Earth Planets Space, Vol. 63 (No. 7), pp. 603-607, 2011
Dun Wang1,2 and Jim Mori1
1Disaster Prevention Research Institute, Kyoto University, Kyoto, Japan
2Institute of Seismology, China Earthquake Administration, China
(Received April 7, 2011; Revised May 13, 2011; Accepted May 21, 2011; Online published September 27, 2011)
We use the back-projection method, with data recorded on the dense USArray network, to estimate the rupture propagation for the Mw 9.0 earthquake that occurred offshore of the Tohoku region, Japan. The results show a variable rupture propagation ranging from about 1.0 to 3.0 km/s for the high-frequency radiation. The rupture propagates over about 450 km in approximately 150 s. Based on the rupture speed and direction, the high-frequency source process can be divided into two parts. The first part has a relatively slow rupture speed of 1.0 to 1.5 km/s and propagates northwestward. In the second part, the rupture progresses southwestward starting with a slow speed of about 1.5 km/s and accelerating to about 3.0 km/s. We see three large pulses at 30 s, 80 s and 130 s. The first two, including the largest second pulse, were located 50 to 70 km northwest of the epicenter. The third occurred about 250 km southwest of the epicenter. The variability of rupture velocity may be associated with significant changes of physical properties along the fault plane. Areas of low/high rupture speed are associated with large/small energy releases on the fault plane. These variations may reflect the strength properties along the fault. Also, locations of the high-frequency radiation derived from the back-projection analysis are significantly different from the areas of very large slip for this earthquake.
Key words: Back-projection, the 2011 Tohoku Earthquake, high frequency energy, rupture velocity.