Earth Planets Space, Vol. 63 (No. 7), pp. 609-614, 2011
Department of Earth & Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138
(Received April 13, 2011; Revised July 13, 2011; Accepted July 14, 2011; Online published September 27, 2011)
The devastating Mw 9.0 earthquake on March 11, 2011 is one of the most complex earthquakes of all the recent large events. Its source region is compact for an earthquake of this size, but it has highly variable amount of energy release from different segments. These conditions prevent conventional back-projection analysis to reveal the details of the rupture process. We incorporate a new metric to assess coherency as a part of back-projection analysis to ensure identification of these weak features. The main features obtained with this new back-projection approach are consistent with previous back-projection results, with strongest energy release downdip and close to the epicentral location. The main rupture propagation is along strike, in southwestern direction giving rise to the length extent of the earthquake. The new coherency function also allows us to investigate rupture characteristics at the beginning of the earthquake, resolving initial updip propagation from the epicentral location. Furthermore, some of very weak energy in the stacks are identified with high coherency. These additional source regions extend the area of the earthquake farther south and north than the region that has been imaged by other back-projection studies.
Key words: Back projection, rupture properties, high frequency, frequency dependence.