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Physiography
Bedrock Formations & Geologic Structure
Unconsolidated Surficial Deposits
Seismicity
Geologic & Seismic Hazards
PHYSIOGRAPHY
This chapter describes in detail the geological resources found in Napa County. Eleven distinct and diverse geomorphic provinces are recognized in California. Each province displays unique, defining features based on geology, faults, topographic relief, and climate. Napa County is located in the Coast Ranges geomorphic province. This province is bounded on the west by the Pacific Ocean and on the east by the Great Valley geomorphic province. The Coast Ranges province extends several hundred miles northward from southern California to near the Oregon border.
A conspicuous characteristic of this province, including Napa County, is the general northwestsoutheast orientation of physiographic features such as valleys and ridgelines. In Napa County, located in the eastern central section of the province, this trend consists of a series of long, linear, major and lesser valleys, separated by steep, rugged ridge and hill systems of moderate relief that have been deeply incised by their drainage systems. This physiography has influenced the local climate (creating several microclimates), the development of soils, and the existence and location of geologic hazards such as landsliding. The combination of physiography, soils, and climate has helped give rise to the production of premium wine grapes and other agricultural products for which Napa County is famous.
BEDROCK FORMATIONS AND GEOLOGIC STRUCTURE
Principal rock units of Napa County involve two key components: (1) an older set of rocks composed of accreted, highly deformed terranes that have been displaced—from hundreds to thousands of kilometers from their position of origin—by plate tectonics (at least in part); and (2) a younger, less deformed set of rocks—lying roughly in their original position (except for San Andreas fault system offsets and smaller localized dislocations)—that overlie the accreted terranes.
The structural geology of the County, like in all of the Coast Ranges, is complex and continues to evolve due to broadly influencing regional forces that act along the North American and East Pacific plate boundary. However, the current governing processes are consistent with events since the Pliocene (about 5–2 million years ago) and Quaternary (last 2 million years), which superimposed compressional deformation on earlier extensional deformation.
UNCONSOLIDATED SURFICIAL DEPOSITS
Unconsolidated surficial soil deposits (clay, silt, sand, gravel, and organic material) in Napa County are geologically young materials lying on bedrock at or near the Earth’s surface. They are typically the product of weathering from bedrock formations and have subsequently been transported and deposited by gravity, sheetwash, streamflow, wind, or other processes. Relative to the underlying bedrock, these deposits are most often loose, weak, and soft, and therefore more susceptible to erosion and landsliding. The soil horizons that have developed on the uppermost part of many of these deposits provide the medium for agriculture, including the County’s valuable vineyard lands.
SEISMICITY
Structural damage from seismic shaking should be anticipated in the County sometime within the next few decades. Older, unreinforced masonry buildings and other buildings constructed before 1930 that have not been seismically retrofitted are most subject to structural failure or collapse.
The chance for a magnitude 6.7 or larger earthquake to occur in the Bay Area by the year 2032 is 62%. Smaller earthquakes (between magnitudes 6.0 and 6.7), capable of considerable damage depending on proximity to urban areas, have about an 80% chance of occurring in the Bay Area by 2032 (U.S. Geological Survey 2003). Depending on the proximity to the County and magnitude of the earthquake, damage could range from nominal to high. Scenarios have been prepared to estimate future earthquake shaking damage in the ten Bay Area counties. Depending on the magnitude considered in the scenario, the estimated damage to buildings in the County could range from $10–$300 million. Most of this damage would be in the southern, more populated part of the County, especially in the deeper alluvium of the lower Napa Valley, which is more susceptible to amplified seismic shaking. It is anticipated that earthquakes on the much longer active and potentially more damaging faults located throughout the Bay Area would result in more groundshaking damage in the County than earthquakes on the shorter active faults within the County.
GEOLOGIC AND SEISMIC HAZARDS
Landsliding, common to the entire Bay Area, is the most potentially damaging geologic hazard in the County. Though often referred to as “mudslides” these more rapid flows may carry various mixtures of debris including boulder to cobble-sized rock fragments, sand, silt, mud, and organic materials. All of the principal ridge and hill systems in the County have experienced at least some landsliding.
| Landsliding is the most potentially damaging geologic hazard in Napa County. Landslide hazards can be reduced by proper land use planning. |
Most landslides present the risk of property damage. Rapidly moving slides such as debris flows and debris avalanches also present the risk of injury and death. Landslide hazards can be reduced by proper land use planning that includes identification of hazard, followed by avoidance measures, or corrective measures.
 Aerial view of Chiles Valley, Napa County |
Surface fault rupture also presents a hazard. The highest potential for surface fault rupture is along the three known active faults in the County: West Napa fault, along the west side of Napa Valley; Green Valley fault, in the southeastern part of the County, and Hunting Creek fault, in the northeastern part of the County. These faults are zoned for special investigation according to the provisions of the Alquist- Priolo Earthquake Fault Zoning Act; human habitation structures cannot generally be built across them.
Ground shaking as a result of future earthquakes, common to other areas of the seismically active San Francisco Bay region, is likely on the three known active faults in the County. The intensity of earthquake motion at the site will depend on the characteristics of the generating fault, distance to the epicenter, the magnitude and duration of the earthquake, and specific geologic conditions. Portions of the County that are underlain by thicker soil deposits could experience slighter ground amplification during seismic activity than upland areas with very shallow bedrock. Severe ground shaking could also trigger secondary effects such as localized failure of slopes and compaction of settlement of loose fills.
Liquefaction refers to the sudden, temporary loss of soil strength during ground shaking. This phenomenon can occur where there is a unique combination of conditions, i.e., clean, saturated, loose granular deposits within depths of about 20–50 feet. Young alluvial soils (geologic map symbols Qhc, Qhay, Qhty, Qha, Qht, Qhf, Qa, Qt, and Qf) are the geologic deposits most likely to contain soils susceptible to liquefaction. The location of these deposits varies widely across the County.
Lateral spreading can occur during strong ground shaking. Lateral spreading generally occurs on slopes and near the tops of slopes where stiff soils are underlain by soft liquefiable deposits. Areas susceptible to lateral spreading are the younger alluvial areas adjacent to the Napa River or other incised rivers within the County.
Lurching and associated ground cracking is generally confined to areas underlain by soft deposits, which are also bordered by steep channel banks or by adjacent hard ground. Areas most susceptible are former and current marsh areas (geologic map symbols Qhbm, Qaf/Qhbm, Qhb) located at the southern end of the County.
Expansive soils and accelerated erosion (such as minor rutting and rilling to extensive gullying) are present at many locations throughout the County. While landslides are generally restricted to hillside areas, the base of slopes, and along steep stream banks, expansive soils and accelerated erosion can occur on both hills and more gently sloping valley areas. While these hazards do not present as high a risk as landsliding, they can be damaging to some kinds of land uses and associated improvements. Geotechnical measures are available to correct expansive soil problems, and accelerated erosion can be avoided by proper erosion control measures.
Subsidence and settlement result from the same physical processes. Subsidence takes place over a long time frame and broad regional area; settlement is usually considered to occur within a relatively short time frame and within a small area, for instance on the project scale. Subsidence/settlement can occur differentially; that is, one area or location subsides or settles more than another. The results of subsidence/settlement, especially when it occurs differentially, can be quite damaging.
Seiches and tsunamis pose a low potential for damage, due to lack of bay front exposure within the County. Some potential may exist for seiche within large bodies of water in the County, such as reservoirs. While presumably low, the risk has apparently not been evaluated. To evaluate seiche risk within large storage tanks requires a site-specific investigation.
