The Chichi earthquake as analogues of earthquakes in the Japan Trench and the Nankai Trough

The area where the abnormal uplift occurred is located at the NW corner of the fault plane. There a river is running from east to west, and the surface becomes more or less flat with the river bed sediment. The GPS data by Dr. S.- B. Yu and Dr. Y.- A. Liou show that the accretionary prism south of the river moved in the NNW direction and collided against the sedimentary wedge (See GPS data support the inelastic deformation).

Traces of surface ruptures by Central Geol Survey

The collision caused the uplift of the northern edge of the sedimentary basin (e.g., the dam uplift), the surface rupture at the peach farm and the fall, and other thrusts. Therefore, the uplift is not due to the accumulation of ductile material from other places stated in the "Why did a large uplift appear at the NW corner?", but the sedimentary wedge between the hanging wall (the southern hill) and the underthrusting wall (the northern hill) was deformed due to the rebound of the accretionary prism.

Because the river bed sedimentary wedge is an analogue of a deep-sea trench sedimentary wedge, we can use the analogy to infer deformation patterns of a trench wedge associated with faulting which is branching from the decollement. I infer that an area of uplift associated with a tsunami earthquake would be the size of the trench sedimentary wedge, from the uplift area of the NW corner. I then revise the defomation pattern shown in figure (b) in "Where do tsunami earthquakes occur" as shown above. This implies that for an earthquake at the trench wedge becomes a tsunami earthquake, motion of the sediment should be blocked by the outer trench slope. This is just seen off Sanriki of the Japan Trench (Iwabuchi, 1980). Therefore, the NW anomalous corner provides a complete analogue to off Sanriku.

On the other hand, the N-S striking part of the surface fault gives a complete analogue of the Nankai Trough thrusts, because the Chelungpu fault is one of the out-of-sequence thrusts (OST) cutting the Plio-Pleistocene consolidated sediment (Suppe, 1981), and the thrusts in the accretionary prism in the Nankai Trough, which are OS, are similar to those observed in the western Foothills in Taiwan (Kuramoto et al., 1999; see the reference in Where do tsunami earthquakes occur? and Suppe, 1981). Both have thin sedimentary covers. As the great earthquakes along the Nankai Trough do not generate abnormal tsunamis, the N-S segment of the Chichi earthquake neither did, since the amount of the vertical uplifts of the sediment is more or less similar to the coseismic uplift revealed by GPS, which is a marked difference from the abnormal uplifts in the NW corner.

References Iwabuchi, Y., Topography of trenches in the adjacent seas of Japan, Marine Geodesy, 4, 121-140, 1980.