High-Precision Elucidation of the 2016 Kumamoto Earthquake's Fault Activity History—Trench Survey Unveils a 15,000-Year Record—

A research team including Associate Professor Daisuke Ishimura from the Graduate School of Science at Chiba University, Assistant Professor Naoya Takahashi from the Graduate School of Science at Tohoku University, Professor Hiroyuki Tsutsumi from the Faculty of Science and Engineering at Doshisha University, and Researcher Toshihiko Ichihara from the Faculty of Social and Cultural Studies at Kyushu University, conducted an excavation survey (fault trench (Note 1) survey) on a surface earthquake fault that appeared during the 2016 Kumamoto earthquake to clarify its active history with high precision (Figure 1). The results revealed that there have been up to eight activities in the past approximately 15,000 years. Furthermore, they succeeded in narrowing down the timing of recent activities to a range of 200-300 years with high precision, and by comparing topographical data from before and after the earthquake, they clarified how the displacement during the earthquake was distributed among multiple faults. The findings of this research are expected to significantly contribute to the understanding of earthquake generation mechanisms and to improve the accuracy of future earthquake prediction.

This research was published online in the academic journal Seismica on April 6, 2026.

(The paper is available here: 10.26443/seismica.v5i1.1713)

Figure 1: Site conditions. (a) Fault trench survey in progress. (b) A distant view of the survey site. (c) Photograph of the trench wall. The fault is indicated by the white curved line. The bright yellow layer is volcanic ash from about 7,300 years ago.

■ Research Background

To evaluate the future probability of an earthquake on an active fault, data on its past activity history is essential. However, a major challenge has been the large errors in dating accuracy due to difficulties in locating faults and the preservation state of sediments. In this study, the team took on the challenge of achieving 'high-precision' historical information by carefully selecting a site where the fault location was clarified by the 2016 earthquake and the resolution of the sediments was high.

■ Key Research Findings

・ Elucidation of Fault Interaction Mechanisms: During large earthquakes, 'secondary' or 'subsidiary' faults can appear around the main fault. In the 2016 Kumamoto earthquake, the Idenokuchi fault, which runs parallel to the southeast of the main Futagawa fault, was also active. Although the displacement of these secondary faults is small, investigating their activity history provides important clues for future earthquake hazard assessment and for more accurately understanding the activity cycle of the main fault.

・ Elucidation of 3D Displacement Field by Topographic Difference Analysis: The research group conducted a comparative analysis of aerial laser survey data from before and after the earthquake to understand the three-dimensional surface movement in detail. The results revealed that the oblique slip underground was completely partitioned at the surface into 'right-lateral strike-slip' on the Futagawa fault and 'vertical movement' on the Idenokuchi fault. This strongly suggests a structural relationship where the Idenokuchi fault is connected to the Futagawa fault at a depth of several kilometers.

・ Narrowing Down the Timing of Activities: The fault trench survey (Figure 1) identified approximately 6 to 8 activities over about 15,000 years. Furthermore, by integrating data from volcanic ash, radiocarbon dating, and surrounding faults (such as the Futagawa fault), the timing of the last two activities was identified with a precision of 200 to 300 years. This is a high level of accuracy compared to previous surveys.

■ Future Outlook

The results of this study show how important it is to investigate not only the main fault but also the smaller surrounding faults. Faults with small displacement per event, like the Idenokuchi fault, have the advantage that past records are more likely to be well-preserved than on the main fault, where strata are easily disturbed by large displacements. Detailed investigation of similar secondary faults in other regions is expected to contribute to improving the accuracy of earthquake prediction and enhancing hazard assessment for complex fault systems. Furthermore, this result is expected to be important information for discussing the linkage and timing relationship with the Hinagu fault zone, which is located on the southern extension of the Futagawa fault.

■ Terminology

Note 1) Fault Trench: A trench excavated to artificially create a fault outcrop.

■ Paper Information

Title: Paleoseismic trenching on slip-partitioned surface ruptures associated with the 2016 Kumamoto earthquake

Authors: Daisuke Ishimura, Naoya O. Takahashi, Hiroyuki Tsutsumi, Shin’ichi Homma, Sakae Mukoyama, Toshihiko Ichihara

Journal: Seismica

DOI: 10.26443/seismica.v5i1.1713

■ About the Research Project

This research was conducted with support from the Grant-in-Aid for Scientific Research (17H04730).