These verses constitute the most devastating threat the Lord had ever made against the seed of Jacob who had polluted the promised land. He is threatening to leave America in a state of total desolation until the latter days unless the people repent. This immediately set the people off to howling and mourning again.
These conditions extended throughout the three days and nights of darkness. During all of this time there seems to have been groaning and residual earthquake aftershocks until the tectonic plates settled into place at the end of three days.