Laser imagery technique is applied to obtain comprehensive features of a typical reverse fault (the Senya fault in NE Japan) and strike-slip fault (the Adera fault in Central Japan) We test several methods for obtaining fault slip from detailed DTM images. Using the visual analyzer“ Geo-Graphia”, a series of three-dimensional laser imagery is processed around the Senya hill, especially along the surface fault ruptures associated with the Rikuu earthquake in 1896. We compared amount of fault slips along the fault ruptures measured by previous field studies with those obtained by automatic and manual topographical profiling using detailed DTM. Three-dimensional laser imagery is also processed along the Adera fault, and slip vectors are restored based on successively faulted terrace risers around Sakashita town.
Marine terraces well developed along the coast of the Shimokita Peninsula are good indicator of late Quaternary crustal deformation. We decipher active reverse faulting on the basis of deformation of the marine terrace surface formed in MIS 5e around Rokkasho village in the southern part of the peninsula. The marine terrace surfaces in the area are classified into two groups of H and M. M1 surface covered by Toya tephra is correlated with that formed in MIS 5e. A 1-2 km wide flexural scarp tilting toward east is found on MI. surface. Vertical offset of M1 surface is over 30 m in the north and 20 m in the south. The flexure extending NNE-SSW for 15 km is consistent with the prevailing Tertiary monocline structure well recorded in seismic reflection record. M1 surface and the Tertiary system have been folded convincingly owing to the activity of west-dipping blind active thrust extending to the depth of several hundred meters beneath the structure. This fault may merge into the extensive submarine fault along shelf edge in the north. It is possible that they compose an over 100-km long active fault.
Since the 2008 Iwate-Miyagi Nairiku Earthquake occurred in the area where detailed active fault studies had not been done before, it is important to define whether the earthquake is related to any existing active fault or inactive fault. We interpret large-scale aerial photographs taken in 1976 as well as taken immediately after the earthquake, in order to compare well-defined surface fault ruptures with pre-existing fault-related landform sporadically distributed along the Koino'oka river in Genbi-cho, Ichinoseki city. In the field, we confirmed that these surface ruptures and deformation partially appeared along the pre-existing fault-related landform. Further, we excavated trenches across the earthquake fault, and found that the fault has been repeatedly moved during Holocene. Thus we confirmed that the cause of the earthquake was strongly related to this newly found active fault.
We excavated paleoseismic trenches across the Yamagata-bonchi fault zone that is a 60-km-long westdipping active reverse fault located in northeast Honshu, Japan. We then recovered the evidence for six to seven surface-rupturing earthquakes in the past ten thousand years from the highly deformed strata and faults exposed on the trench walls in the town of Oishida that located on the northern edge of the fault zone. The most recent paleoseismic event in the town is inferred to have occurred sometime between 1,500 and 2,800 cal. y. B. P. In contrast, trench walls in the town of Yamanobe across the southern part of the fault zone exposed evidence for only one surface-rupturing earthquake in the past ten thousand years. The event age is constrained to have occurred sometime between 4,200 and 5,000 y. B. P. Together with the major fault discontinuity at the center of the fault zone, we conclude that the Yamagata-bonchi fault zone is divided into two 30-km-long behavioral segments, both of which might have produced and will generate M-7.3 earthquakes. An average recurrence interval of 1,400-1,500 years and the long elapsed time since the most recent event on the northern segment allow us to calculate 3-14% of 30-yr conditional earthquake probabilities. Since time range of the event on the southern segment overlaps one of the events recovered from the northern segment, we cannot rule out the near future likelihood of the infrequent worst scenario of 60-km simultaneous multiple ruptures that might produce M-7.5 earthquake.
The Umi fault was first described by Ikeda et al. (2004) as an active fault'of about 9km in length. Fukuoka Prefectural Government organized a research committee for examination and investigation of the Umi fault in 2005. The present study is based on air-photograph analysis, geological and topographical field surveys, soundreflection surveys in the marine areas, drilling and trenching, and tephrochronological studies. Some characters of the Umi fault are clarified through this intensive study. The Umi fault extends father north and south than was previously known, but it does not extend as far north as Hakata Bay and the Genkainada Sea. The length of the fault is 17km at least. The average vertical slip rate in the late Quaternary is estimated to be 0.02-0.03m/kyr (Activity Rank C in Matsuda 1975). This slip rate of the Umi fault is lower than those of the neighboring Kego and Nishiyama faults. The last activity of the fault was confirmed to be younger than 4300y. B. P. by the trenching study at Yamaura. The average recurrence interval of the activity is estimated at 29 kyr. This estimation is based on the assumption that the observed slip at the Yamaura trench (0.6m) occurred in one earthquake event and that the slip at Uenohara (1.9m), which is observed on middle terrace surface, formed immediately after the Aso-4 pyroclastic flow, is a result of repeated earthquake events of the same size slip.
The Kachchh region has suffered from at least four damaging moderate to large earthquakes since the 17th century. However, none of these earthquakes except the 1819 Allah Bund earthquake accompanied surface rupture. Even the recent 2001 Bhuj earthquake with Mw7.6 occurred on a blind fault. Several faults in the Kachchh viz. the Island Belt Fault, the Kachchh Mainland Fault (KMF), and the Katrol Hill Fault were suggested to be active during Late Quaternary time by previous studies. But there is no such supportive evidence available in the historical documents; also none of recent studies except ours (Morino et al.,2007, Malik et al.,2008, and Morino et al.,2008) reported ground truth that these fault are active. We in our earlier paper reported faulting in Late Pleistocene to Holocene age sediment near the Lodai Village along the KMF. To confirm further active faulting along the KMF, paleoseismic investigation near Jhura Village about 30 km west of Lodai revealed an active fault displacing overbank deposits of Kaila River. Two fault strands Fl and F2 were identified in the trench. The northern Fl shows a low-angle reverse fault with inclination of 15° towards the south. At least two faulting events were inferred on the basis of upward fault termination with clear angular unconformity. The net-slip during a single faulting event considering deformation on the hanging wall of Fl fault is over 5 m.
The Atera fault zone that runs along the southwestern border of the Northern Japan Alps, about 70 km in length, is NW-SE and NNW-SSE strikes. We performed geomorphological and geological surveys to clarify the tectonic landform and fault activity of the central part of the Atera fault zone in Kashimo area. We observed remarkable tectonic landforms and outcrops (landslide and fault) in Nagahora located on the left bank of Kashimo river. The landslide along the fault trace in Nagahora is possibly related to the paleo event of Atera fault zone. Horizontal and vertical slip rates, which are calculated from the displacement of the M2 terrace in Nagahora, are more than 0.3-0.4 m/ky and 0.04 m/ky, respectively. Because these slip rates are smaller than that of the other parts along the Atera fault zone, fault displacements are probably divided by several parallel faults in this area. The Owachi fault located to the NE indicates SW uplifting, and the Atera fault located to the SW indicates NE uplifting in this area. The area is located on a horst formed between the right stepping two faults.