main shock (of an earthquake)
Seismological term for the largest earthquake in a sequence, contrasted with foreshocks (前震) and aftershocks (余震).
Even after the main shock, large aftershocks continued.
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.