Seismic Activity Preceding the 2011 M w 9.0 Tohoku Earthquake, Japan, Analyzed With Multidimensional Template Matching

International audience; The observation of a transient slip 1 month before the rupture of the 2011 Tohoku earthquake is a conandrum since the area was supposedly fully coupled. A better understanding of the mechanisms at work during the preseismic phase is thus fundamental. However, the configuration of the Pacific plate and the location of the Tohoku rupture zone 200 km from the coast make it difficult to detect microseismic events. In this study, we use a multidimensional template matching (MDTM) technique to detect earthquakes that are hidden in the noise. The temporal distribution of these 395 newly detected earthquakes provides new insights on the slip history of the megathrust earthquake epicentral zone. The detected events can be separated into two groups: 187 low-frequency detections (below 5 Hz) that well recorded the episodes of earthquake migration prior to the Tohoku earthquake and 208 high-frequency detections (above 10 Hz) that occurred close to the rupture zones of the M ≥ 4.8-6 earthquakes that struck between the 9 March 2011 M7.3 foreshock and the 30 November 2010 Tohoku-Oki earthquake. The seismic rate of these high frequency detection events starts to increase on 30 November 2010 until the Tohoku earthquake.