5:15 PM - 7:15 PM
[SSS14-P15] Geological basement structure around Oita Plain estimated from a microtremor survey
Keywords:Median Tectonic Line, Oita Plain, Geological basement structure, Saganoseki Fault, Microtremor survey
The Median Tectonic Line (MTL), extending from Kanto to eastern Kyushu, is a geological boundary fault that divides southwest Japan into the Inner and Outer Zones. In eastern Kyushu, the Sanbagawa metamorphic rocks are exposed from the Saganoseki Peninsula to the Ono River. In addition, the surface distribution of the Ryoke granitic and metamorphic rocks is also unclear in eastern Kyushu. Therefore, the distribution of the MTL in eastern Kyushu is still under discussion in previous studies.
In Beppu Bay, located in eastern Kyushu, Yusa et al. (1992) imaged the distribution of the MTL (Ryoke granitic and metamorphic rocks / Sanbagawa metamorphic rocks) at a depth of about 3 km in eastern Beppu Bay using seismic reflection surveys. Subsequently, a reinterpretation of the seismic reflection survey data from Yusa et al. (1992) by the Integrated Research for the Beppu–Haneyama Fault Zone (East part of Oita Plain–Yufuin Fault) (2017) suggested that the Oita–Sekinan Group / Sanbagawa metamorphic rocks is not linearly continuous from deep to surface but seem to have a bench-like structure in the shallow part.
Moreover, the Saganoseki Fault is known to extend parallel to the south side of the MTL in eastern Oita Plain. However, the relationship between the Oita–Sekinan Group / Sanbagawa metamorphic rocks and the Saganoseki Fault is not well understood. In this study, comprehensive basement rock depth data were obtained. The onshore continuity of the MTL, as estimated from offshore seismic reflection survey data, was examined, and the relationship between the Oita–Sekinan Group / Sanbagawa metamorphic rocks and the Saganoseki Fault was discussed.
Microtremors can be measured in a short time and easily provide information on basement rock depth. The survey was conducted at approximately 280 points around Oita Plain, including locations from seismic reflection surveys by Oita Prefecture (2000, 2002) and Yoshimi et al. (2007), as well as deep drilling core data reported by Hidaka and Moriyama (2000). The velocity Seismometer KVS-300 (natural period 0.5 seconds) and data logger LS8800 were used. Prior to this measurement, it was verified that the long period components could be recorded with the KVS-300 by comparing the seismogram with the LE-3D/20s (natural period 20 seconds) Then, measurements were taken for more than 30 minutes at each point. The H/V spectral ratio was then calculated.
The peaks of the H/V spectral ratio, corresponding to the basement rock depth, show shorter periods near the Saganoseki Peninsula and longer periods near the estuary of the Oita River. These peaks exhibit a strong correlation with basement rock depth reported in previous studies. Based on this correlation, a preliminary basement rock depth map around Oita Plain was created. Gentle slopes of the basement rock at shallow depth, corresponding to the bench-like structure in Line H, were observed east of the Ono River. The steeply dipping structure of the basement rock extends parallel to the south side of the MTL around the Nyu Hills and bend southward near the Ono River. This steeply dipping structure is interpreted as the Oita–Sekinan Group / Sanbagawa metamorphic rocks. Therefore, the lineament of the Saganoseki Fault may coincide with the distribution of the Oita–Sekinan Group / Sanbagawa metamorphic rocks.
A seismic reflection survey will be conducted to further identify the steeply dipping structure of the basement rock in eastern Oita Plain. Additionally, the bends in geological structures along the Ono River and the distribution of the MTL around Oita Plain will be discussed in this presentation.
In Beppu Bay, located in eastern Kyushu, Yusa et al. (1992) imaged the distribution of the MTL (Ryoke granitic and metamorphic rocks / Sanbagawa metamorphic rocks) at a depth of about 3 km in eastern Beppu Bay using seismic reflection surveys. Subsequently, a reinterpretation of the seismic reflection survey data from Yusa et al. (1992) by the Integrated Research for the Beppu–Haneyama Fault Zone (East part of Oita Plain–Yufuin Fault) (2017) suggested that the Oita–Sekinan Group / Sanbagawa metamorphic rocks is not linearly continuous from deep to surface but seem to have a bench-like structure in the shallow part.
Moreover, the Saganoseki Fault is known to extend parallel to the south side of the MTL in eastern Oita Plain. However, the relationship between the Oita–Sekinan Group / Sanbagawa metamorphic rocks and the Saganoseki Fault is not well understood. In this study, comprehensive basement rock depth data were obtained. The onshore continuity of the MTL, as estimated from offshore seismic reflection survey data, was examined, and the relationship between the Oita–Sekinan Group / Sanbagawa metamorphic rocks and the Saganoseki Fault was discussed.
Microtremors can be measured in a short time and easily provide information on basement rock depth. The survey was conducted at approximately 280 points around Oita Plain, including locations from seismic reflection surveys by Oita Prefecture (2000, 2002) and Yoshimi et al. (2007), as well as deep drilling core data reported by Hidaka and Moriyama (2000). The velocity Seismometer KVS-300 (natural period 0.5 seconds) and data logger LS8800 were used. Prior to this measurement, it was verified that the long period components could be recorded with the KVS-300 by comparing the seismogram with the LE-3D/20s (natural period 20 seconds) Then, measurements were taken for more than 30 minutes at each point. The H/V spectral ratio was then calculated.
The peaks of the H/V spectral ratio, corresponding to the basement rock depth, show shorter periods near the Saganoseki Peninsula and longer periods near the estuary of the Oita River. These peaks exhibit a strong correlation with basement rock depth reported in previous studies. Based on this correlation, a preliminary basement rock depth map around Oita Plain was created. Gentle slopes of the basement rock at shallow depth, corresponding to the bench-like structure in Line H, were observed east of the Ono River. The steeply dipping structure of the basement rock extends parallel to the south side of the MTL around the Nyu Hills and bend southward near the Ono River. This steeply dipping structure is interpreted as the Oita–Sekinan Group / Sanbagawa metamorphic rocks. Therefore, the lineament of the Saganoseki Fault may coincide with the distribution of the Oita–Sekinan Group / Sanbagawa metamorphic rocks.
A seismic reflection survey will be conducted to further identify the steeply dipping structure of the basement rock in eastern Oita Plain. Additionally, the bends in geological structures along the Ono River and the distribution of the MTL around Oita Plain will be discussed in this presentation.