Almost 20 earthquakes were felt around Tokyo last Saturday morning, adding cause for concern as to when the “big one” may hit.

So far, 19 earthquakes and aftershocks, ranging from magnitude 2.2 to 5.5, have been felt in and around the areas near Mount Fuji, south west of Tokyo.  A 5.5 quake is nothing new to in Japan, and many in the area responded on twitter with a voice of annoyance and complacency more than concern.

Twitter reaction to Sat 28/1 earthquake

Twitter user @jljzen reacts nonchalantly to Saturdays earthquake. Picture: Twitter

Reuters reports that the shaking prompted authorities to briefly stop Tokaido bullet train services as a precaution, but these were resumed shortly after.

Although Saturday’s earthquakes resulted in no immediate reports of damage or casualties, it has brought to light concerns of when Japan’s next big earthquake will hit. While last year’s Great Tohoku Earthquake (literally) sent shockwaves around the world, that’s not the area that seismologists are concerned about.

The Global Post reported that the Tokyo University’s Earthquake Research Institute predicts the probability of a quake greater than magnitude 7 hitting Japan’s capital in the next four years could be as high as 70 per cent.

The probability of such a large earthquake increased to 98 per cent when looking over a 30 year period.

And this is why the seismic activity around Mount Fuji is so important.

Seismologists and bureaucrats already think Japan’s next big earthquake will happen somewhere south west of Tokyo, near Mount Fuji. Even though it hasn’t happened yet, they’ve already given it a name: the Tokai Earthquake.

Seismologists are predicting Japan's next big quake will occur near Mount Fuji. Pic: AP.

North east of Mount Fuji lies the Tokai fault, arguably Japan’s most watched fault line. Japan has been warning its citizens of the possibility of a huge earthquake happening along the fault line since the mid 70s. A law was passed in 1978, mandating that preparations must be made and a 14-page pamphlet teaching residents how to prepare for “the inevitable Tokai Earthquake” has also been produced.

But why are people so concerned now?

The Tokai fault line ruptures roughly every 110 years, but it has been almost 160 since the last quake in 1854. And aside from routine earthquakes, scientists also believe a rupture is overdue because of recent activity along the Nankai Trough plate boundary.

The Nankai Trough, running off the coast of Honshu, is the point where the Phillippine plate is subducting, or travelling beneath Japan. Two sections of the plate boundary have already broken, creating large quake, but nothing has happened near Tokai yet.

Keiji Goi at the Tokyo University Earthquake Research Institute explained to the Washington Post that the land near Shizuoka is sinking toward the Nankai Trough at about five millimeters a year, indicating that strain is building up. “The earthquake occurrence is imminent, we believe,” Doi says.

Last week’s Earthquake Research Institute report has noted that the frequency of small earthquakes (magnitude 3 or below) around Tokyo has risen from 343, six months after the March 11 quake, compared to 47 in the six months before, so it appears another big quake could be on the cards.