It’s a common question after an earthquake shakes the Bay Area and jolts people’s attention.
Does this mean a bigger one is coming?
Sometimes. But not usually, experts said Monday, after a 4.3 magnitude quake centered in Berkeley woke up thousands of people across the Bay Area at 2:56 a.m. early Monday morning.
“There is a small chance that this is a foreshock to something larger,” said Angie Lux, a seismologist at the Berkeley Seismology Laboratory. “But there is a less than 1%. It could happen but it is not really statistically significant.”
According to the U.S. Geological Survey, the chance of a smaller quake, with a magnitude 3, occurring in the same area in the next week is 16%. The chances of another magnitude 4 are 2%. And a magnitude 5 or above? Less than 1%.
Larger earthquakes usually produce larger aftershocks, she noted. When an aftershock is larger than the first quake, it becomes the main quake, and the earlier quake is described as a foreshock.
That happened in 2019 during the Ridgecrest Earthquake, which occurred in near the desert town of Ridgecrest in Kern County. On July 4 that year, the first main shock, a 6.4 quake, hit at 10:33 am. It was followed by more than 1,400 aftershocks, many of them small. But the next day there was a 5.4 aftershock, and four hours later, the largest quake, a 7.1 shook the rural area.
“A 4.3 earthquake like the one today is not a big earthquake,” Lux said. “When there is a big quake, large aftershocks are expected because it has affected a large portion of the fault. This affected a smaller area, so there is less likelihood of it triggering a larger quake.”
Sarah Minson, a research geophysicist with the U.S. Geological Survey’s Earthquake Science Center at Moffett Field, noted that quakes 4.0 and above are relatively rare.
During the 56 years from 1967 to 2022, there were 279 earthquakes in Bay Area counties that were magnitude 4 or above, she noted. That’s about 5 per year. Many of those have occurred in rural areas, like northern Sonoma County near the Geysers, or in unpopulated portions of the Diablo Range.
“In most earthquakes, the largest aftershock is one unit lower than the main shock,” she said. “So if you have a 4, the most likely aftershock is going to be a 3 or a 2 that you don’t even feel.”
Monday’s quake, the largest in the Bay Area in nearly three years, caused no serious damage or injuries. It occurred along the Hayward-Rodgers Creek Fault. The last major quake there was in 1868. Historically, geologic studies have shown a major quake has occurred on the fault about every 140 to 170 years on average .
Monday’s event should motivate people to bolt bookshelves to the wall, put putty under valuable vases and china, and set aside some supplies in an emergency kit, like 72 hours of water per person, Lux said.
“Everybody wants to know when the next one is going to happen,” Lux said. “We don’t know. But we know we live in earthquake country. Don’t get scared. Be prepared. One day it will happen.”
