Brief report of the 2016 Kumamoto Earthquake in JAPANIAEG Japan National Group (Japan Society of Engineering Geology)
The 2016 Kumamoto Earthquake occurred on April 14 2016 with following several major
earthquakes in the middle area of Kyushu Island, southwest of Japan. Earthquakes exceeding JMA
Seismic Intensity 6 lower occurred 7 times in three days since April 14. Maximum magnitude was
7.3. JSEG has organized an investigation team just after the Kumamoto earthquake.
Figure 1 shows the general view of Japan. The Japanese Archipelago is comprised of five island arcs
as shown in the figure. The Philippine Sea Plate has been moving to NW direction 4 cm/yr. onaverage and subducting in the Nankai Trough. On the other hand, the Pacific Plate has been movingto WNW direction 9 cm/yr. on average and subducting in the Japan Trench and the Izu-Ogasawara
Trench. Moreover, the Izu-Ogasawara Island Arc have collided with Honshu Arcs and intruded tonorth direction.
These continuing movements on the subduction zones at the boundary of the plates are the maincauses of large earthquakes and tsunamis. And many active faults are distributed in the island arcs.

Japan Sea BasinJapan Trench

Pacific Plate

Nankai TroughKyushuKumamoto

Izu-Ogasawara
Island Arc

Izu-Ogasawara
TrenchOkinawaTroughPhilippine Sea Plate

Ryukyu Trench

Fig.1 General topographic view around Japan

0500kmVolcanic front

The Okinawa Trough which locates north of Ryukyu Arc has been expanding to south direction. The
Beppu-Shimabara rift zone in the Middle Kyushu is considered to be part of this Okinawa Troughzone. Therefore tensional stress of north to south direction is dominant in the middle to south
Kyushu area. (Fig.2 and Fig.3)

North Kyusyu

Middle Kyusyu

South Kyusyu
HistoricEarthquakeActive fault

Kumamoto Earthquake

Fig.2 Active faults and historic Earthquakes inKyushu
Fig.2, 3 The Headquarters for Earthquake
research Promotion, report in 2013
http://www.jishin.go.jp/Fig.3 Average speed vector of displacement in
the 5 years since March 2006
A:Compressive stress caused by Philippine Plate
B: Expanding of Okinawa TroughThe epicenters of the Kumamoto earthquake and aftershocks since April 14 are distributed SW-NE
direction along the southern edge of the Beppu-Shimabara rift zone. (Fig.4) It was reported that thetype and the focal depth of main earthquakes were lateral slip type faults and about 10 km
respectively.

Fig.4
Distribution of the epicenters
since April 14 2016
(At May 10, 14:46)

National Research Institutefor EarthScienceand
Disaster Resilience (NIED)
DisasterInformation
Laboratory
http://www.bosai.go.jp/

There are two right lateral active faults, Hinagu fault and Futagawa fault near Kumamoto City.
According to the analysis of the surface movement by MAI (Multiple Aperture Interferometry), rightslip movement along these faults are clearly shown. (Fig.5) And surface faults along Futagawa Fault
with right lateral slip displacement of maximum 2 m were observed. (Fig.6)

Volcano Aso

Fig.5 MAI (Multiple ApertureInterferometry) analysis using
ALOS-2/PALSAR-2
Geospatial Information Authorityof Japan (GSI)
http://www.gsi.go.jp/cais/

Epicenter
Active fault

Fig. 6 Earthquake surface fault along Futagawa Fault.
Right lateral slip movement was observed.
Geological Survey of Japan, The third report of urgent investigation, April 18, 2016
http://www.gsj.jp/hazards/earthquake/kumamoto2016Futagawa fault continues to the northeast area of the somma and atrio of Volcano Aso. Figure 7
shows a preliminary report of the distribution of landslides and collapses in this area.

A

B

Assumed line ofactive fault
02.5km

Fig.7 Preliminary analysis of landslides distribution and collapses in the west area of Volcano Aso(Assumed fault line is added.)Prof. Chigira and Matsushi, Disaster Prevention Research Institute, Kyoto University

Figure 8 is the photo of steep slope collapse of near the somma just north of Futagawa Fault.
Landslide on gentle slope shown in Fig.9 would be slipped along very soft and high water content
pumice layer of Quaternary tephra.

Fig. 8Large collapse on the steep slope ofnear the somma of Volcano Aso(View from point A in the Fig. 7)

Fig. 9Landslide flows on gentle slope of thelava dome. Landslide slip would haveoccurred in the tephra which coveredon the lava.(View from point B in the Fig.
7)

Fig.8 and Fig.9 are offered by

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PASCO CORPORATION
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