There are several marine terrace surfaces along the western coast of Shimokita Peninsula, northeast Japan. They are classified into H1, H2, M1, M2, A1 and A2 surfaces in descending order around Oma, northwest part of the peninsula. The M1 surface directly covered by Toya tephra (112-115 ka) is correlated with that formed in MIS 5e. A1 and A2 surfaces are Holocene emerged wave-cut benches. The heights of the former shorelines of the Pleistocene marine terrace surfaces around Oma are over 185 m, over 114 m, 60 m and 25 m , respectively. The A1 surface on the Benten-jima Island, to the north of Oma, is 1m higher than that on Oma. Spatially, the H2 and M1 surfaces can be traced for 10 km longshore southward to around Sai. The height of the most continuous M1 former shoreline is in the range of 60 m to less 20 m, indicating distinct southward tilting.
These marine terrace surfaces are planar structural markers that record the actual direction of local tilt. The progressively greater slopes of successively higher marine terrace surfaces clearly record continual crustal tilt in SSW direction around Oma and Sai. The former shorelines of the H2 and M1 surfaces define the planes recording crustal movement. Structural contours on the plane defined by the former shoreline of the M1 surface yield tilting rates of 3.2*10^-5/ka. Emerged wave-cut benches are indicative of abrupt relative sea-lebel changes. They are similar to that produced co-seismically associated with major historical earthquakes. In addition, gradual southward tilt is not documented by geodetic data. Thus, the significant tilt in SSW direction suggests that long-term tilt in this area is not continuous, but episodic. These tectonic features have been convincingly owing to the activity of a south-dipping active submarine fault extending over 40 km along the north coast of the west Shimokita peninsula.

View full abstract

We conducted pit excavation study across the northernmost section of the surface rupture associated with the Mw=6.9 14 June 2008 Iwate-Miyagi Nairiku earthquake to test our hypothesis that the surface break occurred along a pre-existing fault based on topographic observation. The 2.2-km-long northernmost trace of the surface rupture, characterized by a curvilinear trace with N-S-striking section with a west-side up thrust component to the south and NW-SE-striking section having a southwest-side up with significant sinistral slip components to the north, occurred in the westward or southwestward sloping mountain slope, forming the distinct uphillfacing scarps. We dug two pits across the scarp on the NW-SE-trending section where oblique slip with 0.9 m of southwest-side-up vertical and 1.7 m of sinistral offset occurred during the 2008 earthquake. Both pits exposed the steeply dipping rupture zone associated with the 2008 earthquake, which is typical of the strike-slip faulting fracture zone. On the basis of structural and stratigraphic features, including residual stratal deformation after restoration of the 2008 slip, thick accumulation of colluvial and fluvial deposits on upstream side and presence of buried palaeosol mantling over paleo-scarp, we identified two possible events predates 2008, both of which accompanied with uphill-facing scarps similar to the 2008 earthquake. Results of tephra analysis and radiocarbon dating of the strata show that the growth of uphill-facing scarp occurred in the age before 9.5-9.3 ka and another movement might have occurred possibly in the age ranging from 9.5-9.3 ka to 6 ka. Significant erosion in the steep mountain slope during the last glacial period together with long recurrence interval of the earthquakes might result in weak topographic relief on the fault trace in our study area.

View full abstract

We study normal-fault surface rupture associated with the 2011 Fukushima-ken Hamadōri earthquake (Mj 7.0) along the Shionohira fault, Iwaki City. This preliminary report describes the results of observation of the surface rupture (with maximum vertical offset of c.a. 2.2 m), fault outcrop, and borehole sample obtained across the Shionohira fault. We observed row of col, fault-scarp, and fracture zone includes the conglomerate along the Shionohira fault. The borehole sample indicates angular unconformity in the hanging wall of the normal fault in the Manguro area. These results suggest cumulative deformation of the Shionohira fault.

View full abstract

The Mw 7.1 (GNS Science) earthquake (Darfield earthquake) occurred near Christchurch, New Zealand on 4 September 2010. The earthquake happened in the area where active faults had not recognized previously, produced the ground surface rupture (Greendale fault). We surveyed the mode of occurrences of the Greendale fault on 8 days after the Darfield earthquake. Measured dextral and vertical displacements along the fault are ~ 4.2 and ~ 1.5 m (predominantly southern parts up), respectively. The both slip components are distributed roughly symmetrically along the west fault segment. However, the maximum displacement points are perhaps different between dextral and vertical sense based on the survey results. The fault is roughly E–W strike, characterized by many Riedel fractures that show shapes of mole tracks accompanying main dextral displacements. Few active faults had recognized previously on Canterbury plain near Christchurch, whereas distribution of active faults and folds had cleared in the Pegasus Bay area where locates in the eastern part of the Christchurch. Moreover, there are records of a few paleoearthquakes (M>5) near Christchurch and in Pegasus Bay. These data would have been clues to forecast existence of blind active faults on Canterbury plain.

View full abstract