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[SCG52-P16] Geological structure of low gravity anomaly area in the source region of seismic activity in the northeastern part of the Noto Peninsula
Keywords:swarm earthquake, caldera
High seismic activity and continuous crustal movements have been observed since Dec. 2020 in Suzu City in the northeastern part of the Noto Peninsula. This seismic activity occurred in four areas, and the seismic activity started in the southern area. Relocation of hypocenters showed that the hypocentral distribution beneath the southern area has a steeper dip than other areas (Hiramatsu, 2021). The southern area is characterized by a low gravity anomaly (Iizuka low gravity anomaly area) with a shape close to a circle. Nishimura (2021) estimated a spherical pressure source as a source of crustal movement beneath this low gravity anomaly area (Nishimura, 2021).
The Najimi Formation in the middle and late Miocene is widely distributed in the southern area and consists of siliceous and calcareous mudstone. Diatomite, which is the main constituent of the Najimi Formation, has a low density and can cause low gravity anomalies. Furthermore, we may consider that a caldera structure formed by past volcanic activity as the cause of the low gravity anomaly distributed in a relatively circular shape and that the inside of the caldera is filled with sediments with a lower density than the surroundings. Yamasaki and Kono (1973) pointed out that the low gravity anomaly area (Ukai low gravity area) located to the south of the Iizuka low gravity anomaly area might be a caldera structure formed in the Miocene. If the Iizuka low gravity anomaly area would be caused by the caldera structure, there might be dikes and fissures distributed at high angles related to the caldera structure. Fluid movement using such structures could make seismicity higher.
In this study, we estimated a depression structure that can reproduce the observed low gravity anomaly. The purpose of this study is to obtain rough depression structures. We approximated the depression structure to be an elliptical shape in horizontal and is a simple parabolic shape in the depth direction. We set the horizontal center of the depression structure near the center of the low gravity anomaly area, and the center depth is treated as a variable parameter. The boundary of the depression structure was roughly aligned with the distribution of the sedimentary layer containing diatomite on the geological map. In addition to the 1000 kg/m3 density difference between andesite and diatomite, several values are set for the density difference between the inside and outside of the depression structure. We constructed a model of the depression structure with three-dimensional prisms, of which the horizontal dimension is 100 m, and calculated the theoretical gravity value by the set of prisms.
The model calculation with the density difference of -1000 kg/m3, the horizontal dimensions of 7000x5000 m, and the central depth of 300 m provides a value of gravity difference of -11.8 mGal in the Iizuka low gravity anomaly area. The model calculation with the horizontal dimensions of 4500x4000 m, and the central depth of 380 m provides a value of gravity difference of -14.3 mGal in the Ukai low gravity area. These values are close to the observed gravity difference. Applying a density difference of -600 kg/m3 and assuming the horizontal dimensions of the depressions is the same as the above estimations, the central depth and a value of gravity difference are, respectively, 500 m and -11.4 mGal in the Iizuka low gravity area, and 700 m and -14.6 mGal in the Ukai low gravity area.
The Najimi Formation in the middle and late Miocene is widely distributed in the southern area and consists of siliceous and calcareous mudstone. Diatomite, which is the main constituent of the Najimi Formation, has a low density and can cause low gravity anomalies. Furthermore, we may consider that a caldera structure formed by past volcanic activity as the cause of the low gravity anomaly distributed in a relatively circular shape and that the inside of the caldera is filled with sediments with a lower density than the surroundings. Yamasaki and Kono (1973) pointed out that the low gravity anomaly area (Ukai low gravity area) located to the south of the Iizuka low gravity anomaly area might be a caldera structure formed in the Miocene. If the Iizuka low gravity anomaly area would be caused by the caldera structure, there might be dikes and fissures distributed at high angles related to the caldera structure. Fluid movement using such structures could make seismicity higher.
In this study, we estimated a depression structure that can reproduce the observed low gravity anomaly. The purpose of this study is to obtain rough depression structures. We approximated the depression structure to be an elliptical shape in horizontal and is a simple parabolic shape in the depth direction. We set the horizontal center of the depression structure near the center of the low gravity anomaly area, and the center depth is treated as a variable parameter. The boundary of the depression structure was roughly aligned with the distribution of the sedimentary layer containing diatomite on the geological map. In addition to the 1000 kg/m3 density difference between andesite and diatomite, several values are set for the density difference between the inside and outside of the depression structure. We constructed a model of the depression structure with three-dimensional prisms, of which the horizontal dimension is 100 m, and calculated the theoretical gravity value by the set of prisms.
The model calculation with the density difference of -1000 kg/m3, the horizontal dimensions of 7000x5000 m, and the central depth of 300 m provides a value of gravity difference of -11.8 mGal in the Iizuka low gravity anomaly area. The model calculation with the horizontal dimensions of 4500x4000 m, and the central depth of 380 m provides a value of gravity difference of -14.3 mGal in the Ukai low gravity area. These values are close to the observed gravity difference. Applying a density difference of -600 kg/m3 and assuming the horizontal dimensions of the depressions is the same as the above estimations, the central depth and a value of gravity difference are, respectively, 500 m and -11.4 mGal in the Iizuka low gravity area, and 700 m and -14.6 mGal in the Ukai low gravity area.