Earth Planets Space, Vol. 63 (No. 7), pp. 649-654, 2011
Emmy T. Y. Chang1 and Benjamin F. Chao2
1Institute of Oceanography, National Taiwan University, Taipei, Taiwan
2Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan
(Received April 19, 2011; Revised July 3, 2011; Accepted July 4, 2011; Online published September 27, 2011)
On March 11, 2011, Japan's Pacific coast of Tohoku (northeast Honshu) was hit by a strong tsunami set off by a large off-shore magnitude 9.0 earthquake. We employ the method of empirical orthogonal function (EOF) analysis on the dense, continuous GPS data from Japan's GEONET to obtain and examine in detail the spatio-temporal behavior of the 3-D co-seismic and near co-seismic surface deformation of Japan's island of Honshu. For northern Honshu, where the dominant deformation occurred, the co-seismic horizontal displacement and the post-seismic relaxation were toward the east-southeast with a maximum amplitude of nearly 4 meters. The vertical component experienced a subsidence of up to 50 cm on the east side of Honshu toward the epicenter but with a slight uplift to the west. The EOF solutions also detect clear post-seismic relaxation on the ∼20 cm level, of essentially the same deformation pattern but with a larger extent during the hours to days after the mainshock, as well as (3-D) signals from some of the major foreshocks/aftershocks. The co-seismic horizontal strain field derived from the EOF solutions exhibits an interesting spatial pattern that provides further insight towards better understanding of the dynamic and complex seismotectonics of the region.
Key words: EOF, GEONET, co-seismic displacement, post-seismic relaxation, strain field.