Like old style bridges, most older homes where just set on top of their foundations with no fastening of the superstructure to the concrete base. Just the dead weight of the home keeps it in place.
When an earthquake happens it then jeopardizes the integrity of the home when it “shifts off” its foundation.
California’s Earthquake Brace & Bolt is a program that subsidizes part of the costs of doing a home seismic retrofit.
Ah, mitigation at its best!
