Structural Features of Coseismic Surface Ruptures

Abstract

Coseismic surface ruptures created different structural features in each of the four principal rupture segments (Lin 2017). The southwest segment is branched into two zones: Zone-S1 and Zone-2. Zone-S1 is mainly composed of distinct shear faults, left-stepping echelon cracks, and mole tracks occurred mostly along the main segment of the Hinagu fault (Fig. 3). Distinct shear faults striking N10−20°E and dipping 75−90° SE, subparallel to the general trend of the rupture zone, are dominated by right-lateral strike–slip movement. Horizontal slickenside striations observed on shear fault planes, marked by parallel lineations with some grooves and steps in unconsolidated clay, also show strike–slip-dominated movement. In contrast, Zone-S2 is mainly composed of extensional cracks and flexural structures along the southwestern segment of the Futagawa. The surface cracks are distributed over a wide area, and no distinct offset is observed. Liquefaction of sandy material occurred along the extensional cracks, in lowland areas near river channels, and was characterized by boiled sandy material along extensional cracks. Flexural structures formed in a field of vegetables as a waveform pattern, on which the extensional cracks duplicated (Lin et al. 2016; Lin 2017).