Strain accumulation process around the Atotsugawa fault system in the Niigata-Kobe Tectonic Zone, central Japan

Abstract

SUMMARY In order to investigate the strain accumulation process around the Niigata-Kobe Tectonic Zone in central Japan, we estimate a precise GPS velocity field around a representative fault area, the Atotsugawa fault system. The velocity profiles obtained from two dense GPS arrays crossing the Atotsugawa fault demonstrate the existence of a laterally heterogeneous deformation pattern along the fault strike. The West Array suggests the possibility of surface fault creep, which disagrees with inferences gleaned from previous EDM measurements. High shear strain rate, which is calculated from wider velocity field, distributes around not only the Atotsugawa fault system, but also the Takayama-Oppara fault zone, which is located 50 km south from the Atotsugawa fault. In order to consider the effect of interaction between those active faults, we conduct a block-fault model analysis. Our block model consists of nine blocks and 33 fault segments. Relative motion between two blocks at the northern and southern ends of the fault system is calculated to be 9.9 mm yr−1. The intervening region between these two blocks corresponds to the Niigata-Kobe Tectonic Zone with a strain rate of 0.2 ppm yr−1. Slip deficit rates at the Atotsugawa, Ushikubi, and Takayama-Oppara fault zones are estimated to be 2.2, 3.9 and 2.3 mm yr−1, respectively. Although the ratio of slip deficit rate (2.2 : 3.9 : 2.3 ≈ 1 : 2 : 1) of these three faults is similar to that of geological long-term slip rates (1 : 2 : 0.7), the magnitude of geodetic slip deficits is 30 per cent to 50 per cent larger than long-term slip rates. This systematic difference may be due to inelastic deformation of the crust in this region. Since the total slip deficit rate between the Ushikubi and the Takayama-Oppara fault zone is 8.4 mm yr−1, about 85 per cent of the relative block motion, the deformation is mostly accommodated by elastic strain accumulation on these three faults.