More than half of California’s nearly 40 million residents live in counties now under the most restrictive measures for businesses and public gatherings.

SAN FRANCISCO — California residents have become accustomed to receiving cellphone alerts about earthquakes and approaching wildfires. But phones lit up across Los Angeles on Tuesday with a different emergency: Coronavirus cases were on the rise after months of remaining steady. The public safety alert, the first time a virus-related one had been sent to the city’s residents, urged those who had symptoms or suspected they had been exposed to head to a testing site.