A mega-quake advisory by Japan's Meteorological Agency remains in place five days after it was issued. Agency officials say they haven't observed any irregular changes in the earth's crust where the jolt is expected to occur.

The agency put out the advisory on Thursday after a magnitude-7.1 earthquake struck off Miyazaki Prefecture in southern Japan. The tremor triggered tsunami that reached the Kyushu and Shikoku regions, including a 50-centimeter tsunami in Miyazaki City.

Officials say the probability of a massive earthquake is higher than usual along the Nankai Trough, which runs from Shizuoka Prefecture west of Tokyo down to the southern tip of Kyushu.

Thursday's quake registered a maximum intensity of lower 6 on the Japanese scale of 0 to 7 in the city of Nichinan. Intensities of upper 5 were recorded in parts of Miyazaki Prefecture and neighboring Kagoshima Prefecture.

Since Thursday, a number of quakes have been observed in the Hyuganada Sea off Miyazaki and waters off the Osumi Peninsula in Kagoshima.

The agency says minor seismic activity on the Kii Peninsula and in the Ise Bay in western Japan has affected the readings of strain meters placed nearby. But they say this isn't unusual.

The agency says its advisory does not mean that a quake will definitely strike within a certain timeframe. But it has asked 707 municipalities in 29 prefectures -- extending from around Tokyo in the east to Okinawa in the south -- to be prepared for the possibility of disaster.

Authorities say people should make sure furniture is secure as possible, and figure out the locations of their nearest evacuation sites.

Households with young children, the elderly and people with disabilities are advised to consider evacuating voluntarily.