Earth Planets Space, Vol. 63 (No. 7), pp. 687-692, 2011
Thorne Lay1, Charles J. Ammon2, Hiroo Kanamori3, Lian Xue1, and Marina J. Kim1
1University of California Santa Cruz, Santa Cruz, California 95064, USA
2The Pennsylvania State University, University Park, Pennsylvania 16802, USA
3California Institute of Technology, Pasadena, California 91125, USA
(Received April 7, 2011; Revised May 22, 2011; Accepted May 25, 2011; Online published September 27, 2011)
The 11 March 2011 off the Pacific coast of Tohoku (Mw 9.0) Earthquake ruptured a 200 km wide megathrust fault, with average displacements of ∼15-20 m. Early estimates of the co-seismic slip distribution using seismic, geodetic and tsunami observations vary significantly in the placement of slip, particularly in the vicinity of the trench. All methods have difficulty resolving the up-dip extent of rupture; onshore geodetic inversions have limited sensitivity to slip far offshore, seismic inversions have instabilities in seismic moment estimation as subfault segments get very shallow, and tsunami inversions average over the total region of ocean bottom uplift. Seismic wave estimates depend strongly on the velocity structure used in the model, which affects both seismic moment estimation and inferred mapping to slip. We explore these ideas using a least-squares inversion of teleseismic P-waves that yields surprisingly large fault displacements (up to ∼60 m) at shallow depth under a protrusion of the upper plate into the trench. This model provides good prediction of GPS static displacements on Honshu. We emphasize the importance of poorly-constrained rigidity variations with depth for estimating fault displacement near the trench. The possibility of large slip at very shallow depth holds implications for up-dip strain accumulation and tsunamigenic earthquake potential of megathrusts elsewhere.
Key words: 2011 Tohoku Earthquake, megathrust faults, subduction zones, earthquake rupture process, tsunami earthquakes.