TOKYO -- Thousands of people were set to spend a third night in makeshift shelters in northern Japan on Monday, as a series of aftershocks rattled an area where hundreds had been injured in more powerful earthquakes over the weekend.

Around 500 people were injured and a similar number of homes damaged or destroyed in Saturday's quakes in Miyagi prefecture, 300km (186 miles) north of Tokyo. The tremors also triggered landslides and power cuts.

Residents' nerves were frayed by the continuing quakes, although there were no reports of further substantial damage, Reuters reported.

"How long will these earthquakes last? It's frightening really," one elderly woman told domestic broadcaster TV Tokyo at a school gymnasium in the small town of Nango, where evacuees were bundled up in blankets.

More local people joined them on Monday after their previous shelter was damaged by the tremors.

Japan is one of the world's most earthquake-prone nations, sitting on the junction of at least three tectonic plates, slabs on the earth's surface whose movements cause quakes.

In 1995, a strong quake hit the city of Kobe 435km west of Tokyo, killing nearly 5,000 people and injuring 15,000. Tokyo's last major quake in 1923 killed more than 140,000 in the capital and surrounding area, and experts say the capital is long overdue for another. Monday's magnitude 5.0 tremor hit northern Miyagi prefecture, shortly after 04:00 A.M. (1900 GMT Sunday).

That quake came two days after bigger tremors on Saturday. The first, measuring 5.5 on the Richter scale, hit shortly after midnight, followed by another measuring 6.2 about seven hours later.

"Amid the continuing aftershocks, we are very concerned about what is likely to be significant physical and mental strain," Chief Cabinet Secretary Yasuo Fukuda told reporters, referring to the plight of local residents. ---------------Not the "Big One"-------------

The weekend quakes were not thought to be related to the cyclical killer quake which has hit the region every 30-40 years and last struck in 1978, killing 28 people.

Saturday's temblors apparently occurred near a fault line which experts had thought was dormant and hence was no longer being researched, the ***Asahi*** newspaper reported.

"A quake of this relatively small size could occur anywhere in Japan, at any time," said Hiroyuki Fujiwara, an official at the National Research Institute for Earth Sciences and Disaster Prevention.

An increase of one point on the Richter scale means a tenfold increase in amplitude, so a magnitude six quake is 10 times as strong as a magnitude five.

About 2,500 people remained evacuated from their homes as of Monday morning, according to a Miyagi prefecture official, who said the number might rise by nightfall. At least three more aftershocks struck later in the day, including one with an estimated magnitude of about three.

Some victims were stoical. "We are fortunate -- no one was injured, although our house was totally destroyed," another elderly woman at the Nango shelter told TV Tokyo.