9:15 AM - 9:30 AM
[SCG45-38] Observational Efforts toward Understanding the Slow Slip to the Trench in the Subduction Zone of Megathrust Earthquake Source Regions
Keywords:Slow Slip Event, Seafloor borehole observatory, Distributed Optical Fiber Sensing, near the Trench
Subducting oceanic plates, such as the Nankai Trough, exhibit seismogenic behavior (plate interface locking) at certain depths. Understanding the fault slip mechanisms at the plate boundary is an important social issue, considering the occurrence of megathrust earthquakes and the tsunamis associated with them. In the shallower subduction zones, marine sediments accompanying the subducting oceanic plates are accreted to the region, forming thrusts while undergoing plastic deformation, which also leads to the formation of a plate boundary fault structure connected to deeper layers. These shallow plate interfaces, including in the Nankai Trough, cause slip during megathrust earthquakes, which, in turn, trigger megatsunamis. However, slow-slip events have also been repeatedly observed in regions like the Kumano-nada area of the Nankai Trough.
The behavior of the plate boundary in shallow regions appears complex and is not yet fully understood. Questions arise such as: Where exactly does fault slip occur near the trench during a megathrust earthquake? Where exactly does the slow slip occur near the trench? What are the factors controlling the rheology of the boundary that causes slow slip events? What is the relationship between slow earthquakes, known as low-frequency tremors or very-low-frequency earthquakes, and slow slip? To answer these questions, closer proximity and ultra-broadband observations of the dynamics are necessary compared to what we currently have.
We are conducting observational experiments in the Kumano-nada Nankai Trough, where the current state of the plate interface is the best understood from the most dense observations by the DONET and IODP seafloor boreholes, making it the suitable region for understanding shallowest subduction zones.
In April 2016, a relatively large slow-slip event occurred in Kumano-nada following the Mw6.1 earthquake off the southeastern of Mie, and active low-frequency tremor activity also occurred at the subduction front. This suggested the propagation of slow slip toward the trench. Additionally, between December 2020 and January 2021, slow slip was observed again, and observations from the C0006 borehole, which started in 2018, strongly suggested that the slow slip had reached the trench. In this area, slow slip and associated slow earthquake activity have been observed approximately every five years.
To further understand the reality of slow-slip and slow-earthquake events near the trench, we plan to deploy a sufficiently dense observation network near the site of an upcoming slow-slip or slow-earthquake event. Through this observation, we aim to identify the fault structures involved in slow slip and their depths. During the next slow-slip event, we will preemptively drill through the identified fault using the scientific drilling vessel "Chikyu," and establish a dense observation network utilizing fiber optics and other sensors across the fault. This will allow us to directly observe the slip, deformation, and fluid involvement at the fault that causes slow slip.
At present, the depth of faults where slow earthquakes and slow-slip events occur at the subduction tip is still unclear. In September 2024, we deployed an array of five broadband seismometers with a 1.4–2 km mesh above and around the C0006 borehole observatory. By combining long-term seafloor broadband seismic observations by BBOBS and DONET, and crustal deformation observation by the borehole pore pressure and tilt, we aim to determine the distribution of slow earthquakes with the precision necessary to assess the feasibility of fault drilling during the next recurrence.
We are also working on the development of an borehole observation system to be deployed after drilling. By developing a seafloor fiber optic cable capable of separating and observing strain, temperature, and pressure at high spatial resolution, we conducted a verification test in Sagami Bay (Araki et al., JpGU, 2024). In October 2024, we successfully demonstrated fiber optic sensing in the C9038B seafloor borehole offshore Kii Strait, allowing for fiber optic sensing in the borehole. To observe faults at greater depths, we foresee the need for technological development in areas such as high-temperature resistance, fiber optic installation across faults, and the integration of fiber optic sensing into seafloor cable observation networks such as DONET for long-term monitoring.
Furthermore, based on these efforts, we plan to submit a proposal for scientific drilling to the International Ocean Drilling Program (IODP3).
The behavior of the plate boundary in shallow regions appears complex and is not yet fully understood. Questions arise such as: Where exactly does fault slip occur near the trench during a megathrust earthquake? Where exactly does the slow slip occur near the trench? What are the factors controlling the rheology of the boundary that causes slow slip events? What is the relationship between slow earthquakes, known as low-frequency tremors or very-low-frequency earthquakes, and slow slip? To answer these questions, closer proximity and ultra-broadband observations of the dynamics are necessary compared to what we currently have.
We are conducting observational experiments in the Kumano-nada Nankai Trough, where the current state of the plate interface is the best understood from the most dense observations by the DONET and IODP seafloor boreholes, making it the suitable region for understanding shallowest subduction zones.
In April 2016, a relatively large slow-slip event occurred in Kumano-nada following the Mw6.1 earthquake off the southeastern of Mie, and active low-frequency tremor activity also occurred at the subduction front. This suggested the propagation of slow slip toward the trench. Additionally, between December 2020 and January 2021, slow slip was observed again, and observations from the C0006 borehole, which started in 2018, strongly suggested that the slow slip had reached the trench. In this area, slow slip and associated slow earthquake activity have been observed approximately every five years.
To further understand the reality of slow-slip and slow-earthquake events near the trench, we plan to deploy a sufficiently dense observation network near the site of an upcoming slow-slip or slow-earthquake event. Through this observation, we aim to identify the fault structures involved in slow slip and their depths. During the next slow-slip event, we will preemptively drill through the identified fault using the scientific drilling vessel "Chikyu," and establish a dense observation network utilizing fiber optics and other sensors across the fault. This will allow us to directly observe the slip, deformation, and fluid involvement at the fault that causes slow slip.
At present, the depth of faults where slow earthquakes and slow-slip events occur at the subduction tip is still unclear. In September 2024, we deployed an array of five broadband seismometers with a 1.4–2 km mesh above and around the C0006 borehole observatory. By combining long-term seafloor broadband seismic observations by BBOBS and DONET, and crustal deformation observation by the borehole pore pressure and tilt, we aim to determine the distribution of slow earthquakes with the precision necessary to assess the feasibility of fault drilling during the next recurrence.
We are also working on the development of an borehole observation system to be deployed after drilling. By developing a seafloor fiber optic cable capable of separating and observing strain, temperature, and pressure at high spatial resolution, we conducted a verification test in Sagami Bay (Araki et al., JpGU, 2024). In October 2024, we successfully demonstrated fiber optic sensing in the C9038B seafloor borehole offshore Kii Strait, allowing for fiber optic sensing in the borehole. To observe faults at greater depths, we foresee the need for technological development in areas such as high-temperature resistance, fiber optic installation across faults, and the integration of fiber optic sensing into seafloor cable observation networks such as DONET for long-term monitoring.
Furthermore, based on these efforts, we plan to submit a proposal for scientific drilling to the International Ocean Drilling Program (IODP3).