Proceedings of the International Symposium on Engineering Lessons Learned fromthe 2011 Great East Japan Earthquake, March 1-4, 2012, Tokyo, JapanSEISMIC HAZARD ASSESSMENT FOR JAPAN AFTER
THE 2011 TOHOKU-OKI MEGA-THRUST
EARTHQUAKE (Mw9.0)
Hiroyuki FUJIWARA1 and Nobuyuki MORIKAWA2
1

Principal senior researcher, National Research Institute for Earth Science and Disaster Prevention,
Tsukuba, Japan, fujiwara@bosai.go.jp
2
Senior researcher, National Research Institute for Earth Science and Disaster Prevention,
Tsukuba, Japan, morikawa@bosai.go.jp

ABSTRACT: The Tohoku-oki earthquake (Mw 9.0) of March 11, 2011, was the largest
event in the history of Japan. This magnitude 9.0 mega-thrust earthquake initiatedapproximately 100 km off-shore of Miyagi prefecture and the rupture extended 400 - 500
km along the Pacific plate. Due to the strong ground motions and tsunami associated bythis event, approximately twenty thousand people were killed or missing and more than
220 thousands houses and buildings were totally or partially destroyed. This mega-thrustearthquake was not considered in the national seismic hazard maps for Japan that was
published by the headquarters for earthquake research promotion of Japan (HERP). By
comparing the results of the seismic hazard assessment and observed strong groundmotions, we understand that the results of assessment were underestimated in Fukushima
prefecture and northern part of Ibaraki prefecture. Its cause primarily lies in that it failed
to evaluate the M9.0 mega-thrust earthquake in the long-term evaluation for seismic
activities. On the other hand, another cause is that we could not make the functionalframework which is prepared for treatment of uncertainty for probabilistic seismic hazard
assessment work fully. Based on the lessons learned from this earthquake disaster and theexperience that we have engaged in the seismic hazard mapping project of Japan, we
consider problems and issues to be resolved for probabilistic seismic hazard assessmentand make new proposals to improve probabilistic seismic hazard assessment for Japan.
Key Words: probabilistic seismic hazard, long-term evaluation, strong ground motion,
National seismic hazard maps for Japan, Tohoku-oki earthquake

INTRODUCTION
The Tohoku-oki earthquake (Mw 9.0) of March 11, 2011, was the largest event in the history ofJapan. This mega-thrust earthquake was not considered in the national seismic hazard maps for Japanthat was published by the Headquarters for Earthquake Research Promotion of Japan (HERP 2009b).
Based on the lessons learned from this earthquake disaster and the experience that we have engaged in
the seismic hazard mapping project of Japan, we consider problems and issues to be resolved forprobabilistic seismic hazard assessment and make new proposals to improve probabilistic seismic
hazard assessment for Japan.

406

[gal](=cm/s2)

Fig. 1 The peak ground accelerations recorded at K-NET (Δ) and KiK-net ( ) stations.

STRONG GROUND MOTIONThis magnitude 9.0 megathrust earthquake initiated approximately 100 km off-shore of Miyagi
prefecture and the rupture extended 400 - 500 km along the subducting Pacific plate. Due to the large
ground motions and tsunami associated by this event, approximately twenty thousands people were
killed or missing and more than 220 thousands houses and buildings were totally or partiallydestroyed.
The Tohoku-oki earthquake was the first M9-class earthquake that is closely recorded by dense
seismograph network. The ground motions were recorded at more than 1200 K-NET (Fujiwara at al.
2007) and KiK-net (Aoi et al. 2011) stations (Fig. 1) . The peak ground accelerations (PGA) exceeded
1g at 20 sites and the largest PGA, 2933 gals (2933 cm/s2), was observed at the K-NET Tsukidate
station (MYG004).
Huge numbers of large aftershocks and induced earthquakes including M7-class crustal andintraslab earthquakes have been recorded. Those earthquakes occurred not only in the source area but
also several hundreds kilometers away. Some of them were located much nearer to the populated407

urban areas than the main shock and human fatalities as well as severe building damages were causedby large ground shakings.
As shown in Fig. 2, the observed waveforms from the main shock are very complex; depending on
the region, accelerograms show one to three conspicuous and long-duration phases as well as several
phases with smaller amplitudes and shorter durations. A paste-up of accelerograms in Iwate (IWT),
Miyagi (MYG) and Fukushima (FKS) prefectures ordered by latitude from north displays an initialstrong seismic phase first observed at Miyagi stations and then propagated towards the north and south.
A subsequent phase uniformly delayed by approximately 40 s is also clearly observed in the paste-up.
In the southern region, Ibaraki (IBR) and Chiba (CHB) prefectures, only one distinct phase isobserved.
On the other hand, fault rupture inverted by a multi-time window analysis using the strong-motion
waveforms is characterized by one large slip area with maximum slip of 48 m which extends from thearea near the hypocenter toward the shallow part of the fault plane, far off the coast of Iwate, Miyagi
and Fukushima prefectures (Suzuki et al. 2011).

Fig.2 The observed waveforms and distribution of slip on the fault (Suzuki et al. 2011).

COMPARISON BETWEEN THE STRONG-MOTIONS OF TOHOKU EARTHQUAKE AND
THE SEISMIC HAZARD MAPS
Figure 3 shows a comparison between the observed JMA (Japan Meteorological Agency) seismic
intensities of the Tohoku-oki earthquake and JMA seismic intensity distribution for 2% probability ofexceedance in 50 years, which is one of the probabilistic seismic hazard map. In the probabilistic
seismic hazard map, the seismic intensity of 2% probability of exceedance in 50 years has been
evaluated as 6- or 6+ in Miyagi prefecture and in the southern Kanto region, which covers almost theobserved ground motion for the Tohoku-oki earthquake. However, in the northern area of Ibaraki
Prefecture and in Fukushima Prefecture where large earthquakes with high probability of occurrence
had not been expected, the seismic intensity 6+ was observed at the points where seismic intensity 5or 5+ was expected in the seismic hazard map. As you can see from this comparison, predicted ground408

motion level in the probabilistic seismic hazard map was clearly underestimated in Fukushima
Prefecture and the northern part of Ibaraki Prefecture for the Tohoku-oki earthquake (M9.0). This is
primarily because, in the long-term evaluation that has been the basis of the seismicity model for the
probabilistic seismic hazard map, the occurrence of great earthquakes M9.0 has not been evaluated.
On the other hand, the cause of underestimate also lies in the inability to function well the wholeframework of probabilistic seismic hazard assessment methods under the circumstances that many
issues are left unresolved in seismology.

7
6+
65+
54
3 or lessseismic intensity

Fig. 3 Comparison between the observed seismic intensities（○：K-NET, ：KiK-net）of the Tohoku-oki
earthquake and seismic intensity distribution for 2% probability of exceedance in 50 years, which isone of the probabilistic seismic hazard map.

NATIONAL SEISMIC HAZARD MAPS FOR JAPANThe Headquarters for Earthquake Research Promotion of Japan published a new version of thenational seismic hazard maps for Japan in July 2009, which was initialized by the Earthquake
Research Committee of Japan (ERCJ) on a basis of long-term evaluation of seismic activity, and on abasis of strong-motion evaluation. The National Research Institute for Earth Science and Disaster
Prevention (NIED), in the meantime, also promoted a special research project ‘National Seismic
Hazard Mapping Project of Japan’ to support the preparation of the seismic hazard maps (Fujiwara et409

al. 2009). Under guidance of ERCJ, we have carried out the study of the hazard maps.
The hazard maps consist of two kinds of maps. One is a probabilistic seismic hazard map (PSHM)
that shows the relation between seismic intensity value and its probability of exceedance within acertain time period. The other one is a scenario earthquake shaking map (SESM).
The examples of PSHMs are maps of probabilities that JMA seismic intensity exceeds 5-, 5+, 6- and6+ in 30 or 50 years, and maps of the JMA seismic intensity corresponding to the exceedance
probability of 3% and 6% in 30 years and of 2%, 5%, 10% and 39% in 50 years. We classify
earthquakes in and around Japan into three categories such as the characteristic subduction zone
earthquakes, other subduction zone earthquakes, and crustal earthquakes. PSHMs for three earthquake
category are also evaluated. For the PSHM, we use empirical attenuation relation for strong-motion,
which is followed the seismic activity modeling in the basis of long-term evaluation of seismic activity
by ERCJ. Both of peak velocities on the engineering bedrock and on ground surface are evaluated forsites with approximately 0.25km spacing in the basis of the 7.5-Arc-Second Engineering
Geomorphologic Classification Database by Wakamatsu and Matsuoka (2008). The JMA seismic
intensities on ground surface are evaluated from peak ground velocity by using an empirical formula.
The SESMs are evaluated for 485 scenario earthquakes of all major active faults in Japan. Selectionof a specified scenario is essential to make a scenario earthquake shake map. The basic policy of theselection is that we choose the most probable case. We assume several cases of the characteristic
source model and compare the results of them to show deviation of strong-motion evaluation due touncertainties. For the SESMs, based on the source modeling for strong-motion evaluation we adopt a
hybrid method to simulate waveforms on the engineering bedrock and peak ground velocity. The
hybrid method aims to evaluate strong-motions in a broadband frequency range and is a combinationof a deterministic approach using numerical simulation methods, such as the finite difference method,
for low frequency range and a stochastic approach using the empirical or stochastic Green’s function
method for high frequency range. A lot of parameters on source characterization and modeling ofunderground structure are required for the hybrid method. The standardization of the setting
parameters for the hybrid method is studied. We summarize the technical details on the hybrid method
based on the ‘Recipe for strong-motion evaluation’, which are published by the ERCJ.
The national seismic hazard maps for Japan are a comprehensive integration from all of the research
aspects conducted by ERCJ. It contains information of all necessary data for producing the maps. To
cross-check and promote the use of the national seismic hazard maps, an engineering applicationcommittee was established by NIED. Under the committee guidance, we have developed an open web
system to provide seismic hazard information interactively, and name this system as Japan Seismic
Hazard Information Station, J-SHIS (http://www.j-shis.bosai.go.jp/). We aim to distribute a process of
uncertainty evaluation and to meet multi-purpose needs in engineering fields. The information
provided from J-SHIS includes not only results of the hazard maps but also various information
required in the processes of making the hazard maps, such as data on seismic activity, source models
and underground structure.

PROBABILISTIC SEISMIC HAZARD ASSESSMENT IN THE SEISMIC HAZARD MAPS
The methodology of probabilistic seismic hazard assessment was used for preparing theprobabilistic seismic hazard maps for Japan. Probabilistic seismic hazard assessment method is atechnique that has been developed in order to set ground motion level corresponding to certain
probability of exceedance. In probabilistic seismic hazard assessment, the uncertainty for occurrenceof earthquake and level of ground motion are considered. In the national seismic hazard maps forJapan, the map that shows the probability of exceedance for a certain level of ground motion has beenused as a typical map.
To prepare probabilistic seismic hazard maps, seismic hazard assessment methods described below
have been adopted. Seismic hazard assessment analyzes the relationship between the following threeparameters: the ground motion intensity that occur in the future at a given site; the target period; andtarget probability. The brief outline of the procedure of seismic hazard assessment in preparation of the410probabilistic seismic hazard maps is shown below.
(1) Model the earthquake activities around a target site according to the ERCJ's earthquake
classification.
(2) For each earthquake modeled, evaluate the probability of the earthquake magnitude, the probabilityof the distance from the target site, and the probability of earthquake occurrence.
(3) Set a probability model for presuming the ground motion intensity for an earthquake of a given
magnitude and distance. For each modeled earthquake, evaluate the probability of the intensity of theground motions caused by that earthquake within the target period exceeding a certain value. Use
empirical attenuation relations for strong motion evaluation. Specifically, first derive the peak velocity
on the engineering bedrock based on an attenuation relation using the shortest distance from the target
site to the fault plane, then multiply the derived value by the site amplification factor to obtain thepeak velocity, and finally use the relation between the peak velocity and the JMA instrumental seismic
intensity to evaluate the seismic intensity on the ground surface.
(4) Repeat the operation above for the same number of times as the number of the modeled
earthquakes, and sum up the results to obtain the probability of the intensity of the ground motions
occurring within the target period exceeding a certain value by at least one degree, when all
earthquakes are taken into consideration.
In this manner, seismic hazard assessment is conducted for each site, and by fixing any two
parameters of the ground motion intensity, period, and probability, the value of the remaining
parameter are obtained. The probabilistic seismic hazard maps show the distribution of such values.
For stochastic seismic hazard assessment, it is necessary to model all the earthquakes that may
occur in the future. Basically, based on the results on long-term evaluation by HERP, we construct amodel of seismic activity. However, long-term evaluation is intended to be used for general disasterprevention activities and it was focused to assess the earthquakes that are considered likely to occur.
Therefore, the earthquakes had been evaluated are only part of future earthquakes that may occur. To
construct the model needed to evaluate the probabilistic seismic hazard, it becomes necessary to fill
the gap and a model for background earthquakes is required. In probabilistic seismic hazard
assessment, it has become essentially important and difficult issues to model earthquakes that are low
probability of occurrence and have not been assessed in the long-term evaluation by HERP.

SEISMIC ACTIBITY MODEL FOR PROBABILISTIC SEISMIC HAZARD MAPS
In the following, in order to examine more specific issues, we summarized the seismic activity
model that was used for the probabilistic seismic hazard maps.
For the source fault regions of the Tohoku-oki earthquake, the model had been implemented basedon the "Long-term evaluation of seismic activity for the region from the off Sanriku to the off Boso"(HERP 2009a). In the long-term evaluation, the entire area is divided into eight regions and theevaluation for seismic activity had been conducted in each region (Fig. 4 (a)). The results of thelong-term evaluation are summarized in Table 1.
For each region, based on the records for earthquakes occurred in the past, by analyzing the patternof occurrence, the presence of characteristic earthquake was evaluated. For the regions where thepresence of a specific size earthquakes were observed, the size of earthquakes and the interval between
earthquakes have been evaluated. Typical examples include the Miyagi-oki earthquake.
On the other hand, in areas that were difficult to evaluate because no sufficient data were obtained,
probability of earthquake occurrence was calculated by assuming a Poisson process without specifying
where they occur. In addition, for the region where clear evidence of past earthquakes had not beenobtained, for example, the off Boso area, no specific assessment was made and as the evaluation
results, "Unknown" had been shown. Thus, in the long-term evaluation that was conducted before the
Tohoku earthquake, evaluation had been made by the idea that large earthquakes of same size occur
repeatedly in the same area, based on the observed records, historical documents and the results ofgeomorphological and geological surveys. If there was insufficient evidence and data, no assessment
done and "Unknown" had been shown.

411

(a)

(b)

①
②
③④

⑧

⑤
⑥
⑦

Fig. 4 (a) Target areas for "Long-term evaluation of seismic activity for the region from the off Sanriku
to the off Boso". (b) Zoning of background earthquakes for the Pacific plate.
Table 1 Long-term evaluation of seismic activity for the region from the off Sanriku to the off Boso.

Earthquake
Characteristic earthquake in ①
Interplate
earthquakes
othercharacteristic earthquake in ①
Earthquakes in ②
Miyagi-ken-oki earthquake in ③
Interplate earthquakes in ④

Magnitude
Approx. M8.0
than M7.1～M7.6

Unknown
Approx. M7.5
Approx. M7.7
(M8.0 for correlated with
Miyagi-ken-oki earthquake)
Approx. M7.4
Interplate earthquakes in ⑤
(Successive occurrence ofmultiple earthquakes)
Interplate earthquakes in ⑥
M6.7～M7.2
Unknown
Earthquakes in ⑦
Approx. Mt8.2
Tsunami earthquakes in ⑧
Intraplate earthquakes (normal fault type) Approx. M8.2
in ⑧

412

Occur. prob. within
30 years
0.2％～10％
About 90％
Unknown
99％
80％～90％

About 7％ or less

About 90% or more
Unknown
About 20％4％～7％Table 2 Upper limit of magnitude of the background earthquakes for each region in Fig. 4(b).
Region No. in
Fig. 4 (b)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17

Interplate
earthquake
M6.9
M6.9
M7.0
M7.5
M7.0
M7.5
M7.2
M7.1
M7.3
None
None
M6.6
None
M7.0
None
M7.4
None

Intraplate
earthquake
M7.4
M7.4
M7.0
None
M7.0
M7.5
M7.2
M7.1
M7.3
M7.4
M7.1
M7.2
M7.2
M7.0
M7.0
M7.4
M7.0

On the other hand, a significant point in "Long-term evaluation of seismic activity for the region
from the off Sanriku to the off Boso" is that magnitude 8.2 tsunami earthquakes had been expected tooccur with probability of 20% in the next 30 years in the region near the Japan trench from thenorthern off Sanriku to off Boso. If measures had been taken for tsunami earthquakes based on theevaluation, we might be able to reduce some of the victims of this earthquake, especially in south
region from Fukushima prefecture.
In preparation of the probabilistic seismic hazard maps, in addition to the above mentioned
long-term evaluation, earthquakes that are not even mentioned there are considered as "backgroundearthquakes". The background earthquakes of the Pacific plate are shown in Fig. 4 (b). In these regions,interplate earthquakes that occur on the upper boundary of the Pacific plate and intraplate earthquakes
in the Pacific plate have been considered. As shown in Table 2, upper limit of magnitude of the
background earthquakes has been set for each region. This upper limit is determined by the maximum
size of the historical earthquakes that occurred in each area before the Tohoku-oki earthquake,
excluding the large earthquakes that are targets for the long-term evaluation. In this regard, it had been
pointed out that to use the previous maximum value for each region might lead to an underestimate asa result. However, there were many opposing views for setting an upper limit exceeding the previous
largest event.

PROBLEMS IN SEISMIC ACTIVITY MODEL FOR SUBDUCTION ZONE EARTHQUAKES
After the publication of the national seismic hazard maps for Japan in 2005, a series of inland
earthquakes occurred and caused damage in the coastal areas of Sea of Japan. These earthquakes were
not considered in the long-term evaluation. However, these earthquakes had been modeled as the
background earthquakes for probabilistic seismic hazard maps. Framework of the seismic hazard
assessment to complement the seismological knowledge said to be working at least.
However, not only the Tohoku-oki earthquake had not been assessed in long-term evaluation, butalso the earthquake could not be captured in a probabilistic model for assessing the uncertainty in413

seismic hazard assessment, which had been prepared to compensate for the lack of seismologicalknowledge. We could not properly assess the limits of seismological knowledge in a probabilistic
seismic hazard assessment and were unable to reflect the evaluation results to our probabilistic model.
Why could we not take the Tohoku-oki earthquake into account in the probabilistic seismic hazard
assessment? Point of discussion to this question is found in the way of thinking about modeling of low
probability subduction zone earthquakes. Compared with inland active fault earthquakes, subduction
zone earthquakes have short interval of occurrence. Based on the historical experience of the past, thepresence of characteristic large earthquakes that occur repeatedly in the same region was known. For
the subduction zone earthquakes might occur around Japan, we had a delusion of understanding tosome extent about the region, magnitude, and repetition interval of earthquakes.
For the preparedness against subduction zone earthquakes, the imminence of the earthquake wasseen as an important factor in planning measures and measures tended to be taken for earthquake withhigh probability of occurrence.
On the other hand, for earthquakes with a long interval, such as several hundred or several thousand
years, even though its size may be large, priority in earthquake preparedness tended to be relatively
lower estimated.
By comparing the evaluation of subduction zone earthquakes and evaluation of earthquakes for
active faults, the features of the evaluation become clearer. For earthquakes occur at active faults, theso-called characteristic earthquakes usually have repetition interval of thousands of years. Thus, evenwhen evaluated using the BPT distribution, the occurrence probability in 30 years remain at most a
few percent. Using a Poisson process, the probability is around 1% at most. In order to take into
account earthquakes at the fault zone with low degree of seismic activity, it is necessary to evaluate
very infrequently events, such as once in several tens thousands years. If we took into account the low
probability events even for subduction earthquakes similar to characteristic earthquakes at active faults,
in the probabilistic seismic hazard assessment, we might be able to conduct hazard assessment
considering earthquakes of a certain large scale as background earthquakes.

TO PROBABILISTIC SEISMIC HAZARD ASSESSMENT FROM LONG-TERM
EVALUATION
Based on the lessons learned from the Tohoku-oki earthquake, the revision of methodology for the
long-term evaluation has been promoted in HERP. In previous long-term evaluation, based on
observation data, historical records and the results of geological and topographic survey, earthquakes
have been evaluated by using the idea that earthquakes of similar size occur repeatedly in the same
area. In next long-term evaluation in the future, by improving the methodology, it has been aimed totake into account not only earthquakes that can be estimated from seismic data obtained in the past,
but also earthquakes that have not been confirmed by historical records and observations, based on
scientific evidence. After the techniques of long-term evaluation have been improved, we hope thatmany of the earthquakes that may occur in the future will be covered by new long-term evaluation.
On the other hand, it may be difficult to completely evaluate all possible earthquakes in the future
by using the techniques of long-term evaluation that are based on the scientific methodology and thescientific knowledge, such as, observational records, historical records and the results of geologicaland topographic surveys. In probabilistic seismic hazard assessment, a framework for considering theuncertainty of the phenomenon itself and the limits of scientific knowledge, has been prepared using
the stochastic and probabilistic method. It becomes a problem to establish a methodology to function
the framework effectively.
In order to construct a probabilistic seismic activity model that encompasses the seismic activity ofall possible earthquakes, it is necessary for the complement the long-term evaluation to establish adifferent methodology from that of long-term evaluation.

414

Table 3 Seismic activity model for probabilistic seismic hazard maps based on the revision of
long-term evaluation.
RegionNo.

Earthquake
type
Repeating Eq.

Previous
model
None

RevisedModel 1
M=8.4～9.0
P30=0%

RevisedModel 2
M=8.4～9.0
P30=0%

Repeating Eq.

M=8.0
P30=6.3%
M=7.1～7.6
P30=93%(P)1)
Mu=7.0None
None
Mu=7.0
M=7.5
P30=100%
None

M=8.0
P30=7.3%
M=7.1～7.6
P30=88%(P)
Mu=7.0None
None
Mu=8.0/7.52)
M=7.4
P30=55%(P)
M=7.0～7.3
P30=61%(P)
Mu=8.0/7.5
M=7.9
P30=0
M=7.2～7.6
P30=51％(P)
Mu=8.0/7.5
M=7.4
P30=14%(P)None
Mu=8.0/7.5
M=7.0
P30=95%
M=6.9～7.6
P30=69%(P)
Mu=8.0/7.5
None
None
Mu=8.0/7.5
Mt=8.6～9.0
P30=25%(P)M=8.2
P30=5.1%(P)None
Mu=8.0/None

M=8.0
P30=7.3%
M=7.1～7.6
P30=88%(P)
Mu=7.0None
None
Mu=8.2/8.2
None

※

①

Other Eq.

②

Background Eq.
Repeating Eq.
Other Eq.
Background Eq.
Repeating Eq.

③

Other Eq.
Background Eq.
Repeating Eq.

④

Other Eq.
Background Eq.
Repeating Eq.

Other Eq.

Mu=7.5
M=7.4
P30=7.2%(P)None
Mu=7.1
M=7.0
P30=99%None

Background Eq.
Repeating Eq.
Other Eq.
Background Eq.
Repeating
Tsunami Eq.
Repeating Eq.
(Normal fault)
Other Eq.
Background Eq.

Mu=7.3None
None
Mu=7.0M=8.2(6.8)
P30=20%(P)
M=8.2
P30=5.1%(P)None
Mu=7.5⑤
Other Eq.
Background Eq.
Repeating Eq.
⑥⑦⑧

Mu=7.2
M=7.7
P30=81%
None

None
Mu=8.4/8.2
Combinedwith ③

RevisedModel 3G-R model
withPoissonprocess
for total area

Interplate Eq.
Mu=9.5None
None
Mu=8.2/8.2
None

Intraplate Eq.
Mu=8.2None
Mu=8.3/8.2
None
None
Mu=8.3/8.2
Mt=8.6～9.0
P30=25%(P)M=8.2
P30=5.1%(P)None
Mu=8.0/None※ Source fault area for the Tohoku-oki earthquake type.
1) (P) shows that occurrence probability is calculated assuming Poisson process.
2) Mu is the upper limit of magnitude. Mu= Interplate Eq. / Intraplate Eq.

415

To achieve this, it is necessary to evaluate objectively the property of long-term evaluation in themodeling of seismic activity based on the long-term evaluation for the seismic hazard assessment.
Also, it may be necessary to promote proper modeling of background earthquakes that encompasses
all earthquakes without long-term evaluation.
For example, it may be possible to evaluate the magnitude of earthquakes from the area of the plateboundary that can be considered to cause earthquakes, and to assess the frequency of occurrence ofearthquakes by using the Gutenberg-Richter formula that shows the relationship between the numberof earthquakes and their magnitude. In its revision, it is essential to consider a new method of setting
the scale for earthquakes, not limited by the idea of using the historical largest event for each small
region.
After the Tohoku-oki earthquake, HERP had been reviewing the long-term evaluation for the area inwhich the Tohoku-oki earthquake occurred and released the revised version of the "Long-term
evaluation of seismic activity for the region from the off Sanriku to the off Boso" in November 2011.
In this revision, although the revision of the methodology of the long-term evaluation itself
has not yet been made and the most part has remained a traditional evaluation, a newassessment has been made of the Tohoku-oki type earthquake. Based on this evaluation, we
have been making a revision of the seismic hazard assessment. In Table 3, we show theparameters of the seismic activity model for the revision of the probabilistic seismic hazard
maps. In this revision, not only results of the long-term evaluation have been revised, but also theupper limits of background earthquakes have been revised. In addition, here we propose three models
in order to consider uncertainty of seismic activity.

PROBLEMS TO BE SOLVED IN THE FUTURE(1) Modeling of seismic activity with no oversight to low-probability earthquakes
For both subduction zone earthquakes and earthquakes at active faults, it is necessary to aim tomodel seismic activity that can be considered to events about once several thousands or several tens
thousands of years. To achieve this goal, we need to model background earthquakes that include a low
probability of earthquakes by using the Gutenberg-Richter formula or other statistical techniques tocompensate the long-term evaluation.
(2) Preparation of strong ground motion maps considering low-probability earthquakes
In addition to traditional information dissemination to emphasize the urgency of the earthquake
occurrence by showing the probability, by going back to the original purpose of the evaluation ofprobabilistic seismic hazard, we should prepare the maps that show the strong-motion level forearthquake preparedness. For example, based on the averaged long-term seismic hazard assessment,
evaluating strong-motion level for about 10,000 years return period, we should prepare the maps thatshow the distribution of strong-motion level, which represent effect of major earthquakes on active
faults and subduction zone earthquakes with low-probability. Regarding the seismic hazard assessment
for low probability, at present, it is insufficient to evaluate the uncertainty for low probability M8 class
earthquakes and it is necessary to improve techniques for them.
(3) Development of methodology for selecting appropriate scenario earthquakes from probabilistic
seismicity model
In the seismic activity model considering low-probability earthquakes, not only earthquakes withspecified faults, but also earthquakes without specified faults are included. From the seismic activity
model, it is necessary to establish a methodology that can be selected as appropriate scenario
earthquakes for purposes of earthquake preparedness.
(4) Development of methodology for prediction of strong ground motions for mega earthquakes
In order to perform seismic hazard assessment considered the low-probability events until about
10,000 years return period, it is necessary to predict strong ground motions for large earthquakes that416

have not been recorded by the modern seismic observation network. For the "Method for prediction ofstrong ground motion for earthquakes with specified faults (recipe)", which is currently being used forstrong motion prediction, the subduction zone earthquakes up to about M8 and earthquakes on active
fault up to about 80km in length are only verified its scope. The sophistication of techniques that canbe applied to the prediction of strong ground motions for super large earthquakes are required.

ACKNOWLEDGMENTS
This study was conducted as a part of the research on advanced seismic hazard assessment for the
National seismic hazard maps for Japan.

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