3B-P15

The Joint Symposium of Seismic Hazard Assessment 17-19 June, 2013, Sendai, Japan

LONG-PERIOD GROUND MOTION FOR MEGATHRUST EARTHQUAKES AT THE SAGAMI TROUGH:EFFECTS OF SOURCE VARIETY ON GROUND MOTION
Asako IWAKI, Nobuyuki MORIKAWA, Takahiro MAEDA, Shin AOI, and Hiroyuki FUJIWARA
National Research Institute for Earth Science and Disaster Prevention (NIED)

E-mail: iwaki@bosai.go.jp

SUMMARY

This study focuses on long-period ground motion for megathrust earthquakes at the Sagami Trough, the plate boundary of the northeast portion of the Philippine Sea plate lying beneath
the Kanto area, Japan. At the Sagami trough, two megathrust earthquakes are known in the past 1000 years: the 1703 Genroku Kanto earthquake (M7.9-8.2) and the 1923 Taisho Kanto
earthquake (M7.9). The recurrence history of earthquakes eariler than 1703, in terms of the interval times and source areas, still remains unclear. Therefore it is important for seismic
hazard evaluation to consider various source models, more than the already-known models of the past earthquakes.
We aim to demonstrate the variability in long-period ground motion, considering huge uncertainties on the source models. We consider a source region that extends to the outside of thesource areas of the Genroku- and Taisho- type earthquakes where no historical megathrust event is known to have taken place.
Long-perid (3-20 s) ground motion for various scenario cases are computed by 3D ﬁnite-diﬀerence simulations. The variability in peak ground velocity (PGV) and velocity response (Sv) isshown using source models with six source areas with magnitude varying from 7.9 to 8.6, four rupture starting points, and four patterns of asperity locations. It is demonstrated that thevariability in PGV and Sv in Kanto area due to the choice of the rupture starting point is considerably large.

SOURCE MODELSThe source region considered in this study is shown in Fig 1. The boundary of
the source region is determined based on seismological and tectonic data:
the Sagami Trough on the south, the northern limit of Philippine Sea plate(PH)on the north (e.g. Uchida et al. 2010), seismic acvitivy of small repeating
earthquakes on the north and west (e.g. Kimura et al. 2009).
The source area is divided into 6 segments: S1, S2, S3, T1, T2, and D1 (Fig 1).
Each segment composes scenario cases (CS1, CST1, CS12, etc.) by itself or incombination with other segment(s) (Table 1).
CS1 and CS12 roughly correspond to the source areas of the 1923 Taisho and1703 Genroku Kanto earthquakes, respectively.

FKS

100 km
37˚
TCG
GNM

36˚

0000

5IBR00

400

3000

YMN

0S1

TKY

CHB

S320000

S2T1SZO

T2139˚140˚141˚142˚Fig 1. Source area and the segments.
Contours indicate the depth of PH.
Total0CST1CS12

CST12

CST123

CS2

CST2304050

60708036˚35˚139˚140˚141˚142˚37˚37˚

100 km100 km36˚

CST123D

3.0

3.0

3.0

3.0

3.0

3.0

3.0

3.0

4.0

S [km2]

7932

10093

19102

24606

29734

11170

14513

19641

44770

15036

M0 [Nm]

8.70E+20

1.25E+21

3.25E+214.75E+21

6.31E+211.45E+212.15E+21

3.39E+21

1.17E+22

3.03E+21MW

7.89

8.00

8.27

8.38

8.47

8.04

8.16

8.29

8.64

8.25

D [m]

2.71

3.06

4.214.78

5.26

3.22

3.67

4.27

6.45

4.98

139˚140˚141˚

S2

1.12E+10

T1

2.16E+09

T2

3.34E+09

S3

5.13E+09

D1

1.50E+10

○

○

○○○○

○

○

○

○

○○WCDC142˚37˚139˚140˚141˚Fig 3. Rupture starting points W, C, WD
and CD.

100 km

36˚

○035˚

3.0

7.93E+0900
40030000
20000 WD00

10CD1

Δσ [MPa]

S15035˚

CST2300036˚Si [m2]Seg.

20

100 kmTable 1. Outer source parameters for ten scenario cases.
CS11037˚The source parameters are calculated basically following the “recipe” for strong
motion prediction by Earthquake Research Committee (ERC) (2008).
Point sources are distributed every 500 m on the top boundary of PH of the 3D
velocity model of ERC (2012).
Seismic moment M 0 is derived from the equation on circular crack (Eshelby,1957), assuming average stress drop Δσ ~ 3.0 MPa.

KNG

000
1035˚D1

SIT

Other Parameters
Asperities: square-shaped patches of 3 diﬀerent sizes,
10 x 10, 20 x 20, and 30 x 30 km2
Total asperity area: 20 % of the fault area
Slip velocity function: Nakamura and Miyatake (2000)
Rupture propagation velocity : VR = 2700 m/s
Rupture starting point: 4 points (W, C, WD, and CD)
shown in Fig 3.
Direction of fault motion: N140° E (relative motion of
the hanging wall against the foot wall)

Rupture time (s)

○
○

○

○

○

Fig 2. Examples of source models for cases CS1, CS12,and CST123D, showing rupture time and asperity
distributions. Rupture starting point (W) is indicated bythe red star.

○
○

○

○

○

○

○35˚

○

○139˚140˚141˚142˚SIMULATION RESULTS
Comparison between Diﬀerent Scenario Cases
Max Velocity (cm/s)

2010 25 50 100 200 300 400

CST1−W

CS12−W

100 km100 km100 km37˚37˚36˚36˚36˚35˚35˚139˚140˚141˚139˚142˚140˚141˚142˚

CST12−W

CST123−W

CST123D−W

100 km100 km100 km37˚36˚36˚35˚35˚35˚

1000

139˚142˚140˚141˚142˚141˚142˚

CST123−C

CST123D−C

100 km100 km100 km37˚36˚135˚35˚141˚142˚140˚141˚142˚140˚141˚NS

11000

10SZO

10010110Period [s]10SZO101510Period [s]142˚

CS1 (MW7.9) with Variety in Source Parameters

NS10100
10110Max Velocity (cm/s)
02

10 25 50 100 200 300 400

CS1−C

CS1−WD

100 km100 km100 km100 km37˚37˚36˚11000

36˚140˚141˚142˚35˚139˚140˚141˚142˚35˚139˚140˚141˚142˚139˚140˚141˚

Velocity response [cm/s]

139˚

142˚

CS1−W

CS1−W

CS1−W

100 km100 km100 km100 km36˚1000

139˚140˚141˚142˚139˚140˚141˚142˚139˚140˚141˚139˚142˚140˚141˚

1000

Acceleration Response [cm/s/s]

100

101

Fault Distance [km]T=5.0 s103

102104102101

CS12−C
Mw=8.3
T=5.0 s

103

102

101

1010020

Period [s]10001010EW

100101

102

Fault Distance [km]Fig 6. Comparison of 5 % damped Sa at T=5.0 s withthe attenuation relation curves (solid lines) with ±σ(dotted lines) for cases CS1 and CS12 with rupture
starting points W (blue) and C (red).

110102

Fault Distance [km]1020

Period [s]1NS1001011000

10NS

100101100010CHBNS

100

1011000

10NS

1001011000

10NS

100

101510Period [s]

510EW

100

101510EW

100101510Period [s]

SIT

1000

EW

100

101510EW

10010120510Period [s]1Period [s]NS10110NS100

10110FKS

1000

NS

100

101510NIG

1000

NS

100

101510Period [s]Rupture starting point
W

WDCCD

1510001020

Period [s]

TCG

EW

100

10151000

1020

Period [s]

IBR

EW

10010151000

1020

Period [s]

FKS

EW

100

10120

Period [s]EW1020Period [s]

GNM

100

20Period [s]

IBR

1000

1000

20100

52010TCG

1000

20Period [s]

SZO

1000

10520NS

100

520Period [s]

CHB

1000

GNM

1000

20Period [s]

TKY

1000

20Period [s]SZO

120Period [s]

SIT

1020

Period [s]

EW

100

20

Period [s]

TKY

KNG

1000

51000

1020

Period [s]

NIG

EW

100

1020

1
510

Period [s]20Fig 8. 5 % damped Sv for case CS1 with four rupture starting points, W(blue), C (green),WD (orange), and CD (yellow), and four asperity location patterns as shown in Fig 7.
PGV Ratio

Heterogeneity in Rupture TimeReferences

EWNIG

KNG

142˚

Fig 7. (top) PGV distributions for case CS1 with four different rupture starting points, W, C, WD, and CD.
(bottom) PGV distributions for case CS1-W with four different patterns of asperity location.

The results above suggest the strong inﬂuence of directivity eﬀects on
long-period ground motion in Kanto area. The directivity eﬀects may beoverestimated by the unnaturally smooth rupture propagation in our source
models. We introduce heterogeneity in V R as shown in Fig 9, following
Sekiguchi & Yoshimi (2008).
PGV ratio of the heterogeneous model to the original model for CS1-C is shownin Fig 10. The blue colored area (PGV of the heterogeneous model is smaller
than that of the original model) extends toward the direction of rupture
propagation, suggesting that the heterogeneity in rupture propagation process
reduced the eﬀect of directivity.
Fig 11 shows the comparison of Sa (T=5 s) with the attenuation relation curvefor the heterogeneous and the original models.

102

Mw=8.3

100

535˚35˚35˚1011536˚36˚36˚35˚37˚37˚

Velocity response [cm/s]

Asperity
patterns

Velocity response [cm/s]

CS1−W

37˚

FKS

10537˚

20100

537˚36˚35˚101

CS12−W

10102

Fault Distance [km]10535˚

100

EW

Period [s]T=5.0 s

103

100

100

20

Period [s]

100

20

Period [s]

IBR

5NS

10101

104

20

Period [s]

NIG

1000

5120

CS1−W

36˚

Period [s]101000120

100

5CS1−CD

37˚

10FKS

5100

NS

11000

1051020EW

Velocity response [cm/s]

Velocity response [cm/s]

1

Period [s]

IBR

5EW

Period [s]

EW100

20100

20

101000

1000

10TCG

102

Mw=7.9Fig 5. 5 % damped Sv at major cities for the six cases shown in Fig 4,
with rupture starting points W (solid lines) and C (broken lines).

Fig 4. PGV distributions for cases CS1, CST1, CS12, CST12, CST123, and CST123D
with rupture starting points W (top six panels) and C (bottom six panels).

Rupturestarting points

10100

20

Period [s]EWSIT5NS

Period [s]120

Period [s]5101000

1201005105139˚

Period [s]

10CHB

1000

2010035˚

139˚

10SIT

536˚

NS1037˚36˚

140˚140˚

CST12−C37˚139˚

139˚

Velocity response [cm/s]

141˚

Velocity response [cm/s]

140˚

5100

1000

120

Period [s]

CHB

5139˚

1010Velocity response [cm/s]

36˚

Velocity response [cm/s]

5100

1000

20Acceleration Response [cm/s/s]

1

NS

10

Period [s]Velocity response [cm/s]

37˚

10

5T=5.0 s

103

Velocity response [cm/s]

37˚37˚

10TCG

1Mw=7.9

Velocity response [cm/s]

100 km1001000

10CS1−C

Velocity response [cm/s]

100 kmEW

Period [s]CS1−W

10020104

Velocity response [cm/s]

100 km

100TKY

1000

10104

Rupture time (s)01020301040.200.330.500.671.00 1.502.003.005.00
40

original
heterogeneous

50

37˚

Acceleration Response [cm/s/s]

CS12−CNS

5EW

Velocity response [cm/s]

CST1−C

TKY

1GNM

Velocity response [cm/s]

CS1−C

1000

101000Velocity response [cm/s]

142˚

NS

Velocity response [cm/s]

141˚

CST123D

Velocity response [cm/s]

140˚

CST1210020Period [s]CST1Velocity response [cm/s]

139˚

10CST123

Velocity response [cm/s]

142˚

Period [s]5GNM

CS12Velocity response [cm/s]

141˚

1201000

Velocity response [cm/s]

140˚

10Velocity response [cm/s]

139˚

142˚

10100

Velocity response [cm/s]

1

EW

Velocity response [cm/s]

10KNG

1000

Velocity response [cm/s]

141˚

Velocity response [cm/s]

140˚

Velocity response [cm/s]

100

5139˚

NSVelocity response [cm/s]

35˚

KNG

Velocity response [cm/s]

35˚

35˚

1000

Velocity response [cm/s]

36˚

Velocity response [cm/s]

36˚

36˚

Velocity response [cm/s]

Sv (h=5%)

CS1Velocity response [cm/s]

37˚

37˚

Acceleration Response [cm/s/s]

142˚

Acceleration Response [cm/s/s]

141˚

Eﬀects of the inner source parameters are also studied in more detail. Fig 7 and Fig 8 show PGV distributions andSv for CS1 with four diﬀerent rupture starting points and four diﬀerent patterns of asperity location. Variability inlong period ground motion due to rupture starting points is considerably large. Diﬀerence in PGV distribution
between rupture starting points reaches as large as 10 - 20 times at a maximum, while that between asperity
patterns is 1 - 5 times at a maximum.

Velocity response [cm/s]

35˚140˚

10 25 50 100 200 300 400

CS1−W

37˚139˚

2

Velocity response [cm/s]

0Velocity response [cm/s]

10 25 50 100 200 300 400

Velocity response [cm/s]

2

Velocity response [cm/s]

0Long-period ground motion for six scenario cases (with rupture starting points W and C) out of ten listed in Table
1 are computed by 3D FDM (GMS; Aoi & Fujiwara, 1999) in the period range 3-20 s.
Fig 4 and Fig 5 show PGV distributions and 5% damped velocity response spectra (Sv), respectively. The largest
diﬀerence in PGV and Sv among the 6 x 2 cases is several to ten times in this period range. It is suggested that thevariability in PGV and Sv due to rupture starting points can be larger than that due to magnitudes.
Fig 6 shows comparisons of 5 % damped acceleration response spectra (Sa) at period 5 s with the attenuation
relation curves proposed by Morikawa & Fujiwara (abstract # 1B-4) for cases CS1 (MW7.9) and CS12 (MW8.3) withrupture starting points W and C.

Max Velocity (cm/s)

Max Velocity (cm/s)

37˚

100 km100 km36˚

100 km37˚36˚

36˚

35˚

CS1−C
Mw=7.9T=5.0 s

103

102

101

35˚

10035˚

100101

102

Fault Distance [km]139˚140˚141˚139˚140˚141˚Fig 9. Rupture time distribution of the originalmodel (left) and the heterogeneous model (right)
for case CS1-C.

Aoi and Fujiwara (1999), Bul. Seism. Soc. Am., 89, 918-930.
Earthquake Research Committee (2012), http://www.jishin.go.jp/main/chousa/12_choshuki/dat/index.htm.
Kimura et al. (2009), Tectonophysics, 472, 18-27. / Nakamura and Miyatake (2000), Zisin 2, 53, 1-9.
Sekiguchi and Yoshimi (2008), J. Seismology, 12, 185-195. / Uchida et al. (2010), J. Geophys. Res., 115, B07309.

139˚140˚141˚142˚Fig 10. PGV ratio (heterogeneous /original) for CS1-C.

Acknowledgments

Fig 11. Comparison of Sa (T=5.0 s)
with the attenuation relation curve forthe original (red) and heterogeneous(green) models for CS1-C.

This study is done as a part of the project of “Support program for long-period groundmotion hazard map” by Headquarters of Earthquake Research Promotion, Ministry ofEducation, Culture, Sports, Science and Technology (MEXT) of Japan.