Some considerations towards improvement ofthe National Seismic Hazard Maps for Japanafter the Great Tohoku Earthquake

Hiroyuki Fujiwara

National Research Institute for Earth Science and Disaster Prevention

National seismic hazard maps for JapanLong term evaluation

Strong-motion evaluation

Probability of occurrence, magnitude, location

Strong-motion, underground structure

Probabilistic Seismic Hazard Maps

Scenario Earthquake Shaking Maps

・Showing the strong-motion
intensity with a given
probability, or the probability
with a given intensity.
・Considering all possible
earthquakes.

・Showing the strong-motion
intensity around the fault
for a specified earthquake.

Japan Seismic Hazard Information Station

http://www.j-shis.bosai.go.jp
In order to promote the use of the national seismic hazard maps, an engineering applicationcommittee (Chairman: Prof. H. Kameda) was established by NIED. Under the committee guidance,
we developed an open web system to provide information interactively, and named this system asJapan Seismic Hazard Information Station, J-SHIS.
Our products are aimed to meet multi-purpose needs in engineering fields by providing informationof the probabilistic seismic hazard analysis.

Evaluation of occurrence probability of earthquakes
by ERCJFlowchart of PSHA
Modeling of seismic activity

Evaluation of an EQ occurrence probability P(Ei)Probabilistic evaluation of an intensity level P(Yi > y|Ei)

Evaluation of probabilistic seismic hazard
for each earthquake P(Yi > y)= P(Ei) P(Yi > y|Ei)
Evaluation of probabilistic seismic hazard
for all earthquakes P(Y > y)=1-Π[1- P(Yi > y)]Probabilistic Seismic Hazard Maps

Probability in 30 years.
(≧JMA Seismic Intensity 6-）

Seismic Intensity with 3% probability ofexceedance in 30 year.

Comparison with the hazard maps of the past and the ground motion actually occurred(Exceedance probability of JMA seismic intensity 6- within 30 years)

1890～

1920～

Upper: Hazard map

1950～

1980～

Lower: ground motion actually occurredCompared with the hazard maps of the past and the earthquake actually occurred(JMA seismic intensity: Exceedance probability of 3% within 30 years)
１８９０～１９１９
１９２０～１９４９
１９５０～１９７９
１９８０～２００９（Upper: Hazard assessment Lower: Shaking caused by earthquakes）

The 2011 Tohoku-oki earthquake
This magnitude 9.0 megathrust earthquake
initiated approximately 100 km off-shore of
Miyagi prefecture and the rupture extended
500 km along the subducting Pacific plate.
Due to the large ground motions and tsunami
associated by this event, approximately
20,000 people were killed or missing andmore than 220,000 houses and buildings
were totally or partially destroyed.

The Tohoku-oki earthquake was the firstM9-class earthquake that is closely recorded
by dense seismograph network.
The ground motions were recorded at morethan 1200 K-NET and KiK-net.
The peak ground accelerations (PGA)
exceeded 1g at 20 sites and the largest PGA,
2933 gal was observed at the K-NET
Tsukidate station (MYG004).

Paste-up of the observed waveforms
Acceleration EW component

Displacement (0.02-1 Hz) EW component

Normalized acceleration and displacement (0.02 - 1.0 Hz) paste-up of the waveforms observed by K-NET and KiK-net
ordered by latitude which reveal a very complex source process; An initial strong phase originating near the hypocenter isclearly observed. A subsequent seismic phase uniformly delayed by approximately 40s suggests a second event at nearly
the same location. A later seismic phase is strongly observed to the south 50 - 60 s after the initial phase. This phasesuggests that a third strong event took place off-shore of Fukushima-Ibaraki.

Comparison between the hazard maps and observed strong motions
Comparison between the observed seismic intensities（○：K-NET, △：KiK-net）of the Tohoku earthquakeand seismic intensity distribution for 2% probabilityof exceedance in 50 years, which is one of theprobabilistic seismic hazard map.

As you can see from this comparison, predicted
ground 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).

76+
65+
54
3 or lessseismic intensity

Seismic Intensity with 2% probabilityof exceedance in 50 year.

This is primarily because, in the long-term evaluationthat has been the basis of the seismicity model for the
probabilistic seismic hazard map, the occurrence ofgreat earthquakes M9.0 has not been evaluated.
On the other hand, the cause of underestimate alsolies in the inability to function well the wholeframework of probabilistic seismic hazard
assessment methods under the circumstances thatmany issues are left unresolved in seismology.

Seismic activity model for hazard assessment
Earthquakes with specified fault

Earthquakes in major or other fault zones

Return period < 100,000 year
Earthquakes in subduction zones(Interplate or intraplate)

Return period < 500 year
Background earthquakes

Urgency of earthquake occurrence had been too muchemphasized for subduction zone earthquakes!

Seismic activity model for hazard assessment
Earthquakes with specified fault

Earthquakes in major or other fault zones
Earthquakes in subduction zones(Interplate or intraplate)
Background earthquakes
Inland earthquakes

Earthquakes in subduction zones(Interplate or intraplate)
In order to construct a probabilistic seismic activity model that encompasses the seismic activity of all possible
earthquakes, it is necessary for the complement the long-term evaluation to use a different methodology.
It may be necessary to promote proper modeling of background earthquakes that encompasses all earthquakes
without long-term evaluation.

Classification of uncertainty factors● Aleatory Uncertainty
・ Intrinsic uncertainty.

・ It is represented by a random variable in the prediction model.
・ Uncertainty caused by a limitation of model to explain phenomena.

・ Uncertainty used to calculate a hazard curve.

● Epistemic Uncertainty
・ Uncertainty due to a lack of knowledge and data.
・ Uncertainty due to differences in judgment for modeling・ It is evaluated by using a logic tree.
・ Represented by more than one hazard curve.

Modeling of seismic activity in the Pacific plate
before the 311 Tohoku earthquake

M=8.0

M=7.5 >99%6%

Mu=7.0?

Mu=7.0Mu=7.2
Mu=7.5

M=7.7 81%

Mu=7.1

M=7.4 7%

Mu=7.5

M=8.2 5%
Mt=8.2 20%

M=7.0 99%?
M for characteristic earthquakes

Mu=7.3

Mu=7.0Max M for background earthquakes

PSHM taking into account the Tohoku earthquake (M9)■Setting conditions for the Tohoku EQ・Source area： Aftershock area
・The origin of the time：01/01/2011
・Average interval： 600 years
・Latest activity period：
15th century
・Occurrence probability in 30 years：
15.4% (01/01/2011)
0.0% (01/01/2012)
・Ground-motion equation：
Si and Midorikawa(1999)(Saturated with Mw8.3)

Modeling of seismic activity in the Pacific plateafter the 311 Tohoku earthquake

M=8.0

M=7.4 (55%)

M=7.97%

Mu=7.0?

Mu=8.0 or 8.2
Mu=8.0 or 8.4
Mu=8.0 or 8.40%

M=7.4 14%
M=8.2 5%
Mt=8.6-9.0 25%

M=7.0 95%?
M for characteristic earthquakes

Mu=8.0 or 8.2
Mu=8.0Mu=8.0 or 8.3

Mu=8.0 or 8.3

Intraplate
Mu=7.5 or 8.2

Max M for background earthquakes(Model 1 or Model 2)

Seismic activities in the Pacific plate

No.1（G-R law）
No.2

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 ofearthquakes from the area of ​the plate boundary that can beconsidered to cause earthquakes, and to assess the frequencyof occurrence of earthquakes by using the Gutenberg-Richter
formula that shows the relationship between the number ofearthquakes and their magnitude.

Model3 (G-R model)

M7.6～M8.2
mi
7.6
7.7
7.8
7.9
8.0
8.1
8.2
8.3
8.4
8.5
8.6
8.7
8.8
8.9
9.0
9.1
9.2
9.3
9.4
9.5

ν (mi)
9.13E-03
7.42E-03
6.03E-03
4.90E-03
3.98E-03
3.24E-03
2.63E-03
2.14E-03
1.74E-03
1.41E-03
1.15E-03
9.34E-04
7.59E-04
6.17E-04
5.02E-04
4.08E-04
3.31E-04
2.69E-04
2.19E-04
1.78E-04

M8.3～M8.9

Tr(mi)
109.5
134.8
165.8
204.0
250.9
308.7
379.8
467.3
574.9
707.3
870.1
1070.51317.01620.3
1993.42452.43017.13711.84566.55618.0Intraplate （M7.6～M8.2)

M9.0～M9.5)

PSHMs considering the Tohoku type earthquake(a) PSHM2011 (b) PSHM2011＋TohokuM9(a) PSHM2011(b) PSHM2011＋TohokuM9

JMA seismic intensity: Exceedance probability of 2% within 50 years

(a) PSHM2011(b) PSHM2012

PSHMs for 2012
Model 1Model 2Model 3Comparison of hazard curves
Black dashed curve：PSHM2011, Black curve：PSHM2011 with M9Blue line：PSHM2012(Model1), Red curve：PSHM2012(Model2)(a)Fukushima(b) Mito(c) Utsunomiya(d) Saitama(e) Chiba(f) TokyoStrong-motion maps considering
low-probability earthquakes

1,000 year

10,000 year

100,000 year

Based on the averaged long-term seismic hazard assessment, evaluating strong-motion level for
10,000 ~100,000 years return period, we should prepare the maps that show the distribution ofstrong-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.

Problems that must be overcome for
improvement of seismic hazard assessment
1) Modeling of seismic activity with no oversight to
low-probability earthquakes.
2) Preparation of strong ground motion maps
considering low-probability earthquakes.
3) Development of methodology for selecting
appropriate scenario earthquakes from probabilistic
seismicity model.
4) Development of methodology for prediction ofstrong ground motions for mega-thrust earthquakes.

Nankai-trough earthquake

Hyuga-nada

Type1

Type2A

Type2B

Nankai

Tonankai

WeightMw

0.0125

8.9

Depth

TypeA

1

0.01259.0*

B0.01259.0C

0.01259.1*2

D×0.18758.8

A×0.1875

9.0C×0.00625

8.9

A×0.00625

9.0C

××0.01258.8

A××0.0125

8.9C

0.00625

8.9, 8.1

A

0.00625

9.0, 8.2

C×0.0125

8.8, 8.1

A×0.0125

8.9, 8.2

C

0.05

8.7, 8.4

A

0.325

8.6, 8.4

A×0.0125

8.7, 8.3

A×

0.05

8.6, 8.3

A0.0125

8.7, 8.3, 8.1

A

0.05

8.6, 8.3, 8.1

A××

Type3

Tokai×
0.15

1.0

1.0

0.875

1.010モデル
b=0.9

1

0.1

0.01
88.5

Mw

99.5

Hazard curves for Nankai-trough earthquakes
Kochi

Osaka

Shizuoka

Yokohama

NagoyaModel 2010
Model A withreccurence intervalof 88.3 year
Model B withreccurence intervalof 120 yearThank you for attention.