The residents of northeastern Japan live with constant reminders of the powerful March earthquake that sent a deadly tsunami crashing into their shores. Aftershocks occur almost daily. And many survivors sometimes feel the ground shaking even when it is not. Doctors say the sensation is just one of the symptoms of the "earthquake sickness" that many survivors now have to contend with, on top of everything else. Here, a brief guide:

What is "earthquake sickness"?
The term refers to a collection of sensations earthquake survivors sometimes experience, even long after the ground stops trembling. The main symptom is dizziness, which doctors in Japan have seen a lot of lately. The condition is "similar to motion sickness," says Dr. Hideaki Sakata of the Mejiro University Clinic, as quoted by AFP. But for people worried the next deadly temblor could hit at any time, another symptom — the phantom quake — can be the most stressful.

What is a phantom quake?
It's when someone is convinced the earth is rumbling under his feet although, in reality, it is perfectly still. "Sometimes I'm sitting with my friends and I tense up and say, 'Do you feel that?'" says Wakana Oyamada, an office worker in Shibuya, as quoted by The Irish Times, "and they haven't because it was in my imagination." Dr. Sakata says the sensation is similar to the lingering feeling of swaying people sometimes experience when they first get off a boat.

Are there any other symptoms?
Yes—anxiety. Understandably, earthquake survivors commonly worry that another big one could happen at any moment. That concern has been impossible to ignore in northeastern Japan. The area devastated by the quake and tsunami on March 11 has been rattled by more than 400 aftershocks of magnitude 5.0 or greater, with more than 65 of magnitude 6.0 or higher, and at least five of magnitude 7.0. Every one of them had the potential to push a few more people past the breaking point.

Has this hit the Japanese harder than earthquake survivors elsewhere?
Quite possibly. In Japan, every time an aftershock is strong enough to rattle the walls, people wonder whether it will send another tsunami crashing into the damaged Fukushima nuclear plant, releasing a new burst of radiation. "People are getting too sensitive," says Dr. Kazuhiro Soeda, as quoted by The New York Times. "This is something we've never experienced before."

Sources: AFP, Irish Times, NY Times, Allvoices