First-order rupture features of the 2011 MW 9.0 Tohoku (Japan) earthquake from surface waves

• The fault parameters of the 2011 Tohoku earthquake were derived by Rayleigh waves.
• The earthquake was a slow event (∼1.5 km/s) with an asymmetric bilateral faulting.
• The rupture toward the NE had longer source duration than that toward the SE.
• The ruptures of the earthquake were inconsistent with typical tsunami earthquakes.

This study analyzed the rupture directivity of the 2011 Tohoku earthquake by using 100-s Rayleigh-wave travel-times, influenced by the finite source, to derive the fault parameters of the earthquake. The results demonstrated that the earthquake exhibited a slow rupture propagation with a rupture velocity of approximately 1.5–2.0 km/s and asymmetric bilateral faulting. The two rupture directions were N60°E and N127°E, with rupture lengths of approximately 276 km and 231 km, respectively. The rupture toward N60°E had a source duration of approximately 183 s, longer than that toward N127°E (approximately 156 s). Overall, the entire source duration of the earthquake faulting lasted approximately 183 s. Regarding historical seismicity in eastern Japan, the 2011 Tohoku earthquake not only ruptured a locked area in which large earthquakes have rarely occurred, but also ruptured the source regions of several historical earthquakes. With the exception of its slow rupture velocity and generation of a tsunami, the rupture features of the 2011 Tohoku earthquake were inconsistent with those of typical tsunami earthquakes.