May 13, 2016
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On April 14 at 21:26 (JST), there was a magnitude (M) 6.5 earthquake at a depth of approximately 10 km in the Kumamoto region, Kumamoto prefecture. In addition, on April 16 at 01:25 (JST), there was a M7.3 earthquake at a depth of approximately 10 km in the same region. These events had maximum seismic intensity 7 observed in Kumamoto prefecture and caused damage.
* The series of seismic activity prevails from Kumamoto region, Kumamoto prefecture to central Oita prefecture. In Kumamoto region, Kumamoto prefecture seismic activity is high in the region of about 50 km length extending in an NE-SW direction. In addition, in Aso region, Kumamoto prefecture maximum seismic intensity 6 Upper was observed in Kumamoto prefecture due to the M5.8 earthquake on April 16. Also in central Oita prefecture an active seismic activity has been observed just after the occurrence of M7.3 earthquake on April 16: maximum seismic intensity 6 Lower was observed due to another earthquake which occurred just after the M7.3 earthquake.
* The focal mechanism of the M6.5 earthquake on April 14 showed a strike-slip fault type with a tension axis in an NNW-SSE direction. This event occurred within the crust at a shallow depth. The source fault estimated from the distribution of aftershocks and the focal mechanism is a right-lateral strike-slip fault extending in an NNE-SSW direction.
* The focal mechanism of the M7.3 earthquake on April 16 showed a strike-slip fault type with a tension axis in an N-S direction. This event occurred within the crust at a shallow depth. The source fault estimated from the distribution of aftershocks and the focal mechanism is a right-lateral strike-slip fault with normal fault component extending in an NE-SW direction.
* Accompanying these earthquakes, large acceleration values were observed: 1580 gal (synthesis of 3 components) at Masuki observatory of KiK-net in Kumamoto prefecture for the M6.5 on April 14 and 1791 gal (synthesis of 3 components) at the municipal observatory of Otsu-Cho, Kumamoto prefecture for the M7.3 on April 16.
* According to the GNSS observation, crustal deformation associated with the M6.5 earthquake on April 14 and the M6.4 earthquake on April 15 such as a displacement of approximately 20 cm to the north-north-east direction at Jonan observatory in Kumamoto prefecture was observed. Also crustal deformations associated with the M7.3 earthquake on April 16 such as a displacement of approximately 98 cm to the south-west direction at Choyo observatory in Kumamoto prefecture. The analysis of observation data by the Advanced Land Observing Satellite-2 "DAICHI-2" shows an areal extension of crustal deformation, from Kumamoto region, Kumamoto prefecture to Aso region, and large deformation is observed along the Futagawa segment of the Futagawa fault zone and the Takano-Shirohata segment of the Hinagu fault zone. From these crustal deformations, the length of the source fault which slipped was estimated to be about 35 km.
* In the vicinity of the focal areaｓ of the M6.5 earthquake on April 14 and the M6.4 earthquake on April 16, there is the Hinagu fault zone. These events are considered to be due to the activity of Takano-Shirohata segment of it. The Earthquake Research Committee evaluated for the Hinagu fault zone (Takano-Shirohata segment) that a M6.8 or so earthquake is possible to occur at an active time and the probability value of the earthquake occurrence within 30 years is uncertain. In addition, the probability of earthquake occurrence of M6.8 or over for the southern Kyushu area including Hinagu fault zone (Takano-Shirohata segment) was evaluated as 7-18 %.
* In the vicinity of the focal area of the M7.3 earthquake on April 16, there is the Futagawa fault zone. This event is considered to be mainly due to the activity of Futagawa segment of the Ftagawa fault zone. The Earthquake Research Committee evaluated for the Futagawa fault zone (Futagawa segment) that a M7.0 or so earthquake is possible to occur at an active time and the probability value of the earthquake occurrence within 30 years is 0%~0.9% (rather high). In addition, the probability of occurrence of earthquake of M6.8 or over for the southern Kyushu area including Futagawa fault zone was evaluated as 18-27 %.
* According to the field survey, a surface seismic fault of about 28 km length was found along the Futagawa segment of the Futagawa fault zone, and a surface seismic fault of about 8 km length was found along the Takano-Shirohata segment of the Hinagu fault zone. In the neighborhood of Dozono, Mashiki-machi, a right-lateral displacement of maximum 2.2 m occurred. In some segments surface seismic faults with normal-fault component of north decline were found.
* Although the series of seismic activity is decaying as a whole, the activity in the Kumamoto region, Kumamoto prefecture and Aso region is still rather high. The activity in the central Oita prefecture is decaying.
* In cases of the Mid Niigata prefecture Earthquake in 2004 (M6.8) and earthquake in Hamadori, Fukushima prefecture in 2011 (M7.0), aftershocks of M5.0 or so occurred 1 to 2 months after the main shocks. Therefore aftershocks of M5 ~ 6 earthquake (maximum seismic intensity 6 Lower or so) in Kumamoto region, Kumamoto prefecture and Aso region, and aftershocks of M5 or so (maximum seismic intensity 5 Upper or so) in central Oita prefecture may occur and we should pay attention at least for 1 month or so from now on.
* In Kyushu region, as is the case for the earthquake in Aso region, Kumamoto prefecture (M6.1) and earthquake in central Oita prefecture (M6.4), both in 1975, and for the 1997 seismic activity in Satsuma region, Kagoshima prefecture (M6.6 and M6.4), the second earthquake of M of the same order with the first occurred in neighboring region, 2 or 3 months after the first. Therefore in the region extending from Kumamoto prefecture to Oita prefecture, we should be prepared to the possibility of being shaken with seismic intensity 6 Lower or over, which cannot be denied at least for 2 months or so from now on.
# "the 2016 Kumamoto earthquakes" (name after Japan Meteorological Agency) means a series of seismic activity in Kumamoto prefecture which occurred after April 14, 2016 at 21:26.