5:15 PM - 6:45 PM
[U15-P26] Back-projection analysis of 2024 Mw7.6 Noto Peninsula earthquake by using AN-net array: Possibility of supershear rupture on dip-slip faults
Keywords:2024 Noto Peninsula earthquake, back-projection, supershear rupture, AN-net
On January 1st, 2024, a Mw7.6 earthquake occurred in the Noto region of Ishikawa Prefecture, Japan. In this study, we apply the back-projection method (Ishii et al., 2005) to near-field array data to investigate the source process of this earthquake. The back-projection method enables the estimation of the spatiotemporal distribution of seismic radiation intensity directly from observed waveforms, offering a significant advantage over conventional waveform inversion methods by eliminating the need for calculating synthetic waveforms and the assumption such as rupture propagation velocity.
We used data from the array observation network (AN-net) deployed in Niigata Prefecture by Association for the Development of Earthquake Prediction. AN-net is located approximately 100 km east of the mainshock hypocenter, with 40 stations covering an area of 50 km x 30 km. Each station is equipped with strong motion sensors installed on the surface, as well as high-sensitivity seismometers and strong motion sensors installed in boreholes. In this study, we utilized data from the borehole strong motion sensors based on data quality. In the actual analysis, we integrated the acceleration records to obtain velocity waveforms and utilized data filtered within the frequency band of 0.2-1.0 Hz.
In the back-projection method, it is not necessary to assume a fault plane. However, in this study, we assumed a southeast-dipping plane with a strike of 55° and a dip angle of 40° to cover the aftershock area. The characteristics of the source process estimated by the back-projection method are as follows:
1) The rupture initiated in the northeast part of the Noto Peninsula and propagated towards the southwest edge of the aftershock area over a duration of about 15 s, rupturing at a depth of approximately 10 km, although with relatively low energy release.
2) Subsequently, the rupture changed its propagation direction to the northeast, rupturing regions shallower than 10 km. The rupture propagation velocity reached approximately 5 km/s, close to the P-wave velocity, indicating supershear rupture, which lasted for about 20 s (from 15 to 35 s after the rupture initiation). A significant portion of the earthquake's energy was released at this stage.
3) Rupture propagation towards the northeast extended to the northeast edge of the aftershock area, with the rupture propagation velocity slowing down to the S-wave velocity. The overall duration of the mainshock rupture was about 60 s.
The significant energy release in shallow depths is in accord with the occurrence of coastal uplifts of up to approximately 4 m and the generation of Tsunamis. Furthermore, although still at a qualitative study stage, the emergence of strong pulses and duration observed in near-field strong ground motions appear to be generally explained by the supershear rupture propagating in the northeast direction.
The observation of such pronounced supershear rupture on a reverse fault is likely a first observation. Understanding the physical conditions that lead to such supershear rupture and their relationship with the extensive damage observed in the Noto Peninsula are crucial for mitigating future earthquake disasters.
Acknowledgements. We are grateful to Japan Meteorological Agency for the hypocenter list.
We used data from the array observation network (AN-net) deployed in Niigata Prefecture by Association for the Development of Earthquake Prediction. AN-net is located approximately 100 km east of the mainshock hypocenter, with 40 stations covering an area of 50 km x 30 km. Each station is equipped with strong motion sensors installed on the surface, as well as high-sensitivity seismometers and strong motion sensors installed in boreholes. In this study, we utilized data from the borehole strong motion sensors based on data quality. In the actual analysis, we integrated the acceleration records to obtain velocity waveforms and utilized data filtered within the frequency band of 0.2-1.0 Hz.
In the back-projection method, it is not necessary to assume a fault plane. However, in this study, we assumed a southeast-dipping plane with a strike of 55° and a dip angle of 40° to cover the aftershock area. The characteristics of the source process estimated by the back-projection method are as follows:
1) The rupture initiated in the northeast part of the Noto Peninsula and propagated towards the southwest edge of the aftershock area over a duration of about 15 s, rupturing at a depth of approximately 10 km, although with relatively low energy release.
2) Subsequently, the rupture changed its propagation direction to the northeast, rupturing regions shallower than 10 km. The rupture propagation velocity reached approximately 5 km/s, close to the P-wave velocity, indicating supershear rupture, which lasted for about 20 s (from 15 to 35 s after the rupture initiation). A significant portion of the earthquake's energy was released at this stage.
3) Rupture propagation towards the northeast extended to the northeast edge of the aftershock area, with the rupture propagation velocity slowing down to the S-wave velocity. The overall duration of the mainshock rupture was about 60 s.
The significant energy release in shallow depths is in accord with the occurrence of coastal uplifts of up to approximately 4 m and the generation of Tsunamis. Furthermore, although still at a qualitative study stage, the emergence of strong pulses and duration observed in near-field strong ground motions appear to be generally explained by the supershear rupture propagating in the northeast direction.
The observation of such pronounced supershear rupture on a reverse fault is likely a first observation. Understanding the physical conditions that lead to such supershear rupture and their relationship with the extensive damage observed in the Noto Peninsula are crucial for mitigating future earthquake disasters.
Acknowledgements. We are grateful to Japan Meteorological Agency for the hypocenter list.