Characterization of Focal Mechanisms for Small Earthquakes Preceding the Mw 7.5 Noto Peninsula Earthquake

On January 1, 2024, a Mw 7.5 earthquake struck the northern Noto Peninsula in Japan. Prior to the mainshock, an intense seismic swarm persisted for over three years—driven by fluid propagation and accumulation—and led to two large foreshocks: the Mj 5.4 event in 2022 and the Mj 6.5 event in 2023. High-resolution earthquake catalogs are essential for elucidating earthquake nucleation processes, but they provide only a limited view of the detailed faulting mechanisms, fault structures, and spatiotemporal stress changes involved. In contrast, focal-mechanism solutions for small earthquakes (e.g., M < 3)—which are challenging to obtain using traditional methods when station coverage is sparse—provide subtle, qualitative constraints on fault orientations and stress changes. In this study, we aim to deepen our understanding of the Noto Peninsula earthquake’s long-term nucleation by characterizing focal mechanisms of small foreshocks to reveal detailed stress evolution and infer associated fault structures.

We employ the recently developed, cross-correlation–based double-ratio inversion method FocMecDR to determine focal mechanisms of small earthquakes preceding the Mw 7.5 Noto Peninsula earthquake. Analogous to the double-difference concept in earthquake location, FocMecDR uses a reference event with a known mechanism (i.e., strike, dip, and rake) and inverts nearby target events by (1) matching relative polarities for pairs of events via waveform cross-correlation and (2) minimizing the misfit between observed and theoretical P/S amplitude double ratios for pairs of events. We will first briefly introduce this method and validate its effectiveness using the well-studied 2019 Mw 6.4 Ridgecrest earthquake sequence. For the Mw 7.5 Noto Peninsula earthquake sequence and its foreshocks, we focused on the period from January 2020 to March 2024 using the FocMecDR and 135 F-net focal mechanisms as templates, and determined focal mechanisms for more than 1,000 M > 2 earthquakes. The detailed variations in focal mechanisms delineate the dipping angles of fine fault structures and the spatiotemporal evolution of stress. Most interestingly, we found reversed normal-faulting earthquakes following large thrust events. In this presentation, we will report these results and discuss the detailed nucleation process before the Mw 7.5 mainshock.