Proceedings of the ASME 13th International Conference on Environmental Remediation andRadioactive Waste Management
ICEM2010
October
3-7, 2010, Tsukuba,Japan
Proceedings of The 13th International Conference
on Environmental
Remediation
and Radioactive Waste Management
ICEM2010
October 3-7, 2010, Tsukuba, Japan

ICEM2010-00
ICEM2010-40054
RELATIONSHIP BETWEEN HYPOCENTRAL DISTRIBUTION AND GEOLOGICALSTRUCTURE IN THE HORONOBE AREA, NORTHERN HOKKAIDO, JAPANTetsuya Tokiwa
Japan Atomic Energy Agency
Horonobe, Hokkaido, Japan

Koichi Asamori
Japan Atomic Energy Agency
Tono, Gifu, Japan

Naoto Hiraga
Nittetsu Mining Co., Ltd.
Tokyo, Japan

Osamu Yamada
Japan Atomic Energy Agency
Horonobe, Hokkaido, Japan

Hirokazu Moriya
Graduate School ofEnvironmental Studies, Tohoku
University, JapanHikaru Hotta
Construction ProjectConsultants, Inc., Osaka, Japan

Hideharu Yokota
Japan Atomic Energy Agency
Horonobe, Hokkaido, Japan

Itaru Kitamura
Construction Project Consultants, Inc.
Osaka, Japan

ABSTRACTIn this paper, we discuss the relationship between theaccurate hypocentral distribution and three-dimensional (3-D)
geological structure in and around the Horonobe area innorthern Hokkaido, Japan. The multiplet-clustering analysis
was applied to the 421 micro-earthquakes which occurred from
1 September, 2003 to 30 September, 2007. The 3-D geologicalstructure model was mainly constructed from previous seismicreflection profiles and borehole data.
As a result of this analysis, although with slightdifferences in depth between them, the hypocenters were foundto be distributed in the NNW-SSE direction and become deeper
from the west toward the east. The distributed pattern of thehypocenters is similar to that of the geological structure. These
results indicate that the hypocentral distribution may represent
existence of active zone related to the geological structure, andprovide effective information which can contribute toestablishing methods for estimating the future evolution of thegeological environment.
Keywords: hypocenter, seismic, 3-D geological structure,
multiplet-clustering analysis, Horonobe
1. INTRODUCTION
Japanese islands are located in a convergent zone ofseveral continental and oceanic plates, and are one of the most
tectonically active regions in the world. In order to assess thelong-term safety of a geological disposal system in Japan, it isimportant to consider the impact of natural events andprocesses such as earthquake generation and fault movement,volcanism, uplift, and erosion. From these, the potential effectsof earthquakes and fault movements on geological disposal
system include hydrological and geochemical effects (i.e.
destruction of waste package and repository, change ofgroundwater flow and water quality). Thus, understanding thebehavior of earthquakes and fault movements is absolutely
imperative for safety of geological disposal system.
Horonobe area is located at the eastern margin of a
Neogene to Quaternary sedimentary basin which exists at thewestern side of northern Hokkaido (Fig. 1) [1]. Activestructures and seismicity indicate that the Horonobe area is amost active tectonic region in northern Hokkaido [2-5].
The hypocentral distribution and fault movement arehighly related to the geological structure because the geologicalstructure is considered to be formed by accumulation processesof the tectonic events such as earthquake generation and fault
movement. However, very few discussions have been made on
the relationship between them. Recently, Moriya et al. [6]presented a relocation hypocenter method called multipletclustering analysis, which enabled a more accurate estimate ofthe distribution of hypocenters. In addition, recently, theeffectiveness of this method was confirmed by Moriya et al. [7]in the Horonobe area. In this paper, the multiplet-clustering
analysis is applied to shallow earthquakes occurring in theHoronobe area to discuss the relationship between the accurate
hypocentral distribution and three-dimensional (3-D)
geological structure.

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Institute for Earth Science and Disaster Prevention (NIED) andthe observatories of the Japan Atomic Energy Agency (JAEA)
from 1 September, 2003 to 30 September, 2007 (Fig. 2), andthe information on the source location was used from the database of Japan Meteorological Agency (JMA). Most of theevents had magnitudes of less than 3.0. Among a total of 4,614
events located by Hi-net, their events whose P- and S-wave
arrival times were detected at the four observatories of theJAEA system were selected. A total of 421 events for analysis
were analyzed in this study. The hypocenters were calculated
by multiplet-clustering analysis.
2.2 Multiplet-Clustering Analysis
The multiplet-clustering analysis is a method for precise
determination of hypocenters by combining multiplet analysis
with clustering analysis [6]. It is able to relocate the relative
source locations of similar events with quite high accuracy. The
1-D velocity structure proposed by Tamura et al. [4] was used
for this analysis (Table 1).
Table 1. Velocity structure for multiplet-clustering
analysisFig. 1. Location map and Tectonic setting aroundnorthern Japan. Plate boundaries are from Wei andSeno [1]2. HYPOCENTER DISTRIBUTION
2.1 Observation and Data

Fig. 2. Locations of observatory in northern Hokkaido
Fig. 3. Hypocentral distributions estimated by themultiplet-clustering analysis

The earthquake waveform data used in this study werecollected by the Hi-net system [8, 9] of the National Research2Copyright © 2010 by ASME

Fig. 4. Distribution of geological structures around the Horonobe area (modified from Nakata and Imaizumi [3])As a result of this analysis (Fig. 3), hypocenters were
found to be divided into two broad groups (groups A and B).
The hypocenters of group A are not extended in the NSdirection, and are sporadically distributed at the depth from 5 to30 km. These hypocenters become deeper from the west to theeast. The hypocenters of group B are distributed in the NNWSSE direction, and are focused at the depth of 20 km.
3. GEOLOGY
3.1 Geological outline
The sedimentary basin exists in northern Hokkaido, wherethe Horonobe area is located, and is called the Tenpoku
Sedimentary Basin [10]. The total thickness of the Neogene
and Quaternary deposits has reached up to 6,000 m in thisbasin. It is situated at an active foreland fold-and-thrust belt
near the boundary between the Okhotsk and Amurian plates.
The previous seismic reflection surveys suggested that theongoing EW compressive tectonics (neotectonics) in thewestern part of the Horonobe area has begun in 2-3 Ma [11,12].

In this area, the Omagari Fault and the Horonobe Fault arepresented as a geological fault from west to east (Fig. 4). In
addition, the Sarobetsu Flexure Zone (correspond to Teshio
Fault Zone [13] and the Sarobetsu Fault Zone [14]), theHoronobe Fault Zone, and the Toikanbetsu Fault Zone arerecognized as an active fault zone from west to east. In thispaper, we quote the above-mentioned name (Flexure Zone,Fault, Fault Zone) from references without changing.
3.2 3-D geological structure model
In order to construct a 3-D geological structure model, we
attached importance to tectonics that fold-and-thrust belt in thisarea was formed under the influence of an E-W compressivestress. The characteristic geological structures of the fold-andthrust belt include fault-related fold (e.g., fault-propagation fold
and fault-bend fold) and detachment [15]. Therefore, Omagari
Fault and Horonobe Fault are considered faults that branch offfrom the detachment, and the fold structures in this area areconsidered fault-related fold. Fig. 5 shows main geologicalstructure in 3-D.

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Fig. 5. 3-D geological structure model and the cross section

Fig. 6. Hypocentral distribution and 3-D geological structure at low angle view (15 degrees southward dipping)4

Copyright © 2010 by ASME

In northern Hokkaido, several geological cross sections withseveral km in depth had been presented [16-20]. AdvanceIndustrial Science and Technology [20] reported balanced crosssections based on seismic reflection profiles and borehole data.
Hokkaido Promotion Committee of Mining [16] reported crosssections based on seismic reflection profiles and borehole data.
The other cross sections were created by seismic reflection
profiles that velocity sections had not been converted into
depth sections. Therefore, the 3-D geological structure model
was mainly constructed from cross sections proposed byAdvance Industrial Science and Technology [20] and Hokkaido
Promotion Committee of Mining [16]. As for the horizontal
distribution of geology and geological structure, citations weretaken from Geological Survey of Japan [21] and Sugiyama etal. [22] for land area and Ikeda et al. [13] for sea area.
4. DISCUSSION
4.1 Depth of hypocenter
According to deep borehole investigation (ca. 5 km indepth), the geothermal gradient in this study area indicates
2.7 ℃ per 100 m of depth [17]. Thus, the temperature at thedepth of 13 km is expected to exceed 350 ℃, and it is difficult
for brittle deformation to occur deeper than around this depth[23]. However, the hypocenters in this study area distribute up
to 30 km in depth. The heterogeneous seismic velocity
structure is cited as a cause of the difference. The low velocity
layer is established the 1-D seismic velocity model proposed by
Tamura et al. [4] used in this study, and the velocity model issuited for hypocenter determination in this area. However, thelow velocity layer in this velocity model is set from 0 to 2 km
in depth. The low velocity layer is considered thicker than thatof this model because the total thickness of the Neogene andQuaternary deposits in this area has reached up to 6 km. In fact,
it is reported that P-wave velocity at the depth of 4 km is 3
km/s and P-wave velocity of shallower than 1 km are in therange of 1.5 to 2.5 km/s [24, 25]. Therefore, it is highly
possible that hypocenters are located much shallower than theresult of this study. In this study, the depth of hypocenter istreated not as absolute depth but as relative depth. From now
on, construction of the reasonable velocity model will beneeded to clear up the absolute depth.
4.2 Relationship between hypocentral distribution andgeological structure
The multiplet-clustering analysis revealed that thehypocenters are distributed in the NNW-SSE direction (groupB) and become deeper from the west toward the east (group A).
The distributed pattern of the hypocenters is similar to the oneof the geological structure. The hypocenters in group A arelocated in the deep extension part of Sarobetsu Flexure Zone
that is recognized as an active fault zone. Thus, the hypocenter
distribution is consistent with the tectonics. Although, anobvious fault such as Omagari and Horonobe Faults have notbeen inferred in the deep extension part of Sarobetsu Flexure
Zone [20], it is possible that the seismic clusters indicate theexistence of these faults in the crust. Meanwhile, thehypocenters in group B might be related to detachment because
one of this group is concentrated in deep part.
These results indicate that the hypocentral distribution
may represent existence of an active zone related to thegeological structure such as fault in this region. Thus, theanalysis can provide effective information which can contributeto establishing methods for estimating the future evolution ofthe geological environment by discussing the relationshipbetween hypocentral distribution and geological structure.

ACKNOWLEDGMENTS
The earthquake waveform data was downloaded from theweb site of Hi-net owned by National Research Institute for
Earth Science and Disaster Pretension (NIED), Japan, and theinformation on the source location was used from the data baseof Japan Meteorological Agency (JMA) with the Ministry ofEducation, Culture, Sports, Science and Technology. These
data have been provided by the National Research Institute for
Earth Science and Disaster Prevention, Hokkaido University,Hirosaki University, Tohoku University, the University ofTokyo, Nagoya University, Kyoto University, Kochi
University, Kyushu University, Kagoshima University,
National Institute of Advanced Industrial Science andTechnology, and the Tokyo Metropolitan Government. We
would like to thank the above mentioned agencies. This paper
benefited from helpful comments by an anonymous reviewer
and Dr. K. Miyahara, Japan Atomic Energy Agency.

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