| 22 August 2000 |
| Headquarters for Earthquake Research Promotion |
| Earthquake Research Committee |
Seismic activity in the area from Miyakejima to Niijima-Kozushima
Since August 9th, the seismic activity in the area from Miyakejima to Niijima-Kozushima has continued to rise and fall. In particular, in the period from August 15th to the 16th, there was increased seismic activity including a maximal M4.9 event and maximum seismic intensity of 5 lower in an area about 10 km southeast of Kozushima. Also, on August 18th, there was increased seismic activity measuring maximum seismic intensity of 6 lower, and including an M6.0 event about 8 km east of Kozushima, and an M4.9 event about 3 km southwest of Shikinejima. The results of GPS observation of the area showed crustal movement in the area of the M6.0 event. Furthermore, there was increased seismic activity in Sea Zone 2 immediately following this event.
The daily average incidence of M3.5 and greater events in Sea Zone 1 was about 60 events per day during the period from June 26th to July 13th, and about 30 events per day from July 14th to August 8th. During this period, there were repeated occurrences of burst seismic activity (concentrated occurrence of events in the space of a few hours). Additionally, there were 40 events of M5.0 or greater recorded in the three sea zones in the period from June 26th to August 3rd.
However, as the average daily incidence of M3.5 and greater events in the period from August 9th to August 21st was about 10 events per day, and there has only been one event of M5.0 or greater since August 4th, it appears that the trend will be toward longer periods between burst seismic activity than have been seen to date.
*The results of GPS observation of the area show no marked changes in the trends observed in mid-July, namely the movement of Niijima to the northeast and of Kozushima to the southwest, indicating magma activity to the east of Kozushima. In addition, the movement levels in Izu Oshima and the Boso Peninsula have been reducing slightly since mid-August.
*From the above, and considering that the seismic activity and crustal movements are now occurring in previously unseen trends, and that the crustal movements are continuing, it is believed that the possibility of relatively strong earthquakes occurring concentrated in a short period of time remains. Similarly, should another event of similar magnitude (maximum near M6.0) occur, depending on the location, the possibility of seismic intensities of 6 lower cannot be ruled out at present. However, considering the new trend, it is believed that the period of time between occurrences of burst seismic activity will lengthen as the seismic activity levels off.