11:45 〜 12:00
[SCG46-11] Uncovering rupture mechanics and tsunami generation of the Tohoku-Oki earthquake using in-situ seafloor pressure gauges
Ocean-bottom observations are essential for studying earthquake and tsunami processes in the ocean. Traditionally ocean-bottom pressure gauges (PGs) were used to observe tsunamis, while recent studies have revealed that they capture various geophysical phenomena across a wide period range from seconds to years. Utilizing this capability, we have analyzed in-situ PG data, recorded inside the earthquake source region, to reveal the physics of earthquake rupture and massive tsunami generation. In this presentation, we introduce our recent works related to the analyses of the in-situ PG data, which provides important insights into earthquake and tsunami mechanics in the Tohoku subduction zone.
First, we highlight advances in the modeling study of dynamic pressure changes recorded by the in-situ PGs (Kubota et al. 2021). In-situ PGs observe not only tsunamis ranging periods of ~102–103 s but also dynamic pressure changes covering periods of 100–102 s caused by seismic waves (e.g. Filloux 1982), but it was difficult to utilize the dynamic components as the short-period seismic components overlap with the long-period tsunami signals. Applying solid-fluid coupled wave theory (e.g. Saito 2019), we developed a technique to simulate the dynamic pressure fluctuations and successfully decomposed the dynamic component from the broadband in-situ PG data of the 2011 Tohoku-Oki earthquake. We found that the velocity seismograms at stations located just above the epicenter showed two peaks, related to the two dominant moment releases near the epicenter (e.g., Ide et al. 2011).
Next, we present a fault modeling study of the Tohoku-Oki earthquake using the in-situ PGs to explore the mechanics of its large near-trench slip (> 50 m) resulting in a devastating tsunami (Kubota et al. 2022). While the kinematics of this anomalous slip have been studied well, its driving force and underlying physics remain unresolved. Using the in-situ tsunami waveforms recorded by the PGs together with geodetic datasets, we reliably estimated the distributions of slip and shear stress release on the megathrust fault plane. The results showed the near-trench slip (> 50 m) occurred with minimal stress drop (< 3 MPa) at depths < 10 km, while large stress release (> 5 MPa) occurred deeper near the hypocenter ~15 km). This result suggests the near-trench slip occurred without releasing significant pre-accumulated shallow stress but was driven mainly by strain energy releases in the deeper region under the free-surface effects near the trench. This result is consistent with the drilling survey conducted after the Tohoku-Oki earthquake, which showed that the absolute shear stress level at the shallow part after the earthquake was close to zero (Lin et al., 2013; Brodsky et al., 2017) suggesting that the stress level at the shallow part should also be low before and during the earthquake. Our result may suggest that shallow large slips could occur in other subduction zones only if the deeper mechanically coupled areas sufficiently accumulate the strain energy.
Seafloor pressure observations have significantly advanced the modeling of earthquake and tsunami generations. Integrating in-situ pressure observations with solid-fluid coupled wave theory refines the modeling of coseismic slip and stress release and enhances understanding of the physics of massive tsunami generation and its underlying physics. Uncovering the physics of massive tsunami generation is crucial for assessing a wide range of potential future tsunami sources, including megathrust events, tsunami earthquakes, and sequential earthquake rupture scenarios.
First, we highlight advances in the modeling study of dynamic pressure changes recorded by the in-situ PGs (Kubota et al. 2021). In-situ PGs observe not only tsunamis ranging periods of ~102–103 s but also dynamic pressure changes covering periods of 100–102 s caused by seismic waves (e.g. Filloux 1982), but it was difficult to utilize the dynamic components as the short-period seismic components overlap with the long-period tsunami signals. Applying solid-fluid coupled wave theory (e.g. Saito 2019), we developed a technique to simulate the dynamic pressure fluctuations and successfully decomposed the dynamic component from the broadband in-situ PG data of the 2011 Tohoku-Oki earthquake. We found that the velocity seismograms at stations located just above the epicenter showed two peaks, related to the two dominant moment releases near the epicenter (e.g., Ide et al. 2011).
Next, we present a fault modeling study of the Tohoku-Oki earthquake using the in-situ PGs to explore the mechanics of its large near-trench slip (> 50 m) resulting in a devastating tsunami (Kubota et al. 2022). While the kinematics of this anomalous slip have been studied well, its driving force and underlying physics remain unresolved. Using the in-situ tsunami waveforms recorded by the PGs together with geodetic datasets, we reliably estimated the distributions of slip and shear stress release on the megathrust fault plane. The results showed the near-trench slip (> 50 m) occurred with minimal stress drop (< 3 MPa) at depths < 10 km, while large stress release (> 5 MPa) occurred deeper near the hypocenter ~15 km). This result suggests the near-trench slip occurred without releasing significant pre-accumulated shallow stress but was driven mainly by strain energy releases in the deeper region under the free-surface effects near the trench. This result is consistent with the drilling survey conducted after the Tohoku-Oki earthquake, which showed that the absolute shear stress level at the shallow part after the earthquake was close to zero (Lin et al., 2013; Brodsky et al., 2017) suggesting that the stress level at the shallow part should also be low before and during the earthquake. Our result may suggest that shallow large slips could occur in other subduction zones only if the deeper mechanically coupled areas sufficiently accumulate the strain energy.
Seafloor pressure observations have significantly advanced the modeling of earthquake and tsunami generations. Integrating in-situ pressure observations with solid-fluid coupled wave theory refines the modeling of coseismic slip and stress release and enhances understanding of the physics of massive tsunami generation and its underlying physics. Uncovering the physics of massive tsunami generation is crucial for assessing a wide range of potential future tsunami sources, including megathrust events, tsunami earthquakes, and sequential earthquake rupture scenarios.