Brochos.com - a reference database of ברכות

Earthquake

Last Updated On: 9/16/11

When experiencing an earthquake, one should say the Brocho: Oseh Ma'aseh Bereishis

The Brocho is said either during the earthquake, or until Toch Kedei Dibbur (the amount of time it takes to say "Shalom alecha Rebbi") after the earthquake finished.

Source: Sefer Sharei Haberacha (page 475)