2016 Kumamoto Earthquake Report 2 (April 17, 2016): Seismic activities and related information in central Kyushu since April 15, 2016

2016 Kumamoto Earthquake Main Page

Last updated on April 21, 2016

The contents of this website are English translation of the Japanese website of “2016 Kumamoto Earthquakes and related information“, the Geological Survey of Japan (GSJ), AIST.

Seismic activities has been increasing in Kumamoto Prefecture and Oita Prefecture, and landslides occurred in many places of the Minami-Aso Village, near the Aso volcano.

Seismic activity and geological information in central Kyushu

Fig.1

Orange circles denote epicenters of two major earthquakes: M6.5 earthquake at 21:26 JST on April 14 and M7.3 earthquake at 01:25 JST on April 16 determined by Hi-net of the National Research Institute for Earth Science and Disaster Prevention (NIED). Blue circles are aftershocks and other smaller earthquakes from 00:00 JST, April 14 to 07:00 JST, April 16, which were also determined by Hi-net (NIED). Size of the circles is proportional to the magnitude of the earthquakes. Red lines show traces of active faults in this area. Base map is 1:200,000 Seamless Geological Map of Japan (GSJ, AIST). (modified from GSJ, 2016)

Epicenters of the present earthquake sequence show extremely wide distribution in the zone of Beppu-Shimabara Graben from the west to the east coast of the middle Kyushu with a distance of about 200 km and a width of a few tens of km. The earthquake sequence are presently very active at Hinagu Fault Zone and Futagawa Fault Zone in western Kyushu, Aso Volcano area in central Kyushu, and Beppu-Haneyama Fault Zone in eastern Kyushu.

 

Distribution of epicenters plotted on 1:200,000 geological maps in Kumamoto Prefecture

The Hinagu Fault Zone and Futagawa Fault Zone, which were the sources of the M6.5 earthquake on April 14 and the M7.3 earthquake on April 16 respectively, are located between a hilly area of Cretaceous accretionary complex and Jurassic metamorphic formations in the south, and a flatter area of Kumamoto Plain and Yatsushiro Plain in the north. The M6.5 earthquake at 21:26 JST on April 14 was on Hinagu Fault Zone and the M7.3 earthquake at 01:25 JST on April 16 was on Futagawa Fault Zone.

Fig2-1

Distribution of epicenters around the two major earthquakes in Kumamoto plotted on 1:200,000 geological mapsCircles indicate epicenters of earthquakes determined by Hi-net (NIED). Orange circles are the two major earthquakes. Size of the circles is proportional to the magnitude of the earthquakes (modified from GSJ, 2016).

Fig2-2

1:200,000 geological maps of “Kumamoto (Hoshizumi et al.,2004)” and “Yatsushiro and a part of Nomozaki (Saito et al., 2010)” by GSJ, AIST (modified from GSJ, 2016).

 

Distribution of epicenters plotted on 1:200,000 geological maps around Aso Caldera.

Seismic activity has increased around Aso Caldera, Kumamoto Prefecture, after the M7.3 earthquake at 01:25 JST on April 16. The epicenters are located in the northern half of Aso Caldera and aligned along a NE-SW direction.

Fig. 3

Blue circles are earthquakes from 00:00 JST, April 14 to 12:00 JST, April 16, determined by Hi-net (NIED). Size of the circles is proportional to the magnitude of the earthquakes (modified from GSJ, 2016).

 

Distribution of epicenters plotted on 1:200,000 geological maps in Beppu area

Seismic activity has also increased in Beppu City and Yufu City, Oita Prefecture, after the M7.3 earthquake at 01:25 JST on April 16. The epicenters are located along Beppu-Haneyama Fault Zone and are aligned along a ENE-WSW direction.

Fig.4

Blue circles are earthquakes from 00:00 JST, April 14 to 12:00 JST, April 16, determined by Hi-net (NIED). Size of the circles is proportional to the magnitude of the earthquakes (modified from GSJ, 2016).

 

Landslides in Minami-Aso Village in Aso Caldera

Many landslides occurred in the Minami-Aso Village, located within the Aso Caldera, after the M7.3 earthquake on April 16. Among them, large one on the slope above the west of Aso-Ohashi Bridge on Kurokawa River broke the bridge completely. This area is located in the western caldera wall of Aso Caldera and underlain by pre-Aso Caldera lava and pyroclastic rocks erupted 80k to 40k years ago. The caldera wall is steep and can collapse by a strong earthquake motion. Other landslides, including those at the Jigoku Hot Spring, occurred on steep hills of post-caldera lava flow deposits inside the caldera.

Fig. 5

(Taken from GSJ, 2016)

Fig6

(Taken from GSJ, 2016)

 

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