A foreshock and a main quake usually occur within days or weeks of each other, but occasionally in analyzing seismic activity in a region, scientists will identify foreshocks that occurred months or even years before a major quake.
前震和主震往往相隔不过数日或数周，但通过分析一个地区的地震活动情况，科学家们发现，有时，两者间隔的时间长达数月，甚至数年。