Earth Planets Space, Vol. 59 (No. 8), pp. e25-e28, 2007E-LETTER
Tomoya Harada1 and Katsuhiko Ishibashi2
1Graduate School of Science and Technology, Kobe University, Kobe 657-8501, Japan
2Research Center for Urban Safety and Security, Kobe University, Kobe 657-8501, Japan
(Received February 6, 2007; Revised June 21, 2007; Accepted June 29, 2007; Online published July 27, 2007)
The 1994 Hokkaido-Toho-Oki (Mw 8.3) and the 2000 Nemuro-Hanto-Oki (Mw 6.8) earthquakes have been regarded as great/large intraslab events within the subducted Pacific plate along the southern Kurile trench. Determining which nodal planes of their mechanism solutions are actual fault planes and whether or not the 2000 event occurred on the southwestern extension of the 1994 fault plane have been problematic. In order to clarify these points, we examine the relative location of their main-shock and aftershock distributions accurately by simultaneous relocation of their hypocenters using the Modified Joint Hypocenter Determination (MJHD) method. The result clearly shows that the two earthquakes occurred on trench-normal shallow-dipping fault planes within the Pacific slab, apart from and parallel to each other. Therefore, the 2000 earthquake was not a rupture on the deeper extension of the 1994 fault plane, but one on another fault plane. A large intraslab earthquake (Mw 7.8; depth, about 100 km) occurred to the northwest of the 1994 event in 1978, and two trenchnormal fracture zones exist outside the trench to the southeast of the 1994 and 2000 events. Therefore, these three intraslab great/large earthquakes are considered to have been ruptures along pre-existing trench-normal weak zones within the subducted Pacific plate.
Key words: 1994 Hokkaido-Toho-Oki earthquake, 2000 Nemuro-Hanto-Oki earthquake, Kurile trench, intraslab earthquake, simultaneous relocation, trench-normal weak zone.