Superstorm Sandy forced regulators and exchange operators to keep U.S. stock markets closed Tuesday, in the first weather-related shutdown to last more than one day since the Blizzard of 1888.

The decision to close the New York Stock Exchange and other U.S. equity markets for a second straight day—reached by midafternoon Monday—renewed questions about the industry's disaster preparedness. And it underlined the indispensable role humans still play in a market dominated by machines....