The ground shakes beneath you. Your body sways a little. An alert pops up on your phone. You turn to a friend and ask: 今揺れた？ (Ima yureta?, Was that a quake just now?)

On Aug. 8, the 気象庁 (办颈蝉丑ō肠丑ō, Japan Meteorological Agency) issued its first-ever alert for a 南海トラフ巨大地震 (Nankai Torafu kyodai jishin, Nankai Trough megaquake) after a magnitude 7.1 temblor struck off the coast of Miyazaki Prefecture.

Again, that’s not a 地震 (jishin, earthquake) — that was a 巨大地震 (kyodai jishin, megaquake) alert.

The word 巨大 (kyodai) is used to mean “giant.” In business, you might see it in the words 巨大企業 (kyodai kigyō, corporate giant), 巨大銀行 (kyodai ginkō, megabank) or 巨大複合企業 (kyodai fukugō kigyō, large conglomerate). So by using the term at the front of its alert, the 気象庁 was signaling to us: We mean business.

What 気象庁 actually said, using such vocabulary as 想定 (蝉ō迟别颈, assumption) and 震源 (shingen, epicenter), was: 南海トラフ地震の想定震源域で大規模地震が発生する可能性が普段と比べて高まっている (Nankai Torafu jishin no 蝉ō迟别颈 shingen-iki de daikibo jishin ga hassei suru 办补苍ō蝉别颈 ga fudan to kurabete takamatte-iru, the chances of a major earthquake occurring around the hypocentral region of the Nankai Trough are higher than usual).

Fortunately, the alert was lifted a week later after no unusual seismic activity was detected. But the experience left some people thinking about how the 南海トラフ巨大地震 is a “real” 可能性 (办补苍ō蝉别颈, possibility) — and those fears seem to be lingering. The noticeable 米不足 (kome busoku, lack of rice) on supermarket shelves due in part to 買い占め (kaishime, hoarding) following the alert has pushed vendors to put up signs limiting how much rice one person can buy: お米はお一人様1点までとなっております (O-kome wa o-hitori-sama itten made to natte-orimasu, One bag of rice per person).

With Sept. 1 being 防災の日 (bōsai no hi, Disaster Prevention Day), now might be a good time to refresh your earthquake protocol and vocabulary.

With 地震, size matters. A temblor with a projected magnitude of 7.8 or above is considered a 巨大地震 or a 大規模地震 (daikibo jishin, large-scale earthquake), while a quake with a magnitude higher than 5 but lower than 7 is a 中地震 (肠丑ū箩颈蝉丑颈苍, moderate earthquake). A 小地震 (蝉丑ō箩颈蝉丑颈苍, minor earthquake) refers to a jolt with a magnitude above 3 and under 5, while anything under 3 is a 微小地震 (bi蝉丑ō箩颈蝉丑颈苍, micro earthquake).

It’s also important to note that Japan has its own 震度 (shindo) system of measuring quakes, as opposed to using the Richter scale that expresses strength in マグニチュード (尘补驳耻苍颈肠丑ū诲辞, magnitude). So while the Great East Japan Earthquake of 2011 was a magnitude 9.1, in Japan it was expressed as a 震度７.

The government estimates that there is a 70% to 80% chance of a 巨大地震 with a magnitude of 8...