IAG-IASPEI Joint Scientific Assembly 2017
July 30 to August 04, 2017, Kobe, Japan[S07-8] Strong Motion and Seismic Sources 1
10:30-21:00 Wednesday 2 August, 2017

S07-8-05
Near-field long-period strong ground motions
during the 2016 Kumamoto earthquake*Kojiro Irikura, Susumu Kurahashi(Aichi Institute of Technology)

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

Introduction 2
Near-field ground motions• Near-field strong ground motions with high accuracy were recorded by theNIED strong motion network (K-NET and KiK-net) and the JMA and localgovernment seismic-intensity network. In particular, two stations at Mashiki
Town-Hall (93051) station and Nishihara Village-Hall (93048) were locatedwithin 2 km of the surface traces along the Futagawa fault.
• The maximum displacements are about 2.0 m in horizontal (N75E) and about
2.0 in vertical (down) at 93048 and about 1.1 m in horizontal (N65E) andabout 0.7 m in vertical (down) at 93051 [Iwata, 2016].
Rupture models inverted from geodetic data and long-period motion data
• The fault length is more than 40 km and the maximum slip is 4 to 5 m fromthe source inversion due to geodetic data (e.g. Fukahata and Hashimoto,2016; Yarai et al., 2016) and strong motion data (e.g. Kubo et al., 2016b,
Asano and Iwata, 2016, Yoshida et al, 2017).

Displacement Data observed near Earthquake Faults during the 2016
Mw 7.0 Kumamoto earthquake
InSAR image

Very-Near-Field Strong Motion RecordsDisplacement
98051 (Miyazono, Mashiki)

Yoshimi (2016)

93048 (Komori, Nishihara)

Iwata (2016)

Slip Distribution from Inversion
Crustal deformation inversion

Waveform inversion of strong motion dataYarai et al. (2016)

Kubo 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

SMGA source model•••

SMGA1SMGA2•

Fault geometry (strike, dip) based on
Kubo et al. (2016b)
Observed data (KiK-net underground,
Seismic-intensity accelerometernetwork)
Two SMGA model (SMGA1 andSMGA2) best-bit to observed velocity
waveform
Frequency range 0.05 - 1 Hz
SMGA1

SMGA2Str,dip

235°,65°

215°,74°

rake

-145°

-180

M0

1.30×1019Nm

7.49×1018Nm

Dx×Dw

13×5km

9×13kmrisetime

1.6sec1.6secLag time

0.0sec3.0secVr

2.8km/s2.8km/s

North stations, large contribution from SMGA1 South stations, large contribution from SMGA2

Comparison between observed and synthetic waveforms using the SMGA model
93051(Miyazono)

93048(Komori)

KMMH016(Mashiki KiKnet)• Ground motions at very nearfield stations from the fault
traces• 93051 (Mashiki-Miyazono)
about 1.6 km from the fault.
• 93048 (Nishihara-Komori)
about 1 km from the fault.
• KMMH016 (Mashiki-KiK-net)
about 2.4 km from the fault.

LMGA source model•••SMGA1SMGA2LMGA•••

Fault geometry (strike, dip) based on Kubo
et al. (2016b)
LMGA is put to fit the long-period motions
with fling steps and surface breaks.
Simulated displacement and velocity
waveforms are compared with observed
ones at very-near-field stations
Slip velocity time functions are a triangle.
Max slip is given to be 4.0 m.
Frequency range 0.05 – 1 HzLMGAStr,dip235°,65°

rake

-150°

M0

2.84×1018Nm

Dx×Dw

17×3kmrisetime

2.5secLag time

5.0secVr

2.8km/s

Velocity waveform
93051(Miyazono)

93048(Komori)

KiK-net(Mashiki)

Displacement waveform

Modified Characterized Source ModelStrong 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.

SMGA + LMGA
98051

(Miyazono)

98048(Komori)

KMMH016(Mashiki,KiK-net)

Velocity waveform

Displacement waveform

Distribution of Green’s functions (displacement)from individual subfaults to the nearest station
93048 (Nishihara)

NSEW
UD

Contribution from the blue area
to the station
観測
合成

Contribution from the red area
to the station

Distribution of Green’s functions (velocity)from individual subfaults to the nearest station
93048 (Nishihara)
NSEW
UDContribution from the blue area
to the station
観測
合成

Contribution from the red area
to the station

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) except those at very-near-fieldstations less then 2 km.
２．Long-period ground motions (more than 2 sec) with fling-steps at verynear-fault stations are observed.
３．Such 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.