A deep outer‐rise reverse‐fault earthquake immediately triggered a shallow normal‐fault earthquake: The 7 December 2012 off‐Sanriku earthquake ( M W 7.3)

Teleseismic body wave analysis revealed that the 7 December 2012 off‐Sanriku earthquake ( M W 7.3) at the outer rise of Japan Trench consisted of two successive subevents. The first subevent with reverse‐fault mechanism (Event 1, M W 7.1) at 56 km depth was followed by, approximately 20 s later, the second subevent with normal‐fault mechanism (Event 2, M W 7.2) at 6 km depth. Finite‐fault slip models show that the slip of Event 1 was concentrated around the initial rupture point with the maximum of 2.7 m and that Event 2 had two asperities with the maximum of 4.5 m at both sides of the initial rupture point. The static Coulomb Failure Function analyses suggested that Event 1 triggered Event 2 and that both subevents were promoted by the 2011 Tohoku earthquake ( M W 9.1).