15:30 〜 15:45
[S22-14] 動的破壊シミュレーションが明らかにした2024年能登半島地震の滑り及び地殻変動分布における非平面断層形状の効果
2024年M7.6能登半島地震は，事前に記載されていた活断層セグメントを基本的には破壊したことが分かってきた．事前の地形学・地質学的推定（岡村，2009など）と事後の余震活動による推定（Yoshida et al., 2024）により，その断層形状は屈曲や分岐，並走する断層面などによる，顕著な複雑性を示していることが明らかにされた．合成開口レーダ（SAR）観測により，隆起のピークは半島の北西角部に位置する猿山岬地域で生じ約5mに達することが分かった．この地域は，沖合に位置する活断層の屈曲部に一致していることは特筆すべきであり，おそらく活断層の累積変位が半島の形状を決めている要素であろう．今回の地震破壊過程を決めている物理過程を理解するために，観測で制約した断層形状と広域応力場をモデル化した3次元動的破壊シミュレーションを行った．まず地震前の観測で制約したそれらのモデルは，余震分布とよく一致していることを確認した．3次元非平面断層上の破壊過程の数値計算には，高効率な領域分割-境界積分方程式法（FDP-BIEM）を用いた．数値計算の結果の一例を，断層面上の滑りと地表変位分布のスナップショットとして図に示す．計算した地表変位を観測されたSARと比較したところ，その空間分布が非平面断層形状効果としてよく再現されていることが分かった．この一致の良さは，地震時観測変位を用いたパラメタチューニング無しで得られており，モデルに仮定した断層形状とTranspressionの応力場の示している．シミュレーションは，南東傾斜と北東傾斜という共役断層間の動的破壊乗り移りの発生も示す．猿山岬地域での最大隆起は，当該領域での断層屈曲により生じた可能性が大きい．本研究では，観測された強震動と計算結果を比較して，震源効果に解釈も与えた．このように，動的破壊シミュレーションは災害を引き起こす大きな地震時変位や強震動の背後にある物理メカニズムを明らかにする能力があることを示している．
Observations have documented that the M7.6, 2024 Noto Peninsula earthquake primarily ruptured previously documented active fault segments located along the western and northern coast of the peninsula. The preseismic geological/geomorphological inference (e.g., Okamura, 2009) and postseismic aftershock inference (e.g., Yoshida et al., 2024) reveal that this fault system presents remarkable geometrical complexity, including the bends, branches and subparallel strands of the faults. The Synthetic Aparture Radar (SAR) observation found the largest uplift of about 5 m around Cape Saruyama, which marks the northwestern corner of the peninsula. Interestingly, this area is associated with the fault-bend offshore, facing and probably shaping the corner of the peninsula. In order to clarify the underlying mechanism of the earthquake rupture, we conducted the dynamic rupture simulation with the observational constraints of the 3-D fault geometry and regional stress field. We have confirmed their preseismic estimates are consistent with those of postseismic ones based on the spatial distribution and focal mechanisms of the observed aftershocks. The numerically efficient and geometrically flexible Boundary Integral Equation method (FDP-BIEM) is employed to consider the 3-D fault geometry in the dynamic rupture simulation. An example of a snapshot of the dynamic rupture propagation is shown in Figure for the displacement on the fault and the ground surfaces. We successfully reproduce the SAR-detected distribution of the surface displacement as the result of the observationally constrained non-planar fault geometry. The remarkable matching is obtained without postseismic parameter tunings, indicating the validity of the assumed models of the fault geometry and transpressional tectonic stress field. Our simulation also shows that the rupture is transferred between the conjugate faults. We have identified that the fault bend off the Cape Saruyama most probably caused the largest slip and uplift observed there. We also compare the synthetic and observed strong motion waveforms and interpret the source effects. The dynamic rupture simulation demonstrates the ability to illuminate the underlying mechanisms causing the hazardous large coseismic deformations.
Observations have documented that the M7.6, 2024 Noto Peninsula earthquake primarily ruptured previously documented active fault segments located along the western and northern coast of the peninsula. The preseismic geological/geomorphological inference (e.g., Okamura, 2009) and postseismic aftershock inference (e.g., Yoshida et al., 2024) reveal that this fault system presents remarkable geometrical complexity, including the bends, branches and subparallel strands of the faults. The Synthetic Aparture Radar (SAR) observation found the largest uplift of about 5 m around Cape Saruyama, which marks the northwestern corner of the peninsula. Interestingly, this area is associated with the fault-bend offshore, facing and probably shaping the corner of the peninsula. In order to clarify the underlying mechanism of the earthquake rupture, we conducted the dynamic rupture simulation with the observational constraints of the 3-D fault geometry and regional stress field. We have confirmed their preseismic estimates are consistent with those of postseismic ones based on the spatial distribution and focal mechanisms of the observed aftershocks. The numerically efficient and geometrically flexible Boundary Integral Equation method (FDP-BIEM) is employed to consider the 3-D fault geometry in the dynamic rupture simulation. An example of a snapshot of the dynamic rupture propagation is shown in Figure for the displacement on the fault and the ground surfaces. We successfully reproduce the SAR-detected distribution of the surface displacement as the result of the observationally constrained non-planar fault geometry. The remarkable matching is obtained without postseismic parameter tunings, indicating the validity of the assumed models of the fault geometry and transpressional tectonic stress field. Our simulation also shows that the rupture is transferred between the conjugate faults. We have identified that the fault bend off the Cape Saruyama most probably caused the largest slip and uplift observed there. We also compare the synthetic and observed strong motion waveforms and interpret the source effects. The dynamic rupture simulation demonstrates the ability to illuminate the underlying mechanisms causing the hazardous large coseismic deformations.