Identification of Faulting Events based on Radiocarbon Ages of Continuous Black Soil along Active Fault in Trench Wall

Abstract:

This study proposes the exact identification method of faulting events. Radiocarbon ages of typical black soil collected continuously from lower to upper part clarify the timing of faulting events. We sampled black soil at intervals of 3 – 20 cm in a 2.2 m long vertical without space from the excavated trench wall of the Atera Fault, central Japan. Sample preparation and radiocarbon dating were carried out in the JAEA-AMS-TONO of the Tono Geoscience Center, Japan Atomic Energy Agency. Calendar years were obtained by calibrating 14C age using OxCal 4.2.3 with IntCal13 atmospheric curve.
The soil ages in a footwall along the fault varied from 9,500 to 2,000 years ago with depth. The volcanic ash analysis supported the dating results. The soil deposited at an approximately constant rate, 0.1 – 0.2 mm/year. However, there are partially anomalous ages in the footwall. The soil ages at the depth of about 2.0 – 1.7 and 0.4 – 0.2 m were about 7,500 and 2,000 years ago, respectively. The results showed that the soil of a few dozen of cm in thickness deposited in a short term. In addition, the soil ages at the depth of about 1.5 – 1.1 and 0.8 – 0.7 m were about 7,000 and 4,000 years ago, respectively. These ages were older than the just lower soil ages, about 6,000 and 3,500 years ago. These anomalies are probably caused by fault displacements, which means the collapse of a hanging wall with old soil and the forming of lower part at the footwall side. The anomalous ages may suggest that faulting events took place about 7,500, 6,000, 3,500, and 2,000 years ago. Therefore, radiocarbon ages of the continuous black soil along the active fault are effective for identification of the faulting events. In our future research, this method will be applied to the soil of other active fault trench walls to clarify the timing of faulting events.