Magnitude 6.0 Is Not The Big One

A magnitude 6.0 earthquake is big, but not “The Big One.” This blog looks at some interesting aspects of the recent California earthquake, as well as general issues the insurance industry should consider as we await “The Big One.”

The strongest earthquake to strike San Francisco’s Bay Area in 25 years was recorded on Sunday morning. The U.S. Geological Survey (“USGS”) registered a magnitude 6.0 tremor at 3:20 a.m. local time, with an epicenter located 5 miles south/southwest of Napa, California at a depth of 6.6 miles.

The insurance industry is just starting to grasp the complex nature of the Napa earthquake losses, but it is important to note that this might be the first earthquake in California to utilize some of the newest geospatial technologies, allowing companies to immediately understand the risks exposed and produce damage estimates based on the magnitude of shake intensity.

USGS ShakeMap within BMS iVision

Despite the shaking, damage, injuries, and fear, thankfully this earthquake wasn’t “The Big One.” But, the South Napa earthquake provides a good example of how magnitude is important when analyzing an earthquake’s impact to the insurance industry. Earthquake magnitudes are on a logarithmic scale. Each integer number increase in magnitude reflects 10 times more ground motion and 32 times more energy released. While it sounds like a magnitude 6.0 and a magnitude 6.9 are close enough to get lumped into the same category of earthquakes, the impacts of each are dramatically different. For example, the Loma Prieta earthquake (M6.9) of 1989 was more than 22 times stronger than Sunday’s magnitude 6.0 event, illustrated below in a comparison of two ShakeMaps from the USGS.

ShakeMaps from Loma Prieta Earthquake vs. South Napa Earthquake.

All the Potential Faults

It was only 20 years ago, but we often forget that the 1994 Northridge earthquake was on an unknown fault system. Early reports suggest the South Napa earthquake could have come from an unknown fault as well, which emphasizes that the focus should not always be placed solely on the well-known San Andreas Fault.

According to the California State Geologic Survey Map within the Bay Area, the main San Andreas Fault cuts through San Francisco and sections off Point Reyes. However, many other faults within the zone are also prominent and active enough to earn names. The Hayward Fault, Rodgers Creek, San Joaquin and Green Valley are the structural underpinnings of the long valleys characteristic to the region. What may be surprising, however, is that many of the small faults don’t have names at all, especially if they haven’t had major damaging activity in the recent past. These faults should also be considered by not only the modeling companies, but the insurance companies that write the risks. Further, the industry needs to keep in mind that the location of the epicenter is critical to determining expected damage and, so far, most of the major quakes in our lifetime have not been located under major population centers.

Sunday’s earthquake appears to have ruptured on or just west of mapped traces of the West Napa Fault, which has ruptured sometime in the past 11,000 years. The most seismically active areas have been between the longer Rodgers Creek Fault on the west and the Concord-Green Valley Fault to the east. It’s entirely possible that this earthquake occurred where the fault was covered by sediments, with recent movement that we didn’t know about until today. It’s important to reiterate that many faults have been active in the past 2.6 million years. However, numerous more are inactive and countless are still unknown.

California State Geologic Survey Map of Faults.

Massive Flooding

Early reports suggest damage is localized in the region surrounding Napa due to the rupture directivity to the northwest. River valley sediments in Napa Valley likely contributed to the amplification of shaking around Napa. Major river systems in the area are another factor that should be considered when analyzing the potential consequences of California earthquakes. While it seems unfair that California is getting hit with two disasters – the ongoing extreme drought and now a substantial earthquake – this overlap may actually be a good thing. As mentioned in other studies, if this earthquake had happened when water was more abundant, the aging levee system protecting islands within the Sacramento Delta would have been saturated and vulnerable to liquefaction during the earthquake. If those levees succumbed, their inundation would have drawn saltwater from the bay up into the delta system, which could have caused saltwater to reach the California State Water Project intakes. Considering the Delta is the water supply for two-thirds of Californians and supports central valley agriculture, contaminating the water intake would have been disastrous. That’s not the only relationship between the drought and earthquakes. Recently published research suggests that groundwater depletion in the San Joaquin Valley is linked to crustal flexing in the adjacent mountain ranges, potentially increasing seismicity of the region.

“The Big One”

Now, as a forecaster of the weather, sometimes I get asked “what is the latest state of forecasting earthquakes?” This is a common question, particularly after an earthquake such as this. There are currently various unknowns when trying to determine if this earthquake adds or reduces stress for “The Big One.” The bottom line is that it is impossible to predict the exact timing of an earthquake.

About every six years, the USGS updates its hazard maps to incorporate the latest geoscience research. The new USGS hazard map reveals that 16 states are at high risk of damaging earthquakes over the next 50 years, and these states have all historically experienced earthquakes with a magnitude 6.0 or higher. Some of the biggest changes have come in the Pacific Northwest and in California, where research has identified several areas capable of having the potential for larger and more powerful earthquakes than previously believed. A 2008 USGS study determined that the probability of a magnitude 6.7 or larger earthquake occurring within the greater Bay Area in the next 30 years was 63%. When the impact of the South Napa earthquake is included in the next batch of geophysical models for the region, those probabilities are likely going to stay the same. The earthquake released energy, but not enough to appreciably relieve tectonic stress within the region. It would take many more earthquakes of similar magnitude 6.0 to relieve the same amount of energy as just one magnitude 6.7 earthquake.