09:30 〜 09:45
[SSS10-07] 2017年バルパライソ地震の地震学的・測地学的先行現象の解釈に関して
★招待講演
キーワード：2017年バルパライソ地震、核形成、GNSS、群発地震
Numerical simulations and laboratory experiments demonstrate the presence of precursory seismic and aseismic activities of large earthquakes, which contributed to classifying different nucleation scenarios of earthquakes (e.g., McLaskey, 2019). However, a corresponding signal in nature is observed for the limited number of great earthquakes with modern instruments. Even when we identify clear geodetic and/or seismic activities prior to great earthquakes, whether we can directly associate such signals as (part of) the mainshock nucleation needs careful examination, considering our observation limitations. For example, smaller and/or farther events must be hidden in the observation noise, meaning that precursory activities cannot be exhaustively detected. Furthermore, in the context of precursory, geodetic transient is usually directly translated as a representation of aseismic slip, but it can contain deformation caused by both seismic and aseismic slip. These two points may significantly impact the discussion of nucleation models of natural earthquakes because the fraction of seismic and aseismic slip and their temporal evolution modes are the key ingredients to discussing nucleation models. The 2017 Mw 6.9 Valparaiso earthquake is an excellent example demonstrating these limitations, as shown by three papers discussing the Mw 6.9’s nucleation mode.
The 2017 Mw 6.9 Valparaiso earthquake is a megathrust earthquake offshore Chile. Its precursory activities started ~two days earlier. A few GPS stations exhibited transient motion of ~1 cm toward the epicenter and intense seismic swarms, including M 6.1 and M 5.5. Ruiz et al. (2017) identified ~6 cm and ~9 cm of slip from pre-Mw 6.9 repeating earthquakes and the GPS transient displacement, respectively, and concluded that this precursory aseismic slip triggered the Mw 6.9. This would mean that they did not associate it as a direct nucleation phase of the Mw 6.9.
Caballero et al. (2021) pointed out that the precursory GPS transient displacement may contain both displacements associated with seismic and aseismic slip on the interface. They estimated coseismic displacements of these pre-mainshock seismicities, which are potentially hidden in the GPS transient displacement. Furthermore, they evaluated the amount of afterslip possibly induced by the largest M 6.1 foreshock. In the end, they concluded that significant aseismic displacement free from possible afterslip contribution is contained in the GPS transient. Based on the spatial overlap of the aseismic pre-mainshock slip and the foreshock seismicity, Caballero et al. (2021) proposed that this aseismic slip represents a pre-slip as a part of the subsequent Mw 6.9, in contrast with the conclusion of Ruiz et al. (2017).
In contrast with the above two papers, Moutote et al. (2023) analyzed both pre- and post-Mw 6.9 seismicity in a consistent framework. They first densified a regional earthquake catalog with a machine-learning-based phase picker and then applied multiple statistical analyses to the enhanced catalog. Their results showed an anomalously high seismicity rate across the Mw 6.9 occurrence time. Their subdaily GPS and repeating earthquake analyses showed that the pre-Mw 6.9 aseismic slip rate did not significantly increase following the Mw 6.9. Hence, they concluded that the entire sequence spanning the first foreshock, the Mw 6.9 mainshock, and their aftershocks was driven by an aseismic slip. In other words, the transient geodetic deformation prior to the Mw 6.9 is less likely part of the pre-slip (i.e., the nucleation) of the Mw 6.9.
References:
Caballero et al. (2021). https://doi.org/10.1029/2020GL091916
Moutote, Itoh et al. (2023). https://doi.org/10.1029/2023JB026603
Ruiz et al. (2017). https://doi.org/10.1002/2017GL075675
The 2017 Mw 6.9 Valparaiso earthquake is a megathrust earthquake offshore Chile. Its precursory activities started ~two days earlier. A few GPS stations exhibited transient motion of ~1 cm toward the epicenter and intense seismic swarms, including M 6.1 and M 5.5. Ruiz et al. (2017) identified ~6 cm and ~9 cm of slip from pre-Mw 6.9 repeating earthquakes and the GPS transient displacement, respectively, and concluded that this precursory aseismic slip triggered the Mw 6.9. This would mean that they did not associate it as a direct nucleation phase of the Mw 6.9.
Caballero et al. (2021) pointed out that the precursory GPS transient displacement may contain both displacements associated with seismic and aseismic slip on the interface. They estimated coseismic displacements of these pre-mainshock seismicities, which are potentially hidden in the GPS transient displacement. Furthermore, they evaluated the amount of afterslip possibly induced by the largest M 6.1 foreshock. In the end, they concluded that significant aseismic displacement free from possible afterslip contribution is contained in the GPS transient. Based on the spatial overlap of the aseismic pre-mainshock slip and the foreshock seismicity, Caballero et al. (2021) proposed that this aseismic slip represents a pre-slip as a part of the subsequent Mw 6.9, in contrast with the conclusion of Ruiz et al. (2017).
In contrast with the above two papers, Moutote et al. (2023) analyzed both pre- and post-Mw 6.9 seismicity in a consistent framework. They first densified a regional earthquake catalog with a machine-learning-based phase picker and then applied multiple statistical analyses to the enhanced catalog. Their results showed an anomalously high seismicity rate across the Mw 6.9 occurrence time. Their subdaily GPS and repeating earthquake analyses showed that the pre-Mw 6.9 aseismic slip rate did not significantly increase following the Mw 6.9. Hence, they concluded that the entire sequence spanning the first foreshock, the Mw 6.9 mainshock, and their aftershocks was driven by an aseismic slip. In other words, the transient geodetic deformation prior to the Mw 6.9 is less likely part of the pre-slip (i.e., the nucleation) of the Mw 6.9.
References:
Caballero et al. (2021). https://doi.org/10.1029/2020GL091916
Moutote, Itoh et al. (2023). https://doi.org/10.1029/2023JB026603
Ruiz et al. (2017). https://doi.org/10.1002/2017GL075675