Earth Planets Space, Vol. 63 (No. 7), pp. 519-523, 2011
Meredith Nettles, Göran Ekström, and Howard C. Koss
Lamont-Doherty Earth Observatory, Columbia University, USA
(Received April 7, 2011; Revised May 23, 2011; Accepted June 4, 2011; Online published September 27, 2011)
Centroid moment tensors (CMTs) for the March 11, 2011, off the Pacific coast of Tohoku Earthquake and its larger foreshocks and aftershocks are presented. The CMTs are calculated using long-period data from the Global Seismographic Network retrieved in near-real time, and the inversion algorithm employed in the Global CMT Project. Owing to its large size and long duration, the mainshock CMT analysis is performed at periods longer than 300 s. The resulting moment tensor has a scalar moment of 5.3 · 1029 dyne-cm, and a geometry that indicates thrust motion of the Pacific Plate beneath the island of Honshu on a plane dipping 10°. The large scalar moment, translating to moment magnitude MW = 9.1, makes this the fourth largest earthquake in the last 100 years. The mainshock was preceded by several large foreshocks with similar underthrusting mechanisms, the largest of which having MW = 7.4. The aftershock sequence extends over a portion of the plate boundary more than 500 km long, and exhibits an unusual diversity of faulting geometries. Reverse, normal, and strike-slip faulting events with widely varying directions of maximum tension and compression occur throughout the upper plate. A MW = 7.9 aftershock occurred near the southern end of the aftershock zone approximately 30 minutes after the mainshock, and is, at the time of writing, the largest aftershock in the sequence. The second-largest aftershock is a MW = 7.6 normal-faulting outer-rise earthquake that occurred 40 minutes after the mainshock.
Key words: Centroid moment tensor, Tohoku Earthquake, subduction, aftershock.