12:00 〜 12:15
[SVC28-06] 重力異常から推定される鬼界カルデラ周辺の地殻構造
キーワード：鬼界カルデラ、重力異常、地殻構造
Kikai caldera submarine volcano is located at south of Kyushu Island, southwest Japan. For imaging the subsurface crustal structure of this caldera, we have conducted the analysis of the gravity anomaly data. Gravity anomaly is cause by the density heterogeneity due to the difference in subsurface structure, therefore the marine gravity data provides an effective information for investigating three-dimensional crustal structure beneath the seafloor.
The free-air gravity anomaly around Kikai caldera was digitized from the free-air gravity anomaly map published by Japan Coast Guard (Onodera et al., 2009). Bouguer anomaly was calculated from free-air gravity anomalies following the method of Parker (1972). The densities of sea-water, sediment, upper crust, lower crust, and mantle were assumed as 1000, 2100, 2700, 3000, and 3300 kg/m3, respectively. Interface between sea-water and sediment layer was assumed by the acoustic bathymetric data acquired with T/S Fukae-maru, Kobe university. Depth of the interface between upper crustal layer and lower crustal layer, and lower crustal layer and mantle were assumed based on the seismic refraction profile nearby the caldera (Nishizawa et al., 2017). The sediment thickness was not well defined in this area. Bouguer anomalies around Kikai caldera were calculated by subtracting the effects of each interface from the free-air gravity anomalies. Finally, we considered that the bouguer anomalies in this area were caused by the density contrast between the sediment layer and upper crustal layer.
Bouguer anomalies were observed in a range from -5 to 50 mGal around Kikai caldera. Bouguer anomalies with more than 20 mGal were mostly distributed along the caldera rims identified by Tatsumi et al. (2018), indicating the existence of high-density crust or crustal exposure to the seafloor. Bouguer anomalies with less than 15 mGal, which elongated to east-southeast, were observed in southeast area of the rims. Bouguer anomalies with less than 15 mGal were also appeared on whole of the central dome suggested by Tatsumi et al. (2018). The existence of low-density crust, thick sediment, or regional collapse of the crust were implied by the low value of the bouguer anomaly. We will discuss about detailed crustal structure around Kikai caldera in this presentation.
The free-air gravity anomaly around Kikai caldera was digitized from the free-air gravity anomaly map published by Japan Coast Guard (Onodera et al., 2009). Bouguer anomaly was calculated from free-air gravity anomalies following the method of Parker (1972). The densities of sea-water, sediment, upper crust, lower crust, and mantle were assumed as 1000, 2100, 2700, 3000, and 3300 kg/m3, respectively. Interface between sea-water and sediment layer was assumed by the acoustic bathymetric data acquired with T/S Fukae-maru, Kobe university. Depth of the interface between upper crustal layer and lower crustal layer, and lower crustal layer and mantle were assumed based on the seismic refraction profile nearby the caldera (Nishizawa et al., 2017). The sediment thickness was not well defined in this area. Bouguer anomalies around Kikai caldera were calculated by subtracting the effects of each interface from the free-air gravity anomalies. Finally, we considered that the bouguer anomalies in this area were caused by the density contrast between the sediment layer and upper crustal layer.
Bouguer anomalies were observed in a range from -5 to 50 mGal around Kikai caldera. Bouguer anomalies with more than 20 mGal were mostly distributed along the caldera rims identified by Tatsumi et al. (2018), indicating the existence of high-density crust or crustal exposure to the seafloor. Bouguer anomalies with less than 15 mGal, which elongated to east-southeast, were observed in southeast area of the rims. Bouguer anomalies with less than 15 mGal were also appeared on whole of the central dome suggested by Tatsumi et al. (2018). The existence of low-density crust, thick sediment, or regional collapse of the crust were implied by the low value of the bouguer anomaly. We will discuss about detailed crustal structure around Kikai caldera in this presentation.