Establishment of a high-resolution tephro-stratigraphy and chronology based on geological survey

Abstract

This paper reviews the use of high-resolution tephro-stratigraphy and radiocarbon chronology to establish a high-resolution tephrochronology. Tephra formation is distinguished by loam or humic soil layer which presents repose period. Background level and resolution of tephra layers depend on the type of deposits (aeolian, marsh, or lacustrine sediments). Varved sediment from lake bottom is the most reliable recorder. Accurate recgnition of tephra layer in the field should be observed at multiple sites to avoid over counting the numbers of layers. It is recommended to use combination of source volcano and type locality for the tephra name. If numbers are used in the name, they should be given in descending order from the surface.
In the last decade, the development of techniques for ¹⁴C measurement provides high-precision dates. However, high-accurate date requires strict interpretation of geological relationship between dated sample and tephra deposition. Systematic sampling of soil can present high-resolution chronology. On the other hand, ¹⁴C wiigle-matching method is the most reliable technique to determine the eruption age.
An empirical formula, V = 12.2TA (Hayakawa, 1985), is useful to calculate volume of fallout tephra. The case study of the 2000 eruption of Usu volcano allows application of the formula for small-scale eruption. For best results, the most constrained contour should be used for calculation.