17:15 〜 18:30
[SCG45-P10] Estimation of the Active Faults from BSR-derived heat flow by Automated Velocity Analysis in the Nankai Trough, Japan
キーワード：熱流量、速度解析、南海トラフ、反射法地震探査、海底擬似反射面
Along the Nankai Trough southwest Japan, the recurrence of great earthquakes with an interval of approximately 100 years have been identified from historical literature (Satake, 2015). Therefore, it is important to characterize the active faults for causing great earthquakes and tsunami in the Nankai Trough.
Previously, geophysical surveys have observed many faults in the Nankai Trough (e.g., Ashi et al., 2002). A seismic survey is one of the most effective methods to characterize the faults (e.g. Tsuji et al., 2014). Moreover, the joint interpretation of seismic reflection images and heat flow value estimated from the bottom-simulating reflector (BSR) makes it possible to identify the active faults (Yamano et al., 1982). In the Nankai subduction zone, the active faults cutting through accretionary prisms may act as permeable paths for pore fluid flow, and such localized fluid flow transports heat in an advective manner (Yamano et al., 2014). Especially, the active faults that have recently ruptured include open fractures and thus have higher permeability and higher heat flow. Therefore, if we identify the heat flow anomalies, we can infer the active faults there.
Some methods have evaluated heat flow in the Nankai Trough (e.g., heat flow probe (Christoffel et al., 1969); piston core samples (Kinoshita et al., 2008)). One of them is using the information of BSR (e.g., Yamano et al., 1982). On seismic profiles, BSR appears as the high-amplitude reverse-polarity waveforms parallel to the seafloor reflection: BSR indicates CH4 phase boundary between free gas and hydrate (Markl et al., 1970). The heat flow estimation method using BSR uses the depth of BSR from a seismic profile. Then from the depth information, we determine the temperature and the pressure of BSR. The temperature and depth finally estimate the spatial heat flow distribution. However, inaccurate depth information cannot lead us to accurate heat flow values. To obtain an accurate depth model and infer BSR, we have applied automated velocity analysis (Fomel et al., 2013; Chhun et al., 2018). This analysis yielded a high-resolution P-wave velocity structure (Mukumoto et al., 2019). In this analysis, P-wave velocity can be determined to all CMP gathers, and we could obtain accurate depth model for heat flow estimation.
When we applied this method to the seismic data in the Nankai Trough off Kii-suido, the heat flow mapping value was estimated approximately 40-80 (mW/m2). This result is consistent with previous studies (Ohde et al., 2018). Moreover, we interpreted the location of the active faults from the area based on high heat value. The inferred active fault is located in outer ridge region where the strike-slip faults are developed. The presence of such faults supports the interpretation of heat flow anomalies estimated by our approach.
Previously, geophysical surveys have observed many faults in the Nankai Trough (e.g., Ashi et al., 2002). A seismic survey is one of the most effective methods to characterize the faults (e.g. Tsuji et al., 2014). Moreover, the joint interpretation of seismic reflection images and heat flow value estimated from the bottom-simulating reflector (BSR) makes it possible to identify the active faults (Yamano et al., 1982). In the Nankai subduction zone, the active faults cutting through accretionary prisms may act as permeable paths for pore fluid flow, and such localized fluid flow transports heat in an advective manner (Yamano et al., 2014). Especially, the active faults that have recently ruptured include open fractures and thus have higher permeability and higher heat flow. Therefore, if we identify the heat flow anomalies, we can infer the active faults there.
Some methods have evaluated heat flow in the Nankai Trough (e.g., heat flow probe (Christoffel et al., 1969); piston core samples (Kinoshita et al., 2008)). One of them is using the information of BSR (e.g., Yamano et al., 1982). On seismic profiles, BSR appears as the high-amplitude reverse-polarity waveforms parallel to the seafloor reflection: BSR indicates CH4 phase boundary between free gas and hydrate (Markl et al., 1970). The heat flow estimation method using BSR uses the depth of BSR from a seismic profile. Then from the depth information, we determine the temperature and the pressure of BSR. The temperature and depth finally estimate the spatial heat flow distribution. However, inaccurate depth information cannot lead us to accurate heat flow values. To obtain an accurate depth model and infer BSR, we have applied automated velocity analysis (Fomel et al., 2013; Chhun et al., 2018). This analysis yielded a high-resolution P-wave velocity structure (Mukumoto et al., 2019). In this analysis, P-wave velocity can be determined to all CMP gathers, and we could obtain accurate depth model for heat flow estimation.
When we applied this method to the seismic data in the Nankai Trough off Kii-suido, the heat flow mapping value was estimated approximately 40-80 (mW/m2). This result is consistent with previous studies (Ohde et al., 2018). Moreover, we interpreted the location of the active faults from the area based on high heat value. The inferred active fault is located in outer ridge region where the strike-slip faults are developed. The presence of such faults supports the interpretation of heat flow anomalies estimated by our approach.