2017 Annual Meeting Seismological Society of America 18–20 April • Denver, Colorado
Session: Scaling and Empirical Relationships of Moderate to Large Earthquakes:Re-scaling or Re-thinking ?

Examination of Source Scaling Relations
for Crustal Earthquakes in Japan
Kojiro IRIKURA1, #Hiroe Miyake2, Ken MIYAKOSHI3, Katsuhiro KAMAE4,Kunikazu YOSHIDA3, Kazuhiro SOMEI3, and Susumu KURAHASHI1
1: Aichi Institute of Technology, 2: The University of Tokyo, 3: Geo-research Institute,
4: Reactor Research Institute, Kyoto University
This study for the 2016 Kumamoto Eq. is partly published as Irikura et al. (2017, EPS).

Motivation: Source scaling and background physics
Leonard (2014)

HERP (2016) for SHA in Japan2

Motivation: Source scaling and background physics
Leonard (2014)

HERP (2016) for SHA in Japan3

2016 Kumamoto earthquake (Mw 7.0)
Aftershock distribution, InSAR, and fault segment model

F2

F1

F3
H

Triangle shows the seismic stations used in this study. Star shows the rupture starting point of the main shock.
4
Aftershocks occurring within 48 h of the mainshock are plotted.

2016 Kumamoto earthquake
Slip distribution of strong motion waveform inversion (T = 2-20 s)
Averaged
three slip inversions
M0VrLWDS

4.42×1019 Nm
2.4 km/s
44 km
18 km
1.98 m798 km2

Yoshida et al. (2016)
5

Comparison of observed (black) and synthetic (red) velocity
waveform from strong waveform inversion (T = 2-20 s)

Yoshida et al. (2016)

6

Comparison of scaling relationships
between Japan one and other countries
Rupture Length vs. Seismic Moment

Red star:
2016 Kumamoto

Rupture Width vs. Seismic Moment

7

Comparison of scaling relationships
between Japan one and other countries
Rupture Length vs. Rupture Width

Red star:
2016 Kumamoto

Average Slip vs. Seismic Moment

8Comparison of scaling relationships
between Japan and other countries

Rupture Area vs. Seismic Moment
Red star:
2016 Kumamoto

9

SMGA broadband source model for the 2016 Kumamoto Eq.
10



Data

4 K-NET and 14 KiK-net stations
For KiK-net, downhole data were used.
Element event (EGF event)

EGF1: 2016/04/14,23:43 Mw 5.2 for SMGA1&2
EGF2: 2016/04/16,09:48 Mw 4.9 for SMGA3

EGF2 Mw4.9
For SMGA3

Fault geometry

Yoshida et al. (2016): 4 seg.
 Modelling parameters

EGF1 Mw5.2 Area S,

For SMGA1 & 2 Rise time Tr,

Stress drop Δσ,

Main.
Mw7.1



Rupture velocity Vr,
Rupture startling point of SMGA
Location of SMGA

Assumptions

 Only three squared SMGA generate strong
ground motions without background area
 SMGA1 is located including hypocenter.
10

SMGA broadband source model for the 2016 Kumamoto Eq.
11Seg. 3
Seg. 2
Seg. 1

Seg. 4

SMGA1

SMGA2

SMGA3

Area (km2)

51.8

51.8

100.0

M0 (Nm)

2.08×1018

2.08×1018

5.49×1018

Rise time (s)0.60.60.6

Δσ (MPa)

13.6

13.6

13.4

Vr (m/s)

2.8

2.8

2.8

105°
Peak moment rate: Yoshida et al. (2016,SSJ_S15-P02)

11

12

Velocity waveforms (NS component: 0.2-10 Hz) Obs. vs. Syn.

12

13

Velocity waveforms (EW component: 0.2-10 Hz) Obs. vs. Syn.

13

Comparison between observed and synthetic onesof intensity (JMA scale), PGA, and PGV

14 The synthetic ground motions explain well the characteristics of observed ground motionin the broadband frequency range.
 PGA for the station using the surface records could be relatively overestimated due to thenon-linear site effect during the mainshock.

14

Pseudo Velocity Response Spectra (h=0.05) Obs. vs.15Syn.

15

Bias Plots for horizontal and vertical components

16 For short period range (<0.3 s), synthetic
groundmotions

arelarger

thantheobservations.
 It is likely caused by the non-linear site
effect during the mainshock for somestations using the surface records.

N = 10 sites, R<40 km

Mean bias± Standard error
16

2016 Kumamoto earthquake
Slip distribution of strong motion waveform inversion (T = 2-20 s)
Averaged
three slip inversions
M0VrLWDS

4.42×1019 Nm
2.4 km/s
44 km
18 km
1.98 m798 km2
17Yoshida et al. (2016)

Comparison between combined area of asperities from the slip
distribution and that of SMGAs from strong motion simulation

2016Kumamoto(Main)

Miyakoshi et al.
(2015)

18

Acceleration level versus Seismic Moment
Dan et al. (2001)

Empirical relation of the Acceleration level (or “Short period level”)
Acceleration level (A [Nm/s2]): Acceleration source spectral-level

Mo

Seismic moment

AAcceleration source
spectral-level

High frequency range(Short period range)

Sourcespectrum

Acceleration
sourcespectrum19

Combined Area of Asperities (SMGA) vs. Seismic Moment

Combined Area ofAsperities (SMGAs)

20

Stress drop on the SMGA versus seismic moment

21

Stress drop on SMGA is estimated fromthe average stress drop in the rupture area(Δσ) and the inverse ratio of SMGA over
the rupture area (1/ (Sa/S)).
SMGA: Sa = π ・ r2
Rupture area S = π ・ R2
Δσa = S / Sa ・ Δσ (Madariaga, 1979)21
 Red lines indicate the relationship between ΔσSMGA and M0 expected by “Recipe” for inland crustal earthquakes.

Summary
1. The rupture area and asperity area were determined based on slip
distributions obtained from waveform inversion of the 2016 Kumamoto
earthquake observations (Mw 7.0). The relationship between therupture area and the seismic moment for this earthquake follows thesecond-stage scaling within one standard deviation of a three-stage
scaling for crustal earthquakes in Japan.
2. The ground motions of this earthquake are well simulated using acharacterized broadband source model consisting of strong motion
generation areas (SMGAs) based on the EGF method.
3. Scaling relation of rupture area to seismic moment seems to beuniversal for most crustal earthquakes, indicating less regionalvariations of stress drop. On the contrary, scaling relations of rupture
length and width to seismic moment show regional variations.
22

Scaling Relationship for Crustal Earthquakes in Japan
Rupture Area vs. Seismic Moment (Irikura and Miyake, 2001; 2011)

Irikura and Miyake (2011)23

Extended scaling relationship
Rupture Area vs. Seismic Moment (Murotani et al. (2015)Crack model
Mw < 6.5L-model (D ∝ L)
6.5 ≤ Mw ≤ 7.4

W-model (D ∝ W)
Mw > 7.4

Background of this study
W ~ 18 km for L-model
L > 100 km for W-model

Murotani et al. (2015)24