A series of moderate but shallow earthquakes shook the Izu Islands on Thursday, with the strongest jolting Shikine Island with an intensity of upper 5 on the 7-point Japanese intensity scale, the Meteorological Agency said.
The upper 5 tremor jolted the 3.7-sq.-km island and its less than 1,000 inhabitants at 2:32 a.m., followed by three other lower 5 intensity earthquakes at 11:19 a.m., 12:10 p.m. and 12:29 p.m.
Nearby Niijima was hit by a quake measuring 4 on the Japanese scale, while a level 3 temblor rattled Kozu island.
The maximum magnitude of the earthquakes was recorded as 4.9, the agency said.
Rockfalls were reported at two sites triggered by the first quake. There have been no reports of injuries, officials said.
Seismologists said the quakes were shallow and their focus was beneath the seabed off Niijima and Kozu islands.
An official of the Shikine branch of the Niijima village office on central Shikine Island said that when the first quake struck he felt the ground heave upward and then roll for approximately 10 seconds.
The agency warned residents to be cautious, saying that strong quakes with intensities of up to lower 6 may continue to affect the area over the next two weeks.
The islands, located between 140 km and 180 km south of Tokyo, have been shaken by a series of earthquakes since undersea volcanic activity began in the area in late June.
After a quake measuring lower 6 hit Niijima on Saturday there had been a lull in seismic activity, according to the Meteorological Agency. However, since Wednesday morning, that activity has been on the rise again, the agency said.
So far, the only fatality in the series of earthquakes was a man on Kozu Island buried in a landslide after a lower-6 level tremor struck July 1.