Three sub-events composing the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0) inferred from rupture imaging by back-projecting teleseismic P waves

摘要

High-quality vertical component seismograms of teleseismic P waves recorded at 151 stations of the European seismic network have been used to image the rupture process of the 2011 Tohoku Earthquake rapidly by a two-step back-projection method. The spatio-temporal distribution of rupture fronts suggests that the earthquake ruptured northeastwards and southwestwards over a total length of more than 340 km during at least 143 s. The fact that three fault segments ruptured at regions having different lateral heterogeneities implies that the earthquake comprised three sub-events. The first sub-event ruptured northeastwards during the first 25 s, and then turned to the northwest direction. The second sub-event ruptured at a relatively high speed of 2.78～4.70 km/s. The third sub-event ruptured with a direction variation from southwest to southeast near the latitude of 37°. In addition, considering that the first front of the second sub-event appeared at 74.6 s and was about 28 km away from the epicenter, we propose that the second sub-event might have been triggered by the localized increase in tectonic stress in the vicinity of the hypocenter that resulted from the rupture of the first sub-event.