10.1 INTRODUCTION

This chapter contains information on seismic hazards, including a review of the geologic setting of the County, a compilation of active and potentially active earthquake faults in or near the County, and an assessment of the potentially hazardous effects of earthquakes. Also included in this chapter is information on other geologic hazards, including erosion potential, subsidence, landslides, expansive soils and volcanic hazards. This chapter also reviews structural hazards and critical facilities, fire hazards, emergency response, flood hazards and dam safety, solid and hazardous waste management, and airport safety.

10.2 SEISMIC SAFETY

GENERAL GEOLOGIC SETTING OF SUTTER COUNTY

The topography of Sutter County is comprised primarily of the gentle flatlands of the Sacramento River Valley. The only prominent topographic eminence within the County is the Sutter Buttes, a Pliocene volcanic plug which rises abruptly 2,000 feet above the surrounding valley floor. The surface expression of the County's geology governs the extent to which various geologic hazards may (or may not) constitute a threat to life and property within the County.

Sutter County is part of the Great Valley geomorphic province. The Great Valley province is the geologic term for the central valley of California which is drained by the Sacramento River in the North and by the San Joaquin River in the south. The central valley extends nearly 500 miles north and south and averages about 40 miles in width between the Coast Ranges on the west and the Sierra Nevada on the east. Geologically, the Great Valley province is characterized by a great thickness of generally flat-lying sedimentary rocks overlain by alluvial soils. The alluvial soils of the Central Valley range in thickness from a few inches near the foothills to more than 200 feet near the Sacramento River. In Sutter County, the sedimentary rocks are of both marine and continental origin frequently imbedded within tuff-braccias. The continental sediments of the Pleistocene and Recent ages consist of as much as 100 feet of Pleistocene sands and gravels overlaid by 125 feet of recent alluvial fan, flood plain, and stream channel deposits.

The Sutter Buttes, located northwest of Yuba City, form an isolated topographic island within Sutter County and are the most prominent igneous feature in the Great Valley. The Buttes are circular in shape, about 10 miles in diameter. They consist of a central volcanic core of andesite porphyry and tuff surrounded by a ring of sediments, and these sediments are embraced in turn by a ring of andesite tuff and braccia which extends to the Valley alluvium. The volcanic activity that created the Sutter Buttes appears to have occurred in the Early to Middle Pleistocene (between 2.4 and 1.6 million years ago) and the youngest volcanic domes were emplaced by 1.6 to 1.4 million years ago.
FAULT CLASSIFICATIONS

The California Mining and Geology Board has defined active faults as those for which there is evidence of surface displacement within the Holocene epoch; that is, within about the last 11,000 years. Some faults are characterized as active based on surface displacements within historic time, about the last 200 years, while others are characterized as active based on surface displacements in rocks or sediments which are less than 11,000 years old. This definition of active fault does not mean, however, that all faults for which there is no evidence of surface displacement during the Holocene are inactive. Some faults may have been active in this time period, but did not result in identifiable surface displacements, while other faults may still be active although they have not been active during the Holocene. Many recent, damaging California earthquakes including the 1975 Oroville earthquake, the 1983 Coalinga earthquake, and the 1987 Whittier Narrows earthquake occurred on faults not previously recognized as active.

The Mining and Geology Board has defined potentially active faults as those for which there is evidence of surface displacement within the Quaternary period, that is, within about the last 1.6 million years. Faults classified as potentially active faults show no evidence of surface displacements within the past 11,000 years, but this period of time is short geologically and thus such faults are considered potentially active. Faults which do not meet these criteria for being classified as active or potentially active are not necessarily permanently inactive.

Seismic risk is not limited to faults which have been currently identified. A significant fraction of small to moderately large earthquakes typically occur on faults not previously recognized. Such earthquakes are characterized as "background seismicity" or "floating earthquakes" which indicate that the expected sources and locations of such earthquakes are unknown.

Earthquake "magnitude" is a measure of the total amount of energy released in an earthquake. With increasing magnitude (i.e., larger earthquakes) ground motions are stronger, last longer, and are felt over larger areas. Earthquake "intensity" is a measure of the effects of earthquake ground motions on people and buildings. Earthquake intensity is often more useful than magnitude when discussing the damaging effects of earthquakes. The most common intensity scale is the Modified Mercalli Intensity (MMI) scale, which ranges from I to XII.

Table 10.2-1 describes the effects of earthquakes and compares the Richter Scale (magnitude) to the Modified Mercalli Scale (intensity).

Active Faults

No active earthquake faults are known to exist in Sutter County. Regionally, active faults could generate ground motion felt within Sutter County. Figure 10.2-1 is a regional fault map which includes Sutter County in relationship to fault locations. Table 10.2-2 lists key information about important active and potentially active, local and regional faults.

Numerous earthquakes of magnitude M 5.0 or greater have occurred on regional faults, primarily those within the San Andreas Fault System. The west side of the Central Valley is a seismically active region. The greatest historical amount of ground shaking along the west side of the Sacramento Valley resulted from the April 1892 earthquakes in the vicinity of Vacaville and Winters. The 1892 earthquakes are believed to have been produced by the Coast Range - Central Valley blind thrust fault located along the western margin of the valley, parallel to and west of Interstate 5, and about 20 to 30 kilometers west of Sutter County. The estimated magnitude (based on reported intensities) are in the range of M 6.5. For the period 1900-1974, two earthquakes of magnitude M 4.0 and M 4.9 had epicenters just west of Interstate 5 and north of Highway 20 near Williams.

Moderate to large earthquakes in the Foothills Suture Zone along the west slope of the Sierra Nevada are relatively rare. However, a magnitude M 5.7 earthquake occurred in 1975 on the Cleveland Hill Fault in the northern portion of the Zone, in Butte County. This fault up to that time had not been considered active.

Potentially Active Faults

Known fault locations within Sutter County are considered to be potentially active faults. A series of small faults within the Sutter Buttes exhibit evidence of Quaternary motion (within the past 1.6 million years). Generally, movements on these faults were associated with deep-seated volcanism, but may have been partially related to other crust-deformation processes. The faults are not considered active. Refer to Table 10.2-2 for a list of potentially active faults.

PREDICTED EFFECTS OF EARTHQUAKES

Ground Shaking

Based on the known active faults and potentially active faults in the region, Sutter County has the potential to experience low to moderate ground shaking. The intensity of ground shaking at any specific site depends on the characteristics of the earthquake, the distance from the earthquake fault, and on the local geologic and soils conditions. At present there are insufficient data to predict accurately the expected ground motions at various locations within Sutter County.

FIGURE 10.2-1
REGIONAL EARTHQUAKE FAULTS

TABLE 10.2-1
RELATIONSHIP OF MODIFIED MERCALLI SCALE TO RICHTER SCALE

Source: K.V. Steinbrugge, 1982, Earthquakes, Volcanoes and Tsunamis, and Anatomy of Hazards

TABLE 10.2-2
LOCAL AND REGIONAL FAULTS

Most Recent Significant
Earthquake Year;
Fault Richter Scale Magnitude

Sutter Buttes Quaternary

Dunnigan Hills Holocene
(near Arbuckle)

Foothills Suture Zone
Cleveland Hill 1975; M 5.7
Swain Ravine-Spenceville

Midland Quaternary

Unnamed (1892 epicenters 1892; M 6.7
between Vacaville and Winters) 1892; M 6.5

Green Valley-Concord-Calaveras 1984; M 6.1
1979; M 5.9

Rodgers Creek-Hayward 1968; M 6.8
1936; M 7.0

San Andreas 1989; M 7.1 (Loma Prieta)
1906; M 8.3

Eastern Sierra Nevada
Sulphur Creek 1875
Stampede Valley 1966
Genoa Holocene

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Source: Environmental Science Associates, Proposed Ash Landfill EIR, 1992

Liquefaction Potential

Liquefaction, which may occur under strong ground shaking during earthquakes, is the transformation of a granular sediment or fill material from a solid state to a temporarily liquid state. Liquefaction is a serious hazard because buildings on ground which undergoes liquefaction may sink or suffer major structural damage. Evidence of liquefaction may be observed in "sand boils", which are expulsions of sand and water from below the surface due to increased pore-water pressure below the surface. Liquefaction during an earthquake requires strong shaking continuing for a long time period and loose, clean granular materials (particularly sands) that may settle and compact because of the shaking.

Areas paralleling the Sacramento River, Feather River and Bear River which contain clean sand layers with low relative densities coinciding with a relatively high water table are estimated to have generally high liquefaction potential. Granular layers underlying certain areas in the Sacramento Valley have higher relative densities and thus have moderate liquefaction potential. Clean layers of granular materials older than Holocene are of higher relative densities and are thus of low liquefaction potential. Areas of bedrock, including the Sutter Buttes have no liquefaction potential, although localized areas of valley fill alluvium can have moderate to high liquefaction potential.

Seiches

A seiche is a periodic oscillation of a body of water such as a reservoir, river, lake, harbor or bay resulting from seismic shaking or other causes such as landslides into a body of water. The period of the oscillation varies depending on the size of the body of water and may be several minutes to several hours. Depending on the magnitude of the oscillations, seiches can cause considerable damage to dams, levees, and shoreline facilities. The potential for seiches in Sutter County is low as a predicted effect of an earthquake since groundshaking in Sutter County is low to moderate and no reservoirs or dams are located in the County. The County is surrounded by the Feather River and the Bear River on the east and the Sacramento River on the west which could be subject to seiches corresponding to the potential risk of groundshaking.

Landslides

The general potential for landslides in Sutter County is discussed in the Geologic Hazards section of this element. Earthquakes may initiate landslides, particularly during the wet season, in areas of high water or saturated soils. The most likely areas for earthquake-induced landslides are the same areas of high landslide potential discussed in the Geologic Hazards section.

Dam Safety

Earthquakes can endanger dams in several ways, including failure of the foundations or dams themselves due to ground failures. Sutter County does not contain any dams large enough or located such that failure would result in any significant property damage. Dam safety, including seismic safety, is discussed under the Flooding and Dam Safety section of this element and includes a list of dams which could cause varying degrees of inundation in Sutter County if they failed.

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