Absolute Dating of Past Seismic Events Using the OSL Technique on Fault Gouge Material-A Case Study of the Nojima Fault Zone, SW Japan

This study is the first part of a comprehensive paleoseismic study in the Nojima Fault Zone (NFZ), the seismogenic fault of the 1995 M-w 6.9 Kobe (Japan) earthquake, which explores the use of the luminescence dating method for understanding past faulting time and assessing the activity of faults developed in basement rocks. Our approach is focused on methodological aspects of the optically stimulated luminescence (OSL) technique and reports on a series of ages of fault gouge samples. Our analysis revealed that (1) quartz OSL signals have been reset (at least partially) during past seismic faulting events, (2) fault gouge layers bounded by sharp fault planes are younger than gouge layers far from the main fault planes, and (3) the most recent seismic faulting, affecting the different gouge layers on the Nojima fault took place from 62.8 +/- 4.3 ka to 18.5 +/- 1.3 ka, while on the Asano fault from 139.3 +/- 8.9 ka to 45.8 +/- 3 ka, with these ages representing maximum possible ages. In this regard, this is the first successful absolute dating attempt on the NFZ using quartz grains and the only luminescence study, as yet known, producing a series of ages representing neotectonic activity of the Nojima and Asano faults during the late Pleistocene and middle to late Pleistocene, respectively. The present work has indicated that OSL is potentially a promising technique for dating fault gouges and assessing the activity of faults, although more work is needed for further refinement. Plain Language Summary Historical information of past earthquakes is generally inadequate for evaluating the activity of faults in the long past. Paleoseismology approaches this problem by using a number of techniques and methods for identifying and dating earthquake-disturbed materials for which recorded information is not available. To this end, the luminescence dating techniques may provide this information on time scales ranging from some years to even 2 million years. However, there is no systematic effort on their use for such purposes. This study explores the use of a geochronological technique, namely, the optically stimulated luminescence (OSL) technique, and reports on a series of ages of fault rock material, which are directly associated with past fault ruptures, collected from the Nojima and Asano faults (SW Japan). A number of tests performed to validate the reliability of the selected dating technique proved that it could produce accurate results. The derived ages provided evidence for faulting activity (fault ruptures) in the Nojima and Asano faults during the late Pleistocene and middle to late Pleistocene, respectively, with each individual age representing the maximum possible age of a past fault rupture event. Thus, it may be suggested that the selected geochronological technique is potentially reliable for dating past seismic events.