Earth Planets Space, Vol. 63 (No. 7), pp. 697-701, 2011
Kiyoshi Yomogida, Kazunori Yoshizawa, Junji Koyama, and Motohiro Tsuzuki
Earth and Planetary Dynamics, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan
(Received April 11, 2011; Revised May 31, 2011; Accepted June 4, 2011; Online published September 27, 2011)
We present some singular characteristics of the 2011 off the Pacific coast of Tohoku Earthquake in comparison with other megathrust earthquakes, such as the 1960 Chilean and the 2004 Sumatra-Andaman earthquakes. In addition to the conventional along-strike segmentation, along-dip segmentation of the fault area or subduction zone is an important feature for the Tohoku subduction zone, as indicated by the difference in background seismicity: virtually no seismicity in shallow segments but active with large events repeating in deep segments. The interaction between along-dip segments (deep and shallow segments) led to the great 2011 Tohoku earthquake. The along-dip segmentation results in plane or two-dimensional rupture propagation on a coseismic fault. Significant along-strike variability is also important for the 2011 Tohoku earthquake, with segments of both weak (e.g., slow or tsunami earthquakes) and strong plate couplings located adjacent to each other. In contrast, every segment appears to be with strong plate coupling for other megathrust earthquakes. One exception is the 1964 Alaska earthquake that shares a certain degree of common characteristics with the 2011 Tohoku earthquake: two distinct seismogenic zones along the dip direction of the trench, that is, the along-dip segmentation is noticeable. Significant along-strike variability also characterizes the activities in and around the subduction zone of the 1964 Alaska earthquake, including a creeping segment and a tsunami-earthquake segment.
Key words: Megathrust earthquake, along-dip segmentation, seismicity, along-strike variability, 2011 Tohoku earthquake.