Only the hypocenters with high accuracy are being shown. They were determined by the data from not only the seismic stations on land but also the ocean bottom seismograph just above the hypocenters. The site corrections were also applied.
Hypocentral determinaton in the sea area essentially requires observation data obtained just above the hypocenters. We have installed the self-floating ocean bottom seismograph in cooperation with Hydrographic Department Japan Coast Guard,Japan Marine Science and Technology Center. This hypocentral distribution figure was obtained by analyzing some of those data. Although the number of analyzed earthquakes is limited, the accuracy about the depth of hypocenters increased and it showed clear features of the distribution. When you make a cross section by rotating from the north to the west, you can see the distribution on the sheer surface. In depth of more than 7km they are spreading in the area shaped like a sheet 2km thick, and in depth of less than that hypocenters have a wider distribution.
In the northwest of the presently active east coast off the Kozu-shima Island, there was a certain area where few earthquakes were observed. However, since the activity on August 3, some earthquakes have occurred in that area. From the night of 23 to the morning of 24, activity had occurred in the sea area close to the Miyakejima Island. They were spreading in the wide area.
The activity which had begun in the Miyakejima Island moved toward the sea near the Kozushima Island. Hypocenters have moved many times since the M6.4 earthquake.
Hypocentral distribution of past five years, and recent activity Locations of the ocean bottom seismographs during July 2000.
Hypocentral distributuon of recent five days, and all about recent activity Locations of the ocean bottom seismographs from August The Buoytellemeter was installed to enable to collect the real-time data. Mechanisms of major earthquakes are presented.
Enlarged figure of the east coast off the Kozushima Island Time series and distribution of depth The hypocenters of strong activity since the evening of August 3 have been moving to both northwest and the southeast directions the northwest and the southeast
We had installed 24 ocean bottom seismographs in cooperation with JCG and JAMESTEC. It is necessary to upgrade the low accuracy of depthwise, as hypocenters are located in the sea area.
The data obtained just above the hypocenters have increased theaccuracy of depthwise detrmination. Most of the earthquakes are at 10km deep or shallower.
There is a cylindrical hypocentral distribution with a diameter of aprox.1km just below the summit. Seismicity had been active before August 8 when the intermittent eruption began at the summit. Though seismicity has been still active, arrival of P and S waves has become less clear,and so it becomes more difficult to determine hypocenters.
