principal movement; principal shock
Seismological term for the main shaking phase of an earthquake, as opposed to preliminary tremors or aftershocks.
An earthquake early warning was issued before the arrival of the principal shock.
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.