5:15 PM - 6:45 PM
[U15-P28] Characterized source model for evaluating long-period (more than 2s-) ground motions during the 2024 Noto Peninsula Earthquake (Mj7.6)
Keywords:The Noto Peninsula Earthquake, Characterized source model
1. Introduction
The Noto Peninsula Earthquake (Mj7.6), which occurred at 16:10 on January 1, 2024, resulted in severe shaking across the Noto Peninsula, with a maximum seismic intensity of 7. According to GSI, the western part of Wajima City was uplifted by up to 4 meters. According to F-net, the seismic moment was 1.98×1020 Nm (Mw7.5), which corresponds to the third stage of the empirical relationship (S-M0 relationship) between the rupture area and seismic moment in the recipe (HERP, 2020). In Japan, this is the first time that observed waveforms of the third stage earthquake have been recorded near a fault. It is important to construct a characterized source model for such earthquake that can reproduce the observed records near fault, and to increase our knowledge of such a model for future strong ground motion prediction.
In this study, we attempt to construct a characterized source model that can reproduce long-period (more than 2 s) ground motions near fault of the Noto Peninsula Earthquake, and compare the synthetic ground motions evaluated by theoretical methods with observed records.
2. Characterized source model
The geometry of the fault plane is set with reference to Kubo et al. (2024), and asperities are placed on the fault plane. The asperities were adjusted through trial-and-error forward modeling based on heterogeneous slip distributions (e.g. GSI (2024), Kubo et al. (2024), DPRI (2024)). We focused on the reproducibility of velocity waveforms and permanent displacements at five K-NET and KiK-net (surface) stations (ISKH01, ISK001, ISK003, ISKH04, and ISK006) near fault. Synthetic waveforms were evaluated by using the wavenumber integration method (Hisada and Bielak, 2003), and the subsurface structure model was extracted from the J-SHIS subsurface structure model (V4) (Senna et al., 2023) for each station and used as the horizontal stratification structure.
Fig.1 shows the location of the asperities and stations used in this study. The parameters of each asperity are based on the recipe. The slip (Da) is 5.8 m, which is twice the average slip of the entire fault plane evaluated from the S-M0 relationship of Murotani et al. (2010) assuming the rigidity (3.43 × 1010 N/m2). The rupture velocity (Vr) is 2.5 km/s (0.72×β, where β is the S-wave velocity and 3.5 km/s is assumed), the slip velocity time function is Nakamura and Miyatake (2000), the rise time (Tr) is evaluated from 0.5×W/Vr (W is the width of each asperity), and the rake is 90° assuming reverse faulting.
Fig.2 shows comparison of the observed and synthetic velocity waveforms (2-20 s). Time 0s in Fig. 2 corresponds to the origin time of the Mj7.6 earthquake. The figure shows that the pulses observed at ISK003, ISKH04, and ISK006 are generally reproduced. However, the pulses observed in the EW component around 20 s at ISKH04 and ISK006 are not reproduced, and the reproducibility at ISK001 and ISKH01 near the epicenter is also poor. In order to reproduce these pulses, it is necessary to analyze the source inversion results and incorporate these findings into the characterized source model. At ISK001, a relatively large wave packet is observed before the origin time of the Mj7.6 earthquake. This wave packet is considered to be caused by the Mj5.9 earthquake that occurred about 13 seconds before the Mj7.6 earthquake.
Fig.3 shows the synthetic displacement waveforms (more than 2s) including permanent displacement and observed permanent displacement. The horizontal component at ISK003 is slightly underestimated, however the vertical components at ISKH01, ISK001, and ISK003 are comparable comparing with the observed values.
3. summary and issues
The characterized source model developed in this study was generally able to reproduce the characteristic observed waveforms (velocity) with periods of 2 s or longer and permanent displacement near fault, however the reproducibility was not good at several stations. In order to improve the reproducibility, it is necessary to adjust the characterized source model. Furthermore, it is important to verify the characterized source model by using additional information, such as stations which was not used for this study and observed spatial crustal deformation distribution. Additionally, it is necessary to examine the geometry of the fault model. Especially, if surface ruptures are identified, it will be necessary to construct fault model that is consistent with the observation.
Acknowledgements
We used strong motion data from NIED strong-motion seismograph networks (K-NET, KiK-net). We express our appreciation to NIED.
The Noto Peninsula Earthquake (Mj7.6), which occurred at 16:10 on January 1, 2024, resulted in severe shaking across the Noto Peninsula, with a maximum seismic intensity of 7. According to GSI, the western part of Wajima City was uplifted by up to 4 meters. According to F-net, the seismic moment was 1.98×1020 Nm (Mw7.5), which corresponds to the third stage of the empirical relationship (S-M0 relationship) between the rupture area and seismic moment in the recipe (HERP, 2020). In Japan, this is the first time that observed waveforms of the third stage earthquake have been recorded near a fault. It is important to construct a characterized source model for such earthquake that can reproduce the observed records near fault, and to increase our knowledge of such a model for future strong ground motion prediction.
In this study, we attempt to construct a characterized source model that can reproduce long-period (more than 2 s) ground motions near fault of the Noto Peninsula Earthquake, and compare the synthetic ground motions evaluated by theoretical methods with observed records.
2. Characterized source model
The geometry of the fault plane is set with reference to Kubo et al. (2024), and asperities are placed on the fault plane. The asperities were adjusted through trial-and-error forward modeling based on heterogeneous slip distributions (e.g. GSI (2024), Kubo et al. (2024), DPRI (2024)). We focused on the reproducibility of velocity waveforms and permanent displacements at five K-NET and KiK-net (surface) stations (ISKH01, ISK001, ISK003, ISKH04, and ISK006) near fault. Synthetic waveforms were evaluated by using the wavenumber integration method (Hisada and Bielak, 2003), and the subsurface structure model was extracted from the J-SHIS subsurface structure model (V4) (Senna et al., 2023) for each station and used as the horizontal stratification structure.
Fig.1 shows the location of the asperities and stations used in this study. The parameters of each asperity are based on the recipe. The slip (Da) is 5.8 m, which is twice the average slip of the entire fault plane evaluated from the S-M0 relationship of Murotani et al. (2010) assuming the rigidity (3.43 × 1010 N/m2). The rupture velocity (Vr) is 2.5 km/s (0.72×β, where β is the S-wave velocity and 3.5 km/s is assumed), the slip velocity time function is Nakamura and Miyatake (2000), the rise time (Tr) is evaluated from 0.5×W/Vr (W is the width of each asperity), and the rake is 90° assuming reverse faulting.
Fig.2 shows comparison of the observed and synthetic velocity waveforms (2-20 s). Time 0s in Fig. 2 corresponds to the origin time of the Mj7.6 earthquake. The figure shows that the pulses observed at ISK003, ISKH04, and ISK006 are generally reproduced. However, the pulses observed in the EW component around 20 s at ISKH04 and ISK006 are not reproduced, and the reproducibility at ISK001 and ISKH01 near the epicenter is also poor. In order to reproduce these pulses, it is necessary to analyze the source inversion results and incorporate these findings into the characterized source model. At ISK001, a relatively large wave packet is observed before the origin time of the Mj7.6 earthquake. This wave packet is considered to be caused by the Mj5.9 earthquake that occurred about 13 seconds before the Mj7.6 earthquake.
Fig.3 shows the synthetic displacement waveforms (more than 2s) including permanent displacement and observed permanent displacement. The horizontal component at ISK003 is slightly underestimated, however the vertical components at ISKH01, ISK001, and ISK003 are comparable comparing with the observed values.
3. summary and issues
The characterized source model developed in this study was generally able to reproduce the characteristic observed waveforms (velocity) with periods of 2 s or longer and permanent displacement near fault, however the reproducibility was not good at several stations. In order to improve the reproducibility, it is necessary to adjust the characterized source model. Furthermore, it is important to verify the characterized source model by using additional information, such as stations which was not used for this study and observed spatial crustal deformation distribution. Additionally, it is necessary to examine the geometry of the fault model. Especially, if surface ruptures are identified, it will be necessary to construct fault model that is consistent with the observation.
Acknowledgements
We used strong motion data from NIED strong-motion seismograph networks (K-NET, KiK-net). We express our appreciation to NIED.