Initiation of the great Mw 9.0 Tohoku–Oki earthquake

We determined the location, size, mechanism, and the frequency content of the first 4.0 s of the 2011 Tohoku–Oki earthquake. Since the beginning of this earthquake is very small, we develop a comparative approach against a near-by reference earthquake, the master event. We first determined the water depth near the master event using the differential timing between the water phase pwP reflected from the air–water interface and the depth phase pP reflected from the water–crust interface. Then we located the master event using the well-known ocean bathymetry in the area. After calibrating teleseismic arrays (Δ = 30° to 90°) at short periods for timing and amplitude with respect to the master event, we were able to determine the initiation of the main event. It began as a small (Mw = 4.9) thrust event located at 38.19°N, 142.68°E at a depth of 21 km, and, a few seconds later, evolved into a slower extremely large slip event up-dip.