One Year after 2011 Great East Japan Earthquake-International Symposium on Engineering Lessons Learned from the Giant EarthquakeMarch 3-4, 2012, Kenchiku-Kaikan, Tamachi, JapanStrong Ground Motions during the 2011 Pacific
Coast Off Tohoku, Japan Earthquake

Kojiro Irikura(1) and Susumu Kurahashi(2)(1)

Aichi Institute of Technology/Kyoto University(2) Aichi Institute of Technology

Today’s Topics
1. Outline of National Hazard Map in Japan before the 11March 2011 Mw 9.0 earthquake off the Pacific coast ofTohoku
2. Features of strong ground motions from the 2011
Pacific Coast Off Tohoku, Japan Earthquake- PGV and PGA attenuation-distance relation- Why were so large acceleration motions produced ?
3. Source model for generating strong ground motions
4. Summary
Improvement of the recipe of predicting strong groundmotions for mega-thrust earthquakes

Outline of National Hazard Map in Japan before
the 2011 Pacific coast off Tohoku, Japan
earthquake Long-term evaluation of earthquake occurrence (2003)
and seismic hazard map (2010)
 Revision of long-term evaluation of earthquake
occurrence in region from off-Sanriku to off-Boso by
the Earthquake Research Committee (November,2011)

Programs defining the Seismic Hazard in Japan
1. Headquarters for Earthquake Research Promotion
Long-term Evaluation:
Evaluate probabilities of the next occurrence of large earthquakes
for major active faults and subduction-zones along troughs.

Strong Ground Motion Evaluation
Construct seismic hazard maps, probabilistic and deterministic.
Probabilistic hazard map: predicted likelihood of ground motion leveloccurring in a given area within a set period of time.
Shaking map for scenario earthquakes: strong ground motion
from hypothetical source models for specified active faults

2. Central Disaster Management Council
Conduct damage assessments from specific disastrous earthquakes
estimating the extents and sizes of the disasters andtheir impact on individuals and public facilities

Probability of Earthquake Occurrence in 30 years
Inland Crustal-Earthquakes

Subduction Earthquakes

Headquarter of Earthquake Research Promotion (2010)

Long-term evaluation of seismic activity for the region
from the off Sanriku to the off Boso

M9.0 earthquake

Tsunami earthquakeregion

Long-term evaluation of seismic activity
for the region from the off Sanriku to the off Boso

Miyagi-oki earthquake in ③------------------------------------------------------

Sanriku-nanbu earthquake in ④

Tsunami bearthquake in ⑧

Rupture of Mw 9.0 started
from this region.
Earthquake Research Committee (2002)

Large Tsunami
was genarated

Examples of Probabilistic Seismic Hazard Map

Probability of exceedance for morethan seismic intensity 6- in 30 years

Seismic intensity map for 3 % probabilityof exceedence in 30 years

Revision of Probability of Earthquake Occurrencein the Region from the off Sanriku to the off Boso
Probability of earthquake occurrence of earthquakes such as the 11 March 2011
Tohoku earthquake just before the occurrence of the earthquake

Name of past earthquakes
and seismic magnitude

Upper:
Average return
Cumulative Passage
periodProbability*
rateLower:
50 years
Latestoccurrence

Probability of earthquake
occurrence

10 years 30 years

600 years
Off Pacific Coast ofTohoku earthquake

9.0

4 – 6 % 10 – 20 % 20 – 30 % 30- 60 %0.83 –
1.0 about 500 –600 years ago※Cumulative probability is defined as probability by that time.

Earthquake Research Committee (November, 2011)

Strong ground motions from the 2011 Pacific Coast
Off Tohoku, Japan Earthquake Strong ground motions observed along the coast line
near the source fault have several distinctive pulses.
 Attenuation-distance relations of PGA and PGV.
 Very little damage caused by ground motions, althoughaccelerations at some sites near the source area werevery high.

Record Section of Short-Period Motions

After Irikura and Kurahashi (2011)

Comparison of Observed Data and AttenuationRelationships of PGA and PGV
PGAPGVFeatures of Ground Motions Records with ExtremelyHigh Acceleration
 Two Causes of Extremely High Acceleration
1) Ground Motions Records with more than 1000 gals
remarkably deviated from attenuation-distance curves.
Extremely high acceleration motions amplified by local surface
geology, non-linear site effects.

2) Ground Motion Records with relatively high acceleration
almost following attenuation-distance curves.
Distinctive high acceleration pulses seem to propagate
from station to station, attenuating with distance from fault distance.

Stations where more than1000 gals were recorded.
Some PGAs observed at sites near thesource fault seem to be remarkably deviatedfrom attenuation-distance curves.

2011/03/11 14:46 Mw9.0
Str: 201 Dip:9 23.7km

PGA (cm/s/s)

103

102
Obs
Shi Mw9.0
Shi Mw8.0
Shi Mw7.5
Shi Mw7.0

101

100

110100Distance

1000

Volcanic
Plutonic

Qurternary
Tertiary

Surface Geology near Sites
where more than1000gal
were recorded.

Accelerations more than 1000 gals
were recorded at relatively soft soilsites.

NS成分
Closeup ofacceleration
waveforms forground motion
records with morethan 1000 gals

EW成分

H/V spectral ratio of ground motions at Tsukidate(K-NET) between mainshock and small earthquakes
KURIHARA
H/V Spectral Ratio

101

Mainshock
Small earthquake

1000.1

Period[s]1Tsukidate (PGA=2933gal, DR=21.4%)

Stations where relatively high accelerations wererecorded
Relatively high accelerations at stations
near the source fault almost following
attenuation-distance curves.
2011/03/11 14:46 Mw9.0
Str: 201 Dip:9 23.7km

PGA (cm/s/s)

103102Obs
Shi Mw9.0
Shi Mw8.0
Shi Mw7.5
Shi Mw7.0

101

100110100Distance1000

Acceleration Records with remarkable distictive pulses
Horizontal NS

Horizontal EWSource Model for Generating Strong Ground Motions
during the 2011 Pacific Coast off Tohoku, JapanEarthquake The short-period source model proposed by Kurahashi

and Irikura (EPS, 2011) is revised based on re-estimationof locations of strong motion generation areas (SMGAs)
using semblance analysis for estimating azimuths ofseismic waves from the SMGAs.

Slip Distribution of the 2011 Tohoku Earthquake
DPS data including inland andoff-shore observation

Geographical Institute (2011)

Tsunami Waveform Data

Fujii and Satake (2011)

Source Model of the 11 March 2011
off Tohoku, Japan Earthquake
Slip Distributions by the separate inversions of (a) strong motion, (b) teleseismic,(c) geodetic, (d) tsunami datasets.

Yokota et al. (GRL 2011)

Record Section of Short-Period Motions

After Irikura and Kurahashi (2011)

Starting Points of SMGAs Estimated by Back-propagation Method
Location of each SMGA is estimated using a back-propagation method forprevious model (Kurahashi and Irikura, 2011).
SMAG2

SMAG1

SMAG4

SMAG3

SMAG5

Strong Motion Generation Areas by Kurahashi and Irikura(2011) by Forward Modeling of Strong Motion Waveforms

Comparison of Observed and Synthetic Seismograms
Kurahashi and Irikura(2011)
Acceleration
Observed
SyntheticVelocity

Comparison of Source Models for Generating Short-Period
Motions
Kurahashi and Irikura (2011) Kawabe and Kamae (2011)

Asano and Iwata (2011)

Reconsideration of locations of strong motion
generation areas of the 2011 Pacific Coast off Tohoku,
Japan Earthquake
Estimation of locations of SMGA from azimuths of seismic waves from each
SMGA using small arrays.

Re-estimation of Locations of SMGAs from Semblance Analysis
of Wave-Packets seen in Short-Period Seismograms

Wave packet 2

Wave packet 1

Wave packet 3

After Irikura and Kurahashi (2011)

STN3

Semblance Analysis for Wave Packets
using Local Arrays
R1/Vs

STN1

STN2

R2/Vs

Result of semblance analysis for WP1Result of semblance analysis for WP2◆センブランス解析は、平面波入射を仮定しているため、観測点をグループ分けして、仮想点震源（１メッシュ）におけるセンブランスを計算した。（上図）◆各グループにおけるセンブランスを足して、最も大きな仮想点震源（１メッシュ）の部分が破壊域内となる

Result of semblance analysis for WP 3

Estimation of locations of SMGA1 and SMGA3from semblance analysis
Location of SMGA1 from WP1Location of SMGA3 from WP2

Selection of Empirical Green’s Functions (EGFs)
for Simulating Short-Period Motions from SMGAsAcc.(cm/s/s)

Strong motion records of the 2005 Miyagi-oki earthquake
300
200
1000-100-200-300

ONG128 NS

01020304030

4050607080

90

1005060Time(sec)

70

80

90

100Acc.(cm/s/s)

80

MYGH04 NS

400-40-80

01020

The records have clearly two wave-groups from SMGAs. Waveforms of the secondwave-group are used as the EGFs for simulating ground motions from SMGA3 (WP 2).

Fourier Spectra of the EGFs
2007/11/26 22:51

Name

EGF1

EGF2

M

7.2

6.0

Dx,dw

8.5 km7.7km

Stress Drop20MPa

4.2MPaMo

5.23E+18 **7.66E+17** Suzuki and Iwata (2007) only Asp2
108

108

fc=0.3Hz
0508161146

107

106

105

104
0.01

0.1
1
Frequency(Hz)10Unit Distance Source Spectra(cm*s*km)

109
Unit Distance Source Spectra(cm*s*km)

EGF

2005/08/16 11:46

fc=0.3Hz
0512170332
107

106

105

104
0.01

0.1
1Frequency(Hz)10Revised Model

SMGA2
SMGA1
SMGA3

SMGA4

SMGA5L,WMo

Stress drop

SMGA1

34×34

2.68E+2016SMGA2

23.1×23.1

1.41E+2020

SMGA3

42.5×42.5

6.54E+2020SMGA4

25.5×25.5

1.24E+2025.2

SMGA5

38.5×38.5

5.75E+2025.2

Comparison between Observed and Synthetic Motions
Observed
Synthetic

MIYAGI
PrefectureComparison between Observed and Synthetic Motions

FUKUSHIMA
Prefecture

IBARAGI
Prefecture

Observed
Synthetic

Distinctive High Acceleration Pulses

Distinctive High Acceleration Pulses
Borehole station OP -128m at Onagawa NPP
Acc.(cm/s/s)

400

ONG OP-128m NS

2000-200-40001020304050607080Time(sec)Acc.(cm/s/s)

Acc.(cm/s/s)

10011012096

97
98
Time(sec)

130140150

99100400400
2000-200-400
4590

4647
48
Time(sec)

49

50200
0-200-40095

Source Model of Simulating Distinctive Pulses
Heterogeneous stress parametersinside SMGA

⊿σYX
Matsushima and Kawase (2006)

Uniform slip velocity model

Heterogeneous slip velocity model 1 (x2)

Heterogeneous slip velocity model 2 (x4)Distinctive
pulse

DistinctivepulseThis study

Asano and Iwata (2011)

Satoh (2012)

Comparison between SMGAs in this study and source locationsof past earthquakes off the Pacific coast of Tohoku

Strong Motion Generation Area versus Seismic Momentfor Subduction Earthquakes
2011 Tohoku Earthquake

Asano and Iwata (2011)
Kamae and Kawabe (2011)
Irikura and Kurahashi (2012)
Satoh (2012)

After Satoh (2012)

Acceleration Spectral-Level versus Seismic Momentfor Subduction Earthquakes
2011 Tohoku earthquake
Asano and Iwata (2011)
Kamae and Kawabe (2011)
Irikura and Kurahashi (2012)
Satoh (2012)

After Satoh (2012)

Summary 1
Based on the results of our analysis in this study, we improve
the recipe of predicting strong ground motions to be able toapply it to mega-thrust earthquake.
Outer source parameters
1. Source area and seismic moment of a target earthquake:
Source area is set from the tectonic background in theobjective regions.
2. Average stress drop over the entire source area:
Average stress drop is estimated from the empirical scaling
relation of source area versus seismic moment.

Summary 2
Inner fault parameters
3. Segmentation:
The segments of the source area are divided from seismic
activities and geo-morphological setting in the target region.
4. Strong motion generation areas (SMGAs):
SMGA is arranged to assign one per a segment.
5. Average stress parameter for SMGA:
About 25 MPa from empirical relation
6. Heterogeneity of stress parameters inside SMGA
(Matsushima and Kawase, 2006)