Expo 1915 San Francisco
Held just years after the 1906 earthquake the Panama-Pacific exposition had actually been in the planning stages since 1904. Much of the site was built in landfill that partly consisted of partly of debris from the earthquake. It's most visible remnant is the Palace of Fine Arts which was originally made out of plaster and rebuilt with permanent materials in the 1960's.
On opening day, 20 February 1915, President Wilson opened the exposition by pressing a golden telegraph key which sent a wireless message to an antenna on the Tower of Jewels. That started a giant diesel engine starting a fountain.
San Francisco also hosted the 1939-'40 Golden Gate Exposition.