9:30 AM - 9:45 AM
[MIS08-03] Challenges and approaches to conserving fault outcrops - Case study of Miyoshi geopark project -
Keywords:Conservation of fault outcrops, MTL, Sulfate minerals, Geopark, Geoconservation
Conservation efforts for fault outcrops are being conducted in various regions across Japan. Particularly, indoor exposure preservation is carried out in faults where clear seismic faults, such as the Nojima Fault and the Neodani Fault, have emerged in earthquake source areas (Okada., 2011).
However, in many cases, outdoor conservation is predominantly undertaken. In outdoor fault outcrops, natural factors such as erosion and vegetation growth, as well as unintended human-induced destruction, can make conservation and visibility preservation of the outcrops challenging (Kuchitsu., 2022).
In Miyoshi Geopark project area, the Median Tectonic Line (MTL) fault system runs from east to west, with fault outcrops exposed at several locations. This report introduces the current conditions and challenges of the two selected fault outcrops.
Tachino area.This outcrop exposes the crushed zone of the Ikeda Fault, which is one of the faults in the MTL fault system, on the riverbank. Due to its rarity, this outcrop has been designated as a natural monument by the prefectural board of education (Miyoshi Education Committee et al., 2007). However, frequent inundation by the adjacent river and subsequent deposition of sand and mud have caused the outcrop to be submerged regularly. As a result, poor visibility of the outcrop has persisted for many years.
Inohana road Site (Syuzu area in Miyoshi city)This outcrop, exposed on the road cut slope, allows observation of the fault plane of the Ikeda Fault and the surrounding strata on both sides of the fault. This outcrop emerged during the construction of the road and, due to its academic significance, a conservation method was employed involving the application of solidifying agent on the surface and installation of a protective cover (Tanaka., 2011). However, currently, vegetation has proliferated inside the cover, and white needle-like minerals, with a maximum thickness of about 1 cm, have precipitated on the surface of the geological formation. As a result, the distinctive structures of the crushed zone are no longer observable.
Approaches to the conservation of fault outcropsThe fault outcrop in Tachino areaTo identify the timing when the visibility of the outcrop deteriorates, we first carried out a removal operation of sediment and other deposits on the outcrop. Subsequently, due to increased water levels in the Yoshino River, we conducted another removal operation. What became evident during the second operation was that when the discharge from the dam upstream of the Yoshino River exceeded approximately 3000 m3/s, the outcrop became inundated and sediment deposited. With this indicator, it became possible to determine the timing for conservation work.The fault outcrop in Inohana road SiteAt outcrops of the MTL fault system, secondary mineral deposition on geological surfaces has been observed in other regions as well (Itou et a., 2022). To identify and elucidate the formation process of the white needle-like minerals deposited at this outcrop, a partial removal of the outcrop conservation cover was conducted concurrently with a road inspection closure. Several samples of the white needle-like minerals were collected. Subsequently, mineral identification was performed using XRD (Rigaku MiniFlex60) analysis at a Kyoto university.
The analysis revealed that the white needle-like minerals consist of Na-rich sulfate minerals. Reports on sulfate mineral deposition at outcrops in other regions suggest that precipitation increases under dry conditions and decreases under wet conditions (Horiguchi et al., 2000). Therefore, it is believed that the deposition of white needle-like minerals is related to the maintenance of a dry environment on the outcrop surface by the conservation cover.
In this presentation, I would like to report and discuss the findings and future conservation activities in this area.
However, in many cases, outdoor conservation is predominantly undertaken. In outdoor fault outcrops, natural factors such as erosion and vegetation growth, as well as unintended human-induced destruction, can make conservation and visibility preservation of the outcrops challenging (Kuchitsu., 2022).
In Miyoshi Geopark project area, the Median Tectonic Line (MTL) fault system runs from east to west, with fault outcrops exposed at several locations. This report introduces the current conditions and challenges of the two selected fault outcrops.
Tachino area.This outcrop exposes the crushed zone of the Ikeda Fault, which is one of the faults in the MTL fault system, on the riverbank. Due to its rarity, this outcrop has been designated as a natural monument by the prefectural board of education (Miyoshi Education Committee et al., 2007). However, frequent inundation by the adjacent river and subsequent deposition of sand and mud have caused the outcrop to be submerged regularly. As a result, poor visibility of the outcrop has persisted for many years.
Inohana road Site (Syuzu area in Miyoshi city)This outcrop, exposed on the road cut slope, allows observation of the fault plane of the Ikeda Fault and the surrounding strata on both sides of the fault. This outcrop emerged during the construction of the road and, due to its academic significance, a conservation method was employed involving the application of solidifying agent on the surface and installation of a protective cover (Tanaka., 2011). However, currently, vegetation has proliferated inside the cover, and white needle-like minerals, with a maximum thickness of about 1 cm, have precipitated on the surface of the geological formation. As a result, the distinctive structures of the crushed zone are no longer observable.
Approaches to the conservation of fault outcropsThe fault outcrop in Tachino areaTo identify the timing when the visibility of the outcrop deteriorates, we first carried out a removal operation of sediment and other deposits on the outcrop. Subsequently, due to increased water levels in the Yoshino River, we conducted another removal operation. What became evident during the second operation was that when the discharge from the dam upstream of the Yoshino River exceeded approximately 3000 m3/s, the outcrop became inundated and sediment deposited. With this indicator, it became possible to determine the timing for conservation work.The fault outcrop in Inohana road SiteAt outcrops of the MTL fault system, secondary mineral deposition on geological surfaces has been observed in other regions as well (Itou et a., 2022). To identify and elucidate the formation process of the white needle-like minerals deposited at this outcrop, a partial removal of the outcrop conservation cover was conducted concurrently with a road inspection closure. Several samples of the white needle-like minerals were collected. Subsequently, mineral identification was performed using XRD (Rigaku MiniFlex60) analysis at a Kyoto university.
The analysis revealed that the white needle-like minerals consist of Na-rich sulfate minerals. Reports on sulfate mineral deposition at outcrops in other regions suggest that precipitation increases under dry conditions and decreases under wet conditions (Horiguchi et al., 2000). Therefore, it is believed that the deposition of white needle-like minerals is related to the maintenance of a dry environment on the outcrop surface by the conservation cover.
In this presentation, I would like to report and discuss the findings and future conservation activities in this area.