A magnitude 9.0 earthquake, known as the Great East Japan Earthquake,
struck off the Pacific Coast of Japan’s Tohoku region on March 11,2011,shaking the region.
(“ japan:Legal Responses to the Great East Japan Earthquake of 2011”Research and Reports,Law Library, U.S. Library of Congress 11/3/2014)
— often, sometimes, many times.
→(たびたび)地震が起こる。起きてるよ。 — at that time, two years ago, many years ago
→その時、2年前に地震が起きたよ — some day, in ten years
→いつか地震が起こるでしょう(10年内に) — several times(数回) many times(何度も)
An earthquake may happen.
→地震が起こるかも An earthquake can happen.
→地震が起こるはず(可能性を表す) An earthquake is going to happen.
→地震が起こりそうよ(進行状態) An earthquake is sure to happen.
→きっと地震が起こる(確信的な表現) An earthquake is likely to happen.
→地震が起こりそう(傾向を表す表現) An earthquake seems to happen.