A Detailed Source Model for the Mw9.0 Tohoku-Oki Earthquake Reconciling Geodesy, Seismology and Tsunami Records.

Wednesday, 17 December 2014: 8:30 AM
Quentin Bletery1, Anthony Sladen1, Bertrand Delouis1, Martin Vallee2, Jean Mathieu Nocquet1, Lucie Rolland3 and Junle Jiang4, (1)Geoazur - CNRS, Valbonne, France, (2)Institut de Physique du Globe de Paris, Paris, France, (3)Los Alamos National Laboratory, Los Alamos, NM, United States, (4)Caltech, Pasadena, CA, United States
The March 11, 2011, Mw9.0 Tohoku-Oki earthquake was recorded by an exceptionally large amount of diverse data. Many analyses of this mega-earthquake have been made based on different types of these data. In this work, we aim to reconcile them into a single better-resolved source model. To do so, we first present a kinematic 3D finite-fault slip joint inversion including an extended amount of complementary data: teleseismic, strong-motion, high-rate GPS, static GPS and tsunami records. The inversion fits all the data very well and reveals a patchy slip distribution with large-slip (up to 64 m) mostly located up-dip of the hypocenter and near the trench. We compare this co-seismic slip model and its time evolution to geophysical observations independent of the earthquake such as the seismicity before and after the earthquake, slab velocity anomalies imaged by tomography, rheological properties of the megathrust revealed by drilling and geological observations in the footwall. In an attempt to reconcile all these observations, we propose a rupture scenario in which the rupture started as a magnitude 7 like earthquake, in a region where events of this size are common. After 40-50s, the rupture reached an asperity locked for a very long time – possibly since the 869 Jogan earthquake – inducing enough stress perturbation to cause its rupture. Its rupture released an enormous amount of stress and lowered the effective friction of the megathrust located up-dip, allowing the footwall to slip with negligible dynamic friction. This scenario would explain the absence of aftershocks in the footwall and the presence of a large normal fault branching from the megathrust to the surface, which is a mark of particularly low friction on the megathrust. This scenario is supported by a stress-drop distribution derived from our inversion, which reveals a very high stress-drop patch correlated with the normal fault and a slab heterogeneity imaged by tomography.