2016. 12. 19

Japan Society of Civil Engineers
International Activities Center

IAC News ： Special Issue No.10
Quick Report on the 2016 Kumamoto Earthquake
On April 14, 2016, a magnitude 6.5 earthquake
occurred, centered in Kumamoto Prefecture andregistering the maximum intensity of 7 on theJapanese scale in Mashiki Town. On April 16, there
was another earthquake, with a magnitude of 7.3,
which registered an intensity of 7 in Mashiki Town
and Nishihara Village, and caused enormous
damage throughout Kumamoto Prefecture. This wasfollowed by frequent aftershocks, the number ofwhich was the highest ever recorded, and they
showed no signs of subsiding as of May. According
to the records observed at KiK-net Mashiki, in themain shock on April 16, the peak groundacceleration in the EW direction registered 1157 gal(Fig. 1), and the maximum velocity registered 127
kine (Fig. 2). Its seismic motion exhibited massive

Fig. 1: Acceleration time history of the main shockobserved at KiK-net Mashiki

destructive power comparable to that registered inthe 1995 Hyogoken-Nanbu Earthquake. The Japan
Society of Civil Engineers (JSCE) sent anemergency investigation team from the West Branch
and investigation teams from the Earthquake
Engineering Committee and the GeotechnicalEngineering Committee to assess the damage. This
reportprovidesaquicksummaryoftheinvestigation results.
Damage to bridges
Bridges of the Kyushu Expressway and Prefectural
Highway 28 and in the Minamiaso region wereseverely damaged.
On the Kyushu Expressway, the piers of theKiyamagawa Bridge and Akizugawa Bridge, which
had been reinforced against earthquakes, were not

Fig. 2: Speed time history of the main shock observed atKiK-net Mashiki (High Pass Filter: 0.05 Hz)damaged; however, most of the steel bearings were1damaged, and beams were displaced from the bearings, causing anuneven road surface. Also, several overbridges were damaged. TheFuryo Overbridge collapsed onto the expressway following the main
shock early on the morning of April 16 (Photo 1).
On Prefectural Highway 28, there were several damaged bridges that
had been built based on design standards established after the 1995
Great Hanshin-Awaji Earthquake. On the Okirihata Bridge, which isa five-span continuous bridge, rubber bearings were broken at every

Photo 1: Collapse of Furyo Overbridge
onto the Kyushu Expressway

abutment and pier, except for one pier(Photo 2), and as a result, beams moved
in the bridge-axial rectangular direction,and reinforced concrete bridge pierswithunbroken

bearings

sufferedbending damage. Additionally, on theTawarayama

Bridge,therewasdisplacement of beams from the rubber
bearings and buckling of steel beams.
In the Minamiaso area, the Aso-Ohashi

Photo 2: Abutment of Okirihata Bridge with broken rubber bearings

Bridge collapsed and was carried away
downstream due to slope failure (Photo 3). On the Minamiaso Bridge,
which had been reinforced against earthquakes with dampers, parts
mounted with dampers working in the bridge-axial direction weredamaged.
The investigation of damaged bridges indicated that in many cases,
proactive measures such as the installation of a collapse prevention
system had worked; on the other hand, however, there were also
cases in which a structure with lateral displacement constraints was

Photo 3: Collapse of Aso-Ohashi Bridge
due to slope failure

destroyed, exposing its weakness. These investigation results call forfurther study of measures to be taken against the collapse of superstructures.
Damage to stone bridges and the ruins of Kumamoto Castle
These two earthquakes, the foreshock and the main shock, both of which
had a seismic intensity of 7, devastated many of the stone bridges inKumamoto Prefecture and the ruins of Kumamoto Castle. These included
the Tsujunkyo Bridge (constructed in 1854), which is a nationally
important cultural property, the Nagayama Bridge (1878) and Tatekado
Bridge in Kikuchi City, which are designated as cultural property by
Kumamoto Prefecture, and the Yasemegane Bridge (1885) andMonzengawamegane Bridge (1808) in Mifune Town, which are also
designated as cultural property by Kumamoto Prefecture. On these bridges,stone walls collapsed and arch stones were damaged. On the face of theTsujunkyo Bridge, a significant number of cracks with a width of 5–10
cm occurred in the covering soil between the stone wall and the stone2

Photo 4: Deformation of the upper partof the stone wall of Tsujunkyo Bridgepipe portion, and as a result, the upper part of the stone wall
swelled outward (red circle in Photo 4). It was also found thatgaps had developed at many of the joint parts of the stone water
pipes, and extensive damage and flaking were found in theplaster parts, indicating possible water pipe damage. Thesefindings show that further investigation is needed to clarify thedamage status of the water pipes. The photos of Shimotsuru
Bridge (Uki City, Photo 5) show the collapse status of themasonry wall and bridge railings (Photo above), the wide gapsthat developed between the arch stones and the cracking of thosestones (Photo below). The development of gaps between thearch stones was found to be very similar to the arch stone
behavior observed in a vibration experiment using an arch stone
model. According to the report, however, the collapse of archstones did not occur in these major earthquakes. Typical types ofdamage include collapse and swelling of stone walls and fallingof bridge railings. On the other hand, major damage occurred in
the ruins of Kumamoto Castle (Fig. 3) including the complete
collapse of Kitajuhachiken-Yagura, Higashijuhachiken-Yagura,Goken-Yagura, Akazunomon and Nagabei, which are nationallyPhoto 5: Collapse status of the masonry wall ofShimotsuru Bridge (Uki City) and gaps thatdeveloped between the arch stones

important cultural property, damage to26 structures such as turrets (yagura)and gates, and the collapse of stonewalls. Damage was found at 52
locations, and much of it was caused by
the main shock.

Fig. 3: Major damage to the ruins of Kumamoto Castle (Article fromMay 8 Morning Edition of Kumamoto Nichi Nichi Shimbun)

Landslide disasters
The 2016 Kumamoto Earthquake caused landslides in Oita Prefecture, Saga Prefecture, Nagasaki Prefecture,Miyazaki Prefecture and Kagoshima Prefecture as well as in Kumamoto Prefecture; the total number oflandslides has not yet been determined. According to the disaster information from the Ministry of Land,Infrastructure, Transport and Tourism (MLIT), however, the number of landslides was 125 as of May 13, and 94
of them occurred in Kumamoto Prefecture. There were ten victims (one missing person included), who werekilled by mud avalanches. Many landslides occurred in the area including Minamiaso Village, where seismic
motion was strong, and it is assumed that most of the landslides occurred early on the morning of April 16,
when the main shock hit this area.
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Photo 6: Large-scale slope failure thatcaused the collapse of Aso-Ohashi Bridge

Photo 7: Collapsed sediment and rockmass deposited on gently inclined area

Photo 8: Surface failure that occurred on a
gently inclined slope

Photo 9: Weak argillaceous stratum, thelayer that caused the landslide, had formedin the lower part of the akaboku soil
stratum. The Kusasenrigahama pumice fall
deposit can be seen under this layer.

Photo 10: Surface failure on the south
slope of Mt. Eboshidake

Photo 11: Collapsed sediment blocked theoutlet of a valley, downstream of which isthe Sannodani River.

Photo 12: Collapse of the road leading toAso Choyo Ohashi Bridge

Photo 13: Damage and collapse of theaccess road connected to the Aso Choyo
Ohashi Bridge

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The landslide disasters in the earthquake were mostly characterized by ground collapse at areas where thesurface layer consisted of volcanic ash soil. The following are major examples of the collapse: (1) It is assumed
that large-scale slope failure (deep-seated landslides) (Photo 6, 7) occurred when a massive vibration hit theslope that had a weak plane, which had formed under the influence of weathering acting on the lava with cracks
developed in the surface layer of volcanic ash and the basement stratum. Since the upper part of the area where
the collapse occurred was steeply inclined, the massive vibration hitting that part caused cracking, leading to thecollapse of the surface layer. It is considered that this collapse caused the collapse of the weathered lava layer,
which is believed to have supported the surface layer, thus resulting in the large-scale collapse. (2) This was thecollapse of a slope with a gradient of less than 20° (Photo 8). There was a very weak stratum (Photo 9), which
had formed through the argillation of pyroclastic fall material, beneath the relatively thick volcanic ash soil
layer (kuroboku soil/akaboku soil). When hit by the massive vibration, the weak stratum became fluidized andslid off a flattened basement stratum, the Kusasenrigahama pumice layer, resulting in a large-scale mass failure.
It is considered that this was the collapse of volcanic ash soil layers and pyroclastic fall material including very
few boulders. (3) As shown in the surface failure on Mt. Eboshidake (Photo 10), in the area around the central
cone, volcanic ash soil layers of the steeply inclined slope collapsed in a continuous manner, resulting in a greatamount of collapsed sediment. Some of the soil fell into mountain streams and caused mud avalanches; however,most of the soil fell into streams and remained there (Photo 11), raising concerns about more disastrous mudavalanches after a heavy rain or intense rainstorm during the rainy or typhoon seasons in the future.
Furthermore, in any type of slope collapse, large cracks are found on the top or side of the upper parts of thearea where the collapse occurred, which shows that further slope collapse can take place at any time. (4) As
shown in the picture of the road leading to Aso Choyo Ohashi Bridge, roads collapsed and were damaged(Photo 12). This road collapse appears to have been caused by the collapse of volcanic ash soil on lava, since
the inclination of the collapsed part is gentle. Also, collapse and damage of the road constructed in the steeply
inclined area near Aso Choyo Ohashi Bridge were remarkably severe (Photo 13).
Many small-scale slope failures also occurred, and a great amount of soil collapsed into the same mountainstream, and it remains on the slopes or in the stream, threatening to cause mud flows and devastation of thedownstream area after a heavy rainfall. Considering the current situation, and since it takes a lot of time toprepare the hardware aspects of disaster-prevention measures, presently it is most important to enhance thesoftware aspects of measures among the local people so that they can escape disaster immediately and save theirown lives. Furthermore, since deformation such as cracking has developed in some places of the slopes in theareas hit by intense quakes, it is necessary to stay on the alert against slope collapse.
Damage to lifelines
The earthquakes on April 14 and 16 brought about severe
damage to lifelines such as electricity, gas and watersupply (Fig. 4). About 200,000 households temporarily
lost power, and as of the night of April 16, about 80,000
households were still without power, which wasrecovered by April 20. Regarding water supply, up to
370,000 households were without water until one week
later when water could be supplied to 90% of thehouseholds; however, about 4,000 households were still
without water as of May 10. City gas supply wasFig. 4: Restoration status of city gas, electricity and watersupplysuspended for about 100,000 households, but was
gradually restored from April 20 and then completely5

restored by the end of April. Also, many general waste disposal facilities, which are closely linked to daily life,
were damaged. Due to damaged combustion furnaces, four garbage disposal facilities were unusable for a
prolonged time, making it difficult to dispose of disaster waste. As shown in Fig. 5, the number of evacuees,
which once exceeded 180,000, decreased with the restoration of water, gas and electricity supply; however,about 10,000 people were still living in 251 shelters as of May 11.
Traffic-related facilities were also severely damaged.
On the Kyushu Expressway, the section between theUeki Interchange and the Yatsushiro Interchange wasclosed to traffic, but the entire expressway reopened
on April 29. In addition, many roads including
National Routes 3 and 57 and other highways wereclosed to traffic due to damage such as sedimentdisasterandroadcollapse.

Thetransportation

infrastructure was badly damaged, which also affectedpublic transportation. After the derailment of an
out-of-service passenger train, operation of the Kyushu

Fig. 5: Changes in the number of evacuees and shelters

Shinkansen was suspended, and then all other JR Kyushu
train services were also suspended in the prefecture. The Minami Aso Railway was severely damaged bylandslides, and presently it is not clear when the line will resume normal service. Kumamoto Airport was closed
due to the collapse of ceilings in the terminal building and all flights were canceled, and due to the damage tothe movable bridge in Kumamoto Port, all ferry services were canceled. All express bus services and all regularbus services except for those in the Arao City area and the section between Arao and Tamana were canceled,and Kumamoto City Streetcars and Kumamoto Electric Railway suspended operations. The Shinkansen line
partially resumed service on April 20, and service on the entire line resumed on April 27. Kumamoto Airport
resumed operations on April 19, and Kumamoto Port on April 22. Kumamoto City Streetcars partially resumed
service on April 19 and service on the entire line resumed on April 20, and Kumamoto Electric Railway
partially resumed service on April 20 and service on the entire line on April 23. Regular bus services and JR
train services have been progressively resumed.
City halls and town halls, which can also serve as
disaster-prevention centers, became unusable. Photo
14 shows the city hall building of Uto City, in which
the fourth floor of the five-story building nearlycollapsed. Also, Yatsushiro City Hall, Hitoyoshi CityHall, Mashiki Town Hall and Ozu Town Hall became
unusable and could not serve as disaster-preventioncenters. Additionally, foundation hospitals including
Kumamoto City Hospital were not able to takecritical-care patients due to damage to buildings andinterruption of electricity, gas and water supply, whichbrought about confusion at medical sites. SuchPhoto 14: City hall building of Uto City

damage to the disaster-prevention centers hinderedassistance for those affected.
Finally, we express our heartfelt condolences to the families of the victims of the series of earthquakes centeredon the Kumamoto region, and our sincerest sympathy for all those affected by the disasters.
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Acknowledgment:This report was prepared based on the seismic motion records provided by the Kiban Kyoshin network(KiK-net) of the National Research Institute for Earth Science and Disaster Resilience (NIED). We take thisopportunity to express our sincere gratitude.
References:(1) Japan Society of Civil Engineers (JSCE)-West: Emergency disaster report on the 2016 KumamotoEarthquake (First report-tenth report)(2) Investigation Results of the 2016 Kumamoto Earthquake (Materials for Preliminary Reports)(3) Ministry of Land, Infrastructure and Transport (MLIT): Disaster Information on Earthquakes Centered on
the Kumamoto Region of Kumamoto Prefecture (35th Report) http://www.mlit.go.jp/common/001131315.pdf,
at 6:00 on May 13, 2016
The original text of this article is published in JSCE Magazine Vol. 101, No. 7 July.
http://www.jsce.or.jp/journal/thismonth/201607.shtml
The authors are as follows:
Taiji Matsuda: Professor, Kumamoto University Graduate School (JSCE-West, Investigation Team Leader)
Ryuji Kakimoto: Professor, Kumamoto University Graduate School (JSC-West, Investigation Team Member)Motoyuki Suzuki: Professor, Yamaguchi University Graduate School (JSCE Geotechnical Engineering Committee,Investigation Team Leader)
Toshitaka Yamao: Professor, Kumamoto University Graduate School (JSCE-West, Investigation Team Member)
Yoshito Kitazono: Emeritus Professor, Kumamoto University (JSCE-West, Investigation Team Member)
Yoshikazu Takahashi: Associate Professor, Kyoto University Graduate School (JSCE Earthquake Engineering Committee,
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