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[SSS14-P06] Fault outcrops appeared by riverbank erosion near the surface deformation along the Wakayama River in Suzu City, Noto Peninsula
Keywords:Fault outcrop, Surface deformation, Wakayama River, 2024 Noto Peninsula Earthquake
The Noto Peninsula earthquake (M7.6) on January 1st, 2024, caused surface deformations with steep scarps 1—to 2-meter high on the valley plain along the Wakayama River, which flows through Wakayama-machi, Suzu City. Some of those surface deformations appeared along the existing scarp in the Nobutake and Naka areas, suggesting that similar deformations had occurred in the past.
In September 2024, a torrential rainstorm hit the Noto Peninsula, causing extensive damage, mainly to Suzu and Wajima. Heavy rainfall on the Wakayama River caused riverbank erosion and slope failure at many locations, exposing numerous outcrops along the river. The surface deformation intersected the meandering Wakayama River at many places, and fault outcrops were observed at some crossing points. We conducted a field survey along the Wakayama River because many fault outcrops may have appeared. Our field survey revealed several fault outcrops, especially where they intersected surface deformations. We selected three outcrops where fluvial sediments were cut and displaced by the faults, where we could obtain information on past activity could be obtained, and conducted detailed observations. The outcrops were cleaned and photographed. Since they were not flat, the iPhone 15 Pro's LiDAR sensor created three-dimensional shapes. RTK-GNSS measured the position of the outcrop. After observation, samples were collected for radiocarbon dating. The results of the outcrop observation are described below.
Nobutake outcrop (WKG-OC1)
The outcrop is located on the left bank of the south-flowing point, where riverbank erosion has eroded at the crossing point between surface deformation and river, exposing a north-uplifted fault. The uplifted side was mostly mudstone, while humic silt, sand and gravel, and silt mixed with sand and gravel were observed on the south side of the fault. Dragging structures of sand and gravel beds and rotation of gravels were observed near the fault on the downthrown side. As the top of the mudstone on the downthrown side did not appear, a vertical displacement could be at least 2 m. The vertical displacement near the outcrop associated with the Noto Peninsula earthquake was about 30 cm, suggesting the fault has been repeatedly activated.
Naka outcrop (WKG-OC3)
The fault outcrop on the right bank of the SSE-flowing point was exposed due to erosion. A nearly vertical, south-side-up fault was recognized at the bottom of the outcrop. The gravel layer directly overlying the mudstone and the gravel-silt alternation layers overlying them show a convex structure, with the top near the fault. The upper sediment layer at the uplifted side is nearly horizontal and contacts the tilted gravel-silt alternation with an unconformity. This relationship suggests that the unconformity was probably associated with a past movement earlier than the 2024 Noto Peninsula earthquake.
Munesue outcrop (WKG-OC7)
The outcrop is located on the right bank of the NNW-flowing point, where riverbank erosion has continuously exposed terrace deposits for about 200 m. A north-side-up reverse fault was recognized in the middle of the exposure. The fault split into branches at the upper part, and the mudstone on the uplifted side was significantly fractured. The fault was probably moved during the Noto Peninsula earthquake because many cracks and voids were found along it. Under the flood deposits, a drop of about 20 to 30 cm was observed on the past ground surface. On the uplifted side, a fluvial gravel layer directly overlies the mudstone layer, and the vertical difference between the top of the gravel layer and the top of the downlifted side is about 1 m. Silty sediments existed only on the downthrown side, and the Noto Peninsula earthquake could not have caused the vertical difference in the upper boundaries of the gravel beds. These facts suggest that the fault has been repeatedly moved in the past.
Observations of several outcrops suggested that this surface alteration has repeatedly activated and caused similar displacements. Based on the results of outcrop observations and dating, we will discuss the activity of surface deformation.
In September 2024, a torrential rainstorm hit the Noto Peninsula, causing extensive damage, mainly to Suzu and Wajima. Heavy rainfall on the Wakayama River caused riverbank erosion and slope failure at many locations, exposing numerous outcrops along the river. The surface deformation intersected the meandering Wakayama River at many places, and fault outcrops were observed at some crossing points. We conducted a field survey along the Wakayama River because many fault outcrops may have appeared. Our field survey revealed several fault outcrops, especially where they intersected surface deformations. We selected three outcrops where fluvial sediments were cut and displaced by the faults, where we could obtain information on past activity could be obtained, and conducted detailed observations. The outcrops were cleaned and photographed. Since they were not flat, the iPhone 15 Pro's LiDAR sensor created three-dimensional shapes. RTK-GNSS measured the position of the outcrop. After observation, samples were collected for radiocarbon dating. The results of the outcrop observation are described below.
Nobutake outcrop (WKG-OC1)
The outcrop is located on the left bank of the south-flowing point, where riverbank erosion has eroded at the crossing point between surface deformation and river, exposing a north-uplifted fault. The uplifted side was mostly mudstone, while humic silt, sand and gravel, and silt mixed with sand and gravel were observed on the south side of the fault. Dragging structures of sand and gravel beds and rotation of gravels were observed near the fault on the downthrown side. As the top of the mudstone on the downthrown side did not appear, a vertical displacement could be at least 2 m. The vertical displacement near the outcrop associated with the Noto Peninsula earthquake was about 30 cm, suggesting the fault has been repeatedly activated.
Naka outcrop (WKG-OC3)
The fault outcrop on the right bank of the SSE-flowing point was exposed due to erosion. A nearly vertical, south-side-up fault was recognized at the bottom of the outcrop. The gravel layer directly overlying the mudstone and the gravel-silt alternation layers overlying them show a convex structure, with the top near the fault. The upper sediment layer at the uplifted side is nearly horizontal and contacts the tilted gravel-silt alternation with an unconformity. This relationship suggests that the unconformity was probably associated with a past movement earlier than the 2024 Noto Peninsula earthquake.
Munesue outcrop (WKG-OC7)
The outcrop is located on the right bank of the NNW-flowing point, where riverbank erosion has continuously exposed terrace deposits for about 200 m. A north-side-up reverse fault was recognized in the middle of the exposure. The fault split into branches at the upper part, and the mudstone on the uplifted side was significantly fractured. The fault was probably moved during the Noto Peninsula earthquake because many cracks and voids were found along it. Under the flood deposits, a drop of about 20 to 30 cm was observed on the past ground surface. On the uplifted side, a fluvial gravel layer directly overlies the mudstone layer, and the vertical difference between the top of the gravel layer and the top of the downlifted side is about 1 m. Silty sediments existed only on the downthrown side, and the Noto Peninsula earthquake could not have caused the vertical difference in the upper boundaries of the gravel beds. These facts suggest that the fault has been repeatedly moved in the past.
Observations of several outcrops suggested that this surface alteration has repeatedly activated and caused similar displacements. Based on the results of outcrop observations and dating, we will discuss the activity of surface deformation.