noun
principal movement; principal shock
Seismological term for the main shaking phase of an earthquake, as opposed to preliminary tremors or aftershocks.
主要動の到達前に緊急地震速報が発表された。
An earthquake early warning was issued before the arrival of the principal shock.
主要動による揺れは約30秒続いた。
The shaking from the principal movement lasted about 30 seconds.
余震 are aftershocks that occur after the main shock, whereas 主要動 is the main earthquake event itself.
Compound of 主要 (principal) and 動 (movement), a technical term formed from Sino-Japanese elements.