Earth Planets Space, Vol. 57 (No. 6), pp. 521-526, 2005LETTER
Tadashi Maruyama, Yuichiro Fusejima, Toshikazu Yoshioka, Yasuo Awata, and Tabito Matsu'ura
Active Fault Research Center, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8567, Japan
(Received February 10, 2005; Revised May 25, 2005; Accepted May 25, 2005)
The 2004 Mid Niigata Prefecture earthquake sequence produced a nearly 1-km-long, N-S to NNW-SSE-striking, west-side-up surface rupture along a previously unmapped fault at Obiro, Uonuma City, eastern margin of the epicentral region. The maximum vertical displacement is 20 cm. The topographic and geometric features of the surface rupture are indicative of reverse faulting with dip to the west, which is consistent with focal mechanism solutions and aftershock distribution. Along the major active faults and folds that form the framework of landforms in the epicentral region, systematic surface ruptures were not found. Exceptionally small rupture dimension and offset amount of the surface rupture compared with those of the modeled fault suggest that the fault slip appears to have been accommodated by both internal deformation in thick Neogene-Quaternary deposits and growth of the folds and blind thrusts around the epicentral region.
Key words: Surface rupture, reverse fault, 2004 Mid Niigata Prefecture earthquake, tectonics.