Strong Earthquake Strikes Napa Valley, Shakes San Francisco Bay Region
A strong, magnitude (M) 6.0 earthquake occurred at 3:20 a.m. (10:20 a.m. UTC) on the morning of Sunday, August 24, 2014, near Napa, California, shaking the greater San Francisco Bay Region and surrounding areas. This is the largest earthquake to strike the region since the 1989 M6.9 Loma Prieta Earthquake. The earthquake was located 9 km (6 miles) southwest of Napa, CA, at a depth of 11 km (6 mi) on a yet-to-be identified northwest-oriented strike-slip fault production. The San Francisco Bay Region is the second most populous region of California with the region of Napa most famous for its wine.
The strongest ground motions were limited to a radius of approximately 25-50 km of the epicenter. The USGS ShakeMap (version 15) indicates a maximum shaking intensity of IX on the Modified Mercalli Intensity (MMI) Scale, with a perceived level of violent shaking, however, the USGS PAGER report indicates that there was not any population exposure to this level of violent shaking. The greatest felt shaking was severe (MMI VIII) in the immediate epicentral area. Shaking felt by individuals include:
- 60,000 people (20,700 households) felt the greatest level of severe shaking (MMI VIII)
- 86,000 people (29,700 households) were subjected to a very strong (MMI VII) level of shaking
- 153,000 people (52,800 households) subjected to strong shaking (MMI VI)
- 775,000 people (267,200 households) subjected to moderate shaking (MMI V)
- And more than 7.7 million people (2.7 million households) were exposed to light-to-moderate shaking (MMI II-IV)
Geologically, the San Francisco Bay region is underlain by many softer soils, commonly referred to as bay mud. Historically, it has been observed that softer soils amplify ground motions and contribute to a disproportionate amount of damage. The USGS ShakeMap indicates peak ground acceleration (PGA) was approximately 35% the force of gravity in the Napa area. These strong ground motions contributed to the level of observed damage in this region.
USGS ShakeMap: American Canyon
Source: U.S. Geological Survey (USGS)
Probability of Aftershocks
As a general rule, aftershocks can be expected and could reach within one degree of the magnitude of the main earthquake (i.e. in the magnitude 5 range for this event) within one week. The Northern California Seismic System (NCSS) operated by the University of California at Berkeley and the USGS have indicated that there is a 36% probability of a strong and possibly damaging aftershock (smaller than the main shock) in the next 7 days and a 5-10% probability of an earthquake of equal or larger magnitude. Many smaller aftershocks have already been recorded, with the largest registering M3.6 at this time.
Napa Valley is the premier grape growing and vintners region in the United States. Almost the entire valley is covered by a patchwork of wineries and grape growers. Representing 99% of the production of all Napa Valley appellation wines, the 500 member The Napa Valley Vintners trade association notes that Napa County contributes over $13B of economic activity per annum. Record real estate prices for wineries are on the increase according to the Napa Valley Register. Tourism is the second most important industry in the Napa region which is dotted with upscale and luxury boutique hotels, spas and inns.
Initial reports indicate structural damage in the city of Napa and surrounding area. A state of local emergency has been issued in the city of Napa.
The greatest damage has been to the historic buildings. Several buildings in the downtown core of Napa have sustained bricks falling off buildings and some partially collapsed or leaning walls. Structural engineers are assessing the level of damage. At this time, it is not known how much of the structural damage is major and how much is cosmetic. For the time being, the downtown area of Napa has been cordoned off to fully assess damage. Additionally, there have been several reports of non-structural damage such as items falling off shelves, including a number of wine bottles and wine barrels, and substantial sprinkler leakage to many buildings.
Business interruption (BI) losses are a major concern. As this is a very popular tourist area, many businesses – including wineries and restaurants – have sustained damage, both non-structural and structural.
A fire triggered by the earthquake struck a mobile home park, damaging 6 mobile homes, 4 of which were completely lost. It was very fortunate that there was no wind at the time of the fire or losses could have been much worse. There have been reports of broken water mains, which contributed to the fire damage as there were limited resources to fight the fire.
There have been 89 casualties reported, with 3 of these people in critical condition. At this time there are no reports of missing people or fatalities.
The major industry in the Napa region is wine production and sales. A number of wine losses have been reported, which will contribute to the overall financial losses of this event.
According to the USGS, this earthquake was likely located near the well-known West Napa fault and the less well known Carneros-Franklin faults, between two major fault systems: The Hayward-Rodgers Creek Fault system to the west and the Concord-Green Valley Fault system to the east.
The last earthquake to impact the Napa region was a M5.2 in September of 2000. Today’s earthquake was approximately 10 times stronger than the 2000 earthquake. Also, comparatively, the 1989 M6.9 Loma Prieta earthquake was approximately 10 times stronger than today’s Napa Valley earthquake.
RQE Event Proxies
CoreLogic EQECAT model users are advised to consider the following event ID on the RQE platform as a proxy for this earthquake:
The CoreLogic EQECAT US Earthquake model is RQE model region 1. The event ID provided (Event_No 6983) is considered to be the best estimate proxy for this event. The pattern of aftershocks is suggesting this earthquake has occurred on the West Napa fault. As such, this proxy best fits the USGS location of the event. The magnitude of this proxy is slightly higher than the actual event, but it is appropriate given the high ground motions recorded as indicated on the ShakeMap. The location of the recommended proxy is shown in the map below, relative to the USGS ShakeMap.
|1||6983||West Napa Char, M6.25, CA(38.2340, -122.2890)|
Source: CoreLogic EQECAT
It is expected that insured losses for this event could range from $500 million to $1 billion, however, it should be noted that there is a fair amount of uncertainty associated with this given the unknown extent of BI and contents losses, which are still being fully understood. Residential losses are approximately from one half to one quarter of this loss estimate. If the loss exceeds $1 billion it will be from uncertainty in commercial losses. It is anticipated that losses to the wine industry could increase this estimate. At this point in time the Napa Valley wine harvest had already begun. Had this event occurred pre-harvest these losses would have been less.
CoreLogic EQECAT will continue to monitor this event and provide updates as necessary.
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