The Mainichi Shimbun answers some common questions readers may have about a "half-cracked" scenario for a predicted megaquake along the Nankai Trough off western to central Japan.

Question: I've heard that a Nankai Trough quake may occur in two halves. What does that mean?

Answer: That's called a "half-cracked" case. The areas that could potentially be the focus of a Nankai Trough temblor stretch extensively from off the coast of Miyazaki Prefecture to the west to off the coast of Shizuoka Prefecture to the east. If the entire areas slip all at once, that's called a "fully-cracked" case, which would cause enormous damage. Meanwhile, if a magnitude-8 earthquake struck either on the eastern or western side along the trough, it could be followed by another quake on the other side at a different time. That's called a "half-cracked" case.

Q: So earthquakes can occur twice at different locations?

A: It's possible. But the other quake may occur several hours later, several days later, or even several years later.

Q: What kind of situation can we expect in such a case?

A: The initial temblor would naturally topple buildings and cause tsunami among other damage, but because unaffected areas also face a greater seismic probability, it is presumed that delivery of relief supplies to disaster areas and recovery efforts would be stalled. Furthermore, regions on the border of the "half-cracked" model could be rocked by another violent temblor, leaving the areas suffering prolonged damage.

Q: Has a half-cracked earthquake ever hit Japan in the past?

A: The 1944 Tonankai earthquake and the 1946 Nankai quake struck only two years apart during the Showa era (1926-1989), while the Ansei era (1854-1860)'s Tokai quake and Nankai quake occurred just around 30 hours apart in 1854. Meanwhile, Japan last experienced a "fully-cracked" case in 1707 when the Hoei earthquake hit. Over the past roughly 300 years, there were more half-cracked tremors than fully-cracked ones.

Q: Is there any order of quakes occurring between the eastern and western side?

A: According to professor of geodesy Takuya Nishimura at Kyoto University's Disaster Prevention Research Institute, historically earthquakes occurred on the east side first, followed by another on the west side, with areas around the Kii Peninsula being the border. "However, it would be no surprise if a temblor struck on the western side first," Nishimura noted.

(Japanese original by Yukako Ono, Lifestyle, Science & Environment News Department)