Dynamic rupture inversion on the M5.9 pre-event before the 2024 Mw7.6 Noto-Peninsula, Japan, earthquake

Abstract Active seismicity which began since late 2020 under the Noto Peninsula, Japan, led to the Mw7.6 earthquake on the 1st January 2024. This paper analyzes the MJMA 5.9 pre-event that occurred 13 seconds before the Mw7.6 mainshock. Near-field ground motion records clearly distinguish this pre-event from the ground shaking of the mainshock. We then use six near-field ground motions over a 10-second period to obtain first the focal mechanism and then dynamic rupture models by fixing the hypocenter location. We obtain a steep dip angle of 67°, whereas the dip of the M7.6 mainshock is 33° after Japan Metrological Agency catalog. We propose a simplified inversion process that dynamic rupture models are built by varying the location of the target circular patch, the stress level and the fault dipping directions. The preferred model has an up-dip rupture directivity (rake direction) on the south-east dipping fault, i.e. propagating away from the hypocenter of the M7.6 mainshock, and the two events are not aligned on the same planar fault. Uplift of about 20 cm and more is expected on the north coast of the Noto Peninsula in the case of the stress drop of 6 MPa or less. We also analyze another M5.5 pre-event 4 minutes earlier, with a slightly deeper focal depth and a gentle dip angle of 36°. This earthquake has also a rupture directivity in the up-dip direction. This infers that there should have been a preparation process of the Mw7.6 mainshock behind these pre-events and multiple fault segmentations around the hypocenter zone might have played a role.