2:45 PM - 3:00 PM
[SSS12-04] Fault rupture of the Zogahana Fault at the outer rim of the Aso caldera before and after the 2016 Kumamoto Earthquake
Keywords:2016 Kumamoto earthquake, Zogahana Fault, surface displacement, activity history
1. Purpose
Surface displacement associated with the 2016 Kumamoto earthquake that occurred on the Zogahana Fault, which situates the northern end of Aso Caldera, was reported by applying SAR interferometric analysis (Fujiwara et al., 2016) and the field survey (Nakano et al., 2018, MEXT and Kyushu University, 2019) confirmed a N-70 °E right-lateral displacement of a concrete paved road with relative vertical displacement of about 8 cm (northwest drop). The summary report of the first excavation survey of the Zogahana D site conducted by the Ichinomiya Town (now Aso City) Board of Education in 1997 (Ichinomiya Town Board of Education, 1998) pointed out the NE-SW trending, southward dipping active fault with the cumulative displacement of the Akahoya (K-Ah) and Aira-Tn (AT) tephra layers. However, the report does not provide a detailed history of the fault activity. This survey aims to clarify the history of activity of the Zogahana Fault in detail, show the relationship with the activity of the Futagawa fault, which is the source fault of the 2016 Kumamoto earthquake, and discuss the role of this fault in the tectonics of central Kyushu.
2. Method
In this survey, a geomorphic survey using UAV photogrammetry and detailed measurement of surface rupture associated with the 2016 Kumamoto earthquake were conducted, as well as the observation and description of the sediments, faults, and cracks on the outcrop surface revealed by the removal of the surface soil on the slopes along the road. In addition, our simple hand coring and ground-penetrating radar exploration revealed the shallow structure around the fault. Furthermore, to examine the geochronology and the fault activity, dating and tephra identification of samples collected by outcrops and hand coring were performed. Radiocarbon dating was carried out by requesting the Yamagata University High Sensitivity Accelerator Mass Spectrometry Center.
3. Results
The outcrop is a slope with a height of about 2 m and a width of about 10 m, facing the west. The exposed sediments can be divided into 9 layers based on features such as color and viscosity. Of these, the light brown volcanic ash layer that appeared almost in the middle could be identified as a sediment containing K-Ah from the characteristics of the facies and the dating results of the lower black soil layer. The gray volcanic ash layer exposed at the southern end of the outcrop was identified as a sediment containing AT tephra because of the characteristics and the refractive index analysis of volcanic glass.
Over a dozen fresh open cracks occurred around the extension of the surface rupture that appeared on the road surface. The strike is EW to N70 °W. Of these, the clear extension of the crack coincided with the position of a step of several centimeters northward observed on the ground surface above the outcrop, implying that the 2016 Kumamoto earthquake caused these cracks.
In addition, a sharp south-up reverse fault with a strike of N70 °E appears about 2 m south of the crack group. The upper end of this fault does not reach the topsoil, and a slump-shaped normal fault that reaches the topsoil cuts this fault. Furthermore, two slightly unclear faults appear about 2 m south of this fault, and the AT sedimentary layer is in contact with the upper (south) side of the fault. These faults probably existed before the 2016 Kumamoto earthquakes because the positions, shapes, and displacements of the faults match the description in the archaeological site excavation survey report (Ichinomiya Town Board of Education, 1998). It seems that there was almost no displacement at the time of the 2016 Kumamoto earthquake.
Our hand corings revealed multiple subsurface tephras originating from Aso and AT, showing a clear step between north and south across the fault.
4. Discussion
Our investigation confirmed that the Zogahana Fault was activated during the 2016 Kumamoto earthquake, causing displacement on the ground surface and causing numerous opening cracks in the sediments. The displacement was not concentrated on a single fault but diffused into many fractures and shifted 1 to 3 m north of the previous faults that existed before the 2016 Kumamoto earthquake, implying the previous faults did not displace.
In addition, the layer step across the fault suggests that the Zogahana Fault has been accumulating north-down activity at least after the AT tephra. At least five events can be recognized.
This research was supported by JSPS KAKENHI Grant 20K0114.
Surface displacement associated with the 2016 Kumamoto earthquake that occurred on the Zogahana Fault, which situates the northern end of Aso Caldera, was reported by applying SAR interferometric analysis (Fujiwara et al., 2016) and the field survey (Nakano et al., 2018, MEXT and Kyushu University, 2019) confirmed a N-70 °E right-lateral displacement of a concrete paved road with relative vertical displacement of about 8 cm (northwest drop). The summary report of the first excavation survey of the Zogahana D site conducted by the Ichinomiya Town (now Aso City) Board of Education in 1997 (Ichinomiya Town Board of Education, 1998) pointed out the NE-SW trending, southward dipping active fault with the cumulative displacement of the Akahoya (K-Ah) and Aira-Tn (AT) tephra layers. However, the report does not provide a detailed history of the fault activity. This survey aims to clarify the history of activity of the Zogahana Fault in detail, show the relationship with the activity of the Futagawa fault, which is the source fault of the 2016 Kumamoto earthquake, and discuss the role of this fault in the tectonics of central Kyushu.
2. Method
In this survey, a geomorphic survey using UAV photogrammetry and detailed measurement of surface rupture associated with the 2016 Kumamoto earthquake were conducted, as well as the observation and description of the sediments, faults, and cracks on the outcrop surface revealed by the removal of the surface soil on the slopes along the road. In addition, our simple hand coring and ground-penetrating radar exploration revealed the shallow structure around the fault. Furthermore, to examine the geochronology and the fault activity, dating and tephra identification of samples collected by outcrops and hand coring were performed. Radiocarbon dating was carried out by requesting the Yamagata University High Sensitivity Accelerator Mass Spectrometry Center.
3. Results
The outcrop is a slope with a height of about 2 m and a width of about 10 m, facing the west. The exposed sediments can be divided into 9 layers based on features such as color and viscosity. Of these, the light brown volcanic ash layer that appeared almost in the middle could be identified as a sediment containing K-Ah from the characteristics of the facies and the dating results of the lower black soil layer. The gray volcanic ash layer exposed at the southern end of the outcrop was identified as a sediment containing AT tephra because of the characteristics and the refractive index analysis of volcanic glass.
Over a dozen fresh open cracks occurred around the extension of the surface rupture that appeared on the road surface. The strike is EW to N70 °W. Of these, the clear extension of the crack coincided with the position of a step of several centimeters northward observed on the ground surface above the outcrop, implying that the 2016 Kumamoto earthquake caused these cracks.
In addition, a sharp south-up reverse fault with a strike of N70 °E appears about 2 m south of the crack group. The upper end of this fault does not reach the topsoil, and a slump-shaped normal fault that reaches the topsoil cuts this fault. Furthermore, two slightly unclear faults appear about 2 m south of this fault, and the AT sedimentary layer is in contact with the upper (south) side of the fault. These faults probably existed before the 2016 Kumamoto earthquakes because the positions, shapes, and displacements of the faults match the description in the archaeological site excavation survey report (Ichinomiya Town Board of Education, 1998). It seems that there was almost no displacement at the time of the 2016 Kumamoto earthquake.
Our hand corings revealed multiple subsurface tephras originating from Aso and AT, showing a clear step between north and south across the fault.
4. Discussion
Our investigation confirmed that the Zogahana Fault was activated during the 2016 Kumamoto earthquake, causing displacement on the ground surface and causing numerous opening cracks in the sediments. The displacement was not concentrated on a single fault but diffused into many fractures and shifted 1 to 3 m north of the previous faults that existed before the 2016 Kumamoto earthquake, implying the previous faults did not displace.
In addition, the layer step across the fault suggests that the Zogahana Fault has been accumulating north-down activity at least after the AT tephra. At least five events can be recognized.
This research was supported by JSPS KAKENHI Grant 20K0114.