The surface earthquake fault of the 11th April 2011 earthquake in Hamadoori Fukushima pref.
（２）Field survey Report of 11th to 13th April
(By: Tatsuya Ishiyama, Hiroshi Sato (ERI), Tanio Ito (univ. Teikyo Heisei), Nobuhiko Sugito (univ.Nagoya Research Center for Seismology, Volcanology and Disaster Mitigation), Tomoo Echigo (Geo-Research Institute), Naoko Kato (ERI)，Toshifumi Imaizumi (univ. Tohoku)
Introduction
M7.0 (temporary) earthquake with depth 5km occurred around 16:00 11th April at Hamadoori Fukushima prefecture (JMA2011). The surface earthquake fault was found along the active fault, known as Idozawa Fault, by the field survey done from the day after earthquake occurrence (12th to 14th).
Below is the addition and changes from the first report provided on 13th.
Surface earthquake fault of the Fukushima Hamadoori earthquake on 11th April 2011
From 12th to 15th April, a field survey was held to see if there was a surface earthquake fault or not. As a result, intermittent distribution of a high-angle westerly dip or otherwise an almost vertical fault plane was found in the 11km between Tabito and Tabiudo of the Oodaira Tsunaki in Iwaki city. (fig.1). The strike of the tectonic scarp generally indicates in the vicinity of N10ºW, vertical displacement at the lowering of west side is about 0.8-1.8 m, and the right-lateral fault displacement is within/out 30cm. It is assumed that the surface earthquake fault is considered to have right acheleon formation.
Surface earthquake fault appeared along the far most west side fault amongst the several fault lines suggested by Active Fault Research (1991) (fig1 and images). On the other hand, we surveyed the several lines that runs across the fault lines on the east side, id est a geologically defined fault: Izawa fault, but no Surface earthquake fault was found.
Fig.1 Distribution of the surface earthquake fault of the earthquake (M7.0) on 11th April 2011 at Hamadoori Fukushima pref. The red line is the distribution of the surface earthquake fault found by the survey from 12th to 14th April, the red dotted line is the location of the surface earthquake fault assumed mainly from the geological formation, and the green circle indicates the location where surface earthquake fault was confirmed. The orange line is the route of field survey and the black line indicates the location of estimated active fault by Nakada and Imaizumi (2002). The map is 1/25000 topography issued by Geographical Survey Institute.
Proposition of Shionohira fault (provisional title)
Active Fault Research (1991) is defining the several fault lines including the above-mentioned Idosawa fault as “Idosawa fault”in a lump. But on the other hand, faults that distribute along the earthquake fault this time, have an almost vertical structure with cataclasite and fault gouge having Gozaisho metamorphic rocks as protolith, and is obviously a different structure from Izawa fault (Watanabe, Sato, 1935). Therefore, here, we name the normal fault located in the west most side out of “Izawa fault” (Active Fault Research, 1991) as“Shionohira fault”tentatively from Shionohira Tabito-cho, Iwaki city where the maximum vertical displacement was observed. According to Kubo et al.(2007), Shionohira fault will be extending more toward the south than the area where surface earthquake fault appeared this time. Therefore, according to the definition in this, surface earthquake fault this time has appeared along the part of “Shionohira fault”.
Image1: Loc 7. Tsumiki Toono Iwaki city. The surface earthquake fault running across the paddy field on the bottom of the valley. Facing north-northwest. Aperture rift evolves over the top of the cliff surface. The hinge indicated by cliff line of the bending and the aperture rift extends from city road in front to the slope in the back with right echelon formation. The road surface cliff height In front is 50 cm. The amount of horizontal extension measured from the off-set of channel is 6 cm.
Image2. Loc7. Tsumiki Toono Iwaki city. The surface earthquake fault running across the paddy field on the bottom of the valley. Facing north-northwest. An almost vertical fault surface appears along the aperture rift.
Image 3 Loc 8. Tsumiki Toono Iwaki city. The surface earthquake fault cutting across the pavement of a road. The aperture rift echelons to a remarkable extent. Against the strike of the aperture rift generally indicates in the vicinity of N22ºW, the strike of the aperture rift in the middle of the image is N85ºW, which differs in a huge scale. The cliff height is 50 cm. It is near the northern limit of the surface earthquake fault found this time.
Image4 Loc 12. The surface earthquake fault running across the road in the east side of Shionohira village in Shionohira Tabito Iwaki city. Facing east. The cliff height is 1.8 m. The strike of fault surface is N20ºW, the dip is 70Wº. The observed fault striation has two directions:60ºS and 85ºS, and is becoming low angle from the above.
Image 5 Loc 13. Shionohira Tabito Iwaki city. It corresponds to north-north west extension of the surface earthquake fault. The earthquake fault splits in two stria as it forms in right echelon, and runs across the paddy field. The summed vertical displacement at the place of the two stria fault overlapping is about 2.3 m, and the left-lateral fault displacement is within/out 0.5m.
Image6 Loc 13. Shionohira Tabito Iwaki city. The surface earthquake fault extending northwest in image 6. The cliff-height on the pavement of the road is approx:1.3m
Future plans
Hereafter, the plan is to try clarifying the more accurate location of the surface earthquake fault and distribution of co-seismic displacement, taking the above result and tasks into account.
Acknowledgement
I would like to express my gratitude to Lecturer. Tsuyoshi Sato (univ. Teikyo Heisei) and Project Researcher. Toshio Nakayama (ERI) for their contribution at the field survey.
Referrence
- Active Fault Research comp．(1991) Active Faults in Japan, University of Tokyo Press,437pp.
- Japan Metrological Agency (2011) 35th report on 2011 Tohoku Earthquake :http://www.jma.go.jp/jma/press/1104/11b/201104111820.html
- Kubo.K, Yanagisawa.Y, Ymamoto.T, Nakae.S, Takahashi.H, Toshimitsu.S, Sakano.Y, Miyaji.Y, Takahashi.M, Oono.T, Komazawa.M (2007) Seamless Digital Geological Map of “Shirakawa”, Geological Survey of Japan.
- Nakata.K and Imaizumi.T comp. (2002), Katsudansou shousai Digital map, University of Tokyo Press, DVD-ROM2,appended figure 1, 60p.
- Watanabe.H, G.Sato,（1935）Seamless Digital Geological Map of “Nakoso” Geological Survey of Japan