Tsunamis: geology, hazards and risks: introduction

A decade or so ago, if you had asked almost anyone in Europe or North America, they might not have recognized the word ‘tsunami’. The enormous and tragic event that swept across the shores of the Indian Ocean on 26 December 2004, followed only a few years later by the devastating tsunami caused by the March 2011 Great Tohoku earthquake off Japan, both with appalling loss of life, changed all that. Today, the words ‘tsunami warning issued’ seem to appear frequently on international ‘breaking news’, showing the extent to which we have become sensitized to the triggers that launch these deadly, but terrifyingly spectacular, natural events. Yet, great tsunamis and the tectonic events that cause them have not suddenly become more frequent. The historical records of old civilizations contain accounts of major inundations reaching back hundreds or thousands of years and sometimes even warnings to future generations – valuable, if they are heeded. What has changed, and has consequently raised the profile of tsunamis, is the exponential growth in world population over the last few 100 years, the great majority of whom live in coastal areas and are consequently exposed to hazard, along with instant global communication, which brings every large earthquake on Earth's plate margins directly and immediately onto our screens

Increased rates of large-magnitude explosive eruptions in Japan in the late Neogene and Quaternary

Tephra layers in marine sediment cores from scientific ocean drilling largely record high-magnitude silicic explosive eruptions in the Japan arc for up to the last 20 million years. Analysis of the thickness variation with distance of 180 tephra layers from a global dataset suggests that the majority of the visible tephra layers used in this study are the products of caldera-forming eruptions with magnitude (M) >6, considering their distances at the respective drilling sites to their likely volcanic sources. Frequency of visible tephra layers in cores indicates a marked increase in rates of large magnitude explosive eruptions at ~8 Ma, 6–4 Ma and further increase after ~2 Ma. These changes are attributed to major changes in tectonic plate interactions. Lower rates of large magnitude explosive volcanism in the Miocene are related to a strike-slip dominated boundary (and temporary cessation or deceleration of subduction) between the Philippine Sea Plate and southwest Japan, combined with the possibility that much of the arc in northern Japan was submerged beneath sea level partly due to previous tectonic extension of Northern Honshu related to formation of the Sea of Japan. Changes in plate motions and subduction dynamics during the ~8 Ma to present period led to (1) increased arc-normal subduction in southwest Japan (and resumption of arc volcanism) and (2) shift from extension to compression of the upper plate in northeast Japan, leading to uplift, crustal thickening and favourable conditions for accumulation of the large volumes of silicic magma needed for explosive caldera-forming eruptions