San Diego's earthquake threats
The Rose Canyon Fault, and other local faults, are “all over San Diego, but we really don’t have effective mapping of them,” a local professor said. Times of San Diego takes a look at what the experts know about the possibility of a major quake and the impact it would have on the city and region.
Seismic hazards in San Diego
The San Diego chapter of the Earthquake Engineering Research Institute spent five years creating a report, the 2020 Earthquake Planning Scenario, that estimates the impact of a 6.9 magnitude earthquake on the Rose Canyon Fault Zone that threatens San Diego and Tijuana. Residents would face catastrophe, according to the report, with one glaring vulnerability in particular should a major quake hit.
This network of earthquake fault lines comes in from the sea through Coronado, cuts through downtown and runs north below Interstate 5 until going offshore through Mt. Soledad — directly beneath the iconic cross.
Fault lines build up energy gradually as the ground on either side shifts. This happens at about the speed that human fingernails grow, a very slow process. An earthquake occurs when there is a sudden slip along a fault line, releasing the built-up energy in a rupture.
Rose Canyon is connected to other local fault lines, to the north in Orange County and to the south in Baja California. Both have produced damaging ruptures in the last century, including the 2010 Baja California 7.2 magnitude earthquake that shook up San Diego County on Easter.
There are also active offshore fault lines, which also are not well understood but are known to cause 7.0 earthquakes. These led to the 1986 Oceanside earthquake, killing one resident and injuring at least 29.
Professor Thomas Rockwell is a leading geological scientist in San Diego; many of the county’s geologists learned what they know in his San Diego State classroom. Rockwell’s contributions to local research helped form the foundation for the 2020 report.
The last earthquake on the Rose Canyon fault occurred before the California missions were built, likely in the 1700s, according to Rockwell’s observations. On average, the fault has a 6.9 magnitude earthquake about every 700 years.
“We’re probably not going to have one for a couple hundred years, but the fickle thing about recurrence intervals on faults is that they have a fairly high coefficient of variation,” Rockwell said. “That means that, even though the Rose Canyon fault ruptures on average every seven hundred years, it doesn’t mean it couldn’t happen sooner than that.”
“We should still be prepared for an earthquake.”
Background earthquakes are another cause for concern. A magnitude 6 earthquake hit San Diego in 1862, causing a visible rupture in Old Town.
“We can have magnitude 6 on any of these faults anywhere in California as a random background earthquake,” Rockwell said.
Earthquake magnitude scales up based on the size of the rupture and the amount the ground shifts along a fault. For every level of magnitude, there is a tenfold increase in shaking. These background earthquakes are “not considered one of the big ones, but a six would still do a lot of damage,” according to Rockwell.
