The Japan Agency for Marine-Earth Science and Technology(JAMSTEC)conducted a drilling expedition to determine the long-term earthquake history of the Enshunada Sea area on the eastern part of the Nankai Trough from January 5–8, 2020 (Figure 1). This work was performed as part of the Chikyu (Note 1) Shallow Core Program (SCORE, Note 2), which was jointly operated with the Japan Drilling Earth Science Consortium (J-DESC, Note 3). We are pleased to announce the preliminary findings following onboard analyses of drilled samples, conducted by JAMSTEC, the National Institute of Advanced Industrial Science and Technology, and the Atmosphere and Ocean Research Institute and Graduate School of Frontier Sciences of The University of Tokyo.

1．Overview

Historical earthquakes are known from the Enshunada Sea, which forms the eastern end of the Nankai Trough. Earthquakes have occurred in this area at intervals that differ from those in other areas across the trough, where an oceanic plate continues to subduct beneath a continental plate. It is generally thought that earthquakes occur when these plates slide quickly along the boundary. As it is believed that a topographic high between the two plates in the Enshunada Sea suppresses the occurrence of earthquakes, the intervals between earthquakes may be longer than in other Nankai Trough areas. The evidence for this should be recorded in local marine sediments; however, previously recovered samples have not provided a long enough time interval to verify these hypotheses.

On the expedition undertaken in this study, continuous sampling was conducted using a coring system on the research vessel Chikyu, and we were able to successfully collect sediment layers formed by earthquakes (i.e., turbidites, Note 4) at a depression on the seafloor of the Enshunada Sea, where we expected the most continuous geological records to have been preserved. We recovered more than 80 m of continuous sedimentary samples by drilling. In the upper 40-m layer, we found an interval that contained approximately 200 turbidites (Figure 2). Based on the sedimentation rates suggested in previous studies, the period recorded by these 40 m spanned 40,000–50,000 years. As approximately 200 turbidites were found, the average turbidite depositional interval was approximately 200 years. This is the first time that turbidite sequences, which are considered to form during earthquakes, have been collected for such a long period of time (40,000—50,000 years).

As records of land-based tsunami deposits are generally limited in age to tens of thousands of years ago due to changes in sea level, the long-term records we obtained are extremely valuable. We were able to obtain these records because of the Chikyu's hydraulic piston coring system (Note 5), which enabled the sampling of extensive sedimentary intervals with minimal disturbance. In the future, we plan to determine the geologic time period/s in which the turbidites formed and to examine their depositional intervals in greater detail. As these analyses will be based on continuous data from a sufficiently long period of time, we can expect to obtain new insights into the intervals between earthquakes in the Enshunada Sea.