An Active Fault in Western Honshu

A trench in Tsuruga City, Fukui Prefecture, exposes a fault that has been
active in the past 20,000 years. The main fault plane is to the right of
the nearest ladder. For details see Findings, Nosaka Fault .

Last modified 1999/12/16 13:00

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Members

Section Chief	Yuichi SUGIYAMA	sugiyama@gsj.go.jp
Resercher	Koichi SHIMOKAWA	shimo@gsj.go.jp
	Yasuo AWATA	awata@gsj.go.jp
	Kenji SATAKE	satake@gsj.go.jp
	Toshikazu YOSHIOKA	yoshioka@gsj.go.jp
	Futoshi NANAYAMA	nanayama@gsj.go.jp
	Yoshihiko KARIYA	ykariya@gsj.go.jp
	Takashi AZUMA	kero@gsj.go.jp
	Yukari MIYASHITA	yukari@gsj.go.jp
Supporting Researcher	Yuichiro FUSEJIMA	fusejima@gsj.go.jp
Affiliate Reseacher	Taku KOMATSUBARA	komatsub@gsj.go.jp

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Objectives

1. Quantify earthquake hazards by clarifying the location, size, segmentation, and geologically recent activity of faults in Japan.

2. Clarify paleo-earthquakes and tsunami hazards by identifying geologic records of ancient earthquakes and tsunamis.

3. Develop and improve paleoseismic methods, and apply them to keep hazards estimates current.

4. Understand the regional and global tectonic framework for active faults of Japan.

5. Publish findings in timely reports, not only as technical papers and maps for scientists but also through various media that explain the results to government officials and the public.

6. Strengthen cooperation with domestic and foreign organizations concerned with earthquake and tsunami hazards.

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Currernt Research

1. Major active faults in Japan
Apply trenching and archaeology to clarify the geologically recent activity of faults that are more than 15-20 km long and which have average slip rates greater than 0.1 mm/yr. Where appropriate, obtain supporting information by geophysical methods and drilling.

2. Active faults in metropolitan areas
Study active faults in the Tokyo, Osaka, and Nagoya areas, and near other large cities as well. Use methods as in topic 1.

3. Segmentation of active faults
Clarify rupture modes of selected active faults, using displacement and recurrence histories inferred from historical and paleoseismic data.

4. New methods for surveying active faults
Develop aeromagnetic methods for surveying blind faults in the Kinki Triangle of central Honshu.

5. Tsunami
Determine the Holocene history of large tsunamis and related earthquakes in northeastern Hokkaido. Clarify the generation and bathymetric focusing of the 1998 tsunami in Papua New Guinea.

6. Island arc seismotectonics
Put faults and earthquakes in the context of present-day plate tectoncs of the northwestern Pacific region.

7. Public outreach
Compile and publish strip maps showing active fault strands and their activity. Also prepare 1:500,000-scale seismotectonic maps showing active faults, earthquakes, crustal deformation, and volcanoes. Make the data available not only on paper but in various digital formats.

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The followings are main results of active fault and paleoearthquake
researches in the 1998 fiscal year. They are published in the Geological
Survey of Japan Interim Report no. EQ/99/3 (Interim Report on Active Fault
and Paleoearthquake Researches in the 1998 Fiscal Year).

0. Summarized survey result and earthquake potential of major active faults
in the Kinki Triangle, central Japan
Authors: Yuichi Sugiyama, Koichi Shimokawa, Yasuo Awata, Kenji Satake,
Kiyohide Mizuno, Toshikazu Yoshioka, Taku Komatsubara, Futoshi Nanayama,
Yoshihiko Kariya, Takashi Azuma, Yuichiro Fusejima, Eikichi Tsukuda, Akira
Sangawa and Toshihiko Sugai (Univ. Tokyo)
Abstract:After the 1995 Kobe earthquake, the Geological Survey of Japan is
conducting systematic study of major active faults in the Kinki Triangle, a
central part of Japan containing Osaka and Nagoya metropolitan areas. We
have studied 25 active faults, about 75% faults in this region, by the end
of the 1998 fiscal year. We rearranged these faults into 18 seismogenic
faults based on the criteria proposed by Matsuda (1990) for grouping and
dividing faults. We next divided the seismogenic faults into 31 behavioral
segments, using 2-km distance, difference in rupture history, and a value
of 20,000×displacement per event as criteria for segmentation.
Earthquake potential of each segment was assessed based on the elapsed time
ratio and probability for rupture in the coming 100 years. It is made
clear that the 24-km-long Aibano segment with 3 to 5 m unit slip, on the
west coast of Lake Biwa, has 12 to 38% probability of an M≧7 earthquake wit
hin the next 100 years. The 44-km-long Uemachi fault traversing Osaka has
5% probability of a big earthquake in the coming 100 years at the maximum.
On the other hand, at least 14 behavioral segments including the Hokudan
which produced the Kobe earthquake have relatively low earthquake
potential. Their probabilities for rerupturing in the next century are less
than 0.2%. Most of them proved to be causative faults of disastrous
historical earthquakes such as the 1325 Shochu, 1586 Tensho, 1596 Keicho
and 1662 Kanbun earthquakes.

1. Behavioral segments and their cascades for the earthquake occurrence in
the Japanese surface ruptures−Tentative Study−
Author: Yasuo Awata
Abstract: 15 surface ruptures on land in Japan are subdivided into 23
behavioral segments, based on timing of paleofaulting, long-term slip rate,
and fault geometry. Relations between the lengths of behavioral and
earthquake segments, and the maximum displacement are shown as follows:
Lb-seg = 4.9×Dmax and Le-seg = 9.0×Dmax, where Lb-seg is a behavioral segm
ent length, Le-seg is a earthquake segment length in kilometer, and Dmax is
a maximum displacement of segment in meter. The cascade ratio, the ratio of
an earthquake segment length to the length of maximum behavioral segment in
the earthquake segment, is 1.8 for Japanese surface ruptures. These
relationships are applicable to make a segmentation of the active faults,
and to evaluate the cascades for the earthquake occurrence in Japan. In
this application, it is considered that the maximum displacement is 1.5 to
2 times as large as the most-frequently-occurring displacement in the
behavioral segment.

2. Sedimentological study of postulated tsunami deposits along the Kuril
subduction zone, eastern Hokkaido, northern Japan: preliminary study around
the Kiritappu marsh land
Authors: Futoshi Nanayama, Kenji Satake, Koichi Shimokawa, Kiyoyuki Shigeno
(Niigata Univ.) and Ryuta Furukawa
Abstract: Large earthquakes in the Kuril subduction zone have caused
tsunami damage along the Pacific coast of eastern Hokkaido, between Nemuro
and Tokachi coast, northern Japan. In a preliminary study of postulated
tsunami deposits, seven postulated tsunami sand layers (named Ts1 to Ts7 in
descending order) have been found in a peat layer around the Kiritappu
marsh land by sedimentological, tephrochronological and
micropaleontological methods. Ts7, Ts6 and Ts5 are under a regional tephra
B-Tm, which is dated as ca. 9th century. Ts4 is dated 9th to 13th century,
and Ts3 is 13th to 17th century. Ts3 may correspond to a tsunami event
documented in historical records, and it may have been the biggest tsunami
event judging from its wide distribution and thick sand layer along the
eastern coast of Hokkaido. Ts2 and Ts1, which are above a regional tephra
layer Ta-a of ca. 1739 AD, are dated 19th and 20th century, respectively.

3. 1998 rupturing andpaleoseismicity of northern Shizukuishi-bonchi seien fault
Authors: Takashi Azuma, Yasuo Awata, Toshikazu Yoshioka and Yuichiro Fusejima
Abstract: We excavated two trenches on Shinozaki earthquake fault, the
surface rupture generated by the 1998 earthquake in northern Iwate
Prefecture, in order to clarify the underground structure and previous
activities of the fault. A 1 m-wide west-dipping fault zone was exposed
beneath the 1998 flexure scarplet. It is inferred that at least one
faulting event occurred in the period between about 800 B.C. and 1,300 A.D.
from 14C ages of both the layers that were deformed and not deformed by
the faulting prior to the recent rupture. The vertical displacement in
this period is estimated to be two or three times as large as that of the
1998 rupture.

4. Subsurface structure of the Fukaya fault and its southeastern extension
as perceived on seismic reflection profiles
Authors: Kazuo Yamaguchi, Naomi Kano, Takanobu Yokokura, Tsutomu Kiguchi,
Toshiyuki Yokota, Akiko Tanaka, Yoshito Nakashima and Toshiki Ohtaki
Abstract: Several seismic reflection profiles obtained in the northwest
Kanto Plain show that the Fukaya active fault certainly continues
southeastward in the subsurface from Mikajiri to Kumagaya Bridge across the
Ara River. The fault possibly extends further southeastward to the vicinity
of the Yoshimi Hills. The SW-dipping fault plane of the Fukaya fault is
inferred to be high angle at shallow part and low angle at deep part. The
Konan and Kushibiki faults are interpreted as secondary faults of the
Fukaya fault, and their fault planes correspond to the NE-dipping
reflectors.

5. Reevaluation of the displacement per event along the Tsukioka fault,
central Japan, based on the fault geometry in the shallow depth
Authors: Yasuo Awata and Taku Komatsubara
Abstract: The displacement per event of the Tsukioka fault, Niigata
Prefecture, central Japan is reevaluated considering the shallow geometry
of the fault. A 40-meters-deep drilling revealed that the dip of the fault
is 25 degrees beneath the flexure scarp on an alluvial fan of 6 ka. The dip
of the fault in the deep is reported to be 60 degrees. The flexure scarp
with the vertical offset of 3 meters is simulated using the elastic
dislocation theory. From this simulation, the fault displacement is
estimated to be 5-6 meters in the deep, and decreases toward the surface
beneath the flexure scarp. The displacement per event suggests that the
Tsukioka and Muramatsu faults compose a single behavioral segment of 30-35
kilometers long and have a recurrence interval of 10 ky.

6. P-wave seismic reflection survey of the Kakuda-Yahiko fault group in
Niigata Prefecture, central Japan
Authors: Naomi Kano, Kazuo Yamaguchi and Yasuo Awata
Abstarct: Kakuda-Yahiko fault group is a major buried fault along the
western border of the Niigata Plain. We conducted a P-wave seismic
reflection survey across the middle part of the fault group. The survey
line is 7.5 km long, extending from Maki Town to Nishikawa Town in Niigata
Prefecture. Inclined reflectors have been detected at the depth ranging
from nearly 0 m to around 500 m in the western part of the line. Only the
very shallow part could be imaged, however, in the eastern part of the
line. In the middle of the reflection profile obtained, the shallow
subsurface structure has been imaged. It makes possible to estimate the
Quaternary activity of the fault group. The shallow reflectors at the depth
less than about 150m on the east of the distance 500 m are inferred to be
the Holocene deposits. A strong reflector correlative to the top of the
lower Holocene gravel bed gently dips eastward with a relative depth of
about 50 m between the distance 1500 m and 2500 m, while it is almost flat
outside of this portion. Therefore, the eastward-inclined structure of the
Holocene deposits is considered to be a flexure, which is ascribable to the
recent activity of the Kakuda-Yahiko fault group.