Dynamic Stress Estimation During the M 5.7 Remotely Triggered Earthquake Caused by the 2016 M 7.0 Kumamoto Earthquake, Kyushu, Japan, Using Dense Strong-Motion and High-Rate GNSS Data

Abstract The 2016 Kumamoto earthquake of M 7.0 in Kyushu, Japan, was accompanied by an isolated remote earthquake of M 5.7 located 80 km northeast of the mainshock. This event may be one of the largest remotely triggered earthquakes ever reported. Using a new analysis approach, dynamic Coulomb failure stress changes (ΔCFF) at the epicenter of the triggered earthquake were estimated from strong-motion waveforms and high-rate Global Navigation Satellite System data, successfully revealing dynamic stress perturbation. The estimated dynamic maximum ΔCFF reached approximately +730 kPa, significantly exceeding the earthquake-triggering threshold and aligning closely with the result from a full waveform simulation by Miyazawa (2016). The triggered event occurred almost simultaneously with the passage of the maximum stress amplitude. The spatiotemporal distribution of dynamic ΔCFF, which was larger in the northeast and smaller in the southwest of the mainshock fault, reflected radiation and rupture directivity. These dynamic stress characteristics, along with the favorable geological conditions in the focal region, met the required criteria for triggering the event.