2016 Kumamoto Earthquake Sequence and Its Impact
on Earthquake Science and Hazard Assessment

Editorial Board (September 2017)
Editor-in-Chief
Yasuo Ogawa, Tokyo Institute of Technology, Japan

Vice Editors-in-Chief
Masato Furuya, Hokkaido University, Japan
Solid Earth Science

Nozomu Nishitani, Nagoya University, Japan
Space Science

Editors
Valerio Acocella
Ryosuke Ando
Yosuke Aoki
Kimiyuki Asano
Nanan Balan
Stephen Bannister
Paul Bedrosian
Phil Cummins
Koji Fukuma
Thomas Hobiger
Hauke Hussmann
Art Jolly
Attila Komjathy
Alexey Kuvshinov

Juanjo Ledo
Huixin Liu
Takuto Maeda
Koji Matsumoto
Yohei Miyake
Mitsuhiro Nakagawa
Junichi Nakajima
Haruhisa Nakamichi
Yasuhito Narita
Takuya Nishimura
Azusa Nishizawa
Takaaki Noguchi
Masahito Nose
Makiko Ohtake

Keiji Ohtsuki
Kyoko Okino
Taku Ozawa
Severine Rosat
Takeshi Sakanoi
David Shelly
Benoit Taisne
Hiroshi Takenaka
Sunny TamYoshiyuki Tanaka
Yanbin Wang
Yih-Min Wu
Takuji Yamada
Mare Yamamoto

Hiroo Kanamori
Jun'ichiro Kawaguchi
Takafumi Matsui
Hitoshi Mizutani

Toshitaka Tsuda
Zhongliang Wu
Kiyoshi Yomogida

Advisory Board
Benjamin Fong Chao
Bernard Chouet
Yoshimori Honkura
Toshihiko Iyemori

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Earth, Planets and Space (EPS) is the official journal of the Society of Geomagnetism and Earth, Planetary
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Earth, Planets and SpaceHashimoto et al. Earth, Planets and Space (2017) 69:98
DOI 10.1186/s40623-017-0682-7

Open Access

PREFACESpecial issue “2016 Kumamoto
earthquake sequence and its impact
on earthquake science and hazard assessment”
Manabu Hashimoto1*, Martha Savage2, Takuya Nishimura1, Haruo Horikawa3 and Hiroyuki Tsutsumi4
Iwata (2016) assume two planar faults, and Yoshida et al.
(2017) model four planar faults dipping in different directions. Uchide et al. (2016) also present a model of the
rupture process of the mainshock and discuss its relationship with aftershocks. Sawazaki et al. (2016) exploit
high-frequency records to derive temporal variations
in energy release during the period spanning nearly theentire sequence. Fukuyama and Suzuki (2016) estimate
Dc″, a proxy of critical slip-weakening displacement,
based on a near-fault seismogram.
Since its launch in May 2014, the Advanced Land
Observing Satellite 2 (ALOS-2) of the Japan Aerospace
Exploration Agency has been providing us with invaluable information on deformation of the earth’s surface.
ALOS-2 made observations from many tracks with different line-of-sight directions. Using the data from
ALOS-2, several groups present models of coseismic
deformation on faults. Himematsu and Furuya (2016)
exploit pixel offsets as well as phase changes to reveal
deformation. Ozawa et al. (2016) present a model withfour fault segments by fitting the observed interferograms of ALOS-2. Fukahata and Hashimoto (2016) invert
interferograms of ALOS-2 ScanSAR images to simultaneously estimate the slip distribution and dip angle oftwo fault segments. Kobayashi (2017) exploit multiple
aperture interferometry as well as conventional interferometry to reveal coseismic deformation due to foreshocks. Fujiwara et al. (2016) focus on detailed featuresof numerous surface ruptures recognized in ALOS-2
interferograms and present their qualitative mechanism
in relation to faulting and geologic structures. As in thecase of the strong-motion studies, there is a variety in thegeometry and number of faults, reflecting the complexity of the rupture and observed deformations. This wide
variety of fault models indicates a limitation in the abilityof presently available techniques for the study of earthquake sources.

The Kumamoto earthquake sequence that started on
April 15, 2016, resulted in more than 60 fatalities andbrought about serious damage to Kumamoto Prefecture. It is the first time that the seismic intensity of 7 (onthe Japan Meteorological Agency scale) was recorded
twice within 2 days, which unnerved the population in
the affected areas and made the response difficult. Furthermore, seismicity spread out across Kyushu Island,
which enhanced uneasiness in society. The earthquakes
occurred on known active faults where long-term probability of earthquake occurrence was disseminated, butsuch complex activity was unexpected. This uniquesequence will clearly have a significant impact on the fieldof earthquake science and related hazard assessment.
This special issue is a compilation of 43 papers on multidisciplinary research regarding this unique sequence,
the relevant tectonics and associated phenomena withstate-of-the-art techniques as well as traditional ones.
Many papers discuss source processes based on studies from the viewpoint of their own discipline, such as
teleseismic seismograms, strong ground motion records
and surface displacements detected by space geodesy.
Yagi et al. (2016) reveal the temporal evolution of rupture using seismograms from a global network. In contrast, Asano and Iwata (2016), Kubo et al. (2016), Nozu
and Nagasaka (2017) and Yoshida et al. (2017) discuss
the detailed rupture process using strong ground motion
records. Their modeled faults have significant differencesamong them in the number of faults and geometry, butsimilar characteristics in slip distribution and temporal
evolution of rupture are derived. Nozu and Nagasaka(2017) assume a single planar fault, while Kubo et al.
(2016) use a single, smoothly curved plane. Asano and*Correspondence: hashimoto.manabu.7e@kyoto‑u.ac.jp
1
Disaster Prevention Research Institute, Kyoto University, Uji 611‑0011,
Kyoto, Japan
Full list of author information is available at the end of the article© The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License(http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium,provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license,
and indicate if changes were made.

1

2016 Kumamoto Earthquake Sequence and Its Impact on Earthquake Science and Hazard Assessment
Hashimoto et al. Earth, Planets and Space (2017) 69:98

Page 2 of 4

During the Kumamoto earthquake sequence, many
surface ruptures appeared and their origin is controversial. Tajima et al. (2017) discuss the relationship between
surface ruptures found within the Aso caldera and the
distribution of volcanic edifices to derive a possible link
between the source fault and volcanic vents. Sugito et al.
(2016) reported a detailed distribution of surface ruptures associated with the foreshock, some of which also
moved during the mainshock. Shirahama et al. (2016)
also compile a distribution of surface ruptures and conclude that most of them are distributed along previously
identified active faults. Toda et al. (2016) examine surface
ruptures aligned parallel to each other near the Mashiki
town and Nishihara village and present a model of slip
partitioning. Goto et al. (2017) examine subtle geomorphic features of surface ruptures using a digital elevation
model in the city of Kumamoto and discuss their possible
relationship to the earthquake source faults.
Seismicity change prior to a large earthquake is always
an issue. Nanjo et al. (2016) present results of analyses
using four different methods and detected anomalouschange in seismicity. Nanjo and Yoshida (2017) discusschanges in parameters that describe seismicity between
the M6.5 foreshock and M7.3 mainshock. Zhuang et al.
(2017) discuss the effects of newly developed replenishments of the missing data on estimated parameters of theepidemic-type aftershock sequence (ETAS) model. Yanoand Matsubara (2017) relocate hypocenters and discuss
detailed features of seismicity during different periods
such as 2001–2012, between the foreshock and mainshock, and after the mainshock.
A moderate earthquake hit the Yufuin city, Oita prefecture, right after the occurrence of the M7.3 mainshock.
This event is considered to have been triggered by theseismic waves that radiated from the source fault of themainshock. This phenomenon attracted the interest of
many researchers. Miyazawa (2016) carefully examines
the timing of triggering of the earthquake in Yufuin,
exploiting a full waveform simulation of the mainshock.
Enescu et al. (2016) show the activation of seismicity involcanic areas as far away as Hokkaido as well as in nonvolcanic areas such as Noto peninsula. Yoshida (2016)
re-estimates the magnitude of this triggered event using
strong-motion records and GNSS data and concludes
that the preliminary magnitude was underestimated.
Uchide et al. (2016) also examine the magnitude of triggered events. Nakamura and Aoi (2017) estimate a focal
mechanism and location of the triggered event in Yufuin.
Recent development of real-time monitoring for earthquake early warning is amazing. Applications of this
newly deployed technique to the Kumamoto earthquake
sequence are presented in this issue. Kawamoto et al.
(2016) show the ability of the GNSS-based real-time

Since the target region includes the active Aso volcano,which is located along the southern boundary of the
Beppu-Shimabara graben zone, the relationship between
rupture process and subsurface structure is considered
important. Yagi et al. (2016) and Ozawa et al. (2016) discuss the effects of stress changes due to the rupture of
the mainshock on the magma reservoir of Aso volcano.
Aizawa et al. (2017) analyze broadband magnetotelluric
data and reveal a three-dimensional distribution of resistivity to discuss its relationship to the seismicity. Kusumoto (2016) and Matsumoto N et al. (2016) analyze thegravity gradient tensor and examine its relationship to thegeometry of source faults. Mochizuki and Mitsui (2016)
discuss the relationship between heterogeneous structure
and interseismic crustal deformation. Miyakawa et al.
(2016) invert gravity data to reveal a low-density body
beneath the Aso caldera and discuss its possible effecton the termination of rupture. Matsumoto S et al. (2016)
perform a stress tensor inversion of fault plane solutions
and seismic moment tensors in central Kyushu and reveal
a zone of large inelastic strain.
As mentioned above, the high intensity of groundshaking is an important issue in this earthquake
sequence. Zhang et al. (2016) simulate ground motion
records assuming an attenuation structure and site
effect and succeed in reproducing observed groundmotion. Hata et al. (2016) also try to reproduce strong
ground motion, especially near the Shinkansen (bullet train system in Japan) track, with special reference
to site effects. Yamanaka et al. (2016) conduct dense
observation of aftershocks in heavily damaged areas
and find local amplification effects in Mashiki, Nishihara and Kumamoto, but not in Minami-Aso. Chimoto
et al. (2016) conduct array analysis in Mashiki andNishihara and obtain the S-wave velocity structure,
which they consider controls site amplification. On theother hand, Kawase et al. (2017) make a detailed survey of damages and reanalyze strong-motion records
in the Mashiki town, attributing severe structural damages to a strong westward velocity pulse, rather thanto site effects. Tsuno et al. (2017) evaluate site amplification in Kumamoto based on dense observation of
microtremor and conclude that site amplification is
not enough to explain observed ground motion during
the mainshock. Irikura et al. (2017) examine three rupture models of the mainshock of the Kumamoto earthquake sequence and verify the effectiveness of source
scaling relations for crustal earthquakes. Suzuki et al.
(2017) study attenuation on strong-motion records andpresent a feasible method for real-time forecasting ofstrong motions. Nagasaka and Nozu (2017) discuss theapplicability of multiple point sources to strong groundmotion prediction.

2

Earth, Planets and SpaceHashimoto et al. Earth, Planets and Space (2017) 69:98

Page 3 of 4analysis system (REGARD) deployed by the Geospatial
Information Authority (GSI) to detect coseismic displacements and estimate size and location of the source
fault. Kodera et al. (2016) examine the performance ofthe current seismic-based earthquake early warning system operated by the Japan Meteorological Agency (JMA)
and indicate that two improved methods will be implemented in the near future.
This special issue is a compilation of preliminary works
due to the limitation of available data and techniques,but gives deep insights into the generation process of thisunique sequence of earthquakes. As the guest editors of
this issue, we expect that these works will be a milestone
for future studies of this earthquake sequence.

Fukahata Y, Hashimoto M (2016) Simultaneous estimation of the dip angles
and slip distribution on the faults of the 2016 Kumamoto earthquake
through a weak nonlinear inversion of InSAR data. Earth Planets Space
68:204. doi:10.1186/s40623‑016‑0580‑4
Fukuyama E, Suzuki W (2016) Near‑fault deformation and Dc″ during the 2016
Mw7.1 Kumamoto earthquake. Earth Planets Space 68:194. doi:10.1186/
s40623‑016‑0570‑6
Goto H, Tsutsumi H, Toda S, Kumahara Y (2017) Geomorphic features of
surface ruptures associated with the 2016 Kumamoto earthquake inand around the downtown of Kumamoto City, and implications on
triggered slip along active faults. Earth Planets Space 69:26. doi:10.1186/
s40623‑017‑0603‑9
Hata Y, Masaaki Yabe M, Kasai A, Matsuzaki H, Takahashi Y, Akiyama M (2016)
Ground motion estimation for the elevated bridges of the Kyushu
Shinkansen derailment caused by the foreshock of the 2016 Kumamoto
earthquake based on the site‑effect substitution method. Earth Planets
Space 68:199. doi:10.1186/s40623‑016‑0573‑3
Himematsu Y, Furuya M (2016) Fault source model for the 2016 Kumamoto
earthquake sequence based on ALOS‑2/PALSAR‑2 pixel‑offset data:
evidence for dynamic slip partitioning. Earth Planets Space 68:169.
doi:10.1186/s40623‑016‑0545‑7 (Erratum to this article has been
published in Earth, Planets and Space 68:196)
Irikura K, Miyakoshi K, Kamae K, Yoshida K, Somei K, Kurahashi S, Miyake H(2017) Applicability of source scaling relations for crustal earthquakes toestimation of the ground motions of the 2016 Kumamoto earthquake.
Earth Planets Space 69:10. doi:10.1186/s40623‑016‑0586‑yKawamoto S, Hiyama Y, Ohta Y, Nishimura T (2016) First result from the
GEONET real‑time analysis system (REGARD): the case of the 2016
Kumamoto earthquakes. Earth Planets Space 68:190. doi:10.1186/
s40623‑016‑0564‑4
Kawase H, Matsushima S, Nagashima F, Baoyintu Nakano K (2017) The causeof heavy damage concentration in downtown Mashiki inferred fromobserved data and field survey of the 2016 Kumamoto earthquake. Earth
Planets Space 69:3. doi:10.1186/s40623‑016‑0591‑1
Kobayashi T (2017) Earthquake rupture properties of the 2016 Kumamoto
earthquake foreshocks (M j 6.5 and M j 6.4) revealed by conventionaland multiple‑aperture InSAR. Earth Planets Space 69:7. doi:10.1186/
s40623‑016‑0594‑yKodera Y, Saitou J, Hayashimoto N, Adachi S, Morimoto M, Nishimae Y, Hoshiba
M (2016) Earthquake early warning for the 2016 Kumamoto earthquake:
performance evaluation of the current system and the next‑generation
methods of the Japan Meteorological Agency. Earth Planets Space
68:202. doi:10.1186/s40623‑016‑0567‑1
Kubo H, Suzuki W, Aoi S, Haruko Sekiguchi H (2016) Source rupture pro‑
cesses of the 2016 Kumamoto, Japan, earthquakes estimated fromstrong‑motion waveforms. Earth Planets Space 68:161. doi:10.1186/
s40623‑016‑0536‑8
Kusumoto S (2016) Dip distribution of Oita‑Kumamoto Tectonic Line locatedin central Kyushu, Japan, estimated by eigenvectors of gravity gradienttensor. Earth Planets Space 68:153. doi:10.1186/s40623‑016‑0529‑7
Matsumoto N, Hiramatsu Y, Sawada A (2016) Continuity, segmentation andfaulting type of active fault zones of the 02016 Kumamoto earthquake
inferred from analyses of a gravity gradient tensor. Earth Planets Space
68:167. doi:10.1186/s40623‑016‑0541‑y (The Erratum to this article
has been published in Earth, Planets and Space 69:47)
Matsumoto S, Nishimura T, Ohkura T (2016) Inelastic strain rate in theseismogenic layer of Kyushu Island, Japan. Earth Planets Space 68:207.
doi:10.1186/s40623‑016‑0584‑0
Miyakawa A, Sumita T, Okubo Y, Okuwaki R, Otsubo M, Uesawa S, Yagi Y (2016)
Volcanic magma reservoir imaged as a low‑density body beneath Aso
volcano that terminated the 2016 Kumamoto earthquake rupture. Earth
Planets Space 68:208. doi:10.1186/s40623‑016‑0582‑2
Miyazawa M (2016) An investigation into the remote triggering of the
Oita earthquake by the 2016 Mw 7.0 Kumamoto earthquake using
full wavefield simulation. Earth Planets Space 68:205. doi:10.1186/
s40623‑016‑0585‑z
Mochizuki K, Mitsui Y (2016) Crustal deformation model of the Beppu‑
Shimabara graben area, central Kyushu, Japan, based on inversion of
three‑component GNSS data in 2000–2010. Earth Planets Space 68:177.
doi:10.1186/s40623‑016‑0550‑x

Authors’ contributions
All authors of this article served as guest editors for this special issue. All
authors read and approved the final manuscript.
Author details
1
Disaster Prevention Research Institute, Kyoto University, Uji 611‑0011, Kyoto,
Japan. 2 Institute of Geophysics, SGEES, Victoria University of Wellington,
Box 600, Wellington 6140, New Zealand. 3 Geological Survey of Japan, National
Institute of Advanced Industrial Science and Technology, Tsukuba 305‑0046,
Ibaraki, Japan. 4 Faculty of Science and Engineering, Doshisha University,
Kyotanabe 610‑0394, Kyoto, Japan.
Acknowledgements
We express sincere thankfulness to the authors who contributed to thisspecial issue, and reviewers who evaluated the contributions and gave helpfulcomments and suggestions.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims in pub‑lished maps and institutional affiliations.
Received: 10 July 2017 Accepted: 10 July 2017

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4Earth, Planets and SpaceFRONTIER LETTER

Open Access

Rupture process of the 2016 Kumamoto earthquake inrelation to the thermal structure around Aso volcano
Yuji Yagi*, Ryo Okuwaki, Bogdan Enescu, Amato Kasahara, Ayumu Miyakawa and Makoto Otsubo
Earth, Planets and Space 2016, 68:118 doi:10.1186/s40623-016-0492-3
Received: 5 May 2016, Accepted: 15 June 2016, Published: 14 July 2016

Abstract
We constructed the rupture process model for the 2016 Kumamoto, Japan, earthquake from broadband teleseismic body
waveforms (P-waves) by using a novel waveform inversion method that takes into account the uncertainty of Green’s function.
The estimated source parameters are: seismic moment = 5.1 × 1019 Nm (Mw = 7.1), fault length = 40 km, and fault width = 15
km. The mainshock rupture mainly propagated northeastward from the epicenter, for about 30 km, along an active strike-slip
fault. The rupture propagation of the mainshock decelerated and terminated near the southwest side of the Aso volcano; theaftershock activity was low around the northeastern edge of the
major slip area. Our results suggest that the rupture process of thebaa 130˚ 132˚b
34˚mainshock and the distribution of aftershocks were influenced by
the high-temperature area around the magma chamber of Mt. Aso.
Oita Pref.

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uj

Keywords: 2016 Kumamoto earthquake, Rupture process,

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Kumamoto Pref.

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Momemt rate (1018 Nm/s)

Foreshocks and aftershocks, Mt. Aso, Magma chamber

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Miyazaki Pref.

131.0˚
2.28
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mgal/km

131.5˚12

16

Figure 1*Corresponding author: Yuji Yagi, yagi-y@geol.tsukuba.ac.jp

FRONTIER LETTER

Open Access

Source rupture processes of the foreshock and mainshock inthe 2016 Kumamoto earthquake sequence estimated fromthe kinematic waveform inversion of strong motion data
Kimiyuki Asano* and Tomotaka Iwata
Earth, Planets and Space 2016, 68:147 doi:10.1186/s40623-016-0519-9
Received: 17 May 2016, Accepted: 29 July 2016, Published: 31 August 2016

Abstract
The 2016 Kumamoto earthquake sequence started with an MJMA 6.5 foreshock occurring along the northern part of the
Hinagu fault, central Kyushu, Japan, and the MJMA 7.3 mainshock occurred just 28 h after the foreshock. We analyzed thesource rupture processes of the foreshock and mainshock by using the kinematic waveform inversion technique on strong
motion data. The foreshock was characterized by right-lateral strike-slip occurring on a nearly vertical fault plane along thenorthern part of the Hinagu fault, and it had two large-slip areas: one near the hypocenter and another at a shallow depth.
The rupture of the mainshock started from the deep portion of a northwest-dipping fault plane along the northern part of the
Hinagu fault, then continued to transfer to the Futagawa fault. Most of the significant slip occurred on the Futagawa fault, andthe shallow portion of the Hinagu fault also had a relatively large slip. The slip amount on the shallowest subfaults along the
Futagawa fault was approximately 1–4 m, which is consistent with the emergence of surface breaks associated with thisearthquake. Right-lateral strike-slip dominated on the fault segment along the Hinagu fault, but normal-slip components
were estimated to make a significant contribution on the fault segment along theaFutagawa fault. The large fault-parallel displacements recorded at two near-fault
strong motion stations coincided with the spatiotemporal pattern of the fault slip
history during the mainshock. The spatial relationship between the rupture areas of
the foreshock and mainshock implies a complex fault structure in this region.

Keywords: 2016 Kumamoto earthquake, Source rupture process, Kinematic source
inversion, Strong motion data, Futagawa and Hinagu faults

Figure 5*Corresponding author: Kimiyuki Asano, k-asano@egmdpri01.dpri.kyoto-u.ac.jp
52016 Kumamoto Earthquake Sequence and Its Impact on Earthquake Science and Hazard Assessment

FRONTIER LETTER

Open AccessSlip-partitioned surface ruptures for the Mw 7.0 16 April
2016 Kumamoto, Japan, earthquake
Shinji Toda*, Heitaro Kaneda, Shinsuke Okada, Daisuke Ishimura and Zoë K. Mildon
Earth, Planets and Space 2016, 68:188 doi:10.1186/s40623-016-0560-8
Received: 15 July 2016, Accepted: 29 October 2016, Published: 22 November 2016

AbstractN

An ENE-trending ~30-km-long surface rupture emerged during the Mw = 7.0 16 April 2016 Kumamoto earthquake along thepreviously mapped Futagawa and northern Hinagu faults. This included a previously unknown 5-km-long fault within the Aso
Caldera, central Kyushu. The rupture zone is mostly composed of right-lateral slip sections, with a maximum of 2-m coseismic
slip. One of the noteworthy features we observed in the field are ~10-km-long segmented normal fault scarps, dipping to thenorthwest, along the previously mapped Idenokuchi fault, 1.2–2.0 km south of and subparallel to the Futagawa fault. The
maximum amount of coseismic throw on the Idenokuchi fault is ~2 m, which is nearly
Normalequivalent to the maximum slip on the strike-slip rupture. The locations and slip
Right-lateral
fault
strike-slip(Idenokuchi)
motions of the 2016 rupture are also manifested as interferogram fringe offsets infault(Futagawa)
InSAR images. Together with geodetic and seismic inversions of subsurface fault slip,
we present a schematic structural model where oblique motion occurred on anorthwest-dipping subsurface fault and the slip is partitioned at the surface into
strike-slip and normal fault scarps. Our simple dislocation model demonstrates that thisbifurcation into pure strike-slip and normal faults likely occurs for optimally oriented
failure near the surface. The Kumamoto case, with detailed geological observations andgeophysical models, would be the second significant slip-partitioned earthquake
around the globe. It provides an important insight into scale- and depth-dependentstress heterogeneity and an implication to a proper estimate of seismic hazard in
Obliquecomplex and broad multiple fault strands.
faulting
ck
blo
hed

ac
Det

Keywords: 2016 Kumamoto earthquake, Surface rupture, Slip partitioning, Active fault

Figure 7*Corresponding author: Shinji Toda, toda@irides.tohoku.ac.jp

FULL PAPER

Open Access

Small-displacement linear surface ruptures of the 2016 Kumamoto
earthquake sequence detected by ALOS-2 SARinterferometryaN

4

Satoshi Fujiwara*, Hiroshi Yarai, Tomokazu Kobayashi, Yu Morishita, Takayuki Nakano,Basara Miyahara, Hiroyuki Nakai, Yuji Miura, Haruka Ueshiba, Yasuaki Kakiage and Hiroshi Une

5

11

Northwest ofOuter Rim

Earth, Planets and Space 2016, 68:160 doi:10.1186/s40623-016-0534-x
Received: 29 June 2016, Accepted: 6 September 2016, Published: 26 September 2016

Northeast of
Aso

Quaternary
Volcanoes
Arround
Suizenji Park
7

Abstract3

ShirakawaRiver

9Southwest ofOuter Rim
10

Futagawa Fault Zone

We constructed and analyzed the ground surface displacement associated with the 2016 Kumamoto earthquake sequence
using satellite radar interferometry images of the Advanced Land Observing Satellite 2. The radar interferogram generallyshows elastic deformation caused by the main earthquakes, but many other linear discontinuities showing displacement arealso found. Approximately 230 lineaments are identified, some of which coincidewith the positions of known active faults, such as the main earthquake faultsbbelonging to the Futagawa and Hinagu fault zones and other minor faults;
however, there are much fewer known active faults than lineaments. In each area,
the lineaments have a similar direction and displacement to each other; therefore,
they can be divided into several groups based on location and major features.
Since the direction of the lineaments coincides with that of known active faults ortheir conjugate faults, the cause of the lineaments must be related to the tectonic
stress field of this region. The lineaments are classified into the following two
categories: (1) main earthquake faults and their branched subfaults and (2)secondary faults that are not directly related to the main earthquake but whose
slip was probably triggered by the main earthquake or aftershocks.
8Linear surface ruptures detected by
InSAR

InSAR decorrelation zone along
the Futagawa fault zone

Known active faults (Nakata and
Imaizumi 2002; Research group for
active tectonics in Kyushu 1989)

Hinagu Fault
Zone

N

Linear surface ruptures detected by
InSAR

InSAR decorrelation zone along
the Futagawa fault zone

Apr. 16, 2016
Mw 7.0

Apr. 14, 2016
Mw 6.2

Keywords: 2016 Kumamoto earthquake sequence, ALOS-2, SAR interferometry,
Linear surface rupture

0 < depth < 5 km
5 < depth < 10 km

Figure 2*Corresponding author: Satoshi Fujiwara, fujiwara-s2vq@mlit.go.jp
6

Surface ruptures identified by fieldsurvey (Geological Survey of Japan
2016)

Epicenters from Apr. 14 to Apr. 23

Earth, Planets and Space

FULL PAPER

Open Access

Remote triggering of seismicity at Japanese volcanoes
following the 2016 M7.3 Kumamoto earthquake
Bogdan Enescu*, Kengo Shimojo, Anca Opris and Yuji Yagi
Earth, Planets and Space 2016, 68:165 doi:10.1186/s40623-016-0539-5
Received: 7 July 2016, Accepted: 27 September 2016, Published: 20 October 2016

Abstract

Velocity (mm/s)

The MJMA7.3 Kumamoto earthquake occurred on April 16, 2016, in the western part of Kyushu, at a depth of 12 km, on an
active strike-slip fault. Here, we report on a relatively widespread activation of small remote earthquakes, which occurred as
far as Hokkaido, detected by analyzing the continuous waveform data recorded at seismic stations all over Japan. Such
relatively widespread remote seismicity activation, following a large inland earthquake, has not been reported before forJapan. Our analysis demonstrates that the remote events were triggered dynamically, by the passage of the surface waves
from the Kumamoto earthquake. Most of the remotely triggered events in the Tohoku
Time from 01:25:00 (s)cand Hokkaido regions, as well as close to Izu Peninsula, occur at or close to volcanoes,
1 0.01 - 0.2 Hz
1662 km ANNH Vertical0which suggests that the excitation of crustal fluids, by the passage of Rayleigh waves,Rayleigh-1
played an important triggering role. Nevertheless, remote activation in other regions,1
ANNH Radial
0like Noto Peninsula, occurred away from volcanoes. The relatively large-amplitude Love
-1
Rayleighwaves, enhanced by a source directivity effect during the Kumamoto earthquake, may
1
ANNH Transverse0
have triggered seismicity on local active faults. The dynamic stresses in the areas where-1Love
remote activation has been observed range from several kPa to tens of kPa, the1653 km MEAK Vertical
2x10 10 - 30 Hz
0thresholds being lower than in previous dynamic triggering cases for Japan; this might-2x10
relate to a change in the crustal conditions following the 2011 M9.0 Tohoku-oki
400
500
600
700
800
earthquake, in particular at volcanoes in NE Japan.
NS
-4

-4

Keywords: Kumamoto earthquake, Remote triggering, Volcanoes, Active faults,

0.0004 (mm/s)

Dynamic stresses

UD

P

1s

S

Figure 3*Corresponding author: Bogdan Enescu, benescu@kugi.kyoto-u.ac.jp

FULL PAPER

Open Access

Preliminary estimation of high-frequency (4–20 Hz) energy
released from the 2016 Kumamoto, Japan, earthquake
sequence
Kaoru Sawazaki*, Hisashi Nakahara and Katsuhiko Shiomi
Earth, Planets and Space 2016, 68:183 doi:10.1186/s40623-016-0557-3
Received: 25 July 2016, Accepted: 29 October 2016, Published: 23 November 2016

Abstract
We estimate high-frequency (4–20 Hz) energy release due to the 2016 Kumamoto, Japan, earthquake sequence, within a time period
from April 14 to 26 through envelope inversion analysis applied to the Hi-net continuous seismograms. We especially focus on energy
releases after each of the April 14 MJMA6.5 and the April 16 MJMA7.3 earthquakes. The cumulative energy release from aftershocks of
the April 14 event reaches 60% of that from the April 14 event itself by the lapse time of 27 h (pre-April 16 period). On the other hand,the cumulative energy release from aftershocks of the April 16 event reaches only 11 and 13% of that from the April 16 event itself by
the lapse times of 27 h and 10 days (post-April 16 period), respectively. This discrepancy in the normalized cumulative energy release(NCER) indicates that the April 14 event was followed by much larger relative aftershock productivity than the April 16 event. Thus,
NCER would provide information that reflects relative aftershock
productivity and ongoing seismicity pattern after a large earthquake.
ab
pre Apr 16
We also find that the temporal decay of the energy release rate obeys
pre Apr 16
post Apr 16
post Apr 16
the power law. The exponent pE of the power-law decay is estimatedto be 1.7–2.1, which is much larger than the typical p value of the
Omori–Utsu law: slightly larger than 1. We propose a simple
relationship given by pE = βp/b, where p value, b value of the
Gutenberg–Richter law, and β value of the magnitude–energy
release relationship are combined.
1.0

Keywords: 2016 Kumamoto earthquake sequence, Aftershocks,

High-frequency energy release, Normalized cumulative
energy release*Corresponding author: Kaoru Sawazaki, sawa@bosai.go.jp
7normalized cumulative energy release

cumulative energy release (J)

3e+12

2e+12

1e+12

0
0.01

0.1

1

10

0.8

0.6

0.4

0.2

0.0
0.01

lapse time (day)0.11lapse time (day)Figure 9102016 Kumamoto Earthquake Sequence and Its Impact on Earthquake Science and Hazard Assessment

FULL PAPER

Open Access

Stochastic ground-motion simulations for the 2016
Kumamoto, Japan, earthquake
Long Zhang*, Guangqi Chen, Yanqiang Wu and Han Jiang
Earth, Planets and Space 2016, 68:184 doi:10.1186/s40623-016-0565-3
Received: 16 June 2016, Accepted: 5 November 2016, Published: 18 November 2016

Abstract
On April 15, 2016, Kumamoto, Japan, was struck by a large earthquake sequence, leading to severe casualty and building damage.
The stochastic finite-fault method based on a dynamic corner frequency has been applied to perform ground-motion simulations forthe 2016 Kumamoto earthquake. There are 53 high-quality KiK-net stations available in the Kyushu region, and we employed records
from all stations to determine region-specific source, path and site parameters. The calculated S-wave attenuation for the Kyushu
region beneath the volcanic and non-volcanic areas can be expressed in the form of Qs = (85.5 ± 1.5)f 0.68±0.01 and Qs = (120 ± 5)f 0.64±0.05,
respectively. The effects of lateral S-wave velocity and attenuation heterogeneities on the ground-motion simulations were
investigated. Site amplifications were estimated using the corrected cross-spectral ratios technique. Zero-distance kappa filter was
obtained to be the value of 0.0514 ± 0.0055 s, using the spectral decay method. The stress drop of the mainshock based on the USGS
slip model was estimated optimally to have a value of 64 bars. Our finite-fault model with optimized parameters was validated
through the good agreement of observations and simulations at all stations. The attenuation characteristics of the simulated peakground accelerations were also successfully captured by the ground-motion prediction equations. Finally, the ground motions at twodestructively damaged regions, Kumamoto Castle and Minami Aso village, were simulated. We conclude that the stochastic finitefault method with well-determined parameters can
KMMH12
reproduce the ground-motion characteristics of the 2016
Observed [NS]OTIH10Observed [NS]NGSH01
Observed [NS]217.35cm/s
44.54cm/s
38.98cm/s
Kumamoto earthquake in both the time and frequency
domains. This work is necessary for seismic hazard
Simulated
Simulated
Simulatedassessment and mitigation.

Keywords: Ground-motion simulations, Stochastic
finite-fault method, 2016 Kumamoto
earthquake

0

20

40
60
Time(sec)

2

2

2

206.19cm/s2

45.36cm/s2

42.14cm/s2

80

100

0

20

40
60
Time(sec)

80

100

0

20

40
60
Time(sec)

80

100

Figure 8*Corresponding author: Long Zhang, zhanglong98524@163.com

FULL PAPER

Open Access

Crustal deformation associated with the 2016 Kumamoto
Earthquake and its effect on the magma system of Aso volcano
Taku Ozawa*, Eisuke Fujita and Hideki Ueda
Earth, Planets and Space 2016, 68:186 doi:10.1186/s40623-016-0563-5
Received: 22 June 2016, Accepted: 5 November 2016, Published: 22 November 2016 a

Nakadake
Kusasenri

F2

N
F4

Abstract

Assumedmagma system

An MJMA6.5 earthquake (foreshock) and MJMA7.3 earthquake (mainshock) struck Kumamoto Prefecture on April 14, 2016, and April 16, 2016. To
evaluate the effect of crustal deformation due to the earthquake on the Aso magma system, we detected crustal deformation using InSAR and
GNSS. From InSAR analysis, we detected large crustal deformations along the Hinagu Fault, the Futagawa Fault, and the northeast extension of
the latter fault. It extended to more than 50 km, and the maximum slant-range change exceeded 1 m. Although the obtained crustal deformation
was approximately explained by the right-lateral strike-slip on the fault, its details could not be explained by such simple faulting. Additionally,
we found complex surface deformation west of the Aso caldera rim, suggesting that shallow fault slips occurred in many known and unknown
S-DIF (MPa)S-MIN (MPa)SMAX (MPa)faults associated with the earthquake. Most of the crustal deformation could be reasonably explainedby four rectangle
faults located along
the
0.5
0.4
3.5 0.45
2.5 -1.2
Futagawa Fault, in the northeast extension of the Futagawa Fault, alongside the Hinagu
Fault, and in the eastern part of the Futagawa Fault. The first three of faults have high dip
F3
F1
angles and right-lateral slip. The other was a fault with a low dip angle that branched fromb
the shallow depth of the fault along the Futagawa Fault. The normal-dip right-lateral slip
Kusasenri
1.0

was estimated for this segment. Based on the estimated fault model, we calculated thedisplacement and stress field around the Aso volcano by the finite-element method (FEM)
to evaluate the effects on the Aso magma system. In this calculation, we assumed aspherical soft medium located at a 6-km depth beneath the area south of the Kusasenri
region as the magma system and considered only static effects. The result shows complex
distributions of displacements and stresses, but we can notice the following significant
points. (1) The spherical magma system deformed to an ellipsoid, and the total volume
was slightly increased, less than 1%. (2) The differential stress around the upper portion of
the magma system was as large as 3.5 MPa. This is strong enough to open pre-existing
cracks and can cause the migration of magma.

Keywords: Kumamoto Earthquake, Aso volcano, crustal deformation, InSAR, GNSS,
fault model, effect on magma system

2.0

3.0

0.8

1.2

1.6

2.0

8

-0.4

0.0

0.4

N

Nakadake

F4
F2

Assumedmagma system

1.0

0.4

S-DIF (MPa)
2.0

3.0

0.8

SMAX (MPa)
1.2

1.6

2.0

3.5 0.45

2.4

2.5 -1.2

Figure 9*Corresponding author: Taku Ozawa, taku@bosai.go.jp

-0.8

2.4

-0.8

SMIN (MPa)
-0.4

0.0

0.4

0.5

Earth, Planets and Space

FULL PAPER

Open Access

Seismicity prior to the 2016 Kumamoto earthquakes
Kazuyoshi Z. Nanjo, Jun Izutsu, Yoshiaki Orihara, Nobuhiro Furuse, Shoho Togo, Hidetoshi Nitta,
Tomohiro Okada, Rika Tanaka, Masashi Kamogawa and Toshiyasu Nagao*
Earth, Planets and Space 2016, 68:187 doi:10.1186/s40623-016-0558-2
Received: 1 August 2016, Accepted: 29 October 2016, Published: 22 November 2016

Abstract
We report precursory seismic patterns prior to the 2016 Kumamoto earthquakes, as measured by four different methods
based on changes in seismicity that can be used for earthquake forecasting: the b-value method, two methods of seismic
quiescence evaluation, and an analysis of seismicity density in space and time. The spatial extent of precursory patterns differsfrom one method to the other and ranges from local scales (typically, asperity size) to regional scales (e.g., 2° × 3° around thesource zone). The earthquakes were preceded by periods of pronounced anomalies, which lasted in yearly scales (1.5 years), or
longer (>3 years). We demonstrate that a combination of multiple methods detected different signals prior to the Kumamoto
earthquakes. This indicates great potential to reduce the hazard at possible future sites of earthquakes relative to long-term
seismic hazard assessment. We also found that the seismic quiescence in aregional-scale area, detected by using the two methods of seismicaquiescence evaluation, was a common precursor to the 2016 Kumamoto
earthquakes and 2015 Off Satsuma Peninsula earthquake. The result
allows us to interpret both events as the onset that occurred at a section
along the tectonic line from the Okinawa Trough through the Beppu–
Shimabara graben.

Keywords: Crustal deformation, Earthquake, Physical property of rocks,

Time-series analysis, Probabilistic forecasting, Computational
seismology, Statistical seismology, 2016 Kumamoto
earthquakes, Critical phenomenon
Figure 2*Corresponding author: Toshiyasu Nagao, nagao@scc.u-tokai.ac.jp

FULL PAPER

Open AccessFirst result from the GEONET real-time analysis system(REGARD): the case of the 2016 Kumamoto earthquakes
Satoshi Kawamoto*, Yohei Hiyama, Yusaku Ohta and Takuya Nishimura
Earth, Planets and Space 2016, 68:190 doi:10.1186/s40623-016-0564-4
Received: 11 July 2016, Accepted: 5 November 2016, Published: 22 November 2016

Abstract
We present the initial results of rapid fault estimations for the 2016 Kumamoto earthquake on April 16 (Mj 7.3), and coseismic
displacements caused by the two large foreshocks that occurred on April 14 (Mj 6.5) and April 15 (Mj 6.4) from the GEONET
real-time analysis system (REGARD), which is based on a Global Navigation Satellite System (GNSS) kinematic positioning
technique. The real-time finite-fault estimate (Mw 6.85) was obtained within 1 min and converged to Mw 6.96 within 5 min of
the origin time of the mainshock (Mj 7.3). The finite-fault estimate shows right-lateral strike-slip fault along the Futagawa fault
segment, which is consistent with the finite-fault model inferred from post-processed GNSS and InSAR analysis. Furthermore,significant coseismic displacements were observed due to the April 14 and April 15 foreshocks at nearby sites, though theseearthquakes were smaller than the pre-assigned system threshold. Our results also demonstrate the potential for the GNSSbased earthquake early warning system for inland earthquakes.

Keywords: GNSS real-time positioning, Real-time
magnitude estimation, Rapid source
extent estimation, The 2016
Kumamoto earthquakesab
T=343 sec.

T=343 sec.
33°00'

33°00'

0703

0465

0701

32°45'

0703

0465
0704
32°45'

0466
1071

1169

Slip
Obs.
Cal.

0702

0704
0466

1071

2.5 m

1169

0.5 m

2.5 m

Slip
0.1 m

0702

Obs. Cal.
32°30'

32°30'

20 km

20 km
130°15'0701

130°30'130°45'131°00'131°15'131°30'130°15'Figure 4*Corresponding author: Satoshi Kawamoto, kawamoto-s96tf@mlit.go.jp
9130°30'130°45'131°00'131°15'131°30'2016 Kumamoto Earthquake Sequence and Its Impact on Earthquake Science and Hazard Assessment

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Open Access

Earthquake early warning for the 2016 Kumamoto earthquake:
performance evaluation of the current system and thenext-generation methods of the Japan Meteorological Agency
Yuki Kodera*, Jun Saitou, Naoki Hayashimoto, Shimpei Adachi, Masahiko Morimoto, Yuji Nishimae
and Mitsuyuki Hoshiba
Earth, Planets and Space 2016, 68:202 doi:10.1186/s40623-016-0567-1
Received: 13 July 2016, Accepted: 15 November 2016, Published: 1 December 2016

Abstract
The 2016 Kumamoto earthquake (Kumamoto earthquake sequence) is an extremely high-seismicity event that has been occurring across
Kumamoto and Oita Prefectures in Japan since April 14, 2016 (JST). The earthquake early warning system of the Japan Meteorological Agency(JMA) issued warnings for 19 events in the Kumamoto earthquake sequence from April 14 to 19, under some of the heaviest loading conditions
since the system began operating in 2007. We analyzed the system performance for cases where a warning was issued and/or strong motion was
actually observed. The results indicated that the system exhibited remarkable performance, especially for the most destructive earthquakes in
the Kumamoto earthquake sequence. In addition, the system did not miss or seriously under-predict strong motion of any large earthquake from
April 14 to 30. However, in four cases, the system issued over-predicted warnings due to theasimultaneous occurrence of small earthquakes within a short distance, which implies afundamental obstacle in trigger-data classifications based solely on arrival time. We also performed
simulations using the integrated particle filter (IPF) and propagation of local undamped motion(PLUM) methods, which JMA plans to implement to address over-prediction for multiple
simultaneous earthquakes and under-prediction for massive earthquakes with large rupture zones.
The simulation results of the IPF method indicated that the IPF method is highly effective at
minimizing over-prediction even for multiple simultaneous earthquakes within a short distance,since it adopts a trigger-data classification using velocity amplitude and hypocenter
determinations using not-yet-arrived data. The simulation results of the PLUM method
demonstrated that the PLUM method is capable of issuing warnings for destructive inland
earthquakes more rapidly than the current system owing to the use of additional seismometersthat can only be incorporated by this method.
Keywords: Earthquake early warning, 2016 Kumamoto earthquake, Integrated particle filter
Figure 4
method, Propagation of local undamped motion method*Corresponding author: Yuki Kodera, kodera@mri-jma.go.jp

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Open Access

Simultaneous estimation of the dip angles and slip distribution on
the faults of the 2016 Kumamoto earthquake througha weak nonlinear inversion of InSAR data
Yukitoshi Fukahata* and Manabu Hashimoto
Earth, Planets and Space 2016, 68:204 doi:10.1186/s40623-016-0580-4
Received: 1 August 2016, Accepted: 29 November 2016, Published: 19 December 2016

Abstract

0.5

1.5

2.5

5

4

Depth [km]4.5

2

At the 2016 Kumamoto earthquake, surface ruptures were observed not only along the Futagawa fault, where main ruptures
occurred, but also along the Hinagu fault. To estimate the slip distribution on these faults, we extend a method of nonlinear inversion
analysis (Fukahata and Wright in Geophys J Int 173:353-364, 2008) to a two-fault system. With the method of Fukahata and Wright(2008), we can simultaneously determine the optimal dip angle of a fault and the slip distribution on it, based on Akaike’s Bayesian
information criterion by regarding the dip angle as an hyperparameter. By inverting the InSAR data with the developed method, we
obtain the dip angles of the Futagawa and Hinagu faults as 61° ± 6° and 74° ± 12°, respectively. The slip on the Futagawa fault ismainly strike slip. The largest slip on it is over 5 m around the center of the model fault (130.9° in longitude) with a significant normal
slip component. The slip on the Futagawa fault quickly decreases to zero beyond the intersection with the Hinagu fault. On the other
hand, the slip has a local peak just inside Aso caldera, which would be
HinaguFutagawa
a cause of severe damage in this area. A relatively larger reverse fault
NE
SW NNE
SSW-20 -16 -12
-8
-4
0
4
8
12
16
20 [km] -8-40
48
slip component on a deeper part around the intersection with Aso100caldera suggests that something complicated happened there. The
44slip on the Hinagu fault is almost a pure strike slip with a peak of
2
3.5
8
8
3
about 2.4 m. The developed method is useful in clarifying the slip
1.5
1
12
12
distribution, when a complicated rupture like the Kumamoto
0.5
slip [m]earthquake happens in a remote area.
16
16
0

0

8

8

Keywords: The 2016 Kumamoto earthquake, Inversion analysis,

16

InSAR, ABIC, Weak nonlinear inversion

0.5

0.5
error
(strike slip)0.5
[m]

-16

-8

0

error (dip slip) [m]
8

16

-16

-8

0

8

Figure 4*Corresponding author: Yukitoshi Fukahata, fukahata@rcep.dpri.kyoto-u.ac.jp
10

16

-8

0

8

-8

0

8

16

Earth, Planets and Space

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Open Access

Applicability of source scaling relations for crustal earthquakes
to estimation of the ground motions of the 2016 Kumamoto
earthquake
Kojiro Irikura*, Ken Miyakoshi, Katsuhiro Kamae, Kunikazu Yoshida, Kazuhiro Somei,
Susumu Kurahashi and Hiroe Miyake
Earth, Planets and Space 2017, 69:10 doi:10.1186/s40623-016-0586-y
Received: 11 August 2016, Accepted: 7 December 2016, Published: 3 January 2017

Abstract
A two-stage scaling relationship of the source parameters for crustal earthquakes in Japan has previously been constructed, in which source parameters obtained fromthe results of waveform inversion of strong motion data are combined with parameters estimated based on geological and geomorphological surveys. A three-stage
scaling relationship was subsequently developed to extend scaling to crustal earthquakes with magnitudes greater than Mw 7.4. The effectiveness of these scaling
relationships was then examined based on the results of waveform inversion of 18 recent crustal earthquakes (Mw5.4–6.9) that occurred in Japan since the 1995
Hyogo-ken Nanbu earthquake. The 2016 Kumamoto earthquake, with Mw 7.0, was one of the largest earthquakes to occur since dense and accurate strong motion
observation networks, such as K-NET and KiK-net, were deployed after the 1995 Hyogo-ken Nanbu earthquake. We examined the applicability of the scaling
relationships of the source parameters of crustal earthquakes in Japan to the 2016 Kumamoto earthquake. The rupture area and asperity area were determined basedon slip distributions obtained from waveform inversion of the 2016 Kumamoto earthquake observations. We foundthat the relationship between the rupture area and the seismic moment for the 2016 Kumamoto earthquake follows
the second-stage scaling within one standard deviation (σ = 0.14). The ratio of the asperity area to the rupture area
for the 2016 Kumamoto earthquake is nearly the same as ratios previously obtained for crustal earthquakes.
Furthermore, we simulated the ground motions of this earthquake using a characterized source model consisting ofstrong motion generation areas (SMGAs) based on the empirical Green’s function (EGF) method. The locations andareas of the SMGAs were determined through comparison between the synthetic ground motions and observed
motions. The sizes of the SMGAs were nearly coincident with the asperities with large slip. The synthetic groundmotions obtained using the EGF method agree well with the observed motions in terms of acceleration, velocity, anddisplacement within the frequency range of 0.3–10 Hz. These findings indicate that the 2016 Kumamoto earthquake
is a standard event that follows the scaling relationship of crustal earthquakes in Japan.

Keywords: Scaling relationship, Crustal earthquake, Source model, Strong ground motion prediction,
Characterized source model, Strong motion generation area

Figure 9*Corresponding author: Kojiro Irikura, irikura@geor.or.jp

FULL PAPER

Open Access

Anomalous decrease in relatively large shocks and increase in the pand b values preceding the April 16, 2016, M7.3 earthquake
in Kumamoto, JapanK. Z. Nanjo* and A. Yoshida
Earth, Planets and Space 2017, 69:13 /doi:10.1186/s40623-017-0598-2
Received: 1 August 2016, Accepted: 6 January 2017, Published: 14 January 2017

Abstract
The 2016 Kumamoto earthquakes in Kyushu, Japan, started with a magnitude (M) 6.5 quake on April 14 on the Hinagu fault zone (FZ),
followed by active seismicity including an M6.4 quake. Eventually, an M7.3 quake occurred on April 16 on the Futagawa FZ. We investigated if any
sign indicative of the M7.3 quake could be found in the space–time changes in seismicity after the M6.5 quake. As a quality control, we
determined in advance the threshold magnitude, above which all earthquakes are completely recorded. We then showed that the occurrence
rate of relatively large (M ≥ 3) earthquakes significantly decreased 1 day before the M7.3 quake. Significance of this decrease was evaluated by
one standard deviation of sampled changes in the rate of occurrence. We next confirmed that seismicity with M ≥ 3 was well modeled by the
Omori–Utsu law with p ~ 1.5 ± 0.3, which indicates that the temporal decay of seismicity was significantly faster than a typical decay with p = 1.
The larger p value was obtained when we used data of the longer time period in the analysis. This significance was confirmed by a bootstrapping
approach. Our detailed analysis shows that the large p value was caused by the rapid decay of the seismicity in the northern area around the
Futagawa FZ. Application of the slope (the b value) of the Gutenberg–Richter frequency–
magnitude distribution to the spatiotemporal change in the seismicity revealed that the bbavalue in the northern area increased significantly, the increase being Δb = 0.3–0.5.
Significance was verified by a statistical test of Δb and a test using bootstrapping errors.
Based on our findings, combined with the results obtained by a stress inversion analysis
performed by the National Research Institute for Earth Science and Disaster Resilience, wesuggested that stress near the Futagawa FZ had reduced just prior to the occurrence of theM7.3 quake. We proposed, with some other observations, that a reduction in stress might
have been induced by growth of the slow slips on the Futagawa FZ.

Keywords: The 2016 Kumamoto earthquakes, p value of the Omori–Utsu law, b value of

the Gutenberg–Richter law, Hinagu fault zone, Futagawa fault zone, Slow slip

*Corresponding author: K. Z. Nanjo, nanjo@u-shizuoka-ken.ac.jp
11

Figure 42016 Kumamoto Earthquake Sequence and Its Impact on Earthquake Science and Hazard Assessment

FULL PAPER

Open Access

Rupture process of the main shock of the 2016 Kumamoto earthquakewith special reference to damaging ground motions:waveform inversion with empirical Green’s functions
Atsushi Nozu* and Yosuke Nagasaka
Earth, Planets and Space 2017, 69:22 doi:10.1186/s40623-017-0609-3
Received: 1 August 2016, Accepted: 24 January 2017, Published: 31 January 2017

Abstract
In this study, the rupture process of the main shock of the Kumamoto earthquake, particularly the generation of strong ground motions in thefrequency range relevant to structural damage, was investigated based on the inversion of strong ground motions. Strong-motion records inthe near-source region were mainly utilized because the authors were interested in the generation mechanism of damaging ground motionsin the near-source region. Empirical Green’s functions (EGFs) were applied to avoid uncertainty in the subsurface structure model. Four casesof inversions with different combinations of small events were used to investigate the dependence of the inversion results on the selection of
the small events. It was found that the dependence of the final slip distribution and peak slip velocity distribution on the selection of the EGF
events is small. The results clearly indicate that a region of significantly large slip and slip velocity existed approximately 15 km northeast of
the hypocenter. However, no “asperity” was observed between the hypocenter and Mashiki. Thus,
it is not appropriate to conclude that the large-amplitude pulse-like ground motion in Mashiki was
generated by the forward-directivity effect associated with the rupture of an asperity. As far as thesource effect is concerned, the ground motion in Mashiki cannot be interpreted as the worst casescenario. On the other hand, the rupture of the “asperity” 15 km northeast of the hypocenter
should have caused significantly large ground motions in regions close to the asperity. The
significant damage of highway bridges in the region can potentially be attributed to the rupture of
the asperity. The result of this study was compared with an inversion result obtained fromnumerical Green’s functions for a layered half-space. The two results share the main features inspite of the difference of the Green’s functions and stations used. Therefore, it can be concluded
that these two source models capture the main features of the rupture process of the earthquake.

Keywords: The 2016 Kumamoto earthquake, Rupture process, Strong ground motion,

Waveform inversion, Empirical Green’s function, Asperity, Forward directivity

Figure 11*Corresponding author: Atsushi Nozu, nozu@pari.go.jp

FULL PAPER

Open Access

Strong ground motion simulation of the 2016 Kumamoto
earthquake of April 16 using multiple point sources
Yosuke Nagasaka* and Atsushi Nozu
Earth, Planets and Space 2017, 69:25 doi:10.1186/s40623-017-0612-8
Received: 1 August 2016, Accepted: 24 January 2017, Published: 2 February 2017

Abstract
The pseudo point-source model approximates the rupture process on faults with multiple point sources for simulating strong ground motions. A
simulation with this point-source model is conducted by combining a simple source spectrum following the omega-square model with a path
spectrum, an empirical site amplification factor, and phase characteristics. Realistic waveforms can be synthesized using the empirical site
amplification factor and phase models even though the source model is simple. The Kumamoto earthquake occurred on April 16, 2016, with MJMA
7.3. Many strong motions were recorded at stations around the source region. Some records were considered to be affected by the rupture
directivity effect. This earthquake was suitable for investigating the applicability of the pseudo point-source model, the current version of which
does not consider the rupture directivity effect. Three subevents (point sources) were located on
the fault plane, and the parameters of the simulation were determined. The simulated results
were compared with the observed records at K-NET and KiK-net stations. It was found that thesynthetic Fourier spectra and velocity waveforms generally explained the characteristics of theobserved records, except for underestimation in the low frequency range. Troughs in theobserved Fourier spectra were also well reproduced by placing multiple subevents near the
hypocenter. The underestimation is presumably due to the following two reasons. The first is thatthe pseudo point-source model targets subevents that generate strong ground motions anddoes not consider the shallow large slip. The second reason is that the current version of thepseudo point-source model does not consider the rupture directivity effect. Consequently,
strong pulses were not reproduced enough at stations northeast of Subevent 3 such as KMM004,
where the effect of rupture directivity was significant, while the amplitude was well reproduced
at most of the other stations. This result indicates the necessity for improving the pseudo
point-source model, by introducing azimuth-dependent corner frequency for example, so that it
can incorporate the effect of rupture directivity.

Keywords: 2016 Kumamoto earthquake, Strong ground motion simulation, Pseudo
point-source model, Rupture directivity

Figure 1*Corresponding author: Yosuke Nagasaka, nagasaka-y@pari.go.jp
12Earth, Planets and Space

FULL PAPER

Open Access

Data completeness of the Kumamoto earthquake sequence in
the JMA catalog and its influence on the estimation of
the ETAS parameters
Jiancang Zhuang*, Yosihiko Ogata and Ting Wang
Earth, Planets and Space 2017, 69:36 doi:10.1186/s40623-017-0614-6
Received: 5 November 2016, Accepted: 31 January 2017, Published: 27 February 2017

Abstract

0.8
0.6
0.4
0.0

0.2

5
4
3
2

imputation, Relative quiescence, Short-term aftershock
missing1

Keywords: Kumamoto earthquake, ETAS model, Missing data

Magnitudes

6

Empirical magnitudes

7

1.0This study investigates the missing data problem in the Japan Meteorological Agency catalog of the Kumamoto aftershock
sequence, which occurred since April 15, 2016, in Japan. Based on the assumption that earthquake magnitudes are
independent of their occurrence times, we replenish the short-term missing data of small earthquakes by using a bi-scaletransformation and study their influence on the maximum likelihood estimate (MLE) of the epidemic-type aftershock
sequences (ETAS) parameters by comparing the analysis results from the original and the replenished datasets. The results
show that the MLEs of the ETAS parameters vary when this model is fitted to the recorded catalog with different cutoff
magnitudes, while those MLEs remain stable for the replenished dataset. Further analysis shows that the seismicity becomes
quiescent after the occurrence of the second major shock, which
can be regarded as a precursory phenomenon of the occurrence ofbathe subsequent MJ7.3 mainshock. This relative quiescence isdemonstrated more clearly by the analysis of the replenisheddataset.

0

5

10

15

20

0.0

0.2

0.4

0.6

0.8

1.0

Empirical times

Times

Figure 3*Corresponding author: Jiancang Zhuang, zhuangjc@ism.ac.jp

FULL PAPER

Open Access

Effects of the 2016 Kumamoto earthquakes on the Aso
volcanic edifice
Yasuhisa Tajima*, Toshiaki Hasenaka and Masayuki Torii
Earth, Planets and Space 2017, 69:63 doi:10.1186/s40623-017-0646-y
Received: 9 August 2016, Accepted: 14 April 2017, Published: 2 May 2017

Abstract
Large earthquakes occurred in the central part of Kumamoto Prefecture on April 14–16, 2016, causing severe damage to thenorthern segment of the Hinagu faults and the eastern segment of the Futagawa faults. Earthquake surface rupturesappeared along these faults and on the Aso volcanic edifice, which in turn generated landslides. We conducted landformchange analysis of the central cones of Aso volcano by using satellite and aerial photographs. First, we categorized the
topographical changes as surface scarps, arc-shaped cracks, and linear cracks. Field survey indicated that landslides caused
the scarps and arc-shaped cracks, whereas faulting caused the linear
cracks. We discovered a surface rupture concentration zone (RCZ)
formed three ruptures bands with many surface ruptures and landslides
extending from the west foot to the center of the Aso volcanic edifice.
The magmatic volcanic vents that formed during the past 10,000 years
are located along the north margin of the RCZ. Moreover, thedistribution and dip of the core of rupture concentration zone
correspond with the Nakadake craters. We conclude that a strong
relationship exists between the volcanic vents and fault structures in
the central cones of Aso volcano.

Keywords: Kumamoto earthquake, Surface rupture, Central cones of
Aso volcano, Vent location, Magma plumbing system

Figure 10*Corresponding author: Yasuhisa Tajima, yasuhisa79@gmail.com
13

2016 Kumamoto Earthquake Sequence and Its Impact on Earthquake Science and Hazard Assessment

LETTER

Open AccessDip distribution of Oita–Kumamoto Tectonic Line located in central
Kyushu, Japan, estimated by eigenvectors of gravity
gradient tensor
Shigekazu Kusumoto
Earth, Planets and Space 2016, 68:153 doi:10.1186/s40623-016-0529-7
Received: 17 June 2016, Accepted: 27 August 2016, Published: 6 September 2016

Abstract
We estimated the dip distribution of Oita–Kumamoto Tectonic Line located in central Kyushu, Japan, by using the dip of the
maximum eigenvector of the gravity gradient tensor. A series of earthquakes in Kumamoto and Oita beginning on 14 April
2016 occurred along this tectonic line, the largest of which was M = 7.3. Because a gravity gradiometry survey has not been
conducted in the study area, we calculated the gravity gradient tensor from the Bouguer gravity anomaly and employed it to
the analysis. The general dip distribution of the Oita–Kumamoto Tectonic Line was found to be about 65° and tends to behigher towards its eastern end. In addition, we estimated the dip aroundthe largest earthquake to be about 60° from the gravity gradient tensor.
This result agrees with the dip of the earthquake source fault obtained by
Global Navigation Satellite System data analysis.

Keywords: Dip distribution, Oita–Kumamoto Tectonic Line, Kumamoto
earthquake, Gravity gradient tensor, Eigenvector, Futagawa
fault system, Aso caldera, Hohi volcanic zone

Figure 5

Corresponding author: Shigekazu Kusumoto, kusu@sci.u-toyama.ac.jp

LETTER

Open Access

Source rupture processes of the 2016 Kumamoto, Japan,
earthquakes estimated from strong-motion waveforms
Hisahiko Kubo*, Wataru Suzuki, Shin Aoi and Haruko Sekiguchi
Earth, Planets and Space 2016, 68:161 doi:10.1186/s40623-016-0536-8
Received: 1 July 2016, Accepted: 19 September 2016, Published: 3 October 2016

Abstract
The detailed source rupture process of the M 7.3 event (April 16, 2016, 01:25, JST) of the 2016 Kumamoto, Japan, earthquakes
was derived from strong-motion waveforms using multiple-time-window linear waveform inversion. Based on theobservations of surface ruptures, the spatial distribution of aftershocks, and the geodetic data, a realistic curved fault model
was developed for source-process analysis of this event. The seismic moment and maximum slip were estimated as 5.5 × 1019
Nm (Mw 7.1) and 3.8 m, respectively. The source model of the M 7.3 event had two significant ruptures. One rupture
propagated toward the northeastern shallow region at 4 s after rupture initiation and continued with large slips toapproximately 16 s. This rupture caused a large slip region 10–30 km northeast of the hypocenter that reached the caldera of
Mt. Aso. Another rupture propagated toward the surface from the hypocenter at 2–6 s and then propagated toward thenortheast along the near surface at 6–10 s. A comparison with the result of using a single fault plane model demonstrated
that the use of the curved fault model led to improved waveform fit at thestations south of the fault. The source process of the M 6.5 event (April 14,b
2016, 21:26, JST) was also estimated. In the source model obtained for the
M 6.5 event, the seismic moment was 1.7 × 1018 Nm (Mw 6.1), and therupture with large slips propagated from the hypocenter to the surface
along the north-northeast direction at 1–6 s. The results in this study are
consistent with observations of the surface ruptures.

Keywords: The 2016 Kumamoto earthquakes, Source rupture process,
Strong-motion waveforms

Figure 4*Corresponding author: Hisahiko Kubo, hkubo@bosai.go.jp
14Earth, Planets and SpaceLETTER

Open AccessContinuity, segmentation and faulting type of active fault zones ofthe 2016 Kumamoto earthquake inferred from analyses ofa gravity gradient tensor
Nayuta Matsumoto*, Yoshihiro Hiramatsu and Akihiro Sawada
Earth, Planets and Space 2016, 68:167 doi:10.1186/s40623-016-0541-y
Received: 29 July 2016, Accepted: 6 October 2016, Published: 21 October 2016

Abstract
We analyze Bouguer anomalies in/around the focal region of the 2016 Kumamoto earthquake to examine features, such ascontinuity, segmentation and faulting type, of the active fault zones related to the earthquake. Several derivatives andstructural parameters calculated from a gravity gradient tensor are applied to highlight the features. First horizontal and verticalderivatives, as well as a normalized total horizontal derivative, characterize well the continuous subsurface fault structure along
the Futagawa fault zone. On the other hand, the Hinagu fault zone is not clearly
130.5˚
131.0˚detected by these derivatives, especially in the case of the Takano-Shirahata segment,
33.0˚
suggesting a difference of cumulative vertical displacement between the two fault−10
zones. The normalized total horizontal derivative and the dimensionality index
indicate a discontinuity of the subsurface structure of the Hinagu fault zone, that is, asegment boundary between the Takano-Shirahata and the Hinagu segments. The
aftershock distribution does not extend beyond this segment boundary. In other0
words, this segment boundary controls the southern end of the rupture area of theforeshock. We also recognize normal fault structures dipping to the northwest in some 32.5˚
areas of the fault zones from estimations of dip angles.

Keywords: First horizontal derivative, First vertical derivative, Normalized total
horizontal derivative, Dip angle, Dimensionality index, Segment
boundary, Aftershock distribution

−40
0
40
Bouguer Anomaly [mGal]

0

10

km
20

Figure 3*Corresponding author: Nayuta Matsumoto, n.matsumoto@stu.kanazawa-u.ac.jp

LETTER

Open Access

Fault source model for the 2016 Kumamoto earthquake sequencebased on ALOS-2/PALSAR-2 pixel-offset data: evidence fordynamic slip partitioning
Yuji Himematsu* and Masato Furuya
Earth, Planets and Space 2016, 68:169 doi:10.1186/s40623-016-0545-7
Received: 21 July 2016, Accepted: 8 October 2016, Published: 22 October 2016

Abstract
Series of earthquakes including three Mw > 6 earthquakes occurred in Kumamoto prefecture in the middle of the Kyushu
island, Japan. In order to reveal the associated crustal deformation signals, we applied offset tracking technique to ALOS-2/
PALSAR-2 data covering three Mw > 6 earthquakes and derived the 3D displacements around the epicenters. We could identify
three NE–SW trending displacement discontinuities in the 3D displacements that were consistent with the surface location of
Futagawa and Hinagu fault system. We set three-segment fault model whose positions matched the displacement
discontinuities, and estimated the slip distributions on each segment from the observed pixel-offset data. Whereas rightlateral slip was dominant in the shallower depth of the larger segments, normal fault slip was more significant at a greater
depth of the other segment. The inferred configuration and slip distribution of each segment suggest that slip partitioning
under oblique extension stress regime took place during the 2016
Kumamoto earthquake sequence. Moreover, given the consistent focal
mechanisms derived from both the slip distribution model andseismology, the significant non-double couple components in the focal
mechanism of the main shock are due to simultaneous ruptures of bothstrike-slip and normal faulting at the distinct segments.

Keywords: ALOS-2/PALSAR-2, Offset tracking, Crustal deformation,

Triangular dislocation element, Slip partitioning, Non-double
couple component*Corresponding author: Yuji Himematsu, hime.matsu@frontier.hokudai.ac.jp
15

Figure 3

2016 Kumamoto Earthquake Sequence and Its Impact on Earthquake Science and Hazard Assessment

LETTER

Open Access

Surface fault ruptures associated with the 14 April foreshock (Mj 6.5)of the 2016 Kumamoto earthquake sequence, southwest
Japan
Nobuhiko Sugito*, Hideaki Goto, Yasuhiro Kumahara, Hiroyuki Tsutsumi, Takashi Nakata,
Kyoko Kagohara, Nobuhisa Matsuta and Haruka Yoshida
Earth, Planets and Space 2016, 68:170 doi:10.1186/s40623-016-0547-5
Received: 30 July 2016, Accepted: 13 October 2016, Published: 3 November 2016

Abstract
The 2016 Kumamoto earthquake sequence was a rare event worldwide in that the surface ruptures associated with the largest
foreshock (Mj 6.5) of 21:26 (JST), 14 April ruptured again during the mainshock (Mj 7.3) of 01:25 (JST), 16 April. The 14 April Mj 6.5
earthquake produced 6-km-long surface ruptures along the central portion of the Futagawa–
Hinagu fault zone (FHFZ). The mainshock produced 31-km-long surface ruptures along the central
to northeastern part of the FHFZ. Field observations and eyewitness accounts documented thatthe offsets of the ruptures associated with the 14 April foreshock became larger after the 16 Aprilmainshock, suggesting that the same portion of the fault ruptured to the surface twice in the
Kumamoto earthquake sequence. The 6-km-long surface ruptures associated with the largest
foreshock are located near a geometric bend of the FHFZ characterized by ~50° change in strike.
The epicenter of the mainshock is also located near the bend. These observations imply that the
Kumamoto earthquake sequence was initiated due to a stress concentration on the bend of the
FHFZ, and the mainshock was initiated approximately at the same place about 28 h after thelargest foreshock. This foreshock/mainshock sequence of the Kumamoto earthquake is not
successive events on the adjacent different fault zones, because the 6-km-long surface rupturesof the largest foreshock are part of the 31-km-long surface ruptures of the mainshock.

Keywords: 2016 Kumamoto earthquake sequence, Largest foreshock, Mainshock, Futagawa–
Hinagu fault zone, Geometric bend, Surface ruptures

Figure 1*Corresponding author: Nobuhiko Sugito, nsugito@hosei.ac.jp

LETTER

Open Access

Earthquakes in Oita triggered by the 2016 M7.3 Kumamoto
earthquake
Shingo Yoshida
Earth, Planets and Space 2016, 68:176 doi:10.1186/s40623-016-0552-8
Received: 31 July 2016, Accepted: 26 October 2016, Published: 14 November 2016

Abstract
During the passage of the seismic waves from the M7.3 Kumamoto, Kyushu, earthquake on April 16, 2016, a M5.7 [semiofficial
value estimated by the Japan Meteorological Agency (JMA)] event occurred in the central part of Oita prefecture,
approximately 80 km far away from the mainshock. Although there have been a number of reports that M < 5 earthquakes
were remotely triggered during the passage of seismic waves from mainshocks, there has been no evidence for M > 5
triggered events. In this paper, we firstly confirm that this event is a M6-class event by re-estimating the magnitude using thestrong-motion records of K-NET and KiK-net, and crustal deformation data at the Yufuin station observed by the Geospatial
Information Authority of Japan. Next, by investigating the aftershocks of 45 mainshocks which occurred over the past 20 years
based on the JMA earthquake catalog (JMAEC), we found that the delay time of the 2016 M5.7 event in Oita was the shortest.
Therefore, the M5.7 event could be regarded as an exceptional M > 5 event that was triggered by passing seismic waves,
unlike the usual triggered events and aftershocks. Moreover, a search of the JMAEC shows that in the 2016 Oita aftershockarea, swarm earthquake activity was low over the past 30 years compared with neighboring areas. We also found that in thepast, probably or possibly triggered events frequently occurred in the 2016 Oita aftershock area. The Oita area readily
responds to remote triggering because of high geothermal activity and young volcanism in the area. The M5.7 Oita event was
triggered by passing seismic waves, probably because
large dynamic stress change was generated by themainshock at a short distance and because the Oita area
was already loaded to a critical stress state without arecent energy release as suggested by the past low
swarm activity.

Keywords: Dynamic earthquake triggering, Aftershocks
in Oita, The 2016 Kumamoto earthquake

Figure 2

Corresponding author: Shingo Yoshida, shingo@eri.u-tokyo.ac.jp
16Earth, Planets and SpaceLETTER

Open Access

Crustal deformation model of the Beppu−Shimabara graben area,
central Kyushu, Japan, based on inversion of
three-component GNSS data in 2000–2010
Kazuma Mochizuki and Yuta Mitsui*
Earth, Planets and Space 2016, 68:177 doi:10.1186/s40623-016-0550-x
Received: 26 July 2016, Accepted: 18 October 2016, Published: 11 November 2016

Abstract
The 2016 Kumamoto earthquakes, including an Mw-7 right-lateral earthquake on April 15 (UTC), occurred along faults within
the Beppu−Shimabara graben in central Kyushu, Japan. Previous studies showed that the graben area was underheterogeneous stress conditions with north–south T-axes and spreading in a north–south direction. Here, we construct a
detailed crustal deformation model using three-component Global Navigation Satellite System data in 2000–2010 andconsidering the distribution of geological fault traces in this area. Our inversion analysis suggests that the strain accumulation
rate for the right-lateral seismic slip segment (corresponding to the Futagawa fault), where the largest of the 2016 Kumamoto
earthquakes ruptured, was several times smaller than the othersegments in the Beppu−Shimabara graben. Furthermore, we
observe distinct subsidence along the Beppu−Shimabara
graben. Our base model attributes the subsidence to deflationof magma reservoirs beneath volcanoes, but the observed
vertical velocities are poorly fit. In order to improve the fitting
results for the vertical deformation, we need moresophisticated volcano-deformation model (such as a sill-likedeformation source for Mt. Aso) or graben model.

Keywords: The 2016 Kumamoto earthquakes, Kyushu, GNSS,
Crustal deformation, Beppu−Shimabara graben,
Volcano, Block-fault model, Nucleation

Figure 6*Corresponding author: Yuta Mitsui, mit@shizuoka.ac.jp

LETTER

Open Access

The 2016 Kumamoto–Oita earthquake sequence: aftershock
seismicity gap and dynamic triggering in volcanic areas
Takahiko Uchide*, Haruo Horikawa, Misato Nakai, Reiken Matsushita, Norio Shigematsu,
Ryosuke Ando and Kazutoshi Imanishi
Earth, Planets and Space 2016, 68:180 doi:10.1186/s40623-016-0556-4
Received: 22 July 2016, Accepted: 29 October 2016, Published: 15 November 2016

Abstract

Depth [km]Depth [km]The 2016 Kumamoto–Oita earthquake sequence involving three large events (Mw ≥ 6) in the central Kyushu Island, southwest
Japan, activated seismicities in two volcanic areas with unusual and puzzling spatial gaps after the largest earthquake (Mw 7.0)of April 16, 2016. We attempt to reveal the seismic process during the sequence by following seismological data analyses. Our
hypocenter relocation result implies that the large events ruptured different faults of
130.6˚E
130.8˚E
131.0˚E
131.2˚Eb −10 −5 0 5 −10 −a complex fault system. A slip inversion analysis of the largest event indicates a large
0
33.2˚NB
CC’a
5
slip in the seismicity gap (Aso gap) in the caldera of Mt. Aso, which probably released
X’
20 km
5
10
2
accumulated stress and resulted in little aftershock production. We identified that the20
Depth [km]1515
0 4 8 12 16 20
largest event dynamically triggered a mid-M6 event at Yufuin (80 km northeast of the10
33.0˚N20
50epicenter), which is consistent with existence of the 20-km long zone where
0
F
F’
E
−5
0
Mt.
Aso
5
A −1
seismicity was activated and surface offset was observed. These findings will help us
B
10
X C
A’
study the contribution of the identified complexity in fault geometries and theD
32.8˚N
B’
E
15
Slip Model
C’F
geotherm in the volcanic areas to the revealed seismic process and consequently
D’
G
20
E’
H
F’
0
improve our understanding of the seismo-volcano tectonics.
I
G
I
I’
H
Depth [km]

H’ ’
I’

20 km

32.4˚N
0*Corresponding author: Takahiko Uchide, t.uchide@aist.go.jp
17

Depth [km]

c

0

2

4

6

Slip [m]

5

10
15
20
−10 −5 0

Dista

Figure 3

X

X’

0

5

5

10

10

15
20
−25

15

Aso Gap
−20

−15

−10

−5

0

5

Distance [km]5 −10 −

10

15

20

25

20

Depth [km]32.6˚N2016 Kumamoto Earthquake Sequence and Its Impact on Earthquake Science and Hazard Assessment

LETTER

Open Access

Characteristics of the surface ruptures associated with the 2016
Kumamoto earthquake sequence, central Kyushu, Japan
Yoshiki Shirahama*, Masayuki Yoshimi, Yasuo Awata, Tadashi Maruyama, Takashi Azuma,
Yukari Miyashita, Hiroshi Mori, Kazutoshi Imanishi, Naoto Takeda, Tadafumi Ochi, Makoto Otsubo,
Daisuke Asahina and Ayumu Miyakawa
Earth, Planets and Space 2016, 68:191 doi:10.1186/s40623-016-0559-1
Received: 31 July 2016, Accepted: 29 October 2016, Published: 24 November 2016

Abstract
The 2016 Kumamoto earthquake sequence started with a MJ (Japan Meteorological Agency magnitude) 6.5 event on April 14,
and culminated in a MJ 7.3 event on April 16. Associated with the sequence, approximately 34-km-long surface rupturesappeared along the eastern part of the Futagawa fault zone and the northernmost part of the Hinagu fault zone. We carried out
an urgent field investigation soon after the earthquake to map the extent and displacement of surface ruptures with thefollowing results. (1) The rupture zone generally consisted of a series of left-stepping en echelon arrays of discontinuous fault
traces of various lengths. (2) Slip exceeding 100 cm occurred on previously unrecognized fault traces in the alluvial lowland of theKiyama plain and on the western rim of the Aso volcano caldera.
a(3) Large slip with maximum dextral slip of 220 cm wasmeasured throughout the central section of the rupture zone
along the Futagawa segment, and the slip gradually decreased
bilaterally on the adjoining northeastern and southwesternsections. (4) The surface rupture mostly occurred along fault
traces mapped in previous active fault investigations. (5) Mostof the surface ruptures were produced by the mainshock, andsignificant postseismic slip occurred after the mainshock.

Keywords: 2016 Kumamoto earthquake surface rupture,
Futagawa fault zone, Hinagu fault zone, Slip
distribution, Urgent field investigation

Figure 6*Corresponding author: Yoshiki Shirahama, y.shirahama@aist.go.jp

LETTER

Open Access

Near-fault deformation and Dc’’ during the 2016 Mw7.1
Kumamoto earthquake
Eiichi Fukuyama* and Wataru Suzuki
Earth, Planets and Space 2016, 68:194 doi:10.1186/s40623-016-0570-6
Received: 29 June 2016, Accepted: 17 November 2016, Published: 29 November 2016

Abstract

Fault Parallel Velocity [m/s]

Keywords: Near-fault displacement, Slip-weakening distance, Strike-slip fault

Fault Parallel Displacement [m]An Mw7.1 Kumamoto earthquake occurred at 01:25:05 on April 16, 2016 (JST). The earthquake involved a rupture at a shallow
depth along a strike-slip fault with surface breaks. Near-fault ground motion records, especially those of a strike-slip
earthquake, can provide us with direct information on the earthquake source process. During the earthquake, near-fault
seismograms were obtained at KMMH16 station located about 500 m off the fault. The ground displacements were wellrecovered from the double numerical integration of accelerograms at KMMH16 both on the surface and at the bottom of the
252-m-deep borehole. Fault-parallel static displacement was estimated to be about 1.1 m from the acceleration waveforms.
The Dc’’ value, which is defined as double the fault-parallel displacement at peak velocity time, was proposed as a proxy of theslip-weakening distance. Using both the velocity and displacement fault-parallel
2016/04/16 01:25 @KMMH16
waveforms, the Dc’’ value was estimated at about 1 m. This value was between 30
0
and 50% of the total slip on the fault, which is consistent with previous
observations.
0Dc”=1m

-1

0

-0.5
5

Time [s]Figure 4*Corresponding author: Eiichi Fukuyama, fuku@bosai.go.jp
18

10

15

Earth, Planets and SpaceLETTER

Open AccessObservation of earthquake ground motion due to aftershocksof the 2016 Kumamoto earthquake in damaged areas
Hiroaki Yamanaka*, Kosuke Chimoto, Hiroe Miyake, Seiji Tsuno and Nobuyuki Yamada
Earth, Planets and Space 2016, 68:197 doi:10.1186/s40623-016-0574-2
Received: 3 July 2016, Accepted: 22 November 2016, Published: 1 December 2016

Abstract
We have conducted observation of earthquake ground motion due to aftershocks of the 2016 Kumamoto earthquake at 26
temporary stations in damaged areas of Kumamoto city, Mashiki town, Nishihara village and Minami-Aso village (partly in Asocity) in Kumamoto prefecture, Japan. Continuous recordings of ground acceleration were acquired in a period of about 1
month after the occurrence of the main shock on April 16, 2016. This preliminary analysis of the observed records clearly
indicates strong effects of local geological condition in the heavily damaged districts in Mashiki town and Nishihara village.
Spectral ratios of the ground motions at the stations in the severely damaged districts to those at the reference sites arecharacterized by large amplitudes at periods of 0.5–1 s. Peak ground velocities and seismic intensities are also large at thesites. Seismic intensities at the stations in the damaged districts are larger by an intensity of one at the maximum than thoseat the stations with the minor damage. The ground motions at the stations in Kumamoto city are rich in later phases with long
duration suggesting basin effects. However, site amplification effects
could not clearly be identified at the stations in the Minami-Aso area fromthe results in the conventional spectral ratio approach.

Keywords: The 2016 Kumamoto earthquake, Aftershock, Earthquake
ground motion, Strong motion observation, Siteamplification

Figure 1*Corresponding author: Hiroaki Yamanaka, yamanaka.h.aa@m.titech.ac.jp

LETTER

Open Access

Ground motion estimation for the elevated bridges of
the Kyushu Shinkansen derailment caused by the foreshock of
the 2016 Kumamoto earthquake based on the site-effectsubstitution method
Yoshiya Hata*, Masaaki Yabe, Akira Kasai, Hiroshi Matsuzaki, Yoshikazu Takahashi
and Mitsuyoshi Akiyama
Earth, Planets and Space 2016, 68:199 doi:10.1186/s40623-016-0573-3
Received: 11 July 2016, Accepted: 19 November 2016, Published: 1 December 2016

Abstract
An earthquake of JMA magnitude 6.5 (first event) hit Kumamoto Prefecture, Japan, at 21:26 JST, April 14, 2016. Subsequently, an earthquake of
JMA magnitude 7.3 (second event) hit Kumamoto and Oita Prefectures at 01:46 JST, April 16, 2016. An out-of-service Kyushu Shinkansen train
carrying no passengers traveling on elevated bridges was derailed by the first event. This was the third derailment caused by an earthquake inthe history of the Japanese Shinkansen, after one caused by the 2004 Mid-Niigata Prefecture
Earthquake and another triggered by the 2011 Tohoku Earthquake. To analyze the mechanism ofN
EQ-3
this third derailment, it is crucial to evaluate the strong ground motion at the derailment site withhigh accuracy. For this study, temporary earthquake observations were first carried out at a locationnear the bridge site; these observations were conducted because although the JMA Kumamoto
Station site and the derailment site are closely located, the ground response characteristics at these
KKM䠄92䠈6.4䠅
KNK2䠄72䠈5.9䠅
sites differ. Next, empirical site amplification and phase effects were evaluated based on the
MTO䠄139䠈6.6䠅
JKM䠄62䠈5.9䠅
KNK1
obtained observation records. Finally, seismic waveforms during the first event at the bridge site of
SNKinterest were estimated based on the site-effect substitution method. The resulting estimated
KTO䠄Unreleased䠅
acceleration and velocity waveforms for the derailment site include much larger amplitudes than
Foreshockthe waveforms recorded at the JMA Kumamoto and MLIT Kumamoto station sites. The reliability of
MKM䠄50䠈5.6䠅
these estimates is confirmed by the finding that the same methods reproduce strong groundEQ-1
motions at the MLIT Kumamoto Station site accurately. These estimated ground motions will be
EQ-2
useful for reasonable safety assessment of anti-derailment devices on elevated railway bridges.
5 km
0Kyushu Shinkansen Line

Kumamoto Airport

Kumamoto
StationKumamoto Castle

Station name [Abbreviation] 䠄PGV(cm/s)䠈JMA seismic intensity䠅

Keywords: Seismic observation, Site-effect, Response spectrum

Figure 1*Corresponding author: Yoshiya Hata, hata@civil.eng.osaka-u.ac.jp
19

2016 Kumamoto Earthquake Sequence and Its Impact on Earthquake Science and Hazard Assessment

LETTER

Open AccessAn investigation into the remote triggering of the Oita earthquake by
the 2016 Mw 7.0 Kumamoto earthquake using fullwavefield simulation
Masatoshi Miyazawa
Earth, Planets and Space 2016, 68:205 doi:10.1186/s40623-016-0585-z
Received: 21 July 2016, Accepted: 6 December 2016, Published: 19 December 2016

Abstract
High-amplitude seismic waves from the Mw 7.0 Kumamoto earthquake of April 16, 2016, triggered another large earthquake 80
km to the NE roughly 30 s later. The source was located at shallow depths beneath the Yufuin geothermal field, Oita Prefecture,
Japan, and the event magnitude was approximately 5.9. To date, this is one of the clearest known examples of a remotely
triggered large earthquake. The triggered Oita event was followed by significant seismicity, which was distinct from theaftershocks of the Kumamoto earthquake. The Coulomb failure stresschange around the hypocenter, calculated for the passing waves ofathe Kumamoto earthquake by full wavefield simulation, was about 0.7
MPa when the Oita earthquake was triggered, with the static stress
Triggered eventchange being an order of magnitude smaller. The dynamic stress
M5.4
M5.9
changes likely played an important role in triggering. A return to low
seismicity levels 1 month after the triggered earthquake may haveA’
B
important implications for seismic hazard due to dynamic triggering.
130.5˚E

131.0˚E

131.5˚E

33.5˚N33.0˚N

Mw7.0

km
OIT009Keywords: 2016 Kumamoto earthquake, Remote triggering,

M5

A’ Distance (km)

b

10

A

32.5˚N

Corresponding author: Masatoshi Miyazawa, miyazawa@rcep.dpri.kyoto-u.ac.jp

0

A

A

N30W

S30E

M3

Figure3 (km)
Distance
0

10
0

M4

B

10

S60W

M2

20

N60E

0

10

10

10

Open Access
15

20

20

20

Estimation of shallow S-wave velocity structure using microtremor
array exploration at temporary strong motion observation
stations for aftershocks of the 2016 Kumamoto earthquake
c

300

6

200

M

5
4

100

3
2

1

11

APR 2016

21

1

11

Kosuke Chimoto*, Hiroaki Yamanaka, Seiji Tsuno, Hiroe Miyake and Nobuyuki Yamada

21

MAY 2016

311

0

Cumulative Number

LETTER

05

Depth (km)Coulomb failure stress change, Wavefield simulation

0

11

JUN 2016

Earth, Planets and Space 2016, 68:206 doi:10.1186/s40623-016-0581-3
Received: 29 July 2016, Accepted: 3 December 2016, Published: 19 December 2016

Abstract
Shallow S-wave velocity VS profiles were estimated for 26 temporary strong motion observation sites surrounding theepicenters of a sequence of the 2016 Kumamoto earthquake. The microtremor array method was used to gather thedispersion characteristics of Rayleigh waves. VS profiles were obtained by inverting the dispersion curves for each site andthose of three permanent strong motion stations that recorded the sequence of seismic events. The shallow VS profiles near
two of the permanent strong motion stations in the town of Mashiki were almost identical. However, the VS profiles at otherstations varied. The VS profiles were found to have the common feature of the uppermost low-velocity layer being widelydistributed from Mashiki to the village of Minami-Aso, and it
was especially thick in the areas that suffered heavy damage.
This low-velocity layer was a major contributor to the site
amplification. The horizontal-to-vertical spectral ratios of the
microtremors indicate that both the shallow soil and deep
sedimentary layers may control the site response
characteristics over a broad frequency range.

Keywords: Shallow S-wave velocity structure, Microtremor

array exploration, 2016 Kumamoto earthquake,
Site amplification
Figure 4*Corresponding author: Kosuke Chimoto, chimoto.k.aa@m.titech.ac.jp
20Earth, Planets and SpaceLETTER

Open Access

Inelastic strain rate in the seismogenic layer of Kyushu
Island, Japan
Satoshi Matsumoto*, Takuya Nishimura and Takahiro Ohkura
Earth, Planets and Space 2016, 68:207 doi:10.1186/s40623-016-0584-0
Received: 7 July 2016, Accepted: 3 December 2016, Published: 28 December 2016

Abstract
Seismic activity is associated with crustal stress relaxation, creating inelastic strain in a medium due to faulting. Inelastic strain
affects the stress field around a weak body and causes stress concentration around the body, because the body itself has alreadyreleased stress. Therefore, the understanding of inelastic deformation is important as it generates earthquakes. We investigatedaverage inelastic strain in a spatial bin of Kyushu Island, Japan, and obtained the inelasticstrain rate distribution associated with crustal earthquakes, based on the analysis of fault
plane solutions and seismic moments. Large inelastic strains (>10−7 year−1) were found in
the Beppu–Shimabara area, located in the center of Kyushu Island. The strain rate tensor
was similar to that of the stress tensor except the absolute value in the area, implying thatthe inelastic strain was controlled by the stress field. The 2016 Kumamoto earthquake
sequence (maximum magnitude 7.3) occurred in the Beppu–Shimabara area, with the
major earthquakes located around the high inelastic strain rate area. Inelastic strain in thevolume released the stress. In addition, the inelastic strain created an increment of stressaround the volume. This indicates that the spatial heterogeneity of inelastic strain might
concentrate stress.

Keywords: Inelastic strain field, Kyushu, Moment tensor, Seismogenic zone

Figure 5*Corresponding author: Satoshi Matsumoto, matumoto@sevo.kyushu-u.ac.jp

LETTER

Open Access

Volcanic magma reservoir imaged as a low-density body beneathAso volcano that terminated the 2016 Kumamoto earthquake
ruptureAyumu Miyakawa*, Tatsuya Sumita, Yasukuni Okubo, Ryo Okuwaki, Makoto Otsubo,
Shimpei Uesawa and Yuji Yagi
Earth, Planets and Space 2016, 68:208 doi:10.1186/s40623-016-0582-2
Received: 9 July 2016, Accepted: 3 December 2016, Published: 30 December 2016

Abstract

Depth (km)We resolve the density structure of a possible magma reservoir beneath Aso, an active volcano on Kyushu Island, Japan, by
inverting gravity data. In the context of the resolved structure, we discuss the relationship between the fault rupture of the 2016
Kumamoto earthquake and Aso volcano. Low-density bodies were resolved beneath central Aso volcano using a threedimensional inversion with an assumed density contrast of ±0.3 g/cm3. The resultant location of the southern low-density body is
consistent with a magma reservoir reported in previous studies. No Kumamoto aftershocks occur in the southern low-density
body; this aseismic anomaly may indicate a ductile feature due to high temperatures and/or the presence of partial melt.
Comparisons of the location of the southern low-density body with rupture models of the mainshock, obtained from teleseismic
waveform and InSAR data, suggest that the rupture terminus overlaps theSWNE0southern low-density body. The ductile features of a magma reservoir could−2 1 m 2 m 3 m 4 m 5 mhave terminated rupture propagation. On the other hand, a northern low−4−6density body is resolved in the Asodani area, where evidence of current−8−10
volcanic activity is scarce and aftershock activity is high. The northern low−12
1m−14
density body might, therefore, be derived from a thick caldera fill in the
−16
−18
0m
Asodani area, or correspond to mush magma or a high-crystallinity magma
−20
−10
0
10
20
30
40
reservoir that could be the remnant of an ancient intrusion.
Distance (km)Keywords: 2016 Kumamoto earthquake, Density structure, Gravity
inversion, Volcano-tectonic interactions-0.3 -0.2-0.1 0 0.1 0.2 0.3
Density contrast (g/cm3)

Figure 4*Corresponding author: Ayumu Miyakawa, miyakawa-a@aist.go.jp
21

2016 Kumamoto Earthquake Sequence and Its Impact on Earthquake Science and Hazard Assessment

LETTER

Open Access

The cause of heavy damage concentration in downtown Mashiki
inferred from observed data and field survey of the 2016
Kumamoto earthquake
Hiroshi Kawase*, Shinichi Matsushima, Fumiaki Nagashima, Baoyintu and Kenichi Nakano
Earth, Planets and Space 2017, 69:3 doi:10.1186/s40623-016-0591-1
Received: 31 July 2016, Accepted: 17 December 2016, Published: 3 January 2017

Abstract

Acceleraon (cm/s2)

Acceleraon (cm/s2)

To understand the cause of heavy structural damage during the mainshock (on April 16, 2016) of the 2016 Kumamoto
earthquake sequence, we carried out a field survey from April 29 through May 1, 2016, in Mashiki where heavy damage
concentration was observed. The heavy damage concentration in downtown Mashiki could be understood based on theobserved strong motions with the Japan Meteorological Agency instrumental seismic intensity of VII and information collectedby the field investigation. First, the fundamental features of the structural damage in downtown Mashiki were summarized. Then,a distribution map of peak frequencies was derived from horizontal-to1500
IIS_Mashiki_NS
KiK-net KMMH16_NS
vertical spectral ratios of microtremors. We could not see any systematic
1000
correlation between the peak frequencies and spatial distribution of
500
0damage ratios. We also analyzed observed strong motion data at two sites
Time (s)15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31-500
to obtain fling-step-like motions in the displacement time histories through-1000
the double integration of unfiltered accelerograms. It turned out that at-1500
both strong motion observation sites in Mashiki, only the east–west (EW)1500
IIS_Mashiki_EW
KiK-net KMMH16_EW
components had very strong velocity pulses westward before the1000
emergence of the fling-step-like motion eastward, which would be the500
0primary cause of heavy structural damage in downtown Mashiki, not site
Time (s)15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31-500
effects nor the fling-step-like motion itself.

Keywords: Structural damage, HVR, Site effects, Fling-step, Directivity

LETTER

-1500
1500
Acceleraon (cm/s2)*Corresponding author: Hiroshi Kawase, kawase@zeisei.dpri.kyoto-u.ac.jp-1000

IIS_Mashiki_UD

1000

Figure 9 KiK-net KMMH16_UD

500
0
-500

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

Time (s)-1000Open Access-1500

Seismicity controlled by resistivity structure: the 2016 Kumamoto
earthquakes, Kyushu Island, Japan
Koki Aizawa*, Hisafumi Asaue, Katsuaki Koike, Shinichi Takakura, Mitsuru Utsugi, Hiroyuki Inoue, Ryokei Yoshimura, Ken’ichi Yamazaki,
Shintaro Komatsu, Makoto Uyeshima, Takao Koyama, Wataru Kanda, Taro Shiotani, Nobuo Matsushima,
Maki Hata, Tohru Yoshinaga, Kazunari Uchida, Yuko Tsukashima, Azusa Shito, Shiori Fujita, Asuma Wakabayashi,
Kaori Tsukamoto, Takeshi Matsushima, Masahiro Miyazaki, Kentaro Kondo, Kanade Takashima, Takeshi Hashimoto,
Makoto Tamura, Satoshi Matsumoto, Yusuke Yamashita, Manami Nakamoto and Hiroshi Shimizu
Earth, Planets and Space 2017, 69:4 doi:10.1186/s40623-016-0590-2
Received: 29 July 2016, Accepted: 17 December 2016, Published: 3 January 2017

Abstract

Zo

ne

B

The MJMA 7.3 Kumamoto earthquake that occurred at 1:25 JST on April 16, 2016, not only triggered aftershocks in the vicinity of the epicenter,but also triggered earthquakes that were 50–100 km away from the epicenter of the main shock. The active seismicity can be divided into
three regions: (1) the vicinity of the main faults, (2) the northern region of Aso volcano (50 km northeast of the mainshock epicenter), and (3)the regions around three volcanoes, Yufu, Tsurumi, and Garan (100 km northeast of the mainshock epicenter). Notably, the zones between
these regions are distinctively seismically inactive. The electric resistivity structure estimated from one-dimensional analysis of the 247
broadband (0.005–3000 s) magnetotelluric and telluric observation sites clearly shows that the
130.4˚ 130.6˚ 130.8˚ 131˚
131.2˚ 131.4˚ 131.6˚earthquakes occurred in resistive regions adjacent to conductive zones or resistive-conductive
33.4˚
Magnitude
Depth 8 km
Seis. 3transition zones. In contrast, seismicity is quite low in electrically conductive zones, which are
2
3
4interpreted as regions of connected fluids. We suggest that the series of the earthquakes was
M6.4
33.2˚
5
1975
induced by a local accumulated stress and/or fluid supply from conductive zones. Because the6
Seis. 2
relationship between the earthquakes and the resistivity structure is consistent with previous
33˚(Log10 Ωm)
studies, seismic hazard assessment generally can be improved by taking into account the
4
M6.1
1975
resistivity structure. Following on from the 2016 Kumamoto earthquake series, we suggest that32.8˚
3
there are two zones that have a relatively high potential of earthquake generation along theSeis. 1western extension of the MTL.
2
32.6˚

Keywords: Magnetotellurics, Resistivity structure, 2016 Kumamoto earthquake, Futagawa

fault, Hinagu fault, Structural control, Aso volcano, Kuju volcano, Tsurumi volcano,Median Tectonic Line*Corresponding author: Koki Aizawa, aizawa@sevo.kyushu-u.ac.jp
22

32.4˚

Zone A

50 km

Figure 61Earth, Planets and SpaceLETTER

Open Access

Source location and mechanism analysis of an earthquake
triggered by the 2016 Kumamoto, southwestern Japan,
earthquake
Takeshi Nakamura* and Shin Aoi
Earth, Planets and Space 2017, 69:6 doi:10.1186/s40623-016-0588-9
Received: 1 August 2016, Accepted: 9 December 2016, Published: 3 January 2017

Abstract
The 2016 Kumamoto earthquake (Mw 7.0) occurred in the central part of Kyushu Island,
southwestern Japan, on April 16, 2016. The mainshock triggered an event of maximum
acceleration 700 gal that caused severe damage to infrastructure and thousands of homes. We
investigate the source location of the triggered event, and the timing of large energy release,by employing the back-projection method for strong-motion network data. The optimal
location is estimated to be [33.2750°, 131.3575°] (latitude, longitude) at a depth of 5 km, whichis 80 km northeast of the epicenter of the mainshock. The timing is 33.5 s after the origin timeof the mainshock. We also investigate the source mechanism by reproducing observed
displacement waveforms at a near-source station. The waveforms at smaller-sized events,
convolved with the source time function of a pulse width 1 s, are similar to the signature of theobserved waveforms of the triggered event. The observations are also reproduced by synthetic
waveforms for a normal-fault mechanism and a normal-fault with strike-slip components at theestimated locations. Although our approach does not constrain the strike direction well, ourwaveform analysis indicates that the triggered earthquake occurred near the station thatobserved the strong motions, primarily via a normal-fault mechanism or a normal-fault withstrike-slip components.

Keywords: 2016 Kumamoto earthquake, Triggered earthquake, Strong motion,

Back-projection method, Green’s function, Hypocenter determination, Source
mechanism

Figure 4*Corresponding author: Takeshi Nakamura, t_nakamura@bosai.go.jp

LETTER

Open Access

Earthquake rupture properties of the 2016 Kumamoto earthquake
foreshocks (Mj 6.5 and Mj 6.4) revealed by conventional andmultiple-aperture InSAR
Tomokazu Kobayashi
Earth, Planets and Space 2017, 69:7 doi:10.1186/s40623-016-0594-y
Received: 5 July 2016, Accepted: 20 December 2016, Published: 3 January 2017

Abstract
By applying conventional cross-track InSAR and multiple-aperture InSAR (MAI) techniques with ALOS-2 SAR data to foreshocks of the 2016
Kumamoto earthquake, ground displacement fields in range (line-of-sight) and azimuth components have been successfully mapped. The mostconcentrated crustal deformation with ground displacement exceeding 15 cm is located on the western side of the Hinagu fault zone. A locally
distributed displacement which appears along the strike of the Futagawa fault can be
identified in and around Mashiki town, suggesting that a different local fault slip alsoN
contributed toward foreshocks. Inverting InSAR, MAI, and GNSS data, distributed slip
models are obtained that show almost pure right-lateral fault motion on a plane dipping
west by 80° for the Hinagu fault and almost pure normal fault motion on a plane dipping
Futagawa F.
south by 70° for the local fault beneath Mashiki town. The slip on the Hinagu fault reaches
Hinagu F.
around the junction of the Hinagu and Futagawa faults. The slip in the north significantly
extends down to around 10 km depth, while in the south the slip is concentrated near the
ground surface, perhaps corresponding to the Mj 6.5 and the Mj 6.4 events, respectively.
The focal mechanism of the distributed slip model for the Hinagu fault alone shows pureright-lateral motion, which is inconsistent with the seismically estimated mechanism that
includes a significant non-double couple component. On the other hand, when taking
the contribution of normal fault motion into account, the focal mechanism appears
similar to that of the seismic analysis. This result may suggest that local fault motion
occurred just beneath Mashiki town, simultaneously with the Mj 6.5 event, thereby
increasing the degree of damage to the town.
Mashiki
Town

~70°

ral
Right-latefault motion

~ 80°

Mj6.4

Keywords: Foreshocks, Kumamoto earthquake, InSAR, MAI, Crustal deformation,
Fault model

Figure 5

Corresponding author: Tomokazu Kobayashi, kobayashi-t96dv@mlit.go.jp
23

Normalfault motion

?

Mj6.5

2016 Kumamoto Earthquake Sequence and Its Impact on Earthquake Science and Hazard Assessment

LETTER

Open Access

Strong motions observed by K-NET and KiK-net during the 2016
Kumamoto earthquake sequence
Wataru Suzuki*, Shin Aoi, Takashi Kunugi, Hisahiko Kubo, Nobuyuki Morikawa,
Hiromitsu Nakamura, Takeshi Kimura and Hiroyuki Fujiwara
Earth, Planets and Space 2017, 69:19 doi:10.1186/s40623-017-0604-8
Received: 1 August 2016, Accepted: 14 January 2017, Published: 30 January 2017

Abstract
The nationwide strong-motion seismograph network of K-NET and KiK-net in Japan successfully recorded the strong ground motions of the 2016
Kumamoto earthquake sequence, which show the several notable characteristics. For the first large earthquake with a JMA magnitude of 6.5(21:26, April 14, 2016, JST), the large strong motions are concentrated near the epicenter and the strong-motion attenuations are well predictedby the empirical relation for crustal earthquakes with a moment magnitude of 6.1. For the largest earthquake of the sequence with a JMA
magnitude of 7.3 (01:25, April 16, 2016, JST), the large peak ground accelerations and velocities extend from the epicentral area to the northeast
direction. The attenuation feature of peak ground accelerations generally follows the empirical relation, whereas that for velocities deviates fromthe empirical relation for stations with the epicentral distance of greater than 200 km, which can be attributed to the large Love wave having adominant period around 10 s. The large accelerations were observed at stations even in
Oita region, more than 70 km northeast from the epicenter. They are attributed to the
local induced earthquake in Oita region, whose moment magnitude is estimated to be 5.5
by matching the amplitudes of the corresponding phases with the empirical attenuation
relation. The real-time strong-motion observation has a potential for contributing to themitigation of the ongoing earthquake disasters. We test a methodology to forecast theregions to be exposed to the large shaking in real time, which has been developed basedon the fact that the neighboring stations are already shaken, for the largest event of the
Kumamoto earthquakes, and demonstrate that it is simple but effective to quickly makewarning. We also shows that the interpolation of the strong motions in real time isfeasible, which will be utilized for the real-time forecast of ground motions based on theobserved shakings.

Keywords: 2016 Kumamoto earthquake, Strong motion, PGA, PGV, Attenuation

relation, Record section, Induced earthquake, Earthquake early warning,
Real-time interpolation

Figure 1*Corresponding author: Wataru Suzuki, wsuzuki@bosai.go.jp

LETTER

Open AccessGeomorphic features of surface ruptures associated with the 2016 Kumamoto
earthquake in and around the downtown of Kumamoto City, andimplications on triggered slip along active faults
Hideaki Goto*, Hiroyuki Tsutsumi, Shinji Toda and Yasuhiro Kumahara
Earth, Planets and Space 2017, 69:26 doi:10.1186/s40623-017-0603-9
Received: 31 July 2016, Accepted: 14 January 2017, Published: 2 February 2017

Abstract
The ~30-km-long surface ruptures associated with the Mw 7.0 (Mj 7.3) earthquake at 01:25 JST on April 16 in Kumamoto Prefecture appearedalong the previously mapped ~100-km-long active fault called the Futagawa-Hinagu fault zone (FHFZ). The surface ruptures appeared to haveextended further west out of the main FHFZ into the Kumamoto Plain. Although InSAR analysis by Geospatial Information Authority of Japan(GSI) indicated coseismic surface deformation in and around the downtown of Kumamoto City, the surface ruptures have not been clearly
mapped in the central part of the Kumamoto Plain, and whether there are other active faults other than the Futagawa fault in the Kumamoto
Plain remained unclear. We produced topographical stereo images (anaglyph) from5-m-mesh digital elevation model of GSI, which was generated from light detection andranging data. We interpreted them and identified that several SW-sloping river terraces
formed after the deposition of the pyroclastic flow deposits related to the latest large
eruption of the Aso caldera (86.8–87.3 ka) are cut and deformed by several NW-trending
flexure scarps down to the southwest. These 5.4-km-long scarps that cut across downtown
Kumamoto were identified for the first time, and we name them as the Suizenji fault zone.
Surface deformation such as continuous cracks, tilts, and monoclinal folding associated withthe main shock of the 2016 Kumamoto earthquake was observed in the field along the fault
zone. The amount of vertical deformation (~0.1 m) along this fault associated with the 2016
Kumamoto earthquake was quite small compared to the empirically calculated coseismic
slip (0.5 m) based on the fault length. We thus suggest that the slip on this fault zone wastriggered by the Kumamoto earthquake, but the fault zone has potential to generate anearthquake with larger slip that poses a high seismic risk in downtown Kumamoto area.

Keywords: 2016 Kumamoto earthquake, Active fault, Tectonic landform, Surface

rupture, Kumamoto Plain, Suizenji fault zone, Digital elevation model*Corresponding author: Hideaki Goto, hgoto@hiroshima-u.ac.jp
24

Figure 6

Earth, Planets and SpaceLETTER

Open Access

Local site effects in Kumamoto City revealed by the 2016 Kumamoto
earthquake
Seiji Tsuno*, Masahiro Korenaga, Kyosuke Okamoto, Hiroaki Yamanaka, Kosuke Chimoto
and Takeshi Matsushima
Earth, Planets and Space 2017, 69:37 doi:10.1186/s40623-017-0622-6
Received: 1 August 2016, Accepted: 28 February 2017, Published: 7 March 2017

Abstract
To evaluate local site effects in Kumamoto City, we installed six temporary seismic stations along a 6-km north–south survey line in thecity immediately after the 2016 Kumamoto earthquake foreshock (Mj 6.4), which occurred on April 14, 2016. Seismic data from the 2016
Kumamoto earthquake (Mj 7.3), which occurred on April 16, 2016, were successfully recorded at two sites and indicated large amplitudes
in the frequency range of 0.5–3 Hz. Site amplifications estimated from weak ground motion data, with a station at Mt. Kinbo used as areference, are relatively variable along this survey line; however, site amplification factors in the frequency range of 0.5–3 Hz are not large
enough to explain the amplitudes produced by the main shock. Nevertheless, site amplifications estimated from strong ground motion
data recorded at the two sites during the main shock are large in the frequency range of 1–3 Hz. These findings reveal that the strong
ground motions in the frequency range of 1–3 Hz were enhanced by nonlinear behavior of the subsurface soil in Kumamoto City.
Moreover, it is observed that the frequency contents of the main shock data in the frequency range of 0.7–3 Hz differ significantly
between the two sites, despite the proximity of these sites (600-m
interval). Therefore, we also performed single-station microtremor
measurements with an interval distance of approximately 100 m
between these two sites. We confirmed that the peak frequenciesof the horizontal-to-vertical spectral ratios of microtremors havetrends that are similar to those of the site amplification factors
between the two sites. However, these results could not explain the
differences in strong ground motions observed at the two sites
during the 2016 Kumamoto earthquake.

Keywords: Strong ground motion, Local site effect, Nonlinearity,
Kumamoto City, 2016 Kumamoto earthquake

Figure 7

N indicates the*Corresponding author: Seiji Tsuno, stsuno@rtri.or.jp

LETTER

Open Access

Source process of the 2016 Kumamoto earthquake (Mj7.3)
inferred from kinematic inversion of strong-motion records
Kunikazu Yoshida*, Ken Miyakoshi, Kazuhiro Somei and Kojiro Irikura
Earth, Planets and Space 2017, 69:64 doi:10.1186/s40623-017-0649-8
Received: 1 August 2016, Accepted: 25 April 2017, Published: 3 May 2017

Abstract
In this study, we estimated source process of the 2016 Kumamoto earthquake from strong-motion data by using the multiple-time windowlinear kinematic waveform inversion method to discuss generation of strong motions and to explain crustal deformation pattern with aseismic source inversion model. A four-segment fault model was assumed based on the aftershock distribution, active fault traces, andinterferometric synthetic aperture radar data. Three western segments were set to be northwest-dipping planes, and the most easternsegment under the Aso caldera was examined to be a southeast-dipping plane. The velocity structure models used in this study were
estimated by using waveform modeling of moderate earthquakes that occurred in the source region. We applied a two-step approach of theinversions of 20 strong-motion datasets observed by K-NET and KiK-net by using band-pass-filtered strong-motion data at 0.05–0.5 Hz andthen at 0.05–1.0 Hz. The rupture area of the fault plane was determined by applying the criterion of Somerville et al. (Seismol Res Lett 70:59–
80, 1999) to the inverted slip distribution. From the first-step inversion, the fault length was trimmed from 52 to 44 km, whereas the fault
width was kept at 18 km. The trimmed rupture area was not changed in the second-step inversion. The source model obtained from thetwo-step approach indicated 4.7 × 1019 Nm of the total moment release and 1.8
m average slip of the entire fault with a rupture area of 792 km2. Large slip areasawere estimated in the seismogenic zone and in the shallow part corresponding
to the surface rupture that occurred during the Mj7.3 mainshock. The areas of
the high peak moment rate correlated roughly with those of large slip; however,
the moment rate functions near the Earth surface have low peak, bell shape,and long duration. These subfaults with long-duration moment release are
expected to cause weak short-period ground motions. We confirmed that thesoutheast dipping of the most eastern segment is more plausible rather thannorthwest-dipping from the observed subsidence around the central cones of
the Aso volcano.

Keywords: 2016 Kumamoto earthquake, Source process, Kinematic inversion,
Near-fault strong motion

Figure 5*Corresponding author: Kunikazu Yoshida, yoshida@geor.or.jp
25

2016 Kumamoto Earthquake Sequence and Its Impact on Earthquake Science and Hazard Assessment

LETTER

Open Access

Effect of newly refined hypocenter locations on the seismic activity
recorded during the 2016 Kumamoto Earthquake
sequence
Tomoko Elizabeth Yano* and Makoto Matsubara
Earth, Planets and Space 2017, 69:74 doi:10.1186/s40623-017-0656-9
Received: 10 August 2016, Accepted: 16 May 2017, Published: 30 May 2017

Abstract
We present the results of relocating 17,544 hypocenters determined from data recorded during the 2016 Kumamoto Earthquake sequence, during
the interval between April 14, 2016, and August 31, 2016. For this, we used a double-difference relocation method to constrain high-resolution
hypocenter locations by cross-correlation differential times as well as the NIED Hi-net catalog differential times. The sequence included two large
events (on 14 April: MJMA6.5 and on 16 April: MJMA7.3) that occurred in a complicated region where the Hinagu and Futagawa faults meet. By
comparing these high-resolution earthquake locations in three different periods [(P1) between 2001 and 2012; (P2) between MJMA6.5 and MJMA7.3;
and (P3) between MJMA7.3 and August 31, 2016], we present the significant seismicity after the mainshock relative to the background seismicity.
Events during the Kumamoto Earthquake sequence occurred generally within the same sites of known faults and background seismicity. For an
example, the seismicity during period P2 formed a sharp linear shape along the northern part of the Hinagu fault for about 20 km. A series of linear
seismicity events occurred during period P3 along the Futagawa fault to the east (for about 28 km), in the northern part of the Aso caldera, and in
the Oita region around the Beppu–Haneyama fault zone. These events also extended to the mid- and southern parts of the Hinagu fault zone andwere shaped only after the M7.3 event. Moreover, high-resolution hypocenter locations also allowed
us to identify some clusters of events that occurred in regions where background seismicity has notbeen confirmed. For instance, activity on the northwestern edge of the Aso caldera and in small
areas within the Beppu–Haneyama fault zone became apparent with new seismic activity. We also
demonstrate herein the absence of seismicity between the northeast extension of the Futagawa
fault zone and the Aso caldera region, which became clearly shown after the M7.3 event. This
low-seismicity region is located at the boundary of the low- and high-velocity structures anddifferent focal mechanisms, but is also close to the maximum slip area of the M7.3 event.

Keywords: The 2016 Kumamoto Earthquake, Double-difference method, Active fault zone, The
Beppu–Shimabara graben

Figure 3*Corresponding author: Tomoko Elizabeth Yano, tomoyano@gmail.com

26

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Continued from inside cover

Strong motions observed by K-NET and KiK-net during the 2016 Kumamoto earthquake sequence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wataru Suzuki, Shin Aoi, Takashi Kunugi, Hisahiko Kubo,
Nobuyuki Morikawa, Hiromitsu Nakamura,Takeshi Kimura and Hiroyuki Fujiwara

24

Geomorphic features of surface ruptures associated with the 2016 Kumamoto earthquake in and around the downtown
of Kumamoto City, and implications on triggered slip along active faults. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hideaki Goto, Hiroyuki Tsutsumi, Shinji Toda and Yasuhiro Kumahara 24
Local site effects in Kumamoto City revealed by the 2016 Kumamoto earthquake. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Seiji Tsuno, Masahiro Korenaga, Kyosuke Okamoto, Hiroaki Yamanaka,
Kosuke Chimoto and Takeshi Matsushima

25

Source process of the 2016 Kumamoto earthquake (Mj7.3) inferred from kinematic inversion of strong-motion records. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kunikazu Yoshida, Ken Miyakoshi, Kazuhiro Somei and Kojiro Irikura 25
Effect of newly refined hypocenter locations on the seismic activity recorded during the 2016 Kumamoto Earthquake
sequence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tomoko Elizabeth Yano and Makoto Matsubara 26

Cover image: Asano, K. et al., Earth, Planets and Space 2016, 68:147 DOI: 10.1186/s40623-016-0519-9

Continued from back coverContinuity, segmentation and faulting type of active fault zones of the 2016 Kumamoto earthquake inferred fromanalyses of a gravity gradient tensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nayuta Matsumoto, Yoshihiro Hiramatsu and Akihiro Sawada  15
Fault source model for the 2016 Kumamoto earthquake sequence based on ALOS-2/PALSAR-2 pixel-offset data:
evidence for dynamic slip partitioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yuji Himematsu and Masato Furuya  15
Surface fault ruptures associated with the 14 April foreshock (Mj 6.5) of the 2016 Kumamoto earthquake sequence,southwest Japan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nobuhiko Sugito, Hideaki Goto, Yasuhiro Kumahara, Hiroyuki Tsutsumi,
Takashi Nakata, Kyoko Kagohara, Nobuhisa Matsuta
and Haruka Yoshida  16
Earthquakes in Oita triggered by the 2016 M7.3 Kumamoto earthquake. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Shingo Yoshida  16

Crustal deformation model of the Beppu−Shimabara graben area, central Kyushu, Japan, based on inversion of
three-component GNSS data in 2000–2010. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kazuma Mochizuki and Yuta Mitsui  17
The 2016 Kumamoto–Oita earthquake sequence: aftershock seismicity gap and dynamic triggering in volcanic areas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  Takahiko Uchide, Haruo Horikawa, Misato Nakai,
Reiken Matsushita, Norio Shigematsu, Ryosuke Andoand Kazutoshi Imanishi   17Characteristics of the surface ruptures associated with the 2016 Kumamoto earthquake sequence, central Kyushu, Japan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  Yoshiki Shirahama, Masayuki Yoshimi, Yasuo Awata, Tadashi Maruyama,
Takashi Azuma, Yukari Miyashita, Hiroshi Mori, Kazutoshi Imanishi,
Naoto Takeda, Tadafumi Ochi, Makoto Otsubo, Daisuke Asahina
and Ayumu Miyakawa   18Near-fault deformation and Dc’’ during the 2016 Mw7.1 Kumamoto earthquake. . . . . . . . . Eiichi Fukuyama and Wataru Suzuki  18
Observation of earthquake ground motion due to aftershocks of the 2016 Kumamoto earthquake in damaged areas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  Hiroaki Yamanaka, Kosuke Chimoto, Hiroe Miyake, Seiji Tsuno
and Nobuyuki Yamada  19

Ground motion estimation for the elevated bridges of the Kyushu Shinkansen derailment caused by the foreshock of
the 2016 Kumamoto earthquake based on the site-effect substitution method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  Yoshiya Hata, Masaaki Yabe, Akira Kasai, Hiroshi Matsuzaki,
Yoshikazu Takahashi and Mitsuyoshi Akiyama   19An investigation into the remote triggering of the Oita earthquake by the 2016 Mw 7.0 Kumamoto earthquake using
full wavefield simulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Masatoshi Miyazawa  20
Estimation of shallow S-wave velocity structure using microtremor array exploration at temporary strong motion
observation stations for aftershocks of the 2016 Kumamoto earthquake. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  Kosuke Chimoto, Hiroaki Yamanaka, Seiji Tsuno, Hiroe Miyake
and Nobuyuki Yamada  20

Inelastic strain rate in the seismogenic layer of Kyushu Island, Japan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Satoshi Matsumoto, Takuya Nishimura and Takahiro Ohkura   21Volcanic magma reservoir imaged as a low-density body beneath Aso volcano that terminated the 2016 Kumamoto
earthquake rupture. . . . . . . . . . . . . . . . . . . . . . . . . . . . Ayumu Miyakawa, Tatsuya Sumita, Yasukuni Okubo, Ryo Okuwaki, Makoto Otsubo,
Shimpei Uesawa and Yuji Yagi  21

The cause of heavy damage concentration in downtown Mashiki inferred from observed data and field survey ofthe 2016 Kumamoto earthquake . . . . . . . . . . . . . . . . . . . . . . . . . . . Hiroshi Kawase, Shinichi Matsushima, Fumiaki Nagashima, Baoyintu

and Kenichi Nakano  22

Seismicity controlled by resistivity structure: the 2016 Kumamoto earthquakes, Kyushu Island, Japan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  Koki Aizawa, Hisafumi Asaue, Katsuaki Koike, Shinichi Takakura, Mitsuru Utsugi,
Hiroyuki Inoue, Ryokei Yoshimura, Ken’ichi Yamazaki, Shintaro Komatsu,
Makoto Uyeshima, Takao Koyama, Wataru Kanda, Taro Shiotani, Nobuo Matsushima,
Maki Hata, Tohru Yoshinaga, Kazunari Uchida, Yuko Tsukashima, Azusa Shito, Shiori Fujita,
Asuma Wakabayashi, Kaori Tsukamoto, Takeshi Matsushima, Masahiro Miyazaki,
Kentaro Kondo, Kanade Takashima, Takeshi Hashimoto, Makoto Tamura,
Satoshi Matsumoto, Yusuke Yamashita, Manami Nakamotoand Hiroshi Shimizu   22Source location and mechanism analysis of an earthquake triggered by the 2016 Kumamoto, southwestern Japan,
earthquake. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Takeshi Nakamura and Shin Aoi  23
Earthquake rupture properties of the 2016 Kumamoto earthquake foreshocks (Mj 6.5 and Mj 6.4) revealed byconventional and multiple-aperture InSAR. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tomokazu Kobayashi  23

to be continued

Contents
Special issue “2016 Kumamoto earthquake sequence and its impact on earthquake science and hazard assessment”. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Manabu Hashimoto, Martha Savage, Takuya Nishimura,Haruo Horikawa and Hiroyuki Tsutsumi

1

Rupture process of the 2016 Kumamoto earthquake in relation to the thermal structure around Aso volcano. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yuji Yagi, Ryo Okuwaki, Bogdan Enescu, Amato Kasahara,
Ayumu Miyakawa and Makoto Otsubo

Source rupture processes of the foreshock and mainshock in the 2016 Kumamoto earthquake sequence estimatedfrom the kinematic waveform inversion of strong motion data . . . . . . . . . . . . . . . . . . . . . . . . . . . Kimiyuki Asano and Tomotaka Iwata
Slip-partitioned surface ruptures for the Mw 7.0 16 April 2016 Kumamoto, Japan, earthquake. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Shinji Toda, Heitaro Kaneda, Shinsuke Okada,
Daisuke Ishimura and Zoë K. Mildon

556Small-displacement linear surface ruptures of the 2016 Kumamoto earthquake sequence detected by ALOS-2 SARinterferometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Satoshi Fujiwara, Hiroshi Yarai, Tomokazu Kobayashi, Yu Morishita,

Takayuki Nakano, Basara Miyahara, Hiroyuki Nakai,
Yuji Miura, Haruka Ueshiba, Yasuaki Kakiage
and Hiroshi Une

Remote triggering of seismicity at Japanese volcanoes following the 2016 M7.3 Kumamoto earthquake. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bogdan Enescu, Kengo Shimojo, Anca Opris and Yuji Yagi
Preliminary estimation of high-frequency (4–20 Hz) energy released from the 2016 Kumamoto, Japan, earthquake
sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kaoru Sawazaki, Hisashi Nakahara and Katsuhiko Shiomi
Stochastic ground-motion simulations for the 2016 Kumamoto, Japan, earthquake. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Long Zhang, Guangqi Chen, Yanqiang Wu and Han Jiang
Crustal deformation associated with the 2016 Kumamoto Earthquake and its effect on the magma system of Aso
volcano . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Taku Ozawa, Eisuke Fujita and Hideki Ueda
Seismicity prior to the 2016 Kumamoto earthquakes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kazuyoshi Z. Nanjo, Jun Izutsu, Yoshiaki Orihara, Nobuhiro Furuse, Shoho Togo,
Hidetoshi Nitta, Tomohiro Okada, Rika Tanaka, Masashi Kamogawa
and Toshiyasu Nagao

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First result from the GEONET real-time analysis system (REGARD): the case of the 2016 Kumamoto earthquakes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Satoshi Kawamoto, Yohei Hiyama, Yusaku Ohta
and Takuya Nishimura

9

Earthquake early warning for the 2016 Kumamoto earthquake: performance evaluation of the current system andthe next-generation methods of the Japan Meteorological Agency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yuki Kodera, Jun Saitou, Naoki Hayashimoto, Shimpei Adachi, Masahiko Morimoto,
Yuji Nishimae and Mitsuyuki Hoshiba

10

Simultaneous estimation of the dip angles and slip distribution on the faults of the 2016 Kumamoto earthquake
through a weak nonlinear inversion of InSAR data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yukitoshi Fukahata and Manabu Hashimoto 10
Applicability of source scaling relations for crustal earthquakes to estimation of the ground motions of the 2016
Kumamoto earthquake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kojiro Irikura, Ken Miyakoshi, Katsuhiro Kamae, Kunikazu
Yoshida, Kazuhiro Somei, Susumu Kurahashi
and Hiroe Miyake

Anomalous decrease in relatively large shocks and increase in the p and b values preceding the April 16, 2016, M7.3
earthquake in Kumamoto, Japan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . K. Z. Nanjo and A. Yoshida
Rupture process of the main shock of the 2016 Kumamoto earthquake with special reference to damaging groundmotions: waveform inversion with empirical Green’s functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . Atsushi Nozu and Yosuke Nagasaka
Strong ground motion simulation of the 2016 Kumamoto earthquake of April 16 using multiple point sources. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yosuke Nagasaka and Atsushi Nozu
Data completeness of the Kumamoto earthquake sequence in the JMA catalog and its influence on the estimation of
the ETAS parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Jiancang Zhuang, Yosihiko Ogata and Ting Wang
Effects of the 2016 Kumamoto earthquakes on the Aso volcanic edifice. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yasuhisa Tajima, Toshiaki Hasenaka and Masayuki Torii
Dip distribution of Oita–Kumamoto Tectonic Line located in central Kyushu, Japan, estimated by eigenvectors ofgravity gradient tensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Shigekazu Kusumoto
Source rupture processes of the 2016 Kumamoto, Japan, earthquakes estimated from strong-motion waveforms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hisahiko Kubo, Wataru Suzuki, Shin Aoi and Haruko Sekiguchi

Continued on inside back cover

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