foreshock

Definitions

1. A small earth tremor which precedes the mainshock in an earthquake sequence.
2. Any shock or disturbance that precedes an event
   figuratively

Equivalents

Examples

“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.”

CEFR level