M6.0 South Napa, California Earthquake – August 24, 2014
The M6.0 South Napa, California Earthquake was the largest earthquake to strike the San Francisco Bay Area in over 25 years. It caused significant damage to over 100 structures and was widely felt across the region.
What We Have Learned
In the wee hours of Sunday morning at 3:20am, on August 24, 2014, the residents and wineries of Napa Valley were shaken by a magnitude 6.0 earthquake, the largest in the San Francisco Bay Area in over 25 years, and the first since the M7.9 1906 earthquake to rupture on the surface. Residents in this area feel small earthquakes often, and they generally pause for a moment and then go back to whatever they were doing. But this was no small earthquake, and as the small representative sample of Jawbone© data shows, many residents had trouble going back to sleep after this one.
In the days, weeks and months that followed, scientists collected and analyzed data from instruments that recorded the event. Many others spread out across the Napa Valley and surrounding area to collect additional data such as fault rupture expressed on the ground surface, as well as shaking-induced damage to buildings and infrastructure. Data collected from airborne LIDAR were used to look for effects on the ground surface. In addition, airborne and ground-based LIDAR data were collected in areas known to have surface cracks to measure and describe what happened.
Tectonic Setting
To put the South Napa earthquake in context, the West Napa fault, which was the causative fault, is part of the large San Andreas Fault Zone (SAFZ) system. The West Napa fault extends along the western margin of the basin that underlies much of Napa Valley. The relative motion on this fault is not well known but is thought to be about 1 mm/yr (0.04 in/yr). The SAFZ is a complex collection of smaller fault segments that collectively accommodate the horizontal movement between the North American Plate and the Pacific Plate.
Sometimes a single fault segment will slip and cause an earthquake, and other times many fault segments slip in unison, causing a larger earthquake. The South Napa earthquake included a few small segments that ruptured together to produce the M6 earthquake. The only other noteworthy earthquake in the Napa area in the recent past (since 1973) was the M4.9 Yountville earthquake on September 3, 2000. The epicenter of that quake was 20 km (12 mi) NNW of the South Napa epicenter, and also happened on the West Napa fault system. In addition, a M3.0 foreshock occurred on August 5, 2014, 19 days before the M6.0 mainshock.
Immediate Impact
Many California residents have the Did You Feel It? (DYFI) webpage bookmarked, so as soon as the shaking stopped, they started reporting their experience of the shaking on DYFI. Within an hour of the earthquake, over 25,000 citizen scientists entered their experience on the website, creating a map of the distribution of shaking that they felt. The highest intensities were MMI VII-VIII in the city of Napa and extending a littler further north. Shaking intensities of VII-VIII are described as strong to severe, and very likely woke up all but the most heavy of sleepers. Ultimately, more than 40,000 citizens shared their experiences, with the farthest felt report 300 km (186 mi) to the east in Reno, NV. The computed instrumental intensities agreed well with those reported by residents.
The shaking damaged some wood-frame houses and caused significant damage to some commercial buildings in downtown Napa, including the 1870 courthouse. Damage to wood-frame houses included broken or cracked chimneys and failure of cripple walls (see Additional Resources below) and short walls in the crawl space between the foundation and the floor. A few homes even shifted off of their foundations, which can be caused by the failure of cripple walls, which drop the floor to the foundation and sometimes allow houses to slide off their foundation.
Some residents reported a difference in the amount of water being discharged from natural springs. The change occurred within an hour of the earthquake, but the discharge amount returned to normal within a few days. This is not an unusual phenomenon in areas with natural springs and geothermal features. The shaking causes the shifting of below-surface features that may temporarily (and sometimes permanently) change the flow from a spring or even stop the flow. When the pressure adjusts and equilibrates with the water table, the flow will often return. For further details, see Groundwater Effects from Earthquakes.
In the month following the M6.0 mainshock, there were 80 aftershocks of magnitude greater than or equal to 1.8. The three largest aftershocks included a M3.9 on August 26, a M3.2 a week after the mainshock on August 31, and then a M4.1 10 months later on May 22, 2015. The number of aftershocks was less than usual for an earthquake of this size, 80 during the first month following the mainshock, compared to 320 following the M6.0 in Parkfield in 2006.
Fieldwork and Data Analysis
Scientists found that some of the fault segments that ruptured on the morning of August 24 had been previously mapped as part of the West Napa fault system, but the rupture also extended northward where no faults had been mapped before. The length of the rupture was about 12.5 km (8 mi), and scientists measured up to 46 cm (18 in) of slip. No other surface features were found, such as evidence of liquefaction or landslides. This lack of ground failure was attributed to the lack of rainfall in the area—a shallower water table would have made the surface material less stable and more prone to ground failure.
Shallow slipping across the fault, known as afterslip, continued after the mainshock and lasted for several months. Afterslip is slow movement on the fault, and it is unclear whether it is associated with, or independent of, the aftershocks. An additional 14+ cm (5.5+ in) of slip was measured during the first 24 hours, and up to 20 cm (8 in) after 48 hours, mainly on southern half of the main fault strand. Afterslip causes additional damage to manmade structures, such as roads, but does not release seismic waves or cause shaking.
Inspection of buildings after the earthquake yielded 100+ red-tagged structures, meaning immediate rehabitation is too dangerous. Engineers and scientists determined that the worst-damaged buildings resulted from a combination of:
- the rupture direction along the fault - to the north and updip towards Napa. (see directivity)
- the sedimentary basin - soft sediments amplify the shaking.
- commercial buildings were pre-1950 masonry and had not been built to current code standards or retrofit.
- wood-frame houses that were damaged were older ones that had not been retrofit (see links below for retrofitting information).
Broader Impacts
The 2014 earthquake spurred earth scientists to further investigate the West Napa fault, which had been little-studied prior to 2014. Several paleoseismic trenching investigations have been carried out by USGS and CGS scientists. These studies suggest that earthquakes on this fault are fairly infrequent, although precise timing of past events remains elusive. The map of Quaternary active faults has been updated to include the 2014 surface ruptures, and mapping of the section of fault north of the 2014 ruptures is currently being improved based on LIDAR data and field examination. Preliminary results suggest that the fault extends along the southwest side of Napa Valley farther north than previously recognized, past the town of Calistoga. The 2014 earthquake serves as a reminder that the numerous less-active faults throughout the Bay Area in California nevertheless pose significant hazards and are worthy of scientific investigation and public awareness. While each individual fault may produce earthquakes only once every few thousand years, collectively these less-active faults account for a 13% chance of an earthquake above magnitude 6.7 before 2043, contributing to an overall 72% chance of such an earthquake from all known faults.
Concluding Thoughts
This earthquake was unusual for its size, because it had a relatively long surface rupture of 12.5 km (8 mi) with a significant amount of surface slip (up to 46 cm, or 18 in). It also had a large amount of afterslip, and fewer aftershocks than average for an earthquake of its size. However, knowledge gained provides new insights into the influence of the sedimentary basin under the Napa region, which can be used in future hazard maps to more accurately forecast earthquake hazards in the area.
Additional Resources
- USGS Event page - ShakeMap, Did You Feel It? and more details about the Napa Earthquake
- Earthquake outlook for the San Francisco Bay region 2014–2043
- Protecting Your Family From Earthquakes (English/Spanish; .pdf)
- Protecting Your Family From Earthquakes (Asian-languages; .pdf)
- Earthquake Strengthening of Cripple Walls in Wood-Frame Dwellings (.pdf)
- Quantifying Business Interruption, Downtime, and Recovery following the 2014 South Napa Earthquake and Identifying the Causes (.pdf)
- Additional Publications
- Earthquake Country Alliance Bay Area
Scientific Staff
Slide show of multimedia items associated with this project.
Damage to the brick exterior of the downtown Napa Post Office. Photo take September 11, 2014
Damage to the brick exterior of the downtown Napa Post Office. Photo take September 11, 2014
USGS summer geology interns Alexandra Pickering and Nikita Avdievitch collecting topographic data with a three-dimensional laser scanner on August 26. Surface rupture from the 2014 South Napa Earthquake is visible in the foreground
USGS summer geology interns Alexandra Pickering and Nikita Avdievitch collecting topographic data with a three-dimensional laser scanner on August 26. Surface rupture from the 2014 South Napa Earthquake is visible in the foreground
USGS volunteer Whitney DeLong and geology intern Alexandra Pickering collecting topographic data with a three-dimensional laser scanner along Cuttings Wharf Road in Napa County, CA on August 26, 2014. In the background are engineers inspecting the earthquake surface rupture where is passes through a horse paddock.
USGS volunteer Whitney DeLong and geology intern Alexandra Pickering collecting topographic data with a three-dimensional laser scanner along Cuttings Wharf Road in Napa County, CA on August 26, 2014. In the background are engineers inspecting the earthquake surface rupture where is passes through a horse paddock.
Damage to a building in downtown Napa, CA caused by the South Napa earthquake. Photo taken by USGS from California Highway Patrol helicopter on August 24, 2014
Damage to a building in downtown Napa, CA caused by the South Napa earthquake. Photo taken by USGS from California Highway Patrol helicopter on August 24, 2014
Surface rupture in soil near the intersection of state Highway 12 and Cuttings Wharf Road in Napa, CA on August 24, 2104
Surface rupture in soil near the intersection of state Highway 12 and Cuttings Wharf Road in Napa, CA on August 24, 2104
Surface rupture across Los Carneros Ave just northwest of South Ave in Napa County, CA. Photo taken by USGS from California Highway Patrol helicopter on August 24, 2014
Surface rupture across Los Carneros Ave just northwest of South Ave in Napa County, CA. Photo taken by USGS from California Highway Patrol helicopter on August 24, 2014
Surface rupture across State Highway 12 at Cuttings Wharf Road - Photo taken by USGS from California Highway Patrol helicopter on August 24, 2014
Surface rupture across State Highway 12 at Cuttings Wharf Road - Photo taken by USGS from California Highway Patrol helicopter on August 24, 2014
The M6.0 South Napa, California Earthquake was the largest earthquake to strike the San Francisco Bay Area in over 25 years. It caused significant damage to over 100 structures and was widely felt across the region.
What We Have Learned
In the wee hours of Sunday morning at 3:20am, on August 24, 2014, the residents and wineries of Napa Valley were shaken by a magnitude 6.0 earthquake, the largest in the San Francisco Bay Area in over 25 years, and the first since the M7.9 1906 earthquake to rupture on the surface. Residents in this area feel small earthquakes often, and they generally pause for a moment and then go back to whatever they were doing. But this was no small earthquake, and as the small representative sample of Jawbone© data shows, many residents had trouble going back to sleep after this one.
In the days, weeks and months that followed, scientists collected and analyzed data from instruments that recorded the event. Many others spread out across the Napa Valley and surrounding area to collect additional data such as fault rupture expressed on the ground surface, as well as shaking-induced damage to buildings and infrastructure. Data collected from airborne LIDAR were used to look for effects on the ground surface. In addition, airborne and ground-based LIDAR data were collected in areas known to have surface cracks to measure and describe what happened.
Tectonic Setting
To put the South Napa earthquake in context, the West Napa fault, which was the causative fault, is part of the large San Andreas Fault Zone (SAFZ) system. The West Napa fault extends along the western margin of the basin that underlies much of Napa Valley. The relative motion on this fault is not well known but is thought to be about 1 mm/yr (0.04 in/yr). The SAFZ is a complex collection of smaller fault segments that collectively accommodate the horizontal movement between the North American Plate and the Pacific Plate.
Sometimes a single fault segment will slip and cause an earthquake, and other times many fault segments slip in unison, causing a larger earthquake. The South Napa earthquake included a few small segments that ruptured together to produce the M6 earthquake. The only other noteworthy earthquake in the Napa area in the recent past (since 1973) was the M4.9 Yountville earthquake on September 3, 2000. The epicenter of that quake was 20 km (12 mi) NNW of the South Napa epicenter, and also happened on the West Napa fault system. In addition, a M3.0 foreshock occurred on August 5, 2014, 19 days before the M6.0 mainshock.
Immediate Impact
Many California residents have the Did You Feel It? (DYFI) webpage bookmarked, so as soon as the shaking stopped, they started reporting their experience of the shaking on DYFI. Within an hour of the earthquake, over 25,000 citizen scientists entered their experience on the website, creating a map of the distribution of shaking that they felt. The highest intensities were MMI VII-VIII in the city of Napa and extending a littler further north. Shaking intensities of VII-VIII are described as strong to severe, and very likely woke up all but the most heavy of sleepers. Ultimately, more than 40,000 citizens shared their experiences, with the farthest felt report 300 km (186 mi) to the east in Reno, NV. The computed instrumental intensities agreed well with those reported by residents.
The shaking damaged some wood-frame houses and caused significant damage to some commercial buildings in downtown Napa, including the 1870 courthouse. Damage to wood-frame houses included broken or cracked chimneys and failure of cripple walls (see Additional Resources below) and short walls in the crawl space between the foundation and the floor. A few homes even shifted off of their foundations, which can be caused by the failure of cripple walls, which drop the floor to the foundation and sometimes allow houses to slide off their foundation.
Some residents reported a difference in the amount of water being discharged from natural springs. The change occurred within an hour of the earthquake, but the discharge amount returned to normal within a few days. This is not an unusual phenomenon in areas with natural springs and geothermal features. The shaking causes the shifting of below-surface features that may temporarily (and sometimes permanently) change the flow from a spring or even stop the flow. When the pressure adjusts and equilibrates with the water table, the flow will often return. For further details, see Groundwater Effects from Earthquakes.
In the month following the M6.0 mainshock, there were 80 aftershocks of magnitude greater than or equal to 1.8. The three largest aftershocks included a M3.9 on August 26, a M3.2 a week after the mainshock on August 31, and then a M4.1 10 months later on May 22, 2015. The number of aftershocks was less than usual for an earthquake of this size, 80 during the first month following the mainshock, compared to 320 following the M6.0 in Parkfield in 2006.
Fieldwork and Data Analysis
Scientists found that some of the fault segments that ruptured on the morning of August 24 had been previously mapped as part of the West Napa fault system, but the rupture also extended northward where no faults had been mapped before. The length of the rupture was about 12.5 km (8 mi), and scientists measured up to 46 cm (18 in) of slip. No other surface features were found, such as evidence of liquefaction or landslides. This lack of ground failure was attributed to the lack of rainfall in the area—a shallower water table would have made the surface material less stable and more prone to ground failure.
Shallow slipping across the fault, known as afterslip, continued after the mainshock and lasted for several months. Afterslip is slow movement on the fault, and it is unclear whether it is associated with, or independent of, the aftershocks. An additional 14+ cm (5.5+ in) of slip was measured during the first 24 hours, and up to 20 cm (8 in) after 48 hours, mainly on southern half of the main fault strand. Afterslip causes additional damage to manmade structures, such as roads, but does not release seismic waves or cause shaking.
Inspection of buildings after the earthquake yielded 100+ red-tagged structures, meaning immediate rehabitation is too dangerous. Engineers and scientists determined that the worst-damaged buildings resulted from a combination of:
- the rupture direction along the fault - to the north and updip towards Napa. (see directivity)
- the sedimentary basin - soft sediments amplify the shaking.
- commercial buildings were pre-1950 masonry and had not been built to current code standards or retrofit.
- wood-frame houses that were damaged were older ones that had not been retrofit (see links below for retrofitting information).
Broader Impacts
The 2014 earthquake spurred earth scientists to further investigate the West Napa fault, which had been little-studied prior to 2014. Several paleoseismic trenching investigations have been carried out by USGS and CGS scientists. These studies suggest that earthquakes on this fault are fairly infrequent, although precise timing of past events remains elusive. The map of Quaternary active faults has been updated to include the 2014 surface ruptures, and mapping of the section of fault north of the 2014 ruptures is currently being improved based on LIDAR data and field examination. Preliminary results suggest that the fault extends along the southwest side of Napa Valley farther north than previously recognized, past the town of Calistoga. The 2014 earthquake serves as a reminder that the numerous less-active faults throughout the Bay Area in California nevertheless pose significant hazards and are worthy of scientific investigation and public awareness. While each individual fault may produce earthquakes only once every few thousand years, collectively these less-active faults account for a 13% chance of an earthquake above magnitude 6.7 before 2043, contributing to an overall 72% chance of such an earthquake from all known faults.
Concluding Thoughts
This earthquake was unusual for its size, because it had a relatively long surface rupture of 12.5 km (8 mi) with a significant amount of surface slip (up to 46 cm, or 18 in). It also had a large amount of afterslip, and fewer aftershocks than average for an earthquake of its size. However, knowledge gained provides new insights into the influence of the sedimentary basin under the Napa region, which can be used in future hazard maps to more accurately forecast earthquake hazards in the area.
Additional Resources
- USGS Event page - ShakeMap, Did You Feel It? and more details about the Napa Earthquake
- Earthquake outlook for the San Francisco Bay region 2014–2043
- Protecting Your Family From Earthquakes (English/Spanish; .pdf)
- Protecting Your Family From Earthquakes (Asian-languages; .pdf)
- Earthquake Strengthening of Cripple Walls in Wood-Frame Dwellings (.pdf)
- Quantifying Business Interruption, Downtime, and Recovery following the 2014 South Napa Earthquake and Identifying the Causes (.pdf)
- Additional Publications
- Earthquake Country Alliance Bay Area
Scientific Staff
Slide show of multimedia items associated with this project.
Damage to the brick exterior of the downtown Napa Post Office. Photo take September 11, 2014
Damage to the brick exterior of the downtown Napa Post Office. Photo take September 11, 2014
USGS summer geology interns Alexandra Pickering and Nikita Avdievitch collecting topographic data with a three-dimensional laser scanner on August 26. Surface rupture from the 2014 South Napa Earthquake is visible in the foreground
USGS summer geology interns Alexandra Pickering and Nikita Avdievitch collecting topographic data with a three-dimensional laser scanner on August 26. Surface rupture from the 2014 South Napa Earthquake is visible in the foreground
USGS volunteer Whitney DeLong and geology intern Alexandra Pickering collecting topographic data with a three-dimensional laser scanner along Cuttings Wharf Road in Napa County, CA on August 26, 2014. In the background are engineers inspecting the earthquake surface rupture where is passes through a horse paddock.
USGS volunteer Whitney DeLong and geology intern Alexandra Pickering collecting topographic data with a three-dimensional laser scanner along Cuttings Wharf Road in Napa County, CA on August 26, 2014. In the background are engineers inspecting the earthquake surface rupture where is passes through a horse paddock.
Damage to a building in downtown Napa, CA caused by the South Napa earthquake. Photo taken by USGS from California Highway Patrol helicopter on August 24, 2014
Damage to a building in downtown Napa, CA caused by the South Napa earthquake. Photo taken by USGS from California Highway Patrol helicopter on August 24, 2014
Surface rupture in soil near the intersection of state Highway 12 and Cuttings Wharf Road in Napa, CA on August 24, 2104
Surface rupture in soil near the intersection of state Highway 12 and Cuttings Wharf Road in Napa, CA on August 24, 2104
Surface rupture across Los Carneros Ave just northwest of South Ave in Napa County, CA. Photo taken by USGS from California Highway Patrol helicopter on August 24, 2014
Surface rupture across Los Carneros Ave just northwest of South Ave in Napa County, CA. Photo taken by USGS from California Highway Patrol helicopter on August 24, 2014
Surface rupture across State Highway 12 at Cuttings Wharf Road - Photo taken by USGS from California Highway Patrol helicopter on August 24, 2014
Surface rupture across State Highway 12 at Cuttings Wharf Road - Photo taken by USGS from California Highway Patrol helicopter on August 24, 2014