Abstract: On 11 March 2011, the M_W9.1 earthquake struck Tohoku, Japan. In order to investigate the temporal and special changes of apparent stress in the rupture volume of the mainshock, we selected 563 earthquakes (5.0≤M_S≤6.9) located within the rupture volume which occurred between Jan., 1996 and Jun., 2016 to analyze the temporal variation and spatial distribution of apparent stress. It was found that the apparent stress started to increase at the middle of 2002, and reached to the highest value of 1.76 MPa from 0.6 MPa at the beginning of 2009, which increased about 3 times. Until when the 2011 Tohoku M_W9.1 earthquake was drawing closer, the apparent stress kept above 1.5 MPa. From the occurrence of the Tohoku M_W9.1 earthquake to Jun.2016, the apparent stress manifested a gradually dropping but kept above 1.5 MPa with the higher values. It showed that there were only individual higher values of apparent stress on the ground or the focal fault plane from 1996 to 2005, while there were lots of higher values of apparent stress in the rupture zone, from 2006 to Feb, 2011. In the three months after the occurrence of the Tohoku M_W9.1 earthquake, the apparent stress was at a high value, and then, gradually dropped with a slow rate. Since the apparent stress is a lower bound for the average stress on the fault plane, higher values of apparent stress could show higher values of average stress on the fault plane. Therefore, it can be said that, before the Tohoku M_W9.1 earthquake, the average stresses on fault planes of the rupture zone had been increasing for about 8.5 years. As a result, the stress in the rock of the rupture zone increased gradually before the Tohoku M_W9.1 earthquake, which cause faulting and occurrence of the earthquake.

Key words: Apparent stress, Tohoku M_W9.1 earthquake, Rupture zone