Seismic Activity in Aki Nada

The focal mechanism was a normal fault type with an east-west tension axis. The aftershock zone extends a little more than 20 km to the north and south, and is distributed at depths from 40 to 50 km. From these data it is believed that the fault moved along a north-south strike. The spread of the aftershocks also roughly correspond to the magnitude of the event.

The source of this event was located within the subducting Philippine Sea Plate, facing northwest from the Nankai Trough situated approximately 100 km south of Shikoku in the northern tip of the seismically active zone. The same plate suddenly tilts to the west (toward Kyushu) near the earthquake source.

From these data, it is believed that this earthquake was caused when the Philippine Sea Plate was pulled and twisted to the west, damaging it.

*The results of GPS observation of the area showed horizontal crustal movement, accompanying the occurrence of the earthquake. The movement was southward in the area north of the epicenter and northward to its south, and corresponded to the earthquake's focal mechanism.

*Aftershock activity to this event is at a generally low frequency, but includes a somewhat higher than average number of strong events. A look at past incidence for the Aki Nada area shows that there was an M7.3 event, the Geiyo Earthquake, in 1905, and an M6.2 earthquake in 1949. There were three aftershocks of M6.0 or greater (including a maximal M6.2 aftershock 6 months after the main shock) following the 1905 event.

It is believed that an aftershock on the order of M5 to this event, would result in a seismic intensity of around 4 in central Hiroshima Prefecture.

*Further study and research is needed into the seismic activity occurring with the subducting plate.

• Seismic activity in Aki Nada(1)
• Seismic activity in Aki Nada(2)
• Seismic activity in Aki Nada(3)
• Movement vector chart for fixed zone 950385
• Figure of the Philippine Sea Plate and the current earthquake