Rupture process of the 9 March, 2011 Mw 7.4 Sanriku-Oki, Japan earthquake constrained by jointly inverting teleseismic waveforms, strong motion data and GPS observations

The slip history of the 2011 Mw 7.4 Sanriku-Oki, Japan earthquake, which occurred fifty-one hours before the Mw 9.1 Tohoku earthquake, is constrained by jointly inverting waveforms of teleseismic body waves, long period surface waves and local strong motions as well as GPS observations, after first relocating its hypocenter using a double difference approach and teleseismic P waves. The inverted results indicate that the rupture of this Sanriku-Oki earthquake was dominated by the failure of an elliptical shape asperity, elongating roughly along the plate motion direction. The rupture initiated at the southeast corner of this asperity and propagated mainly in the west-northwest direction with a rupture velocity of 3.1 km/s in the beginning 15 s and 1.1 km/s in the next 40 s. It released a total seismic moment of 1.6 × 10²⁰ Nm, with 82% occurring in the first 25 s. The rupture had an average slip of 1 m and produced an average stress drop of 0.9 MPa. The Sanriku-Oki earthquake did not break the hypocenter region of the Mw 9.1 Tohoku earthquake but slightly increased the Coulomb stress there. A correlation between the high slip region and the high Vp/Vs ratio of the overriding plate right above the plate interface has been found, which suggests the Sanriku-Oki earthquake and its frequent predecessors might have broken a relatively weaker patch within a large strongly coupled asperity.