17:15 〜 19:15
[SSS10-P05] 阿蘇カルデラ内における2016年熊本地震の断層破壊停止：マルチビームバックプロジェクション法と強震記録を用いた解析
キーワード：2016年熊本地震、震源破壊過程、阿蘇カルデラ、広帯域地震動
In a previous study (SSJ 2024), I used the multi-beam seismic back-projection method and a dense array of near-source strong motion records from the K-NET/KiK-net networks to estimate the broadband frequency (0.1–10 Hz) fault rupture process of the 16 April 2016 Kumamoto earthquake (M7.1). In this study, I focus on the rupture termination process of the earthquake within the Aso caldera, applying the same methodology while extending the imaging target domain to encompass the caldera.
I selected 104 stations within 100 km of the hypocenter and grouped them into multiple subarrays, evenly distributed in angular regions around the hypocenter. For the back-projection target area, I constructed a 3D grid mesh with a 500 m spacing, covering a volume of 50 × 26 × 20 km from the Hinagu fault to the Aso caldera. Using the 3D S-wave velocity model of the Kyushu region (Matsubara et al., 2022), I calculated travel times for each grid-station pair. Beam power for each grid and subarray was obtained by stacking the normalized velocity envelopes at the arrival times, incorporating both source and travel times. Grid energy was evaluated at 0.25 s intervals across the 3D mesh by averaging the envelope amplitude within a ±1 s window around the arrival times. Maximum grid energies were determined as the product of the combined subarray beam energies. I investigated the source process of the Kumamoto earthquake across four frequency bands (0.1–0.5, 0.5–1, 1–5, and 5–10 Hz). My results indicate that rupture lasted for a total of 22s, propagating 45 km from the Hinagu fault to the northeastern region of Aso caldera.
High frequency ground motion radiation
High-frequency (HF) ground motions (1–10 Hz) were strongly radiated from 3 to 10 s across a broad region within the hanging wall wedge of the Hinagu fault, spanning depths of approximately 15 km to the surface. Intermediate-frequency radiation (0.5–1 Hz) was concentrated above the southern edge of the Futagawa fault at a depth of ~5 km from 6 to 9 s (Fig. 1). The strong HF ground motion radiation estimated in this study largely overlaps with areas where JMA intensity 6 or higher were observed during the Kumamoto earthquake. This suggests that HF radiation may have been a key factor contributing to the intense shaking and widespread building damage in Kumamoto City. Additionally, the location of strong HF radiation indicates the possibility that off-fault radiation played a significant role in generating high-frequency ground motions.
Low frequency ground motion radiation
Strong low-frequency (LF) radiation (0.1–0.5 Hz) occurred north of the HF radiation zone at depths shallower than 5 km between 11 and 22 s (Fig. 1). From 11 to 16 s, LF radiation was primarily concentrated along the Idenokuchi fault, a normal fault subparallel to the Futagawa fault, where approximately 10 km of surface rupture was identified in field surveys (Fig. 1). A strong motion record of the Kumamoto earthquake observed at Nishihara Mura Komori (JMA), showed a prominent velocity pulse at about 0.2 Hz. This record was observed in close proximity to the Futagawa main fault trace and is therefore likely to reflect the shallow strong low-frequency radiation region identified in this study.
Fault rupture termination and the Aso caldera underground structure
Two distinct episodes of strong LF radiation occurred within the Aso caldera between 17 and 22 s. The first episode (17–18 s) originated beneath Nakadake volcano at depths of 1–3 km (Fig. 1), coinciding with the shallow magma chamber of the volcano, located ~1 km beneath the post-caldera central cone, as identified by ambient noise tomography of the Aso caldera (Huang et al., EPS, 2018). The second episode (18–22 s) occurred at depths of 4 km to the surface, closely aligning with the location of the Miyaji faults northeast of the Aso caldera, where surface ruptures were observed (Fig. 1). This episode likely reflects the stopping phases of the earthquake.
The location of these strong LF radiation episodes suggests that fault rupture propagated through the Aso caldera and terminated at the Miyaji faults northeast of the caldera (Fig. 1). This rupture may be linked to the extension of the Okayama-Kumamoto Tectonic Line (OKTL) through the caldera, as indicated by strong seismic reflectors aligned with the OKTL, identified in a 3D seismic reflection survey across the central cones of Aso volcano (Tsutsui et al., JVGR, 2004).
I selected 104 stations within 100 km of the hypocenter and grouped them into multiple subarrays, evenly distributed in angular regions around the hypocenter. For the back-projection target area, I constructed a 3D grid mesh with a 500 m spacing, covering a volume of 50 × 26 × 20 km from the Hinagu fault to the Aso caldera. Using the 3D S-wave velocity model of the Kyushu region (Matsubara et al., 2022), I calculated travel times for each grid-station pair. Beam power for each grid and subarray was obtained by stacking the normalized velocity envelopes at the arrival times, incorporating both source and travel times. Grid energy was evaluated at 0.25 s intervals across the 3D mesh by averaging the envelope amplitude within a ±1 s window around the arrival times. Maximum grid energies were determined as the product of the combined subarray beam energies. I investigated the source process of the Kumamoto earthquake across four frequency bands (0.1–0.5, 0.5–1, 1–5, and 5–10 Hz). My results indicate that rupture lasted for a total of 22s, propagating 45 km from the Hinagu fault to the northeastern region of Aso caldera.
High frequency ground motion radiation
High-frequency (HF) ground motions (1–10 Hz) were strongly radiated from 3 to 10 s across a broad region within the hanging wall wedge of the Hinagu fault, spanning depths of approximately 15 km to the surface. Intermediate-frequency radiation (0.5–1 Hz) was concentrated above the southern edge of the Futagawa fault at a depth of ~5 km from 6 to 9 s (Fig. 1). The strong HF ground motion radiation estimated in this study largely overlaps with areas where JMA intensity 6 or higher were observed during the Kumamoto earthquake. This suggests that HF radiation may have been a key factor contributing to the intense shaking and widespread building damage in Kumamoto City. Additionally, the location of strong HF radiation indicates the possibility that off-fault radiation played a significant role in generating high-frequency ground motions.
Low frequency ground motion radiation
Strong low-frequency (LF) radiation (0.1–0.5 Hz) occurred north of the HF radiation zone at depths shallower than 5 km between 11 and 22 s (Fig. 1). From 11 to 16 s, LF radiation was primarily concentrated along the Idenokuchi fault, a normal fault subparallel to the Futagawa fault, where approximately 10 km of surface rupture was identified in field surveys (Fig. 1). A strong motion record of the Kumamoto earthquake observed at Nishihara Mura Komori (JMA), showed a prominent velocity pulse at about 0.2 Hz. This record was observed in close proximity to the Futagawa main fault trace and is therefore likely to reflect the shallow strong low-frequency radiation region identified in this study.
Fault rupture termination and the Aso caldera underground structure
Two distinct episodes of strong LF radiation occurred within the Aso caldera between 17 and 22 s. The first episode (17–18 s) originated beneath Nakadake volcano at depths of 1–3 km (Fig. 1), coinciding with the shallow magma chamber of the volcano, located ~1 km beneath the post-caldera central cone, as identified by ambient noise tomography of the Aso caldera (Huang et al., EPS, 2018). The second episode (18–22 s) occurred at depths of 4 km to the surface, closely aligning with the location of the Miyaji faults northeast of the Aso caldera, where surface ruptures were observed (Fig. 1). This episode likely reflects the stopping phases of the earthquake.
The location of these strong LF radiation episodes suggests that fault rupture propagated through the Aso caldera and terminated at the Miyaji faults northeast of the caldera (Fig. 1). This rupture may be linked to the extension of the Okayama-Kumamoto Tectonic Line (OKTL) through the caldera, as indicated by strong seismic reflectors aligned with the OKTL, identified in a 3D seismic reflection survey across the central cones of Aso volcano (Tsutsui et al., JVGR, 2004).