5:15 PM - 6:45 PM
[U15-P03] 2024 Noto earthquake and earthquake swarm
Keywords:Noto earthquake, earthquake swarm, stress field
It is believed that in places where earthquake swarms occur, the degree of fracture in the Earth's crust is high, making it difficult for large-scale destruction to occur. On the other hand, it has been thought that the Earth's crust is relatively homogeneous in places where large earthquakes occur, so large ruptures can occur. In that sense, the Noto earthquake is a rare case in which a M7 earthquake occurred in a place where an earthquake swarm is occurring. The Noto earthquake swarm has a very long active period, and the Matsushiro swarm is the only other earthquake swarm that lasts for such a long period of time. The Noto earthquake swarm is estimated to be caused by water intrusion from deep underground due to crustal deformation (Kato, 2023). Similarly, the Matsushiro swarm is a known example of an earthquake swarm caused by the inflow of water from deep underground. Let us consider the difference between the two from the perspective of the stress field in areas where earthquake swarms occur. First, if the fluid rises from deep underground, the direction chosen is along the main compression axis of the region. Looking at the stress field, based on the focal mechanism analysis data of the earthquake, the area of the Matsushiro swarm was of a horizontal displacement type with east-west compression, so the direction of the minimum principal stress axis was horizontal. On the other hand, the stress field in the northeastern part of the Noto Peninsula was a reverse fault type with a compression axis in the NW-SE direction, and the direction of the minimum principal stress axis was vertical. The fluid spreads in a plane perpendicular to the direction of the tension axis. In the case of the Matsushiro swarm, the tension axis is in the north-south direction, so the surface on which the fluid spreads is a vertical east-west direction. In this case, fluid from deep underground can enter the vertical cracks and quickly move to the upper crust. The fluid that reached the shallows then spread along the NE-SW trending active faults in the area. It is known that during the Matsushiro swarm, a large amount of water overflowed to the earth's surface during the seismic activity.
On the other hand, during the Noto earthquake swarm, fluid from deep within the reverse fault field was rising. Then, even though the compression axis is horizontal, which is the same as in the Matsushiro area, the tension axis is vertical, so the direction in which the fluid spreads is horizontal. In this stress field, fluid cannot easily flow into shallow areas. Therefore, it is thought that the period of activity was prolonged because fluid from deep inside could not flow to the surface. Similar to the Matsushiro swarm, the earthquake swarm occurred due to fluid rising from deep underground, but the stress field in the crust around the Noto Peninsula was of a reverse fault type, so the direction of the minimum principal stress axis was vertical. For this reason, the cracks in the earth's crust are oriented horizontally, preventing fluid from overflowing to the earth's surface, and the water spreads horizontally, accumulating strain in a wide area within the earth's crust, causing major earthquakes.
If the aftershock area of the Noto earthquake is plotted together with the epicenter area of past large earthquakes on the eastern margin of the Japan Sea, a new seismic gap can be seen in the right figure. The epicenter areas of past large earthquakes that occurred on the eastern margin of the Japan Sea were shown and the seismic gaps(X, A, A', B) that was pointed out in the past, and the new seismic gap S (Ishikawa & Bai, 2024). The new seismic gap lies off the western coast of Sado. The figure on the left shows the repetition of active and quiet periods of earthquakes in the eastern margin of the Japan Sea, as shown by Ishikawa (1994), and it was said that there was an seismic gap C in Toyama Bay (Hatori,1984). It is believed that the 1614 earthquake caused tsunami damage to Takada, and it was thought that the earthquake occurred in Toyama Bay, but later, due to a lack of data, the epicenter was unknown. It is hoped that the 1614 earthquake will be reexamined.
On the other hand, during the Noto earthquake swarm, fluid from deep within the reverse fault field was rising. Then, even though the compression axis is horizontal, which is the same as in the Matsushiro area, the tension axis is vertical, so the direction in which the fluid spreads is horizontal. In this stress field, fluid cannot easily flow into shallow areas. Therefore, it is thought that the period of activity was prolonged because fluid from deep inside could not flow to the surface. Similar to the Matsushiro swarm, the earthquake swarm occurred due to fluid rising from deep underground, but the stress field in the crust around the Noto Peninsula was of a reverse fault type, so the direction of the minimum principal stress axis was vertical. For this reason, the cracks in the earth's crust are oriented horizontally, preventing fluid from overflowing to the earth's surface, and the water spreads horizontally, accumulating strain in a wide area within the earth's crust, causing major earthquakes.
If the aftershock area of the Noto earthquake is plotted together with the epicenter area of past large earthquakes on the eastern margin of the Japan Sea, a new seismic gap can be seen in the right figure. The epicenter areas of past large earthquakes that occurred on the eastern margin of the Japan Sea were shown and the seismic gaps(X, A, A', B) that was pointed out in the past, and the new seismic gap S (Ishikawa & Bai, 2024). The new seismic gap lies off the western coast of Sado. The figure on the left shows the repetition of active and quiet periods of earthquakes in the eastern margin of the Japan Sea, as shown by Ishikawa (1994), and it was said that there was an seismic gap C in Toyama Bay (Hatori,1984). It is believed that the 1614 earthquake caused tsunami damage to Takada, and it was thought that the earthquake occurred in Toyama Bay, but later, due to a lack of data, the epicenter was unknown. It is hoped that the 1614 earthquake will be reexamined.