For the 30th anniversary of the Loma Prieta earthquake—the 6.9 tremor struck October 17, 1989 shortly before game three of the World Series between the Oakland Athletics and the San Francisco Giants—Curbed SF will feature quake-related coverage looking back at that fateful day, and what you can do to prepare for the next big one.
A magnitude 6.4 earthquake hit Southern California on July 4, near the town of Ridgecrest, roughly 75 miles east of Bakersfield.
One day later, a magnitude 7.1 quake followed in the same area.
That’s not the end of it—the U.S. Geological Survey (USGS) warned that there was a two percent chance of “one or more aftershocks that are larger than magnitude 7.1” in the following week. A potentially confusing piece of information since the same report cautions that “no one can predict the exact time or place of any earthquake.”
Seismologists can, however, predict the general likelihood of an earthquake based on existing data—and the best kind of data is other recent earthquakes.
After the smaller July 4 quake, Lucy Jones, a seismologist at Caltech, told CBS News, “There is about a one in 20 chance” of a larger earthquake in the same area within days.
Sure enough, an earthquake came along within hours of the 6.4 shaker.
What does this mean for the Bay Area? Only that what seismologists have warned about for years still rings true: “The big one” is coming. Although we can’t predict where or when it will hit, data can narrow down the variables to a disconcerting window of likelihood.
According to a USGS fact sheet, first published in 2014 and later updated in 2016, there’s a 72 percent probability of one or more magnitude 6.7 earthquakes from 2014 to 2043 in the San Francisco Bay Area region.
The nearby Calaveras Fault, which extends to Danville and San Ramon, is the next most dangerous one, with a 26 percent chance. The San Andreas Fault sits at 22 percent, with the Concord Fault at 16 percent, and the San Gregorio Fault at six percent.
There’s also a 13 percent chance of smaller and less noteworthy faults—including ones yet to be discovered—being the trigger.
But no matter which fault plays host to the next massive quake, it will spell trouble for the entire region: “Earthquakes this large are capable of causing widespread damage; therefore, communities in the region should take simple steps to help reduce injuries, damage, and disruption,” according to the report.
Another lesson from July’s SoCal shakeup: Be wary about the danger of foreshocks. These smaller precursors of larger quakes are much more rare than aftershocks, but still common enough that residents should anticipate them.
According to a USGS FAQ: “Worldwide the probability that an earthquake will be followed within three days by a large earthquake nearby is somewhere just over six percent. In California, that probability is about six percent. This means that there is about a 94 percent chance that any earthquake will NOT be a foreshock. In California, about half of the biggest earthquakes were preceded by foreshocks; the other half were not. At this time, we cannot tell whether or not an earthquake is a foreshock until something larger happens after it ... so only in retrospect.”
Six percent is not likely in the context of any one quake. But considering that California suffers tens of thousands of temblors each year, it’s a number that should give Bay Area residents pause. In other words, treat every earthquake as if it could be a precursor to a larger one.
Even after a small tremor, take a few simple steps, such as:
- IPut together an earthquake kit. If you already have a kit, check to make sure that it’s intact and whether any old materials need replacement.
- Develop an emergency communication plan with loved ones. The California Academy of Sciences cautions, “Do not rely on cell phones or other devices that require electricity. Develop a plan for reuniting after the disaster in case family members are separated from one another during an earthquake” without these assets.
- Train yourself how to react during a quake. The Department of Homeland Security’s disaster guidelines include, “If in a vehicle, pull over and stop. If in bed, stay there. If outdoors, stay outdoors. Do not get in a doorway. Do not run outside.” The best thing to do is get underneath the nearest available shelter and cover your head and the back of your neck until the shaking stops.
According to a USGS model published in 2018, Estimates of casualties in the San Francisco Bay could include “800 deaths and 18,000 nonfatal injuries from building and structural damage” when the day finally comes.