July 10, 2018
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Earthquake in the Northern Osaka Prefecture on June 18, 2018

* On June 18 at 07:58 (JST), there was a magnitude (M) 6.1 earthquake at a depth of approximately 15 km in the northern Osaka prefecture. The maximum seismic intensity 6 Lower was observed in Osaka prefecture and damage was caused by this event. The focal mechanism had a compression axis in a WNW-ESE direction. This event occurred within the crust. In the area around the event, there are Arima-Takatsuki fault zone, extending in an E-W direction, and Ikoma fault zone and Uemachi fault zone, both extending in an N-S direction.

* The largest aftershock until July 10 at 16 o'clock, is an M4.1 earthquake on June 19 with maximum seismic intensity 4 observed. The aftershock activity, being confined in the area of about 5 km length in EW and about 5 km length in NS, is decaying but still continues. Aftershocks are distributed on the plane dipping to east in the northern part of the aftershock area, and on the plane dipping high to south-east in the southern part. The focal mechanisms are mixed with strike-slip fault type and reverse fault type. Aftershocks with strike-slip fault type occur in the whole aftershock area, those with reverse fault type occur in the northern part.

* The seismic fault of the M6.1 earthquake, estimated from the focal mechanism and the distribution of aftershocks, is generally composed of two northern and southern faults. The northern fault is a reverse fault dipping to east and the southern fault is a right-lateral strike-slip fault dipping high to south-east.

* According to the GNSS observation, very slight crustal deformation accompanying this event is reported.