A scaling law for the initiation of earthquakes and the specific case of the giant Tohoku event

The M 9.0, 2011 Tohoku earthquake yielded a complex broadband rupture extending southwards over 600 km along strike and triggering a large tsunami that ravaged the East coast of North Japan. Strong motion high-rate continuous GPS data and teleseismic data, indicated a complex frequency dependent rupture. To image the earthquake rupture in detail, we applied a backprojection technique to waveforms from local accelerometer networks. The earthquake began as a small-size twin rupture, slowly propagating mainly updip and triggering the break of a larger-size asperity at shallower depths, resulting in up to 50 m slip and causing high-amplitude tsunami waves. For a long time the rupture remained in a 100–150 km wide slab segment delimited by oceanic fractures, before propagating further to the southwest. The occurrence of large slip at shallow depths likely favored the propagation across contiguous slab segments and contributed to build up a giant earthquake. Through numerical simulations, we investigated possible structural control on the broadband rupture process of the Tohoku earthquake, in terms of the rupture velocity, seismic radiation and slip/stress distribution along the subduction interface, exploring a number of initial stress and interface behavior to capture the main features of the rupture and its radiation pattern. Finally we compared the initial phase of the Tohoku earthquake with the one of several other earthquakes in Japan, with magnitude ranging between 4 and 8, indicating a possible scaling of the slope of the moment rate function associated to an increase of the slip-weakening distance with magnitude.