Abstract
The 2011 Mw 9.0 Tohoku-oki earthquake is one of the world’s
best-recorded ruptures. In the aftermath of this devastating event, it
is important to learn from the complete record. We describe the state of
knowledge of the megathrust earthquake generation process before the
earthquake, and what has been learned in the decade since the historic
event. Prior to 2011, there were a number of studies suggesting the
potential of a great megathrust earthquake in NE Japan from geodesy,
geology, seismology, geomorphology, and paleoseismology, but results
from each field were not enough to enable a consensus assessment of the
hazard. A transient unfastening of interplate coupling and foreshock
activity were recognized before the earthquake, but did not lead to
alerts. Since the mainshock, follow-up studies have (1) documented that
the rupture occurred in an area with a large interplate slip deficit,
(2) established large near-trench coseismic slip, (3) examined
structural anomalies and fault-zone materials correlated with the
coseismic slip, (4) clarified the historical and paleoseismic recurrence
of M~9 earthquakes, and (5) identified various kinds of
possible precursors. The studies have also illuminated the heterogeneous
distribution of coseismic rupture, aftershocks, slow earthquakes and
aseismic afterslip, and the enduring viscoelastic response, which
together make up the complex megathrust earthquake cycle. Given these
scientific advances, the enhanced seismic hazard of an impending great
earthquake can now be more accurately established, although we do not
believe such an event could be predicted with confidence.