Three-Dimensional Splay Fault Geometry and Implications for Tsunami Generation
G. F. Moore,1,2*
N. L. Bangs,3
A. Taira,1
S. Kuramoto,1
E. Pangborn,3
H. J. Tobin4
Megasplay faults, very long thrust faults that rise from the subduction plate boundary megathrust and intersect the sea floor at the landward edge of the accretionary prism, are thought to play a role in tsunami genesis. We imaged a megasplay thrust system along the Nankai Trough in three dimensions, which allowed us to map the splay fault geometry and its lateral continuity. The megasplay is continuous from the main plate interface fault upwards to the sea floor, where it cuts older thrust slices of the frontal accretionary prism. The thrust geometry and evidence of large-scale slumping of surficial sediments show that the fault is active and that the activity has evolved toward the landward direction with time, contrary to the usual seaward progression of accretionary thrusts. The megasplay fault has progressively steepened, substantially increasing the potential for vertical uplift of the sea floor with slip. We conclude that slip on the megasplay fault most likely contributed to generating devastating historic tsunamis, such as the 1944 moment magnitude 8.1 Tonankai event, and it is this geometry that makes this margin and others like it particularly prone to tsunami genesis.
1 Center for Deep Earth Exploration, Japan Agency for Marine Earth Science and Technology, 3173-25 Showamachi Kanazawa-ku, Yokohama, Japan.
2 Department of Geology and Geophysics, University of Hawaii, Honolulu, HI 96822, USA.
3 University of Texas Institute for Geophysics, J. J. Pickle Research Campus, Building 196, 10100 Burnet Road (R2200), Austin, TX 78758–4445, USA.
4 Department of Geology and Geophysics, University of Wisconsin, Madison, WI 53706, USA.
* To whom correspondence should be addressed. E-mail: gmoore{at}jamstec.go.jp
