foreshock

esijäristys puhekieltä A small earth tremor which precedes the mainshock in an earthquake sequence. Not all mainshocks have foreshocks.
Any shock or disturbance which precedes an event
1991, D. Roderick Kiewiet, ‎Mathew D. McCubbins, The Logic of Delegation - Page 88:

The FY 1981 increases in defense — something of a "foreshock" to the rapid rise in defense spending over the next several years — were prompted by the fall of the Shah in Iran and the Soviet invasion of Afghanistan.