We conducted a borehole survey across the western strand of the Itozawa fault at Shionohira, Iwaki City, Fukushima Prefecture where normal faulting surface rupture occurred at the April 11, 2011 Fukushima-ken hamadori earthquake (Iwaki earthquake, Mj7.0). Extracted core samples expose bedrock conglomerate, fluvial sand and gravel deposits, and artificial fill, from lower to upper, in ascending order. We found evidence for cumulative vertical displacement on the top of pre-2011 artificial fill, river channel gravel layer, and bedrock. Radiocarbon age was yielded from the organic-rich unit deposited on downthrown side. It suggests that one of the pre-2011 surface rupturing earthquakes occurred about 50,000 years ago. Small amount of vertical displacement at the pre-2011 earthquakes imply that coseismic slip and its deformation style at the paleo-earthquakes are different from those of the 2011 Iwaki earthquake.
For faults without overlaying Quaternary sediments where conventional basic methods such as geomorphology survey and excavation of trenches cannot be applied to obtain the paleoseismic data for seismic hazard assessment, the physical and chemical features of exhumed fault fracture zones developed in basement rocks may provide valuable clues to infer the seismic activity. We conducted literature survey on faults which cuts granite basement and have been inactive since the Late Pleistocene in order to compare the properties of the fault fracture zones to those of well-known active faults and establish the feasibility of paleoseismic investigation for basement faults. Through the cross-check mainly using three literature sources, the survey extracted only three faults (southern part of the Futaba fault, the Chimata fault and the Uchihata fault system) from more than 4200 faults in Japan including duplicate ones. The three faults features relatively low seismicity along them and the difference of fault type from the surrounding active faults.
We conducted seismic reflection survey in the coastal sea area of the Ishikari lowland, southern Hokkaido to reveal the offshore extension of the southern part of the active fault along the eastern margin of the Ishikari lowland. As a result, five depositional sequences (1 to 5 in descending order) and two anticlines (A and B) were recognized. The depositional sequences 1, 3 and 5 show downlap patterns and depositional sequences 2 and 4 show onlap patterns. So these deposits show cyclic process of regression and transgression. Therefore they are considered as the upper Pleistocene to the Holocene deposited under the influence of glacial sea-level change. The anticlines deform the upper Pleistocene and the scale of the deformation rate is 10-1 m/ky. Below the scarp which considered as the offshore extension of the active fault by the previous report, there are no deformation nor fault. The scarp is sedimentary structure formed after the last glacial maximum. These anticlines are comparable to the anticlines reported by previous survey along the coastline and offshore area. Then the southern part of the active fault extends to the Mukawa-oki Anticline and the length is extended to 86 km.
The Philippine fault is a ~1250-km-long, left-lateral strike-slip fault extending NNW parallel to the Philippine archipelago. This fault has been very active in the past 200 years with several destructive earthquakes accompanied by surface rupture. However, there was no large-scale map of the Philippine fault, which is essential for mitigating seismic hazard from future earthquakes. We mapped the surface trace of the Philippine fault on 1:50000-scale topographic maps based mainly on interpretation of ~1:30000-scale aerial photographs. We then compiled these fault trace data on a Geographic Information System to produce the first digital active fault map of the Philippine fault. These 1:50000-scale active fault maps are available from the website of Philippine Institute of Volcanology and Seismology (PHIVOLCS). These maps reveal that there are notable along-strike variations in fault trace geometry and magnitudes of historical seismicity of the Philippine fault. The Philippine fault in central Luzon and Mindanao Islands are well segmented and produced large (M≥7) earthquakes. In contrast, the fault in Masbate and Leyte Islands are more continuous and produced only moderate earthquakes in the past 400 years. There are geomorphic and geodetic evidence of aseismic creeping on the Philippine fault in northern and central Leyte. These observations suggest that the Philippine fault may be comparable to the San Andreas fault in that both of the faults are composed of locked, transition and creeping segments as previously suggested.