South Napa Earthquake – One Year Later

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One year ago, the largest earthquake in over 25 years hit the San Francisco Bay Area, causing significant damage in California’s famous Napa Valley. The magnitude 6.0 earthquake occurred early in the morning on August 24, 2014, on the West Napa Fault.

Hydrograph showing stream flow in cubic feet per second on USGS streamgage on Sonoma Creek near Agua Caliente, from about August 23 – September 13, 2014. The sharp rise starting on August 24 reflects an increased streamflow due to the South Napa Earthquake.

One year ago, the largest earthquake in over 25 years hit the San Francisco Bay Area, causing significant damage in California’s famous Napa Valley. The magnitude 6.0 earthquake occurred early in the morning on August 24, 2014, on the West Napa Fault. A smaller magnitude 5.0 earthquake on the same fault had damaged the city of Napa in 2000.

This anniversary of the event is a time to look back at what the U.S. Geological Survey and its scientific partners have learned from the South Napa Earthquake.

The South Napa Earthquake caused extensive damage through both ground shaking and surface cracking (rupture). Ongoing fault movement along the surface rupture, called afterslip, continued for several months, and caused further damage to foundations and structures. The earthquake was unusual for the length of surface rupture (8 miles), the amount of surface slip (up to 18 inches), and the large afterslip that followed the earthquake (up to 14 inches).

With each earthquake experienced, scientists learn more about the hazards, and how the natural and built environment will respond. We can use this increased knowledge to make our communities safer and more resilient in future quakes. For example, the surprising amount of afterslip observed gave scientists a new way to look at and forecast continuing hazards in the weeks to months, after the earthquake occurred.

Shaking Effects

Pavement buckling and tented sidewalk resulting from compressional forces at north end of main fault rupture, Sandybrook Lane, Napa California.

The earthquake fault rupture extended northward from the epicenter, directing much of the seismic energy toward the city of Napa.

The ground shaking was very strong along the fault and in the Napa Valley. Yountville, Vallejo, and American Canyon were also damaged. The shaking was lighter in Sonoma, Glen Ellen, and St Helena, where there was little damage. The earthquake woke people throughout northern California, in cities as far away as Sacramento and Santa Cruz

The city of Napa was very strongly shaken: many historical masonry buildings and older residences were damaged. Damage to structures decreased with distance from the rupture. Older structures tended to experience more damage than more recently built structures Earthquake retrofitting of older structures can help to minimize damage in future earthquakes.

The numbers of aftershocks following the earthquake was relatively low for an earthquake of this size, and in contrast to other large Californian earthquakes, the aftershock locations do not clearly outline the main fault rupture surface. Previous smaller earthquakes in the area also triggered relatively few aftershocks for their size, suggesting that this area doesn’t produce many aftershocks. Much of the fault appears to be smoothly slipping (afterslip) with no stuck spots that would usually fail in aftershocks.

The South Napa earthquake was very well recorded and produced a number of strong ground motion recordings that are of considerable interest to the engineering community. These recordings are being used, in part, to better understand the performance of residences and older masonry buildings that have been retrofitted.

 
An Instrumental Intensity “ShakeMap,” depicts the ground shaking produced by the 2014 South Napa earthquake. Warmer colors represent stronger shaking.
 
 
“Did You Feel It?” community intensity map indicating the severity of shaking felt by people in central California during the South Napa Earthquake in 2014.
Damaged unreinforced masonry building on Main St. in downtown Napa.

Drought-Related Effects

The timing of the earthquake was not only near the end of California’s normally dry season, but also during a multi-year-long severe drought. The drought-induced low water table and dry ground inhibited landslides and liquefaction that can occur during earthquakes of this size, sparing the area greater damage than could have been.

Rivers and streams in the area were flowing at record lows at the time of the quake because of the continuing drought, so when water began to flow again in some previously dry surrounding creeks and streams in the aftermath of the South Napa Earthquake, it prompted many nearby residents to scratch their heads. The discharge of springs and groundwater to these streams began within an hour after the earthquake, increasing intermittently from 0.1 to nearly 3 cubic feet per second within a couple of weeks. This kind of hydrogeologic response to earthquakes is well known to scientists. Within days of reaching their peak, the water flows in surrounding creeks returned to pre-earthquake levels.

Investigations After the Quake

To assist the Federal Emergency Management Agency (FEMA) with response and recovery after the quake, the USGS and its scientific partners prepared a report with details of fault afterslip, shaking and damage in the city of Napa downtown area, and fault hazards of the West Napa Fault System, as well as associated geospatial information and imagery.

Initially, the fault afterslip was rapid and extended into at least the southern part of the Browns Valley neighborhood in the city of Napa. USGS issued a forecast for the continuing afterslip for some homes that was included in the report for FEMA. Afterslip rates decreased dramatically in the months following the earthquake and is still being monitored by the USGS.

Hydrograph showing stream flow in cubic feet per second on USGS streamgage on Sonoma Creek near Agua Caliente, from about August 23 - September 13, 2014. The sharp rise starting on August 24 reflects an increased streamflow due to the South Napa Earthquake.
Afterslip forecast hazard map of the Browns Valley neighborhood and surrounding area in Napa, Calif. Fault traces and other lineaments show levels of afterslip hazard. All of the faults and/or imagery lineaments shown as heavy green lines on this map may be considered to have a low level of afterslip hazard.

Later investigations confirmed the shaking recorded on seismic instruments was consistent with the observed damage in the downtown area of Napa. There was no evidence for the shaking being strongly amplified in any particular pockets of the downtown area.

The seismic hazard posed by the West Napa Fault System is still being evaluated. Some fault strands that broke in residential areas and damaged homes, had not previously been mapped. Also, fault strands that had been mapped prior to this earthquake were not thought to be highly hazardous. The California Geological Survey (CGS) is currently remapping the West Napa Fault System in concert with fault excavation (trenching) studies carried out by the USGS and CGS to evaluate the prehistoric record of rupture on the various fault strands.

USGS Geologists inspecting fault trace in a trench dug across one of the ruptures from the 2014 South Napa earthquake.

To determine the continuity and extent of the faults that ruptured during the South Napa earthquake and its aftershocks, and to search for possible interconnections with other mapped faults, USGS recorded aftershocks using temporary seismograph arrays that were positioned across the rupture zone and mapped faults located north and south of the rupture. Observing how seismic waves travel along a complex fault zone can reveal how fault segments might be connected at depth. Results suggest the West Napa Fault and the Franklin Fault (to the southeast) may be continuous at depth. Existing fault maps show that the Franklin Fault extends southward to the Calaveras Fault zone and the West Napa Fault extends north of the seismic array. Assuming a continuous fault zone, the West Napa – Franklin Fault zone could be capable of generating a much larger magnitude earthquake than the M 6.0 that occurred on August 24, 2014.

Technology

Recent advances in technology let to more rapid assessment of the nature and effects of this quake, showcasing significant advances since the previous big quake in the San Francisco Bay area in 1989 – the magnitude 6.9 Loma Prieta earthquake.

The California Integrated Seismic Network detected the South Napa earthquake immediately, and the first ShakeMap, issued four minutes after the earthquake, depicted the violent shaking in the immediate area. ShakeMaps provide near-real-time maps of ground motion and shaking intensity following significant earthquakes. These maps are used by federal, state, and local organizations, both public and private, for post-earthquake response and recovery, public and scientific information, as well as for preparedness exercises and disaster planning.

The first ShakeCast was issued 11 minutes after the earthquake. ShakeCast is an application for automating ShakeMap delivery to critical users and for facilitating notification of shaking levels at user-selected facilities. For example, the California Department of Transportation (Caltrans) received an automated report that they could use to determine which bridges and overpasses experienced the greatest level of shaking and were most at risk to damage so they could prioritize inspections.

The first PAGER Assessment (providing fatality and economic loss impact estimates) was issued 13 minutes after the earthquake, and accurately forecast a low level of human casualties, but significant economic losses in the area. There was one fatality due to the earthquake, and at least $500 million in economic losses.

USGS PAGER Assessment for the 2014 South Napa Earthquake with initial estimates of fatalities and economic loss.

Over 41,000 people went online to report what they felt during the event. The "Did You Feel It?" maps compiled from these personal accounts, reflected peak shaking intensities up to a “severe” level, just a little bit less than the “violent” shaking calculated in the instrument-generated ShakeMaps.

The prototype earthquake early warning “ShakeAlert” system provided about 5 seconds of warning to test users 23 miles away in Berkeley and a 9-second warning to San Francisco, 31 miles away. Once fully implemented in the western U.S., the advance warnings of a few seconds up to a minute or more provided by the ShakeAlert system can be enough to slow public transit systems, open fire-house doors, and allow people to “Drop, Cover and Hold On.” Although there were no trains running at 3:20 a.m. when the earthquake happened, the Bay Area Rapid Transit (BART) system’s automated train-stopping system did successfully activate when the warning was received.

USGS geologists and geophysicists, along with their colleagues from federal, state, and local government agencies and academia, started conducting field work immediately after the earthquake. They walked across the landscape to trace where the fault had ruptured the Earth’s surface, and mapped the earthquake surface rupture and post-earthquake afterslip, in great detail using mobile laser scanning (a truck-mounted Lidar system), alignment arrays (such as creepmeters that directly measure extremely small fault movements), GPS measurements, and satellite-based radar surveys (InSAR and UAVSAR). These activities resulted in a comprehensive understanding of fault motion and ground response, in ways that could not have been imagined the last time (1989) there was a large quake in the San Francisco Bay Area. This detailed assessment of the South Napa earthquake and the West Napa Fault system helps scientists assess future earthquake hazards in the area so communities can better prepare for them.

With one fatality, and approximately a half billion dollars of economic damage, the city of Napa is still recovering from last year’s quake. Our better scientific understanding of the 2014 earthquake sheds light on the nature of earthquake faults and hazards in the region, and will enable communities to be better prepared and better able to withstand the next earthquake.