Tectonic interpretation of the Taiwan earthquake on Sept. 21, 1999

-A subduction zone earthquake exposed on land-

     Tetsuzo Seno

Professor, Geodynamics Division, Earthquake Research Institute, University of Tokyo

Fig 1 Plate boundaries and tectonic elements near Taiwan

Taiwan is in the same tectonic position where the South China Sea is subducting beneath the Luzon arc. Since the collision with the continental slope of China started only 4 million years ago, only the sediment has been accreted.

Fig 2  Cross-section of the structure of Taiwan (Seno, 1994; modifiled from Suppe, 1981)

In the matured collision zone like Himalaya, the mechanical plate boundary (the Main Boundary Fault in Himalaya) is located far from the suture zone (the Indus-Zhanpo suture zone In Himalaya). Similarly in Taiwan, most of the plate motion is consumed at the thrusts and the decollement beneath the Western Foothills. The suture zone, i. e. the Longitudinal Valley, however has not died yet.

The structure of the Coastal Plain - Western Foothills should be regarded as a subduction zone, rather than a collision zone. Therefore 1999 Taiwan earthquake should be regared as a subduction zone earthquake. The slip direction of the Harvard CMT solution is different only by 5 degrees from the Eurasian - Philippine Sea plate motion expected from Seno et al. (1993).


Seno. T., S. Stein, and A. E. Gripp, A model for the motion of the Philippine Sea plate consistent with NUVEL-1 and geological data, J. Geophys. Res., 89, 17941-17948, 1993.

Seno, T., Tectonics in the Taiwan region, Jisin, 46, 461-477,1994 (in Japanese with a English abstract).

Suppe, J., Mechanics of mountain building and metamorphism in Taiwan, Mem. Geol. Soc. China, 4, 67-89, 1981.

 Updated Oct. 30, 1999  Report of the visit to the surface ruptures Seno's Homepage ERI's Homepage