Foreshock Acceleration Linked to Slow Earthquakes Before a Large Earthquake: Implications for Two-Stage Aseismic Processes

Shukei Ohyanagi , Yuta Ito, Kai Koyama, Tomoaki Nishikawa

Foreshocks are important for understanding the initiation process of large earthquakes. It has long been suggested that the acceleration of foreshock activity is driven by precursory aseismic fault slip. However, observational evidence supporting this relationship remains limited. Furthermore, while slow earthquakes—diverse low-velocity fault slip phenomena—frequently occur at plate boundaries, their relationship with foreshock acceleration has not been fully elucidated. Here we report the first instance in which both localized, short-term foreshock acceleration and tectonic tremor bursts were directly observed simultaneously in the same region. Prior to the Mjma 6.9 interplate earthquake that occurred on November 9, 2025, in the Japan Trench subduction zone off the coast of Iwate, an exceptionally pronounced acceleration of foreshocks was recorded. We quantitatively characterized this exceptional acceleration using an epidemic-type aftershock sequence model with an acceleration term. Using data from ocean-bottom seismometers located directly above the source region, we found that tremor bursts occurred concurrently with this acceleration, spanning a range of approximately 100 km along-strike, including the foreshock source area. This tremor activity strongly suggests the progression of slow slip along the plate interface. The foreshock activity accelerated sharply starting three days before the mainshock within the asperity that ruptured during the 1968 Mw 7.0 earthquake. In contrast, the nearby tremor activity showed no comparably strong acceleration. Based on these results, we interpret that slow slip accompanied by tremor bursts had been progressing along the plate interface for about one week prior to the mainshock. This slow slip may have promoted accelerating aseismic slip within the asperity (i.e., preslip), leading to the intense foreshock acceleration and the subsequent large earthquake.