–20/02/2021) in the study region. a Earthquake distribution (\(M_{{\text{j}}} \ge 4.0\), ≤ 100 km depth) before the 2011 Tohoku-Oki earthquake (01/10/1997–11/03/2011). b Earthquake distribution (\(M_{{\text{j}}} \ge 4.0\), ≤ 100 km depth) after the 2011 Tohoku-Oki earthquake (11/03/2011–20/02/2021). The yellow star is the epicenter of the 2011 Tohoku-Oki earthquake. Dashed lines denote the trench axes of the Japan Trench and the Sagami Trough. Red and blue rectangular regions denote the downdip and updip regions, respectively, that are mentioned in the discussion. c Magnitude–time plot of the earthquakes (01/10/1997–20/02/2021) that occurred in the study region. The \(M_{{\text{j}}} \ge 4.0\) earthquakes are shown in green. The yellow star denotes the 2011 Tohoku-Oki earthquake