5:15 PM - 7:15 PM
[SVC32-P03] Hypocenter Determinations for Three-Dimensional Seismic Velocity Structure in Usu Volcano Using JMA Picking Data
Keywords:Usu volcano, hypocenter determination, three-dimensional velocity structure
Introduction
At Usu volcano, nine eruptions of felsic magma have been recorded since 1663 up until the latest eruption in 2000. In the case of a summit eruption, there is a risk of explosive magmatic eruptions that can cause damage in all directions, and if it occurs at the foot of the mountain, there is a possibility of eruptions in residential and tourist areas. Therefore, it is crucial to issue Volcanic Forecast/Warnings appropriately and evacuations based on them before eruptions. From the perspective of issuing Volcanic Forecast/Warnings, the precursory phenomena of eruptions are characterized by significant seismic activity and crustal deformation associated with magma intrusion, making the understanding them important. In addition, swarm earthquakes occurred in 2010, 2015, and 2021, particularly the 2021 swarm earthquake, which was reminiscent of magma intrusion accompanied by shallowing the hypocenters and tilting changes. While reflecting on these attempted eruption events, decision criteria of the Volcanic Alert Level criteria have also been revised (Fuwa and Miyamura, 2024).
At the volcano, the three-dimensional P-wave velocity structure of the shallow area was investigated by an artificial seismic survey conducted in 2001 (Onizawa et al., 2007). The introduction of three-dimensional structures for the hypocenter determinations deepens the understanding of phenomena occurring underground in terms of improved location accuracy and relationships with subsurface structures. Thus, it is believed to contribute to the monitoring and assessment of volcanic activity by the Japan Meteorological Agency (JMA). Here, we will present the application of the determination of the hypocenter using the JMA picking data for the three-dimensional velocity structure.
Three-dimensional Velocity Structure
The P-wave velocity was estimated using the initial arrival time data obtained from artificial earthquake surveys conducted in 2001. Regarding the S-wave velocity, during the determination of the hypocenter of the precursor earthquakes before the 2000 eruption, a simple relationship of Vp = aVs + b was assumed based on the P-wave velocity estimated from artificial seismic exploration, and the constants a and b were determined through numerous hypocenter calculations using a grid search approach. The adopted values are a=1.45 and b=0.9 (Onizawa et al., 2007). The velocity structure that can be estimated by such ways is limited to shallow areas up to a few kilometers from the surface to the upper crust, but it can contribute to the determination of the hypocenters of earthquakes that occur regularly and those caused by magma intrusion.
Hypocenter determination Using JMA picking Data
This focuses on the seismic sources related to the JMA picking data from October 12, 2002, to May 14, 2024. Routine analysis by JMA using one-dimensional structures recognizes that earthquakes continue in the shallow part at around 1-km depth beneath the Usu volcano, while sporadic earthquakes are also occurring beneath Showa-Shinzan. The former is thought to be related to the subsidence of the O-Usu cryptodome that grew during the eruptions from 1977 to 1982 (Aoyama et al., 2009). In contrast, the swarm earthquakes that occurred in 2010, 2015, and 2021 were determined to be deeper than 2 km below the volcano, clearly distinguishing them from the regularly occurring earthquakes. In issuing the Volcanic Alert, this difference in depth becomes one of important indicators to recognize magma intrusion (Fuwa and Miyamura, 2024).
These basic positional relationships remain unchanged even for the hypocenters for the three-dimensional structures. However, compared to the routine hypocenters by JMA, the hypocenters for three-dimensional structures are determined to shift southward and to shallower depths. This is thought to be reflected in the deepening basement toward the south and the large Vp/Vs above the basement by the introduction of a three-dimensional structure. The precursor earthquakes before the 2000 eruption are characterized by earlier phase activity due to the rise of magma within the Pre-Neogene basement and by a later expansion to the south. In 2010, 2015, and 2021, when the hypocenters of the swarm earthquakes are overlaid with the hypocenter distribution of the precursory earthquakes before the 2000 eruption, they almost coincide with the area within the Pre-Neogene basement. From the perspective of the occurrence locations, this supports the idea that these swarm earthquakes were caused by magma intrusion.
At Usu volcano, nine eruptions of felsic magma have been recorded since 1663 up until the latest eruption in 2000. In the case of a summit eruption, there is a risk of explosive magmatic eruptions that can cause damage in all directions, and if it occurs at the foot of the mountain, there is a possibility of eruptions in residential and tourist areas. Therefore, it is crucial to issue Volcanic Forecast/Warnings appropriately and evacuations based on them before eruptions. From the perspective of issuing Volcanic Forecast/Warnings, the precursory phenomena of eruptions are characterized by significant seismic activity and crustal deformation associated with magma intrusion, making the understanding them important. In addition, swarm earthquakes occurred in 2010, 2015, and 2021, particularly the 2021 swarm earthquake, which was reminiscent of magma intrusion accompanied by shallowing the hypocenters and tilting changes. While reflecting on these attempted eruption events, decision criteria of the Volcanic Alert Level criteria have also been revised (Fuwa and Miyamura, 2024).
At the volcano, the three-dimensional P-wave velocity structure of the shallow area was investigated by an artificial seismic survey conducted in 2001 (Onizawa et al., 2007). The introduction of three-dimensional structures for the hypocenter determinations deepens the understanding of phenomena occurring underground in terms of improved location accuracy and relationships with subsurface structures. Thus, it is believed to contribute to the monitoring and assessment of volcanic activity by the Japan Meteorological Agency (JMA). Here, we will present the application of the determination of the hypocenter using the JMA picking data for the three-dimensional velocity structure.
Three-dimensional Velocity Structure
The P-wave velocity was estimated using the initial arrival time data obtained from artificial earthquake surveys conducted in 2001. Regarding the S-wave velocity, during the determination of the hypocenter of the precursor earthquakes before the 2000 eruption, a simple relationship of Vp = aVs + b was assumed based on the P-wave velocity estimated from artificial seismic exploration, and the constants a and b were determined through numerous hypocenter calculations using a grid search approach. The adopted values are a=1.45 and b=0.9 (Onizawa et al., 2007). The velocity structure that can be estimated by such ways is limited to shallow areas up to a few kilometers from the surface to the upper crust, but it can contribute to the determination of the hypocenters of earthquakes that occur regularly and those caused by magma intrusion.
Hypocenter determination Using JMA picking Data
This focuses on the seismic sources related to the JMA picking data from October 12, 2002, to May 14, 2024. Routine analysis by JMA using one-dimensional structures recognizes that earthquakes continue in the shallow part at around 1-km depth beneath the Usu volcano, while sporadic earthquakes are also occurring beneath Showa-Shinzan. The former is thought to be related to the subsidence of the O-Usu cryptodome that grew during the eruptions from 1977 to 1982 (Aoyama et al., 2009). In contrast, the swarm earthquakes that occurred in 2010, 2015, and 2021 were determined to be deeper than 2 km below the volcano, clearly distinguishing them from the regularly occurring earthquakes. In issuing the Volcanic Alert, this difference in depth becomes one of important indicators to recognize magma intrusion (Fuwa and Miyamura, 2024).
These basic positional relationships remain unchanged even for the hypocenters for the three-dimensional structures. However, compared to the routine hypocenters by JMA, the hypocenters for three-dimensional structures are determined to shift southward and to shallower depths. This is thought to be reflected in the deepening basement toward the south and the large Vp/Vs above the basement by the introduction of a three-dimensional structure. The precursor earthquakes before the 2000 eruption are characterized by earlier phase activity due to the rise of magma within the Pre-Neogene basement and by a later expansion to the south. In 2010, 2015, and 2021, when the hypocenters of the swarm earthquakes are overlaid with the hypocenter distribution of the precursory earthquakes before the 2000 eruption, they almost coincide with the area within the Pre-Neogene basement. From the perspective of the occurrence locations, this supports the idea that these swarm earthquakes were caused by magma intrusion.