10th Asian Regional Conference of IAEG (2015)

Mimic active faults – the Quaternary deformations by bedrock expansion
Shuichi HASHIMOTO (1), Tadashi MIWA(2), Yuji TORIGOE(2),Masanori NIIYAMA(3) and Kunio TAKANO(3)(1) Tohoku Electric Power Co., Inc., Japan
E-mail:hashimoto.shuichi.sp@tohoku-epco.co.jp(2) Tohoku Electric Power Co., Inc., Japan,(3) Dia Consultants co., Ltd., Japan
Abstract
It is important to evaluate seismogenic active faults in order to proper seismic designfor nuclear facilities. We should carefully perform geological surveys and judge the fault
activity from the viewpoint of earthquake generation, because it is often difficult todistinguish seismic from nonseismic deformation features. We are liable to interpret “theQuaternary deformations along to the geologic fault planes” as the evidence of active faults.
In case of the Higashidori Nuclear Power Plant site, there are several sets of characteristic
deformations in the late Quaternary terrace deposit (younger than 120ka) covering theweathered Miocene bedrock with some geologic faults. Careful geological surveys clarify
that these deformation features are not consistent in the continuity. For clear example, theshear planes of some faults often disappear in the dip direction and strike direction.
Furthermore, these features have no accumulation in the late Quaternary. These facts
indicate that these deformation features occurred in shallow bedrock which has been
suffered from weathering or deterioration. Therefore, they should not be generated bytectonic or seismogenic. Another physicochemical analysis indicates that these features areoften better explained by the volumetric expansion of the weathered or deteriorated
bedrock.
Keywords: active fault, non-tectonic features, Quaternary deformation, volumetric expansion
1.

Introduction

It is very important to consider and evaluate thefault activities in order to assure the seismic designfor the electric facilities, particularly nuclear power
plants.
Detailed geological surveys were performed in
the site of Higashidori Nuclear Power Plant (NPP).
2.

Outline of the study area

Higashidori NPP site is located on the Pacific
Ocean side of the Shimokita Peninsula, northeast endof Tohoku District, Northeast Japan (Fig. 1). The sitecovers the area 3.5 km long from north to south and 1
km wide from west to east. The study area is incontact with the Shimokita mountainous area to thewest and is facing the Pacific Ocean to the east.
The study area is underlain by Early to Middle
Miocene sedimentary and pyroclastic rocks covered
with several Late Pleistocene terrace deposits.

TohokuDistrict

Shimokita PeninsulaFig. 1 Location map of the study area1

10th Asian Regional Conference of IAEG (2015)3.

Geomorphology

The Gamanosawa Formation unconformably
overlies the Tomari Formation and is restricted in
distribution by major faults. Elongated shape bodies
from north to south of this formation lie in NNE
trending fault contact with the Tomari Formation.
The Gamanosawa Formation is composed ofconglomerate, sandstone, mudstone and tuff. Thediatom age indicates 9.9-14.9 Ma (NPD 5C-4A zone).

Geomorphologic surface studies about altitude,
distribution pattern, clearness, index tephras, etc.
show that the on-site terrace are divided mainly into
M1, M1’, M2, and L1 surface, which are comparedwith MIS5e, 5e, 5c and 5a, respectively (Fig.2).
The results also indicate that lineaments or
tectonic landform suggesting fault activity are notobserved.
4.

4.2 Quaternary
The Middle Pleistocene terrace deposits
unconformably cover the Neogene basements andwidely distributed in the study area.
These terrace deposits consist mostly of graveland marine sand and are overlain by loam. Theseterrace deposits contain some intercalated index
tephras, such as Toya (112-115ka), Aso-4 (85-90ka),
To-Rd (80ka) and so on. These terrace deposits aredivided into M1, M1’, M2 and L1 terrace based on
above mentioned index tephras and the altitude oftheir planes.

Regional geology

4.1 Neogene
Neogene basement rocks have no outcrops in thesite because they are widely overlain by Pleistocene
terrace deposits. So, the basement geology has beenstudied in detail by drilling, tunneling and trenching
method as a result of nuclear energy assessment.
The Neogene geological map and the location ofgeological surveys are shown in Fig.3. The Neogeneof the study area is divided into the Sarugamori,Tomari and Gamanosawa Formations in ascending
stratigraphic order.
The Sarugamori Formation is the lowest
formation and crops out the north of the site. Thisformation is composed of mudstone, sandstone,
conglomerate and pumiceous tuff.
The Tomari Formation, mainly conformably
overlies the Sarugamori Formations, is widelydistributed in the site. It is composed of andesite lava,
tuff breccia, lapilli tuff and tuff. Tuff breccia rarely
contains silicified wood (fossil trunk). K-Ar age ofthe lava indicate 13-15 Ma (Watanabe et al., 1993).

5.

Geological structure

The structure of Miocene strata in the study area
is characterized by several NNE-SSW trending major
faults (Fig.3). Although gently dipping to eastward
generally, the Miocene strata are faulted by above
mentioned NNE trending normal faults. The throw ofthese major faults are estimated up to several hundred
meters in maximum. It is considered that thesestructures were formed during the late Miocene
tectonics of WNW trending tension stress field asknown in Northeast (Tohoku) Japan.

Fig. 2 Geomorphic surface map in the study area

210th Asian Regional Conference of IAEG (2015)

Fig. 3 Geological sketch map of the study area
6.

Quaternary deformations

are shown in Fig. 4.
Minor reverse-like s-19 fault (type 1) isobserved at the south wall of the trench. Fault plane
trends NNW and dips west in the deeply weathered
tuff breccia. Although the hanging wall side of thefault was elevated one meter, displacement isdiminishing westward and it dies out in the berm.
Although hanging wall side rises asymmetrically
only the range of ten meters, there is no differencein the altitude of the basement surface from east towest. On the other hand, when we trace a fault line
to the north wall in the strike direction, there
become only convex form(type 2). Furthermore, wecan observe several convex form of bedrock surface
which wave length is ten meters in the same trenchwall (Photo.1，Fig.5).
Bedrock around here is highly weathered andturned dark brown color. And strongly deteriorated
zone of the bedrock in the hanging wall side of thes-19 fault is rather thicker than in the foot wall side(Fig.4). A number of low dipping sheeting joints
are developed in the basements (Fig.5).
Black fine precipitates adhere to their joint
planes, and slickensides are often observed on them.
These facts indicate the result of rocks friction
between joints.

A wide variety of the Quaternary deformations
occur in the study area. They are often observed at
the surface of weathered basements. “TheQuaternary deformations” can be divided into threetypes. The characteristics of each deformation are
as follows;
Type 1: small displacement or step along fault
plane
Site and features; in the basement of Quaternary
deposits or in the lamina of Quaternary deposits.
Size; several centimeters to tens of centimeters
Type 2: convex upward form
Site and features; upside of the weathered
bedrock surface and the shape is concordant to theheaving bedrock surface. Size; wave length isseveral meters, heaving height is several tens ofcentimeters
Type 3: clay injection into Quaternary deposits
Site and features; upside of major fault zone,
the soft clay is originated from shear zone ofgeologic major fault. Size; tens of centimeters.
7.

Example of Quaternary deformation at
Trench 20’-1

Examples of the Quaternary deformation anddistribution of weathered bedrocks at Trench 20’-13

10th Asian Regional Conference of IAEG (2015)

Fig. 4 Bedrock deterioration rank of Trench 20’-1 south wallca. 25m

Photo1 Photo of Trench 20’-1(from east to west)

west

convex form

convex form

convex form

east
Toya Tephra
M1 terrace deposit

Tomari Formation

Fig. 5 Layout of Trench 20’-14

10th Asian Regional Conference of IAEG (2015)8.

Discussion

change and the timing of heaving for the future.

In this example of reverse-like s-19 fault, 1
meter displacement near the bedrock surface along
the shear plane is disappeared within a few meters
depth. Although very local zone (less than 10mwide) of upheaval around the s-19 fault was formed,
there is no vertical separation in whole wall of thetrench.
In addition, distribution of weathered bedrocks(Fig. 4) strongly corresponds to “the Quaternarydeformations”. Other trenching data show thatmany convex upward forms with wavelength of ten
meters are commonly distributed in weatheredbedrocks.
The phenomena of volumetric expansion ofrocks during weathering are reported concerning
various types of rocks (Chigira and Nakata, 2013).
In case of Higashidori site, the measurements ofvolumetric expansion were carried out for tuffbreccia using titanium known as immobile element.
The measurement obtained the result of 20 percent
expansion at highly weathered part in comparison
at fresh part (Torigoe et al., 2013).
From the above-mentioned observations and themeasurements, we think these deformations are notformed by tectonic origin at least, but formed by
the volumetric change of weathered rocks.
Concerning type 3, “heaving bedrock”
phenomena which can form shear planes by slow
speed deformation is reported (Noe, et al., 2007).
9.

References
Chigira, M. and Nakata, E. (2013): Volumetric
expansion during weathering of various rocks
and its geological significance, Abstracts of the120th annual meeting of the geological Societyof Japan, p.151 (in Japanese).
Noe, D. C., Higgins, J. D. and Olsen, H. W.
(2007): Steeply Dipping Heaving Bedrock,
Colorado: Part 1—Heave Features and PhysicalGeological Framework, Environmental andEngineering Geoscience, Vol.13, No.4,
pp.289-308.
Torigoe, Y., Miwa, T., Nakata,E. and Chigira, M.
(2013): The volume expansion of rocks by theweathering, a case study of Higashidori Nuclearpower station, Abstracts of the 120th annual
meeting of the geological Society of Japan,
p.303 (in Japanese).
Watanabe, N., Takimoto, T., Shuto, K. and Itaya, T.
(1993): K-Ar ages of the Miocene volcanic
rocks from the Tomari area in the Shimokita
Peninsula, Northeast Japan arc, J. Min. Petr.
Econ. Geol., vol.88, pp.352-358.

Conclusions

The Quaternary deformations can be divided
into three types. The characteristics of eachdeformation are as follows;
Type 1 (“fault like planes”) mainly exist in
weathered rocks and go extinct at shallow depth. Itmeans “rootless”.
Type 2 (convex upward deformation ofQuaternary deposits) exist in weathered bedrock
beneath it and often accompanied with no faults.
Type 3 (injected clay) often has irregular flow
structure in it with no sharp shear planes. Thesefacts indicate that the injection was not coseismic
event with high-speed shearing during earthquake.
On the whole, the Quaternary deformations are
not consistent in the continuity and orientations,and have no accumulation of displacements
respectively.
All of the facts suggest these phenomena
occurred in shallow weathered or deteriorated zone
and should not be tectonic and seismogenic faults.
Because the research cases are rare, it is not stillclear the mechanism of volumetric change of rocks
completely. A few issues to be solved are left. We
should more discuss a quantification of volumetric5