August 2026

Installation of the measurement equipment at the Midorikawa Lower Reaches Branch Office. We began Distributed Acoustic Sensing (DAS) observations along the optical fiber cables running along the Midorikawa River in Kumamoto Prefecture on August 21, 2026. DAS is an observation technique that uses optical fiber cables themselves as sensors to acquire vibration data at intervals of several meters along cables extending for several tens of kilometers. Compared with conventional seismic observations, DAS provides exceptionally dense spatial sampling of seismic data. Our center previously conducted DAS observations at the same location from February to March this year. The optical fiber cables are installed along the Midorikawa and Mifune Rivers and cross the Hinagu Fault along their route. Following the occurrence of the 2026 Kumamoto Earthquake, we resumed temporary observations at the site. The earthquake is considered to have involved rupture of the Hinagu Fault, and the optical fiber cable crosses directly over the fault.DAS observations using an optical fiber cable crossing a fault before and after an M7-class earthquake are extremely rare worldwide. In addition to capturing the ongoing aftershock activity in detail, we hope that comparison with the measurements conducted before the earthquake will allow us to detect changes in the heterogeneous structure of the fault zone caused by the 2026 Kumamoto Earthquake. We would like to thank everyone at the Kumamoto River and National Highway Office and the Midorikawa Lower Reaches Branch Office of the Kyushu Regional Development Bureau, Ministry of Land, Infrastructure, Transport and Tourism, for their cooperation in conducting these observations. Example of an aftershock recorded by the 13-km-long optical fiber cable along the Mifune River. The vertical axis represents time, and the horizontal axis represents the measurement location. Redder colors indicate stronger ground motion. The horizontal signal appearing at around 23:29 indicates the occurrence of an aftershock. The diagonal lines correspond to vibrations generated by vehicles traveling on the road. Layout of the optical fiber cables. The blue line indicates the cable along the Midorikawa River, and the green line indicates the cable along the Mifune River. The red lines indicate the locations of active faults. Conceptual image by ChatGPT

Temporary DAS Observation Following the 2026 Kumamoto Earthquake