April 15, 2016
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On April 14 at 21:26 (JST), there was a magnitude (M) 6.5 (provisional) earthquake at a depth of approximately 10 km in the Kumamoto region, Kumamoto prefecture. This event had a maximum seismic intensity 7 observed in Kumamoto prefecture and caused damage. The seismic activity follows the pattern of main shock-aftershocks sequence. The maximum aftershock, as of April 15 at 18 o'clock, is a M6.4 (provisional) earthquake that occurred on April 15 at 00:03, the maximum seismic intensity 6 Upper being observed. Although the aftershock activity seems to be declining, it is still active. The aftershock area extends approximately 20km in an NE-SW direction.
* The focal mechanism 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 of the main shock is a right-lateral strike-slip fault extending in an NNE-SSW direction.
* Accompanying this event, large acceleration values were observed, such as 1580 gal (synthesis of 3 components) at Masuki observatory of KiK-net in Kumamoto prefecture.
* According to the GNSS observation, crustal deformations associated with the main shock and the maximum aftershock were observed, such as a displacement of approximately 20 cm to the north-north-east direction at Jonan observatory in Kumamoto prefecture (provisional).
* In the vicinity of the focal area, there is the Hinagu fault zone. The recent seismic activity is 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 M 6.8 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 %.