SAN FRANCISCO—A 4.1 preliminary magnitude earthquake shook up San Francisco and the surrounding Bay Area on Tuesday, July 20. The quake struck at approximately 2:41 a.m. and was centered about 1.5 miles northeast of Fremont, with a depth of 5-7 miles.
A series of 14 smaller aftershocks followed the initial quake, the two largest measuring 2.7 and 2.6 in magnitude. No injuries or property damage was reported, though police officials in the areas affected received a number of calls from startled residents.
“That earthquake definitely woke me up last night,” tweeted one woman in South San Francisco. Fatima Sarker of Fremont tweeted “I can’t believe I slept through that earthquake.”
Last August, the region was hit with the South Napa earthquake, a magnitude 6.0 quake that killed one woman and caused over $400 million in damage. It was the largest earthquake in the Bay Area in 25 years, since the Loma Prieta earthquake in 1989.
The earthquake on Tuesday is believed to have been caused by the Hayward fault line, which runs through the cities of Berkeley, Oakland, and Fremont. According to the USGS, the last major earth quake to have originated from the fault line occurred at about 7:53 a.m. in October 1868.
Though the exact magnitude of that quake is not known, it is considered one of the most destructive earthquakes in the history of California, having killed 30 people and caused extensive damage. In 1906, a San Francisco earthquake killed more than 3000 people.
The 2003 Working Group for California Earthquake Probability predicted that the Hayward-Rodgers Creek Fault system had a 27 percent chance of producing a magnitude 6.7-plus earthquake in the next 30 years.