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On the Izu Peninsula of Japan, the Sagiriko fault group is located to the south of the Kita-Izu fault zone, which ruptured during the 1930 Kita-Izu earthquake (M jma = 7.3). A NS-trending strand of the Sagiriko fault group was ruptured during the 1930 earthquake, but conjugate EW-trending strands were not ruptured, and no Holocene activity on them has been documented. We surveyed the near-surface structure across the Sagiriko-b fault, one of the EW-trending strands, by ground penetrating radar profiling and radiocarbon dating of sediments obtained from two drilling cores. The results revealed a right-lateral displacement of 5.5 m of fluvial terraces around the survey site that formed after about 10 ka. The right-lateral slip-rate was calculated to be more than 0.55 mm/yr. This rate, which is significantly smaller than the crustal contraction rate in this area, suggests that the crustal contraction was probably spread across several EW-trending active faults parallel to the Sagiriko-b fault. [ABSTRACT FROM AUTHOR]

The Median Tectonic Line (MTL) is an arc-parallel strike-slip fault that accommodates much of the arc-parallel component of the oblique convergence of the Philippine Sea and Eurasian plates at the Nankai Trough. The MTL in Shikoku is one of the fastest-slipping faults in Japan, with a late Quaternary right-lateral slip rate of 5–10 mm/yr. To estimate the right-lateral slip amounts of the past faulting events on the MTL, we acquired 2D and pseudo-3D ground-penetrating radar (GPR) sections across the ENE-trending Ikeda fault of the MTL in eastern Shikoku. We conducted the GPR surveys at the Higashi-Miyoshi site, where two terrace riser offsets mark the active fault trace. The 2D lines were about 28–64 m long, and the pseudo-3D data were sized 20 m × 30 m with a 0.5-m inline spacing. We used 50 MHz GPR antennas and conducted wide-angle measurements to estimate the electromagnetic wave velocity. We identified three paleochannels on the final depth-converted GPR sections, and two of them are deflected by the fault. A paleochannel at 0.6–1.4 m depth is observed on all inline sections of the pseudo-3D GPR data. We built a 3D model of this paleochannel and estimated the right-lateral and vertical displacements of ~ 3.5 m and ~ 0.5 m, respectively. This paleochannel offset is probably caused by the most recent surface-rupturing earthquake on the Ikeda fault, which may be the 1596 Keicho-Fushimi earthquake. This study demonstrates the usefulness of the GPR surveys to identify geological features displaced laterally and vertically by the most recent surface-rupturing earthquake. [ABSTRACT FROM AUTHOR]