main shock (of an earthquake)
Seismological term for the largest earthquake in a sequence, contrasted with foreshocks (前震) and aftershocks (余震).
本震の後も、大きな余震が続いた。
Even after the main shock, large aftershocks continued.
気象庁は、今回の地震を本震と判断した。
The Japan Meteorological Agency determined that this earthquake was the main shock.
本震 is the main, largest earthquake in a sequence, while 余震 refers to the smaller quakes that follow it.
Compound of 本 (main) and 震 (earthquake), a straightforward technical term.