Abstract
Most large earthquakes occur along an oceanic trench, where an oceanic
plate subducts beneath a continental plate. Massive earthquakes with
a moment magnitude, Mw, of nine have been known to occur in only
a few areas, including Chile, Alaska, Kamchatka and Sumatra. No historical
records exist of a Mw = 9 earthquake along the Japan trench, where
the Pacific plate subducts beneath the Okhotsk plate, with the possible
exception of the AD 869 Jogan earthquake, the magnitude of which
has not been well constrained. However, the strain accumulation rate
estimated there from recent geodetic observations is much higher
than the average strain rate released in previous interplate earthquakes.
This finding raises the question of how such areas release the accumulated
strain. A megathrust earthquake with Mw = 9.0 (hereafter referred
to as the Tohoku-Oki earthquake) occurred on 11 March 2011, rupturing
the plate boundary off the Pacific coast of northeastern Japan. Here
we report the distributions of the coseismic slip and postseismic
slip as determined from ground displacement detected using a network
based on the Global Positioning System. The coseismic slip area extends
approximately 400 km along the Japan trench, matching the area of
the pre-seismic locked zone. The afterslip has begun to overlap the
coseismic slip area and extends into the surrounding region. In particular,
the afterslip area reached a depth of approximately 100 km, with
Mw = 8.3, on 25 March 2011. Because the Tohoku-Oki earthquake released
the strain accumulated for several hundred years, the paradox of
the strain budget imbalance may be partly resolved. This earthquake
reminds us of the potential for Mw\~9 earthquakes to occur along
other trench systems, even if no past evidence of such events exists.
Therefore, it is imperative that strain accumulation be monitored
using a space geodetic technique to assess earthquake potential.
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