Dating of tsunami boulders from Ishigaki Island, Japan, with a modified viscous remanent magnetization approach

Reworked boulders are expected to acquire a viscous remanent magnetization (VRM) approximately parallel to Earth's modern magnetic field. The magnitude of such a VRM depends on several factors, including the time since reworking and ambient temperature, for which there are well-known theoretical relationships. VRM unblocking temperature can, therefore, be a powerful tool for determining the reworking age of boulders and can be used to assess the timing of geological hazards such as landslides and tsunami events. In this study, VRM unblocking temperatures for twenty-seven samples from four coral tsunami boulders on Ishigaki Island, Japan, are compared with three candidate time–temperature relationships for VRM acquisition. For the Pullaiah nomogram, which is applicable to single-domain magnetite particle assemblages, nineteen samples from four boulders agree well with the expected unblocking temperature derived from previously reported ¹⁴C ages. Two samples have low unblocking temperatures and six samples appear to have anomalously high VRM unblocking. The Walton nomogram, which is used for lognormal grain-size distributions, relates acquisition temperatures required to produce equal magnetic (paleo)intensities and cannot explain the high unblocking temperatures; it produces younger predicted ages than the youngest boulder ¹⁴C age. We find that an alternative time–temperature relationship, defined by a stretched exponential law, has the potential to yield reworking ages for the anomalous boulders that are consistent with the ¹⁴C ages. We suggest that future VRM dating can be undertaken using a combination of the Pullaiah nomogram and the stretched exponential law.