10th Asian Regional Conference of IAEG (2015)

Seismic and displacement hazard assessments on active faults in Japan
Shinji TODA(1)(1) International Research Institute of Disaster Science, Tohoku University, Japan
E-mail:toda@irides.tohoku.ac.jp
Abstract
Here I briefly review 20-yr progress in active fault study in Japan and raise several
new issues. In terms of seismic hazard assessment, the Headquarters of Earthquake
Research Promotion successfully published the first probabilistic national seismic hazard
map in 2005 based on the recent intensive surveys on the major active faults. The hazard
map is roughly consistent with the historical intensity distribution affected by the shortly
repeated subduction earthquakes. However, underestimates and unpredictability occurredinland areas due to unmapped blind faults. In contrast, overestimates for a large fault system
might be another issue (e.g., 2014 Nagano-ken-hokubu earthquake of M6.7 on the
Itoigawa-Shizuoka Tectonic Line). Surface displacement is the other aspect of hazard on
active faulting. Recent development of remote sensing techniques demonstrates that thesurface ruptures not only occurred along the pre-existing mapped scarps but also involved
numerous distributed faults away from the main rupture zone. It requires further study ofprobabilistic fault displacement hazard analysis for strict assessment such as nuclear safety.
Keywords: seismic hazard map, fault segmentation, blind fault, rupture complexity
1.

Introduction

2.1 First national seismic hazard map
The first national seismic hazard map for Japan
was published in 2005 based on probabilities of large
earthquakes along subduction zones, onshore active
faults, and background seismicity (HERP, 2005). The
probabilities and expected magnitudes are calculated
from historical, paleoseismological and geological
data as well as instrumental seismicity data. A vast
amount of paleoseismological data, mostly obtained
after the Kobe earthquake, is newly incorporated into
the detailed calculations taking the conditional
probability into account. Time-dependent seismic
hazard considering conditional probabilities on mostmajor active faults provide us with more realistic
estimates (Fujiwara et al. 2009). These outputs areavailable to the general public and researchers on the
portal web named as ‘J-SHIS’, a browser-based GIS(http://www.j-shis.bosai.go.jp/map/?lang=en)toenhance utilization of the seismic hazard maps, aswell as giving the fundamental information of fault
locations, subsurface and surface geologic conditions(site amplifications), exposed population, and othermodels. Active fault database is also accessible fromtheGeological
SurveyofJapan,
AIST(https://gbank.gsj.jp/activefault/index_e_gmap.html).

It has been 20 years since the January 17, 1995
Kobe (Hyogo-ken-nambu) earthquake of M 7.3.
Because the Nojima fault (Photo 1), one of the
mapped active faults (Mizuno et al., 1990), caused
the Kobe earthquake, not only scientists but also thegeneral public have become aware of the importanceof active fault as a potential source of a destructive
inland earthquake. Under such a background, theHeadquarters for Earthquake Research Promotion(HERP) has been established in 1995 and leading ahuge amount of surveys including drilling, trench
excavation, seismic reflection, and other geophysicalexplorations have been deployed across more than
100 major active faults. The accumulated data havebeen effectively contributed to making seismic hazard
map.
During the recent 20 years, several destructive
earthquakes including the March 2011 great Tohoku
earthquake occurred and have affected our studies.
Here I review the recent progress and several issues
associated with seismic and displacement hazard
assessments on active faults in Japan.
2.

Seismic hazard

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10th Asian Regional Conference of IAEG (2015)

The map, a result of a probabilistic seismic hazard
analysis (PSHA), shows the probability ofexceedance of the Japan Meteorological Agency(JMA) intensity 6- (roughly equal to MMI IX-X) in
Japan for a 30-year period (Fig. 1). Since thehigh-probability regions are highly affected by the
subduction zone earthquakes, regional differences forinland earthquakes are invisible. In other words,
tremendous efforts and data from active faults are noteffectively utilized in the map. Thus, the HERP has
started to make three versions of the hazard map,
categorizing all the sources into repeated subduction
megathrust earthquakes (category I, Fig. 1a),unknown sources of subduction earthquakes(category II), and inland active faults (category III,
Fig. 1b). Now if one wants to see the hazard of theM~7 class inland shocks, a PSHA for category III isappropriate and avoids confusing regional differencesof the hazard with the extreme high probabilities dueto the subduction earthquakes.
Validation of the hazard map is a new issue. A
simple spatial comparison with the epicenters of large
shocks superimposed on the hazard map, falsifying
the hazard map (e.g., Geller, 2011), is inappropriate.
Miyazawa and Mori (2009) compared the HERP’snational seismic hazard map with maximum seismic
intensity maps during the past 500 years, and thenconcluded that both are very similar except the casefor onshore crustal earthquakes. Ishikawa et al.
(2011) reached the similar conclusion with their more
rigorous tests for the recent 120-year data. Both tests
point out that there may be still a deficiency of data
for the inland active faults that have recurrence timesof thousands of years.

Photo 1. Fresh rupture scarp emerged along theNojima fault on Awaji Island at the 1995 Kobe
earthquake.

2.2 Blind fault and class C fault issue
During the recent 20 years since the Kobe
earthquake, several large inland earthquakes havestruck the regions off the major active faults chosen
by the HERP. As already mentioned, however,palaeoseismic trenching studies and other approaches
to extract active fault parameters largely rely on
surface-rupturing processes. In other words, if nosurface rupture had taken place, no large earthquake
would have been identified. From that viewpoint,
Toda (2013) re-examined the surface-rupturing
earthquakes since 1923 when the Japan
Meteorological Agency (JMA) officially started theircatalog recording. Only 20% of M≥6.5 and 44% ofM≥7.0 shallow earthquakes left the surface breaks
that correspond to their source fault dimension (Fig.
2). He thus speculates that the number of potential
destructive earthquakes of M6-7 estimated from the
major active faults would be significantly
underestimated. He also performed numericalcalculations with active fault data and their assigned
probabilities in the report also largely underestimate

Fig. 1. 30-yr probabilities of strong ground motion of
JMA intensity ≥ 6 lower caused by seismic sources ofCategory I (subduction-zone earthquakes withspecified seismic source faults) and Category III(shallow earthquakes inland area and in sea area).
(HERP, 2010).

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10th Asian Regional Conference of IAEG (2015)

the number of observed earthquakes. Both
independent analyses suggest that there would be far
more minor active faults hidden beneath Japaneseislands.

length, displacement and thus magnitude within acertain interval. The characteristic earthquake does
not consider any significant change in size of therepeating earthquakes. Thus the HERP’s estimates
was that the largest future earthquakes along thedifferent segments of the Japan trench were expected
to have magnitudes between 7 and 8 with different
inter-event times. The Tohoku earthquake thus forced
us to reconsider the conventional segmentation basedon the characteristic earthquake model.
Proper assessment of multiple segment rupture forinland earthquakes, which determines the size of theearthquake, is also a serious issue fordesign-and-construction decisions. As already noted,
the HERP basically follows the definition of“seismogenic active fault” provided by Matsuda(1991) in which the seismogenic faults are chosen
and connected within a gap smaller than 5 km
(Wesnousky 1988; 2006). However, the strategy only
calculates the maximum size of large earthquakes andtheir frequency, ignoring the more frequent
segment-based smaller ruptures. As well as theadvanced palaeoseismic studies along the San
Andreas fault in California (e.g., Scharer et al. 2014),
numerous palaeoseismic excavations across the ISTL
(Fig. 3) already show that it is more likely thatfrequent M~7 shocks have occurred, along withcomplex combinations of multiple segment ruptures.
The 22 November 2014 Nagano-ken-hokubu
earthquake (M=6.7, Mw=6.2) also demonstrated aclear difficulty in estimating size of an earthquake.
More than 9 km of complex surface faulting occurredon the previously mapped N-NW trending Kamishiro
fault, one of the segments of the 150-km-long
Itoigawa-Shizuoka Tectonic Line (ISTL) active fault
system (Figs 3 and 4). The surface rupture lies ~5 km
west of the epicentre, juxtaposing a basin with a
NNW-trending mountain range. Surface rupture
mostly observed as warped and buckled the groundsurface, corresponds to vertical and contractional
deformation associated with a shallow east-dipping
thrust fault (east side up), accommodating NW-SE
compression. The maximum vertical displacement
and contraction measured were 80 cm and 50 cm
respectively (Fig. 4). The 2014 earthquake is the firstsurface-rupturing earthquake to strike on one of the
110 major active faults evaluated by the HERP since1995. Numerous palaeoseismic data, in particular on
the most active central ISTL, allowed the HERP toforecast a 14% 30-yr probability of an M=8.3
earthquake if the entire ISTL ruptured. One of theother rupture scenarios was an Mw =7.4 event on thenorthern half of the ISTL, involving the Kamishiro
fault, which was also largely overestimated. In
addition, the observed 80-cm-vertical displacement ismuch smaller than the expected 3-4-m slip from a
1,000-1,500 yr elapsed time and a vertical slip rate of
3 mm/yr of the Kamishiro fault. Thus, given theFig. 2. Distribution of inland earthquakes which haveoccurred in Japan since 1923.
Asada (1991) has already pointed out the so called‘class C fault issue’ which raises a question about
hidden active faults with slip rates of slower than 0.01
mm/yr. He claimed that “since the recent destructive
inland earthquakes are occupied equally by all theclasses (1 mm/yr ≤ class A < 10 mm/yr, 0.1 mm/yr ≤class B < 1 mm/yr, 0.01 mm/yr ≤ class C < 0.1
mm/yr), the number of class C faults must be 100
times of the class A faults.” But in the catalogue of
the Research Group for Active Faults in Japan (1991),
the estimated numbers of class A, B and C faults are
103, 884, and 660 respectively, which suggests thatmajority of the slowly moving class C faults are
hidden, possibly due to fast rates of erosion andsedimentation associated with Japan’s climate andsurface processes.
2.3 Fault segmentation and site-based evaluation
The size and rupture dimension of the 11 March2011 Tohoku earthquake surprised many earthquake
scientists in Japan. The Tohoku earthquake broke five
pre-defined segments, yielding a magnitude 9.0
earthquake involving ~50 m coseismic slip. Suchsegmentation along the Japan Trench is based on the‘characteristic earthquake model’ (Schwartz &Coppersmith 1984) in which each segment hasrepeatedly produced a characteristic size of rupture3

10th Asian Regional Conference of IAEG (2015)

evidence so far available, it can be concluded thateither the 2014 earthquake was not a characteristic
event, or the palaeoseismic data are extremely
incomplete.

Fig. 3. Space-time diagram for large magnitude
surface-rupturing earthquakes on the ISTL during thepast 12,000 years (Maruyama et al. 2010). Horizontal
dark gray bar indicates a range of palaeo-earthquake
occurrence. Horizontal light gray bar denotes a time
range across which no datable strata were recovered.
Fig. 4. (a) Map of the 2014 surface ruptures along theKamishiro fault (Okada et al., 2015) superimposed onaninterferogram
computedfrom
ALOS-2/PALSAR-2(Geospatial
Information
Authority of Japan, 2015). Solid line and dashed line
indicate confirmed and inferred rupture traces
respectively. Distribution of (b) vertical slip and (c)fault normal contraction of the surface rupture zones
along the Kamishiro fault (simplified from Okada etal., 2015).
3.

Displacement hazard

The other hazard associated with active faulting isground displacement that brings damage any
manmade structures. There are numerous cases thesignificant fault slip destroyed major roads, railways,
private houses, school buildings, dam reservoirs andothers. To prevent such disaster in advance, for4

10th Asian Regional Conference of IAEG (2015)

example, the Aliquist-Priolo Earthquake Fault Zoning
Act, which forces people not to construct new
structure along fault zone 500 feet away from most
mapped faults and 200 feet away from less significant
faults, has been enforced since 1972 in California. No
potential active faulting is definitely required for the
crucial constriction such as nuclear power plant.
Although mapping fault traces is not easy task inJapan unlike San Andreas fault system in California,
majorities of rupture locations at future earthquakes
are believed to occur along the mapped fault traces
based on the surface rupture maps during the past a
few hundred years. These surface rupturing
earthquakes also lead to the empirical relations
between magnitude and displacement, and between
fault length and displacement (e.g., Matsuda, 1975;
Wells and Coppersmith, 1994), allowing us to predictamount of slip at a specific site along a fault. Up tothe present, except a local ordinate of Tokushima
Prefecture along the Median Tectonic Line in
Shikoku, we have not had any regulation associatedwith the predicted surface displacement along anactive fault. However, concerning about theproximity of nuclear power plants (NPP) to active
faults (e.g., Chapman et al., 2014), re-evaluation ofminor but suspect faults have become a main issue
with respect to fault displacement hazard.

engineering elaborations. In addition, since some ofthese distributed faults are proved to have moved
during the late Quaternary period, one could map
such subsidiary faults by intensive geological survey
in advance.

Fig. 5. Surface fault offsets across discontinuity
traces identified in the interferogram from aperspective
view
in
the
2011
Fukushima-ken-hamadori (Iwaki) earthquake of
M=7.0 (Fukushima et al., 2013). The maximum offset
of 1.5 m was found along the western trace of the
Itozawa fault (Ia). Note that there are many secondary
faults away from the two main rupture traces of Ia
and Yb (Yunodake fault).

3.1 Probabilistic analysis and rupture complexity
Takao et al. (2013) proposed an evaluation
formula of the probabilistic fault displacement hazard
analysis (PFDHA) based on 19 surface-rupturing
earthquakes since 1891 Nobi earthquake in Japan.
Despite limited number of distributed fault data, they
concluded that significant displacement associatedwith engineering concerns mostly occurs on principal
fault traces and the effect of the distributed faults isextremely small.
However, recent development of remote sensing
techniques, in particular SAR interferometry andairborne LiDAR, enables us to map accurate andcomplex features of surface ruptures associated with
the inland earthquakes. Several significant cases wereobserved in the 2007 Niigata-ken-Chuetsu earthquake,2008 Iwate-Miyagi-nairiku earthquake, 2011
Fukushima-ken-Hamadori earthquake (Fig. 5), and2014 Nagano-ken-hokubu earthquake (Fig. 4) inJapan, and the 2014 South Napa earthquake in
California, and the 2010 El Mayor-Cucapah
earthquakein
Mexico.
Theseearthquakes
demonstrate that the surface ruptures not only
occurred along the pre-existing mapped scarps but
also involved numerous distributed faults a few
kilometers away from the main rupture zone. A few
cases also show remotely triggered aseismic surface
slips. However, the amounts of slip on these
distributed subsidiary faults are generally small as
Takao et al. (2013) pointed out, and might be handled

3.2 Site-based fault identification
Relicensing process for existing NPPs in Japanafter the Fukushima-Daiichi NPP disaster at the 2011
Tohoku earthquake raised an issue to identify active
faults, most of which are small faults, near and/or
beneath their facilities. The definition of “active
fault” given by the Japan’s new Nuclear Regulatory
Authority (NRA) is based on paleoseismologicalevidence of faulting during the late Pleistocene(120-130 ka). If no young overlying sediments thatcan be examined to determine the last fault movement,
NRA requires investigators to seek evidence over the
past 400,000 years. As mentioned, the problem is nota principle fault strand but small faults or distributed
fault traces.
Such suspect minor faults exposed at theirconstructions and new investigation sites are often
originated from non-tectonic processes. A suspect
fault at the Ohi NNP operated by the Kansai ElectricPower Co. Inc. had been intensively investigated andthen evaluated as inactive by NRA. Several minor
faults exposed on their trench walls, claimed as active
faults by two NRA’s expert geoscientists, were also
indeed non-tectonic faults associated with a massive
paleo-landslide. Another non-tectonic faults would be
generated from differentially heaving bedrock due toexpansive weathered clay-rich Tertiary units at theHigashidori NNP, the Tohoku Electric Power Co. Inc.
A similar non-tectonic faulting case is already5

10th Asian Regional Conference of IAEG (2015)

reported from Noe et al. (2007) near the Denver
metropolitan area in Colorado. Numerous small faults
displaced the late Quaternary strata and claimed as agroup of active faults by the NRA’s expert
geoscientists, were exposed in and around the facility.
However, most of the faults are only observableshallower than several meters from the surface.
Furthermore, their strikes, dips and locations arerather random, and nothing to do with current stressregime and locations of suspect geomorphologicalscarps the expert scientists claim.
However, recent cases of the surface rupturing
earthquakes and large local stress perturbations (e.g.,
Oskin et al., 2012) suggest that displacement hazard
is not easily evaluated. Thus it might be important to
explore “what-if” scenarios in case, incorporating
expert, evidence-based judgments on the likelihoodof movement and possible magnitudes ofdisplacement, and suggest a probabilistic fault
displacement hazard analysis (PFDHA) (Chapman
et al., 2014).
4.

W., Cluff, L. and Kawamura, H. (2014): Active
faults and nuclear power plants, EOS, American
Geophysical Union, Vol. 95, No. 4, 28 January 2014,
pp. 33-34.
Fujiwara, H., Morikawa, N., Ishikawa, Y., Okumura,T., Miyakoshi, J., Nojima, N. and Fukushima, Y.
(2009): Statistical comparison of National
Probabilistic Seismic Hazard Maps and frequencyof recorded JMA seismic intensities from theK-NET strong-motion observation network in Japan
during 1997-2006. Seismological Research Letters,Vol. 80, pp. 458-464.
Geller, R. J. (2011): Shake-up time for Japaneseseismology, Nature, Vol. 472, pp. 407-409.
Headquarters for Earthquake Research Promotion(2005): Seismic Activity in Japan,
http://www.hp1039.jishin.go.jp/eqchreng/eqchrfrm.
htm.
Headquarters for Earthquake Research Promotion(2010): National seismic hazard maps for Japan
2010, Earthquake Research Committee (K. Abe,chair), Headquarters for Earthquake Research
Promotion, available fromwww.jishin.go.jp/main/chousa/10_
yosokuchizu/index.htm (in Japanese).

Conclusions

Here I briefly reviewed 20-yr progress in active
fault study in Japan and raised several issues. In termsof seismic hazard assessment, the HERP successfully
published the first national seismic hazard map in2005 based on the recent intensive surveys of the
major active faults, which is roughly consistent withthe historical intensity data associated with thesubduction earthquakes. However, underestimatesoccurred in inland areas due to unmapped hidden
faults and so called class C faults. In contrast,
overestimates might be an issue for a large fault
system (e.g., 2014 Nagano-ken-hokubu earthquake
along the Kamishiro fault, a part of the Itoigawa
Shizuoka Tectonic Line). Regarding to thedisplacement hazard, recent development of remotesensing techniques demonstrates that the surface
ruptures not only occurred along the pre-existing
mapped scarps but also involved numerous
distributed faults away from the principal fault strand.
It requires further study of probabilistic fault
displacement hazard analysis.

Ishikawa, Y., Okumura, T., Fujikawa, S., Miyakoshi,
J. Fijiwara, H., Morikawa, N. and Nojima, N.
(2011): Validation of the probabilistic seismic
hazard maps for Japan, Journal of Japan Association
for Earthquake Engineering, Vol. 11, No. 4, pp.
68-87 (in Japanese with English abstract).
Maruyama, T., Toda, S., Okumura, K., Miura, D.,
Sasaki, T., Haraguchi, T. and Tsuji, Y. (2010):
Historical and paleoseismic investigations along the
Itoigawa-Shizuoka Tectonic Line active fault
system, 2009 report of the comprehensive study ofthe Itoigawa-Shizuoka Tectonic Line active fault
system, Ministry of Education, Culture, Sports,
Science and Technology (MEXT), pp. 230-254 (inJapanese).
Matsuda, T. (1975): Magnitude and recurrence
interval of earthquakes from a fault. Journal of
Seismological Society of Japan (Zisin), Vol. 28, pp.
269-283 (in Japanese with English abstract).

Acknowledgements

Matsuda, T. (1991): Seismic zoning map of JapaneseIslands, with maximum magnitudes derived from
active fault data. Bulletin of Earthquake ResearchInstitute, University of Tokyo, Vol. 65, pp. 289-319(in Japanese with English abstract).

I am grateful to Masahiro Chigira, who kindly
invited me to 10th Asian Regional Conference of
IAEG.
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