Yoshihisa Iio (ERI)
Velocities of the Koudushima, Shikinejima, and Niijima islands measured by the GEONET (GSI) are not constant but vary in time from June to August 2000. Further, magnitudes of the velocities of the Niijima Island are usually larger than those of the Shikinejima Island, although they are not true in some periods. It is inferred from a comparison between the time-dependent velocities and seismic activity that the velocities reflect the following two factors.
1. Which part does a dyke intrusion occur in, north or south of the fault
of the M6.4 earthquake occurred on July 1, 2000?
2. Does a large earthquake of M>=6 occur at a distance more than ten
kilometers from the area of the dyke intrusions?
Fig.1.
Displacements in the NS and EW directions of the Koudushima (8), Shikinejima (6), and Niijima (5) islands to the Minamiizu GPS station measured by the GEONET (GSI). Arrows indicate occurrences of large earthquakes of M>=6. The velocities are not constant but vary in time. It is seen the tendency that velocities of the Niijima island become small when those of the Shikinejima island become large. All the velocities become small for a few days after the occurrences of the earthquakes of July 15, and July 30, of which hypocenters are determined at a distance more than ten kilometers from the area of the dyke intrusions.
Fig.2a and 2b.
Velocities per day composed from the NS and EW components. Fig.2b is magnified one in the vertical direction. The characteristics of the velocities indicated above are clearly seen. Although the velocities from Aug. 10 to 18 show the same tendency as those after the occurrences of the earthquakes of July 15, and July 30, no large earthquake occur in the period at a distance from the area of the dyke intrusions. Instead, the Miyakejima volcanoes erupted in this period.
Fig.3.
Ratios of the composed velocities of the Shikinejima Island to those of the Niijima Island. It is found that the ratios become large after the occurrences of the earthquakes of M>=6 of July 1, July 9, and Aug. 18. The ratios also become large after July 24. This is perhaps related to the occurrence of the earthquake of July 24 6:52 Mw5.6(HVD). It is inferred from an epicentral distribution (ERI and JMA) that dyke intrusions occurred south of the fault of the earthquake of July 1 when these earthquakes occurred. The dyke intrusion south of the fault is likely to generate seismic and/or aseismic slips on the fault (Nagoya university, 2000) and the slips increase the velocities of the Shikinejima Island more than the Niijima Island.
