Hundreds of aftershocks have rocked the ground and frayed nerves in the five weeks since Japan's massive earthquake and tsunami, forcing survivors to relive the terror almost daily.

The incessant rumbling of the Earth's stressed crust has held back relief work, imperilled already dangerous operations to contain a nuclear crisis and fuelled fears far beyond the coast that was devastated by the giant wave.

Many now complain of "earthquake sickness" -- the sensation that the ground is swaying beneath their feet even when it is not -- a condition blamed on confused inner-ear balance receptors and a heightened state of anxiety.

For the tens of thousands living in spartan and crowded evacuation shelters in and near the tsunami wastelands, the creaking of already weakened buildings and the risk of another killer wave spark mortal fears.

"We are almost getting used to the aftershocks, yet every time one of them strikes, we are reminded of the terror we felt the day of the tsunami," said Kenichi Endo, 45, who lost his fisherman father at sea to the monster wave.

"I become afraid that maybe it will return," said Endo, now one of 790 people holed up in an elementary school turned evacuation centre in the devastated port of Onagawa in Miyagi prefecture. "I have flashbacks."

In Tokyo too, where buildings have been shaken and trains halted by quakes, millions are put on edge every time a shrill seismic early-alert tone sounds on TV or their mobile phones, warning of a fresh threat.

Since the 9.0-magnitude quake shifted the seafloor by 24 metres (yards) and sent a huge wave crashing into Japan, more than 400 quakes above magnitude 5.0 have hit, most below the sea but many beneath Japan itself.

Maps show their epicentres spread out like an angry rash across the Pacific seafloor east of Japan, one of the world's most quake-prone and volcanic places on the intersection of several tectonic plates.

Geophysicists agree that the jolts and rumbles will not stop any time soon. They only differ on whether they will go on for months, years or even a decade.

A powerful 7.1-magnitude aftershock struck on April 7 followed by a series of shocks above 6.0 this week, with the biggest one prompting a tsunami scare, one of several issued and lifted since the monster quake.

At the Onagawa evacuation centre, the lights went off on April 7 and hundreds poured out of the buildings where they shivered in the cold for about an hour until the tremors subsided and they were allowed back inside.

"I thought the ceiling was going to collapse," said evacuee Keiko Katsumata, 57, who said she had been in poor health. "Just when I thought my life was starting to move forward again, little by little, these aftershocks came."

She said the rumbles bring back bad memories and sickening feelings of guilt: "I think of the time when I should have urged my friends to flee. If I could relive March 11, I think I would try to do more to help them escape."

Town official Kiyoto Abe said that "the aftershocks are adding another layer of stress for evacuees. In neighbouring towns, I've heard evacuation centres were damaged by the aftershocks and needed to relocate to other areas.

"It's been a kind of double evacuation."

The effect of the jolts has not just been psychological -- they have also set back brick-and-mortar efforts to rebuild the lives of traumatised people.

The 7.1-quake triggered deadly landslides, cracked buildings, shattered windows and weakened the terrain, and it also delayed the scheduled construction of new temporary housing, Abe said.

"We are checking which areas will be approved for construction," he told AFP as work was ongoing for just 57 temporary houses, with plans for another 169 buildings currently on hold because of safety concerns.

Aftershocks centred near the tsunami-damaged Fukushima nuclear plant have also repeatedly forced emergency crews there to evacuate, and led to fears the charred reactor buildings and water pipes may be further compromised.

Radiation fears have also haunted some in the Onagawa evacuation centre, which lies just five kilometres (three miles) from a coastal nuclear plant of the same name that was also damaged on March 11 but went into full shutdown.

"I am afraid that another tsunami may knock out the Onagawa nuclear plant and cause it to leak radioactive material, like in Fukushima," said Endo, one of the evacuees, adding that he was worn out from many sleepless nights.

"If that happens, we'll have nowhere to flee. We are at the frontline."