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A major earthquake in California is likely to knock out many communications services for days or weeks, including the vast majority of cellphones in the areas closest to the epicenter, according to a landmark new analysis by the U.S. Geological Survey.

The widespread disruption would imperil the public’s access to 911 operators and lead to delays in reporting fires and calls for medical help.

Cell towers are vulnerable to sustained power outages. The same goes for cellular equipment on power poles and buildings at risk of extreme shaking, liquefaction and fire, the USGS said. California’s cellphone networks have been notoriously unreliable during blackouts that occur during life-threatening emergencies, such as during wildfires in 2019, where wide swaths of the San Francisco Bay Area were cut off from cell service for significant periods.

In a grim estimate of the challenges, a magnitude 7 earthquake that struck on the Bay Area’s Hayward fault could leave Alameda County — its likely hardest hit area — able to provide only 7% of the demand for voice and data service after the quake. That is identical to the communications service failure in New York City after the 9/11 attacks in 2001, when 93% of cellphone calls failed.

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