Traces of paleo-earthquakes and tsunamis along the eastern Nankai Trough and Sagami Trough, Pacific coast of central Japan

Abstract

Great earthquakes of M8 and above and accompanying tsunamis have repeatedly occurred in the Nankai and Sagami Trough regions. These events have caused severe damage to the coastal areas close to the troughs. As part of the response to the 2011 off the Pacific Coast of Tohoku Earthquake (or the Great East Japan Earthquake) and tsunami, the Cabinet office of the central Japanese Government proposed new guidelines for assessing the risk of similar earthquakes and tsunamis affecting the Nankai and Sagami Trough regions. These new guidelines call for the largest possible class of earthquake and tsunami to be taken into account even if the probability of such an event is low. Large earthquakes and tsunamis in this region would affect an area with high concentrations of population and industrial infrastructure. As a result of these changes, the last 2 years have seen a high public awareness of disaster mitigation measures in the region. One of the results has been that some local governments have begun upgrading their existing disaster prevention infrastructure, such as raising the height of existing dikes and reinforcing refuges to help protect the population in the case of future great earthquake and tsunami events.
Paleoseismological studies have been carried out in the Nankai and Sagami Trough regions to help establish the recurrence history of great earthquakes and tsunamis. The sources of information are both historical documents and tsunami deposits. However, our present level of knowledge remains insufficient to be able to confidently reconstruct the size and recurrence intervals of past earthquakes and tsunamis. This information is key to developing predictive models for the timing and size of future events as well as formulating disaster prevention measures, and there is a need for more studies.
In this excursion, we will visit historical sites that preserve evidence for past natural disasters and geological sites that preserve records of seismic-related uplift and great tsunamis. We will also observe the current state of tsunami disaster mitigation measures, such as the plan for tsunami evacuation, in Shizuoka Prefecture. Through this excursion, we also hope to offer the participants an opportunity to discuss the significance of geological information in paleoseismology, to assess its importance in informing and guiding plans for disaster mitigation and to explore likely future trends in the field of paleoseismology.