There were “significant reports” of fires caused by gas leaks, as well as breaks to power, water and communications lines in the region, he told a press conference.
California Governor Gavin Newsom said he had requested emergency federal assistance from the White House, and state resources have been activated at their “highest level”.
Newsom later declared a state of emergency for San Bernardino County, expanding on an earlier announcement covering Kern County, near the epicentre of both the quakes this week.
The latest quake was 11 times stronger than the 6.4-magnitude quake “foreshock” the previous day, according to the United States Geological Survey.
The two major quakes, along with multiple aftershocks, have revived fears of the “Big One” – a powerful tremor along the San Andreas Fault that could devastate major cities in Southern California.
“This is an earthquake sequence. These earthquakes are related,” said Caltech seismologist Lucy Jones.
There was a 10% chance of Friday’s quake being followed by another magnitude 7.0 or higher quake in the next week, she added.
In Kern County, where the quake struck near the town of Ridgecrest, some 1,800 residents were without power, fire department chief David Witt said.
“As for injuries reported, nothing but minor injuries and such as cuts and bruises, by the grace of God,” he said.
Numerous gas leaks were reported near the epicentre, including the towns of Trona and Argus.
“Firefighters have secured leaks where possible and evacuated residents from homes with leaks that cannot be secured,” the San Bernardino fire department tweeted.
In Los Angeles, the fire department deployed vehicles and helicopters, and reported fallen power lines and localised electricity outages.