New Seismic Activity Model of Large
Earthquakes along Nankai Troughfor Probabilistic Seismic Hazard Maps

Toshihiko Okumura (Shimizu Corp.)
Hiroyuki Fujiwara (NIED)

Plate Tectonics around Japan

Pacific Plate

Philippine Sea Plate
after Japan Coast Guard

Source Area of Damaging Earthquakes

2011 Tohoku
Earthquake

From 1885 to 1995

Showa Tonankai, 1944
Showa Nankai, 1946

Earthquake Research Committee (1999):Seismic Activity in Japan

Earthquakes in and around Japan
Depth

1904～1995
M≧4.5

Seismic activity of small tomiddle size earthquakes
along Nankai Trough is
NOT highDepthEarthquake Research Committee (1999):Seismic Activity in Japan

Large Earthquakes along Nankai Trough・ Large earthquakes (M≧8)haveoccurred repeatedly withrecurrence interval 90-150 yrs
・ One huge earthquake or two
successive earthquakes withinterval of 1day to 3yrs
・ Source area often reaches toTokai area
Lack ofdata?

・ Tokai area has never caused
earthquake independently

M8.2-8.5
M8.2-8.5
M7.9-8.2
M8.6

Interval: 90-150 years(Average=117yrs)

M8.4
M8.4
M7.9
M8.0HyugaNankaiTonankai

TokaiEarthquake Research Committee (2013)

Affected Area of Nankai Trough EQs
Affected area includes:
・ Large cities©名古屋太郎Osaka: 2.7mill.
Nagoya: 2.3mill.
Hamamatsu: 0.8mill.
Shizuoka: 0.7mill.

・ Industrial belt
・ Important traffic lines
Cabinet Office(2004)©W236©TC411-507©BatholithLong-Term Evaluation (2001)
Evaluation of occurrence potentials for Nankai and Tonankai earthquakes

Time predictable mode:
Tokai

TonankaiNankaiNankai Earthquake・ M≒8.4
・ Tr=90.1yrs
・ P30≒60% (2012)
Tonankai Earthquake・ M≒8.1
・ Tr=86.4yrs
・ P30≒70-80% (2012)
Nankai + Tonankai
・ M≒8.5
・ P30 : not evaluatedTokai Earthquake

Earthquake Research Committee,
Headquarters for Earthquake Research Promotion (2001)

not evaluatedAddition to Long-Term Evaluation 2001
In order to perform probabilistic seismic hazard
analysis, the followings were assumed:
 Evaluation of Tokai area
 Average recurrence interval: 118.8 yrs(renewal process) Elapsed time from the latest
event: 157.0 yrs (1854)⇒ P30=88% (as of Jan. 2012) M=8.0NankaiTonankai

TokaiAddition to Long-Term Evaluation 2001
In order to perform probabilistic seismic hazard
analysis, the followings were assumed:
 Probability that adjoining areas slip
simultaneously to generate larger earthquakes
 When both of adjoining two areas cause earthquakes
within 30 years, probability that they cause one large
earthquake is 1/2.

Model for Hazard Assessment
No.

Nankai
M=8.4
P30=62%

Tonankai
M=8.1
P30=72%

Tokai
M=8.0
P30=88%Probability
in 30 years

(1)

×××

1.3%

××

2.2%

×

3.4%

(2)
(3)

×(4)

×9.3%

×(5)

×

2.8%

(6)

×2.8%

(7)

×15%

(8)

×12%

(9)

×12%

(10)

9.8%

(11)

9.8%

(12)

9.8%

(13)

9.8%
Total

two or more areas slip simultaneously

these patterns are notobserved

100%{(1-0.62)×0.72×0.88}/2=0.12(0.62×0.72×0.88)/4=0.098

Seismic Hazard Map 2012
Prob. that JMA seismic intensity is 6-Lower or greater in 30 years from 2012

Problems in Old Model
1. Three areas, Nankai, Toankai and Tokai areevaluated independently• large probability assigned to odd occurrence patterns

2. Possibility of occurrence of larger earthquakes
is eliminated• need to incorporate lessens learned from Tohoku
earthquake of 2011
• evidence of large tsunami of 2000 years ago has
been found recently

Revised Long-term Evaluation (May, 2013)
Earthquake Research Committee has publishedrevised version of long-term evaluation of Nankai
trough earthquakes
1. Possibility of larger earthquakes (up to M=9.1)• potential source area is expanded

2. Evaluation of earthquake(s) along whole theNankai trough, not for individual areas• P30=66.5% ⇔ Nankai:62%, Tonankai:72%, Tokai:88%

3. Diversity of future earthquakes• probability of occurrence of individual case is notshownSource Area of Maximum Earthquake Source area is expanded to west(Hyuga area), south(shallow tsunami
generating zone: 0-10km) and north(deep zone: 25-35km)

Hyuga

Earthquake Research Committee (2013)

Variation of Earthquakes

Earthquake Research Committee (2013)

Variation of Earthquakes
Assumptions to make a model:
・ Prob. of 1EQ or 2EQs is even・ Nankai and Tonankai always slip
・ when 2 EQs occur, source zone
is divided between Nankai andTonankai
・ Prob. that source area reaches
Tokai is 0.75
・ Prob. of maximum EQ (Hyuga toTokai) is 0.05

HyugaNankai

Tonankai

TokaiEarthquake Research Committee (2013)

Variation of Earthquakes
WeightMw

Depth

0.01258.8-M-0.01259.0-MD0.01259.0SM-

0.01259.1

SMD×0.1625

8.7

-M-

×0.1625

8.9SM-

×0.01258.8-M-×0.0125

9.0SM-

××0.0258.7-M-

××0.025

8.9SM-

××0.05

8.4

S--

0.05

8.7, 8.3

-M-

0.325

8.5, 8.3-M-

×0.025

8.7, 8.2-M-×0.1

8.5, 8.2

-M-

0.75

1.0

Hyuga

0.5

0.5

1EQ2EQs

tentativeNankai Tonankai

Tokai××0.15

1.0

1.0

Depth S: 0-10km, M: 10-25km, D: 25-35km

0.05

0.325
0.025

0.10

Source Area of Earthquakes
One Large Earthquake

Hyuga + Nankai + Tonankai + Tokai

Nankai + Tonankai + Tokai

Nankai + Tonankai

Source Area of Earthquakes
Two Successive Earthquakes

Difference (2013 model-2012 model)
Prob. that JMA seismic intensity is 6-Lower or greater in 30 years

A・ extended source area to Hyuga
B・ Probability of Nankai area
62% → 66.5%

CBADC・ Probability of Tonankai & Tokai
Tonankai: 72% → 66.5%
Tokai:
88% → 66.5%
D・ larger earthquake(Tokai was 8.0 in old model)

Seismic Hazard Curves

Summary
1. Seismic activity models for earthquakes along
Nankai Trough are presented• Old model based on Long-term evaluation 2001
• New model based on Long-term evaluation 2013

2. Major points of model revision are expressed• Consideration of Up to M9.1 earthquake• Evaluation of occurrence probability of whole area,
not for individual area• Variation of earthquakes

3. Seismic hazard maps based on both models arecompared