Complex dynamics of the 2024 magnitude 7.6 Noto Peninsula earthquake in Japan – A long-term earthquake swarm and its preceding foreshocks

The study, published in the journal Earthquake Research Advances, links the 2024 magnitude 7.6 Noto Peninsula earthquake to a large swarm of earthquakes that began underground in Japan’s Noto Peninsula in November 2020.

The study revealed that the magnitude 7.6 earthquake that struck Japan’s Noto Peninsula on 1 January 2024 was preceded by a series of foreshocks, including three larger earthquakes (M5.5, M4.6, and M5.9). These foreshocks occurred just seconds to minutes before the main shock.

The final notable foreshock (M 5.9) occurred just 14 seconds before the mainshock, but other studies have considered it to be the rupture initiator of the M 7.6 mainshock. This concentration of foreshocks highlights the complex change in behavior from a prolonged swarm to a burst of foreshock activity just before the mainshock.

High-frequency teleseismic back-projection of the mainshock’s P waves, performed in Wuhan by co-author Dr. Wang Dun from China University of Geosciences, revealed a protracted initial rupture process lasting about 25 seconds, which then propagated to both sides. This slow initial rupture process, occurring close to the preceding earthquake swarm region, likely reflects a complex relationship between dynamic earthquake rupture and fluid flow along multiple sub-parallel faults.

Dr. Lei Xinlin of Japan’s National Institute of Advanced Industrial Science and Technology (AIST) carefully reviewed seismic activity since 2018, particularly earthquakes of magnitude 4 or greater. The research team found that the 2024 M7.6 mainshock likely ruptured a reverse fault above a parallel fault associated with the May 2023 Suzu earthquake (M6.5) beneath the northeastern Noto Peninsula.

However, all of the preceding foreshocks, including the initial rupture of the magnitude 7.6 mainshock, initiated around deeper near-parallel faults, and the rupture likely traveled between these near-parallel faults during the first 25 seconds.

The aftershocks of the 2024 mainshock have spread over a range of about 160 km from the northeast to the southwest. The study found that the aftershocks were initially concentrated on the southwest side of the peninsula, but spread in both directions as logarithmic time passed since the mainshock, likely due to continued aftershocks triggered by the rupture of the mainshock.

“While other recently published studies have focused on one aspect of the Noto earthquake sequence, such as the rupture of the mainshock or the relocation of smaller earthquakes, this study combines results from different angles, including the relocation of all seismic activity since 2018,” says Professor Zhigang Peng of the Georgia Institute of Technology. “It is therefore likely one of the most complete analyses of this sequence to date.”

The results of this study highlight the importance of monitoring earthquake swarms with seismometers and geodetic instruments and understanding fluid transport patterns, which could enhance predictive models of future earthquakes in this region and elsewhere.