A method of estimation of the stress level in the focal region of an earthquake is proposed in this paper. The stress level in the focal region before and after the December 26, 2004 M9.0 Sumatra earthquake is estimated. The result shows that the stress value before this major earthquake occurred was about 2.79 MPa and dropped to 1.51 MPa after its occurrence with a decrease of about 46%. Therefore, this large earthquake occurred under a relatively lower stress level.
A major earthquake of M9.0 occurred off the west coast of Northwestern Sumatra on December 26, 2004, and caused strong tsunami waves resulting in the loss of several hundred thousand people. It could be the one of the largest earthquakes in the world history since 1900, and has therefore, attracted the attention many of scientists (Gower, 2005; Ji, 2005a; 2005b; Ni et al., 2005; Park, 2005; Stein and Okal, 2005; Yagi, 2005; and Yamanaka, 2005). But the stress level in the focal region of this large earthquake still remains to be investigated.
In seismology, the stress state of an earthquake could be reflected by its focal mechanism, the stress drop and the apparent stress. Although, the focal stress level could be estimated by the stress drop under the assumption that the focal stress level is directly proportional to the stress drop, the assumption by itself is questionable, because a high stress drop does not necessarily denote the existence of high stress level. Apparent stress is defined as the product of seismic efficiency and average stress, and could be used as an indicator of the focal stress level. |
