JpGU-AGU Joint Meeting 2017
May 20 to 25, 2017, Makuhari, Chiba, Japan[S-CG70] Analysis and Prediction of Near-Source Strong Ground Motions:
Present Status and Future Perspective

Long-period strong ground motions near the source faultof the 2016 Kumamoto earthquake*Kojiro Irikura, Susumu Kurahashi(Aichi Institute of Technology)

Introduction 1Surface ruptures:• Surface rupture mostly occurred along fault traces mapped in previous
active fault investigations. Approximately 34-km-long surface rupturesappeared along the eastern part of the Futagawa fault zone and thenorthernmost part of the Hinagu fault zone.
• Large slip with maximum dextral slip of 2.20 m was measured throughout thecentral section of the rupture zone along the Futagawa segment, and the slip
gradually decreased bilaterally on the adjoining northeastern andsouthwestern sections.
• Slip exceeding 1 m occurred on previously unrecognized fault traces in thealluvial lowland of the Kiyama plain and on the western rim of the Aso
volcano caldera.
After Shirahama et al. (2016)

Introduction 2
Near-field ground motions• Near-field strong ground motions with high accuracy were recorded by
the NIED strong motion network (K-NET and KiK-net) and the JMA andlocal-government seismic-intensity network. In particular, two stations atMashiki Town-Hall (93051) station and Nishihara Village-Hall (93048)
were located within 2 km of the surface traces along the Futagawa fault.
• The maximum displacement at 93051 is about 1.1 m in horizontal (N65E)
and about 0.7 m in vertical (down).
The maximum displacement at 93048 is about 2.0 m in horizontal (N75E)
and about 2.0 in vertical (down)(Iwata, 2016)

Examples of slip distributions from the waveform inversion ofstrong motion data
Kubo et al. (2016)

久保モデル

Asano and Iwata (2016)

Yoshida et al. (2016)

Characterized source models with SMGAs(Irikura et al. 2017)

Three SMGA model based on the slip
distribution of Yoshida et al. (2016)

A single SMGA model based on theslip distribution of Kubo et al. (2016a)

Comparison of the observed (black) and synthetic (red) ground motions of
three components (east–west, north–south, and up–down)
A Single SMGA model(Irikura et al., 2017)
Empirical Green’s method

Model 1: A single flat fault plane model with a SMGA
• Irikura et al. (2017) based on slip
distribution of Kubo et al. (2016a)

Empirical Green’s function method
KMMH16（KiK-net Mashiki）
Acceleration(cm/s/s)

NSVelocity(cm/s)

Irikura et al.(2017)

EWUD

Model 2: A curved fault plane model with a SMGA
• Outer fault parameters are estimated based on Kubo et al.(2016b)
Empirical Green’s function method
KMMH16（KiK-net Mashiki)
Acceleration(cm/s/s)

NSVelocity(cm/s)

Kubo et al.(2016)

EWUD

Comparison between synthetic motions by Model 1 and Model 2
Model 1 : Irikura et al.(2017)
Acceleration(cm/s/s)

NSVelocity(cm/s)

EWUD

Model 2: SMGA model based on Kuboet al . (2016)
Acceleration(cm/s/s)

NSVelocity(cm/s)

EWUD

Long-period ground motion from SMGA

Assume a single SMGA in the seismogenic zone

SMGA

Velocity structure
Very shallow: Borehole logging
at KiK-net and K-NET sites
Shallow: J-SHIS model V2
Deeper: Fukuyama et al. (2000)
SMGA model
Computation method:KMMH06
Discrete wavenumber method(Bouchon,1981）
Slip velocity time functions:Nakamura and Miyatake function
with rise time of 5.0 sec
M0 of SMGA：2.16×1019Nm
Area of SMGA：12km×10km
Stress drop: 13.9 MPa

Long-period ground motions (velocity) from SMGA
93051
93048

SMGA

KMMH16
KMMH06

KMMH06
KMM005Synthetic long-period motions from SMGA at near-fault stations are smaller than the observed.

93048

1Frequency(Hz)Synthetic long-period motions from SMGA
have nearly the same level as observed atstations except very near-field stations
93048 and 93051.

Fourier Spec.(cm/s*s)

103
102 NS10110010-110-2 KMM005
10-30.01
0.1
1Frequency(Hz)

103
102
101
100
10-1
10-2 KMMH06
10-30.01
0.1
1Frequency(Hz)

93051

103
102
101
100
10-1
10-2 LOGNHR
10-3
0.01
0.1

93048

1Frequency(Hz)103
102 EW
101
100
10-1
10-2 KMM005
10-30.01
0.1
1Frequency(Hz)
103
102
101
100
10-1
10-2 KMMH06
10-3
0.01
0.1
1Frequency(Hz)Fourier Spec. (cm/s*s)

3210
101
100
10-1
10-2
KMMH16
10-3
0.01
0.1
1Frequency(Hz)
103
102
101
100
10-1
10-2 LOGMSK
10-30.01
0.1
1Frequency(Hz)Fourier Spec.(cm/s*s) Fourier Spec. (cm/s*s) Fourier Spec. (cm/s*s)

103
102
101
100
10-1
10-2 LOGNHR
10-30.010.1

Fourier Spectra 10 UD

103
102 EW
101
100
10-1
10-2
KMMH16
10-3
0.01
0.1
1Frequency(Hz)
103
102
101
100
10-1
10-2 LOGMSK
10-3
0.01
0.1
1Frequency(Hz)

93051

103
102
101
100
10-1
10-2 LOGNHR
10-3
0.01
0.1

93048

1Frequency(Hz)103
102 UD
101
100
10-1
10-2 KMM005
10-3
0.01
0.1
1Frequency(Hz)Fourier Spec.(cm/s*s)

KMMH0693051

Fourier Spec.(cm/s*s) Fourier Spec. (cm/s*s) Fourier Spec. (cm/s*s) Fourier Spec. (cm/s*s)

SMGA103102 NS
101
100
10-1
10-2
KMMH1610-30.01
0.1
1Frequency(Hz)
103
102
101
100
10-1
10-2 LOGMSK
10-3
0.01
0.1
1Frequency(Hz)Fourier Spec.(cm/s*s)

Obs.
Syn.

Fourier Spec.(cm/s*s) Fourier Spec. (cm/s*s) Fourier Spec. (cm/s*s) Fourier Spec. (cm/s*s)

Long-period motions
from SMGA

103
102
101
100
10-1
10-2 KMMH06
10-3
0.01
0.1
1
Frequency(Hz)Source model by Kubo et al. (2016 b)

Slip distribution

Moment-rate distribution

Slip distribution of the Kumamoto earthquake from a weak nonlinear
inversion of InSAR data (Fukahata and Hashimoto, 2016)
NishiharaMashiki

Long-period ground motions from LMGA
Assume a LMGA in surface layers
above the seismogenic zone
Computation method:
Wavenumber integration method(Hisada,2003）

LMGA
KMMH06

Slip velocity time function:
triangle function withrise time of 5.0 sec
M0 of LMGA: 2.88×1018Nm
Area of LMGA: 12 km x 2 km
Slip: 4.0 m
Depth of top of LMGA: 0 km

Long-period ground motions (displacement) from LMGA
LMGA-Model a: Length 12 km, Width 2 km
93051

LMGA
LMGA

93048
KMMH06
KMMH16

Black: observed
Red: Synthetic

Long-period ground motions (displacement) from LMGA
LMGA-Model b: Length 20 km, Width 2 km
93051

LMGA
LMGA

93048
KMMH06
KMMH16

Black: observed
Red: Synthetic

Long-period ground motions (displacement) from LMGA
LMGA-Model c: Length 12 km, Width 2 km
93051

LMGA93048
KMMH06
KMMH16

Black: observed
Red: Synthetic

Long-period ground motions (displacement) from LMGA
LMGA-Model d: Length 5 km, Width 2 km

LMGA93051
93048
KMMH06
KMMH16Black: observed
Red: Synthetic

Modified Characterized Source Model
Strong Motion Generation Areas (SMGAs)+Long-period Motion Generation Areas (LMGAs)
SMGAs are located inside the seismogenic zone on the source fault plane
having slip time functions such as Kostrov-type (e.g. Nakamura-Miyatake ,regularized Yoffe functions)
LMGAs are placed is inside near-surface layers above the seismogeniczone on the source fault plane having slip time functions such assmoothed ramp functions.

Long-period ground motions from SMGA + LMGASMGALMGAKMMH06• SMGA
M0：2.16×1019 Nm
Area：12km×10km
Slip velocity time function:
Nakamura-Miyatake function
with rise time of 5.0 sec
Stress drop: 13.9 MPa• LMGA
M0：2.88×1018 Nm
Area: 12 km ×2 km
Slip velocity time function
Triangle function with rise time of 5.0 sec
Slip: 4.0 m

Long-period ground motions from SMGA + LMGA
93051

Velocity

93051

93048

93048

KMMH16

KMMH16

Displacement

Summary
１．The ground motions of the 2016 Kumamoto earthquake (Mw 7.0) are
well simulated using a characterized source model consisting of strong
motion generation areas (SMGAs).
２．However, long-period ground motions (more than 3 sec) at very nearfault stations from SMGA are underestimated compared with observed
motions.
３．The long-period ground motions including fling steps at very near-fault
stations are well simulated when the long-period generation areas(LMGA) in near-surface areas of earthquake fault are taken into accountin addition to the SMGAs.