Concord Fault Is Reminding People of the Danger beneath Our Feet

Decades have gone by since the Bay Area felt the power of a strong earthquake and each passing year without a major incident creates a greater sense of complacency. However, Monday night served as a wakeup call – a fairly gentle reminder that ‘it’s not if, but when.’ Those who lived through the 1989 Loma Prieta earthquake, 30 years ago Thursday, know that adage to be all too true, but for people who were born or arrived here after, the seismic events of this week may be a bit unsettling. A little discomfort may be a good thing.

A magnitude 4.5 earthquake struck less than a mile south of Pleasant Hill at 10:33 p.m. Monday. It was a noticeable jolt but lacked the power to do any real damage, aside from some tragic reports of broken beer bottles. Perhaps more alarming than the one decent shake were the dozens of others that occurred in the same general area within a little more than a 24-hour period.

The U.S. Geological Survey reports that 37 earthquakes hit the Pleasant Hill area between Monday and Tuesday evening, with an additional outlier near Pacheco. They ranged in magnitude from .9 to 4.5, with the most recent 1.4 magnitude at 11:20 p.m. Tuesday. It’s worth noting the strengths have not increased or decreased in any linear fashion — a magnitude 3.4 struck Tuesday at 7:11 p.m., followed by a M1.1 and then the latest at M1.4 — making it hard to predict whether the risk is growing or if the swarm is dying down.

Despite years of lull, this is normal life during earthquake season in California, that particular time of year in late summer or early fall when the groundwater dissipates from aquifers and relieves weight on the faults, making them more likely to slip.

Although the San Andreas Fault, and Hayward to a lesser degree, attracts the most attention in Northern California, the swarm felt Monday and Tuesday erupted from the quarter-mile-wide strike-slip Concord Fault that cuts beneath a large urbanized population and critical infrastructure. It stretches approximately 11 miles from the Mount Diablo foothills to the edge of Martinez and connects to the longer Green Valley Fault across the Carquinez Strait.

The Concord Fault slices between Walnut Creek and Clayton, between Pleasant Hill and Concord and through the cities of Pacheco and Martinez before it drops off into the water and picks back up as the Green Valley Fault in Benicia. According to U.S. Census Bureau 2017 estimates, cities the Concord Fault pass through or between are home to more than 285,000 people – more than half of that population resides in Concord.

The Concord Fault stretches approximately 11 miles, from the Mount Diablo foothills to the Carquinez Strait. Map courtesy of USGS.

The USGS calculates a 72 percent probability that a magnitude 6.7 or greater earthquake will strike somewhere within the San Francisco Bay Region between now and 2043, with a 16 percent chance it could happen on the Concord Fault stretch. Although potential for a big quake on the San Andreas or Hayward faults is considerably larger, the Concord Fault is relatively active and hasn’t experienced a significant shake, M5.4, since October 1955.

Sperling’s BestPlaces, a national organization that compiles city resource and demographic data, shows that Concord housing development boomed between 1950 and 1979, with a current median home age of 47 years. The majority of the area’s housing construction occurred prior to strict building codes enacted in 1981 and more stringent earthquake retrofit codes that emerged after the 1989 Loma Prieta catastophe.

Another concern among experts is the presence of oil refineries and storage facilities on or near the Concord Fault and age of infrastructure that supports them.

According to the San Francisco Bay Area Planning and Urban Research Association, five major refineries in the Bay Area supply approximately 25 percent of the state’s crude oil-to-gas refining capacity, producing about 800,000 barrels per day. The five refineries are located in Richmond, Rodeo, Benicia and Martinez — there are several other energy-related storage operations located in the area. SPUR adds that four of those refineries were built prior to 1915 and the fifth came along in the late 1960s. Although extensive work has been performed to modernize and seismic safety is assessed routinely at the five facilities, the fact remains that older infrastructure is susceptible to occasional failure and increased risk during strong earthquakes.

Two ethanol tanks exploded and burned for several hours Tuesday, October 16, 2019. A magnitude 4.5 earthquake occurred on the nearby Concord Fault the night before. Photo courtesy of Contra Costa Fire Protection District.

Unfortunately, we became acutely aware Tuesday of the dangers associated with energy facility failures as two tanks containing 250,000 gallons of ethanol, used as a gasoline additive, exploded at NuStar Energy in Crockett. Fire crews stopped the disaster at the storage facility before additional nearby tanks followed the same fate, but residents were forced indoors with a shelter-in-place order, Highway 80 was shut down in both directions for several hours and a firefighter was injured in the process. The cause of the explosion is still under investigation, but Monday night’s earthquake is being considered as a possibility.

The impact of that one facility being out of operation will be felt throughout the state with gas prices soaring beyond the untenable levels they’ve recently reached. The safety hazard of just one facility explosion drew support from emergency personnel across dozens of different, widespread departments. The effort to contain the fire was remarkable and continues to be as they maintain a “foam blanket” on the area that could reignite.

But what we saw at NuStar was just one facility after a relatively moderate magnitude 4.5 earthquake. Whether the quake was the cause or not, it’s cause for thought about how the area would be impacted by an event on the Concord Fault anywhere near the level we saw 30 years ago.