chapter 6 6 earthquake - stevens county, · pdf filestevens county, washington multi ... the...

Stevens County, Washington Multi - Hazard Mitigation Plan pg 127

Chapter 6

6 Earthquake An earthquake is trembling of the ground that results from the sudden shifting of rock beneath the earth’s crust. Earthquakes may cause landslides and rupture dams. Severe earthquakes destroy power and telephone lines, gas, sewer, or water mains, which, in turn, may set off fires and/or hinder firefighting or rescue efforts. Earthquakes also may cause buildings and bridges to collapse.

By far, earthquakes pose the largest single natural hazard exposure faced by Washington. They may affect large areas, cause great damage to structures, cause injury, loss of life and alter the socioeconomic functioning of the communities involved. The hazard of earthquakes varies from place to place, dependent upon the regional and local geology.

Earthquakes occur along faults, which are fractures or fracture zones in the earth across which there may be relative motion. If the rocks across a fault are forced to slide past one another, they do so in a stick-slip fashion; that is, they accumulate strain energy for centuries or millennia, then release it almost instantaneously. The energy released radiates outward from the source, or focus, as a series of waves - an earthquake. The primary hazards of earthquakes are ground breaking, as the rocks slide past one another, and ground shaking, by seismic waves. Secondary earthquake hazards result from distortion of the surface materials such as water, soil, or structures.

Ground shaking may affect areas 65 miles or more from the epicenter (the point on the ground surface above the focus). As such, it is the greatest primary earthquake hazard. Ground shaking may cause seiche, the rhythmic sloshing of water in lakes or bays. It may also trigger the failure of snow (avalanche) or earth materials (landslide). Ground shaking can change the mechanical properties of some fine grained, saturated soils, whereupon they liquefy and act as a fluid (liquefaction). The dramatic reduction in bearing strength of such soils can cause buried utilities to rupture and otherwise undamaged buildings to collapse.

The earth’s crust breaks along uneven lines called faults. Geologists locate these faults and determine which are active and inactive. This helps identify where the greatest earthquake potential exists. Many faults mapped by geologists, are inactive and have little earthquake potential; others are active and have a higher earthquake potential.

When the crust moves abruptly, the sudden release of stored force in the crust sends waves of energy radiating outward from the fault. Internal waves quickly form surface waves, and these surface waves cause the ground to shake. Buildings may sway, tilt, or collapse as the surface waves pass.

Ground shaking from earthquakes can collapse buildings and bridges; disrupt gas, electric, and phone service; and sometimes trigger landslides, avalanches, flash floods, fires, and huge, destructive ocean waves (tsunamis). Buildings with foundations resting on unconsolidated landfill and other unstable soil, or trailers and homes not tied to their foundations are at risk because they can be shaken off their mountings during an earthquake. When an earthquake occurs in a populated area, it may cause deaths and injuries and extensive property damage.

Aftershocks are smaller earthquakes that follow the main shock and can cause further damage to weakened buildings. Aftershocks can occur in the first hours, days, weeks, or even months after the quake. Be aware that some earthquakes are actually foreshocks, and a larger earthquake might occur.


Ground movement during an earthquake is seldom the direct cause of death or injury. Most earthquake-related injuries result from collapsing walls, flying glass, and falling objects as a result of the ground shaking, or people trying to move more than a few feet during the shaking (FEMA 2004).

Figure 6.1. Earthquake Profile for United States (USGS 2007).

6.1 Measuring an Earthquake Earthquakes are measured in two ways. One determines the power, the other describes the physical effects. Magnitude is calculated by seismologists from the relative size of seismograph tracings. This measurement has been named the Richter scale, a numerical gauge of earthquake energy ranging from 1.0 (very weak) to 9.0 (very strong). The Richter scale is most useful to scientists who compare the power in earthquakes. Magnitude is less useful to disaster planners and citizens, because power does not describe and classify the damage an earthquake can cause. The damage we see from earthquake shaking is due to several factors like distance from the epicenter and local rock types. Intensity defines a more useful measure of earthquake shaking for any one location. It is represented by the modified Mercalli scale. On the Mercalli scale, a value of I is the least intense motion and XII is the greatest ground shaking. Unlike magnitude, intensity can vary from place to place. In addition, intensity is not measured by machines. It is evaluated and categorized from people's reactions to events and the visible damage to man-made structures. Intensity is more useful to planners and communities because it can reasonably predict the effects of violent shaking for a local area.

Table 6.1. Modified Mercalli Earthquake Intensity Scale.

Intensity Description I. Only instruments detect the earthquake II. A few people notice the shaking III. Many people indoors feel the shaking. Hanging objects swing.


Table 6.1. Modified Mercalli Earthquake Intensity Scale.

Intensity Description IV. People outdoors may feel ground shaking. Dishes, windows, and doors rattle. V. Sleeping people are awakened. Doors swing, objects fall from shelves. VI. People have trouble walking. Damage is slight in poorly-built buildings. VII. People have difficulty standing. Damage is considerable in poorly-built buildings. VIII. Drivers have trouble steering. Poorly-built structures suffer severe damage, chimneys may fall. IX. Well-built buildings suffer considerable damage. Some underground pipes are broken. X. Most buildings are destroyed. Dams are seriously damaged. Large landslides occur. XI. Structures collapse. Underground utilities are destroyed. XII. Almost everything is destroyed. Objects are thrown into the air.

(IGS 2004)

6.2 Seismic Shaking Hazards The U.S. Geological Survey has gathered data and produced maps of the nation, depicting earthquake shaking hazards. This information is essential for creating and updating seismic design provisions of building codes in the United States. The USGS Shaking Hazard maps for the United States are based on current information about the rate at which earthquakes occur in different areas and on how far strong shaking extends from quake sources. Colors on the map below show the levels of horizontal shaking that have a 1 in 10 chance of being exceeded in a 50-year period. Shaking is expressed as a percentage of “g” (g is the acceleration of a falling object due to gravity). This map is based on seismic activity and fault-slip rates and takes into account the frequency of occurrence of earthquakes of various magnitudes. Locally, this hazard may be greater than that shown, because site geology may amplify ground motions.

Figure 6.2. Shaking Hazard Map for Washington State (USGS 2007).

The International Building Code (IBC), a nationwide industry standard, sets construction standards for different seismic zones in the nation. IBC seismic zone rankings for Washington are among the highest in the nation. When structures are built to these standards they have a better chance to withstand earthquakes.


Structures that are in compliance with the 1970 Uniform Building Codes (UBC), which are now replaced by the International Building Code, are generally less vulnerable to seismic damages because that was when the UBC started including seismic construction standards to be applied based on regional location. This stipulated that all structures be constructed to at least seismic risk Zone 2 Standards. The State of Washington adopted the UBC as its state building code in 1972, so it is assumed that buildings built after that date were built in conformance with UBC seismic standards and have a lesser degree of vulnerability. Obviously, issues such as code enforcement and code compliance are factors that could impact this assumption. However, for planning purposes, establishing this line of demarcation can be an effective tool for estimating vulnerability. In 1994, seismic risk Zone 3 Standards of the UBC went into effect in Washington, requiring all new construction to be capable of withstanding the effects of 0.3 times the force of gravity. More recent housing stock is in compliance with Zone 3 standards. In 2003, the state again upgraded the building code to follow International Building Code Standards.

The Washington State Legislature has also adopted the 2006 version of the International Residential Code (2006 WBCC) as the official state building code starting on July 1, 2007. The 2006 IRC governs the new construction of detached one- and two-family dwellings and multiple single-family dwellings (townhouses) not more than three stories in height with separate means of egress. Provisions in the 2006 IRC for earthquake structural and foundation design are determined by the seismic design category of a proposed structure as defined in Figure R301.2(2) of the 2006 IRC. Nearly all of Stevens County is Seismic Design Category B.

Studies of ground shaking in Washington during previous earthquakes have led to better interpretations of the seismic threat to buildings. In areas of severe seismic shaking hazard, older buildings are especially vulnerable to damage. Older buildings are at risk even if their foundations are on solid bedrock. Areas shown in Figure 6.2 with high seismic shaking hazard can experience earthquakes with high intensity where weaker soils exist.

6.3 Local Earthquake Hazards and Risk

6.3.1 Earthquake Profile in Washington More than 1,000 earthquakes occur in the state annually. Washington has a record of at least 20 damaging earthquakes during the past 125 years. Large earthquakes in 1946, 1949, and 1965 killed 15 people and caused more than $200 million (1984 dollars) in property damage. Most of these earthquakes were in western Washington, but several, including the largest historic earthquake in Washington (1872), occurred east of the Cascade crest. Earthquake histories spanning thousands of years from Japan, China, Turkey, and Iran show that large earthquakes recur there on the order of hundreds or thousands of years. Washington's short historical record (starting about 1833) is inadequate to sample its earthquake record. Using a branch of geology called paleoseismology to extend the historical record, geologists have found evidence of large, prehistoric earthquakes in areas where there have been no large historic events, suggesting that most of the state is at risk (Walsh et al. 2006).


Figure 6.3 Geologic Setting in Washington.

Washington is situated at a convergent continental margin, the collisional boundary between two tectonic plates. The Cascadia subduction zone, which is the convergent boundary between the North America plate and the Juan de Fuca plate, lies offshore from northernmost California to southernmost British Columbia. The two plates are converging at a rate of about 3-4 centimeters per year (about 2 inches per year); in addition, the northward-moving Pacific plate is pushing the Juan de Fuca plate north, causing complex seismic strain to accumulate. Earthquakes are caused by the abrupt release of this slowly accumulated strain.

Intraplate, or Benioff zone, earthquakes occur within the subducting Juan de Fuca plate at depths of 15 to 60 miles, although the largest events typically occur at depths of about 25 to 40 miles. The largest recorded event was the magnitude 7.1 Olympia quake in 1949. Other significant Benioff zone events include the magnitude 6.8 Nisqually quake of 2001, the magnitude 5.8 Satsop quake in 1999, and the magnitude 6.5 Seattle-Tacoma quake in 1965. Strong shaking lasted about 20 seconds in the 1949 Olympia earthquake and about 15 to 20 seconds during the 2001 Nisqually earthquake. Since 1900, there have been five earthquakes in the Puget Sound basin with measured or estimated magnitude 6.0 or larger, and one of magnitude 7. The approximate rate for earthquakes similar to the 1965 magnitude 6.5 Seattle-Tacoma event and the 2001 Nisqually event is once every 35 years. The approximate reoccurrence rate for earthquakes similar to the 1949 magnitude 7.1 Olympia earthquake is once every 110 years.


Subduction zone, or interpolate, earthquakes occur along the interface between tectonic plates. Scientists have found evidence of great magnitude earthquakes along the Cascadia Subduction Zone. These earthquakes were very powerful (magnitude 8 to 9 or greater) and occurred about every 400 to 600 years. This interval, however, has been irregular, as short as 100 years and as long as 1,100 years. The last of these great earthquakes struck Washington in 1700.

Shallow crustal earthquakes occur within about 20 miles of the surface. Recent examples occurred near Bremerton in 1997, near Duvall in 1996, off Maury Island in 1995, near Deming in 1990, near North Bend in 1945, just north of Portland in 1962, and at Elk Lake on the St. Helens seismic zone (a fault zone running north-northwest through Mount St. Helens) in 1981. These earthquakes had a magnitude of 5 to 5.5. Scientists believe the 1872 magnitude 6.8 earthquake near Lake Chelan was shallow and may be the state’s most widely felt earthquake. The 1936 magnitude 6.1 earthquake near Walla Walla also was shallow. Because of their remote locations and the relatively small population in the region, damage was light from these two quakes. Recurrence rates for earthquakes on surface faults are unknown; however, four magnitude 7.0 or greater events occurred during the past 1,100 years, including two since 1918 on Vancouver Island.

The state’s two largest crustal earthquakes felt by European settlers occurred in Eastern Washington – the 1872 quake near Lake Chelan and the 1936 earthquake near Walla Walla. Residents of Spokane strongly felt a swarm of earthquakes in 2001; the largest earthquake in the swarm had a magnitude of 4.0. The recent Spokane earthquakes were very shallow, with most events located within a few miles of the surface. The events occurred near a suspected fault informally called the Latah Fault; however, the relation between the fault and the swarm is uncertain. Geologists have mapped the Spokane area, but none confirmed the presence of major faults that might be capable of producing earthquakes. State geologists continue to investigate the geology and earthquake risk near Spokane.

Elsewhere in Eastern Washington, geologists have uncovered evidence of a number of surface faults; however, they have not yet determined how active the faults are, nor determined the extent of the risk they pose to the public. One fault, Toppenish Ridge, appears to have been the source of two earthquakes with magnitudes of 6.5 to 7.3 in the past 10,000 years (EMD 2004).


Figure 6.4. Seismicity of Washington from 1990 – 2001 (USGS 2007).

6.3.1.1 Past Events in Washington

6.3.1.1.1 Lake Chelan, December 14, 1872

This magnitude 6.8 (estimated) earthquake occurred about 9:40 pm and was felt from British Columbia to Oregon and from the Pacific Ocean to Montana. It occurred in a wilderness area, which in 1872 had only a few inhabitants—local Indian tribes, trappers, traders, and military men. Because there were few man-made structures in the epicenter area near Lake Chelan, most of the information available is about ground effects, including huge landslides, massive fissures in the ground, and a 27 foot high geyser.

Extensive landslides occurred in the slide-prone shorelines of the Columbia River. One massive slide, at Ribbon Cliff between Entiat and Winesap, blocked the Columbia River for several hours. A field reconnaissance to the Ribbon Cliff landslide area in August 1976 showed remnants of a large landslide mass along the west edge of Lake Entiat (Columbia River Reservoir), below Ribbon Cliffs and about 3 kilometers north of Entiat. Although the most spectacular landslides occurred in the Chelan-Wenatchee area, slides occurred throughout the Cascade Mountains.

Most of the ground fissures occurred in the following areas: at the east end of Lake Chelan in the area of the Indian camp; in the Chelan Landing-Chelan Falls area; on a mountain about 12 miles west of the Indian camp area; on the east side of the Columbia River (where three springs formed); and near the top of a ridge on a hogback on the east side of the Columbia River. These fissures formed in several locations. Slope failure, settlements, or slumping in water-saturated soils may have produced the fissures in areas on steep slopes or near bodies of water. Sulfurous water was emitted from the large fissures that formed in the Indian camp area. At Chelan Falls, a “great hole opened in the Earth” from which water spouted as much as 27


feet in the air. The geyser activity continued for several days, and, after diminishing, left permanent springs.

In the area of the epicenter, the quake damaged one log building near the mouth of the Wenatchee River. Ground shaking threw people to the floor, waves were observed in the ground, and loud detonations were heard. About two miles above the Ribbon Cliff slide area, the logs on another cabin caved in.

Damaging ground shaking of intensity VI extended to the west throughout the Puget Sound basin and to the southeast beyond what is now the Hanford Site. Individuals in Montana, Idaho, Oregon, and Canada felt the earthquake. Aftershocks occurred in the area for two years.

6.3.1.1.2 State Line (Walla Walla), July 15, 1936

This magnitude 6.1 earthquake occurred at 11:05 am, with an epicenter about 5 miles south-southeast of Walla Walla. It was widely felt through Oregon, Washington, and northern Idaho, with the greatest shaking occurring in Northeast Oregon. Property damage was estimated at $100,000 (about $1.35 million in 2004 dollars) in this sparsely populated area. The earthquake moved small objects, rattled windows, and cracked plaster in the communities of Colfax, Hooper, Page, Pomeroy, Prescott, Touchet, Wallula, and Wheeler; most of the impact and damage was in the Walla Walla area.

The earthquake alarmed residents of Walla Walla, many of whom fled their homes for the street. People reported hearing moderately loud rumbling immediately before the first shock. Standing pictures shook down, some movable objects changed positions, and doors partially opened. The earthquake was more noticeable on floors higher than the ground floor. It knocked down a few chimneys and many loose chimney bricks; damaged a brick home used by the warden at the State Penitentiary that was condemned and declared unsafe; and damaged the local railroad station. Several homes moved an inch or less on their foundations. Five miles southwest of Walla Walla, the earthquake restored the flow of a weakened 600 foot deep artesian well to close to original strength; the flow had not diminished after several months. Walla Walla residents reported about 15 to 20 aftershocks.

6.3.1.1.3 Spokane Swarm, 2001

The Spokane, Washington area experienced a swarm of earthquakes starting with a magnitude 3.7 quake on June 25, 2001 followed by more than 30 recorded tremors. The Spokane quakes were shallow, sometimes only a mile or two deep. High frequency vibrations during the quakes caused loud booms, which had many residents concerned.

The series of temblors is called a swarm because they are close in magnitude, as opposed to a single large quake followed by aftershocks. It's been difficult for researchers to determine just what's going on in Eastern Washington because seismology recording devices there are few and far between.

Quake damage has been limited mostly to listing chimneys, cracked walls and crumbling mortar, but the state emergency management division has been keeping an eye on the events. Local seismologists reported that the chance that the small quakes were a precursor to something larger was low. There have been small quakes in Spokane intermittently from 1915 to 1962.


6.3.2 Stevens County Earthquake Profile Washington ranks second in the nation after California among states vulnerable to earthquake damage according to a Federal Emergency Management Agency study. The study predicts Washington is vulnerable to an average annual loss of $228 million per event (FEMA 2001). Earthquakes in eastern Washington are typically shallow, crustal type, and are the least understood of all earthquake types.

Figure 6.5 depicts an assessment of earthquake probability and known fault lines in Stevens County.


Figure 6.5. Earthquake Probability and Fault Lines in Stevens County.


6.3.2.1 Vulnerability Assessment

As seen in Figure 6.5, the probability for Stevens County to experience a magnitude 5 or higher earthquake in the next 50 years is approximately between 5% and 6% for the northernmost region of the county and between 6% and 7% for most of the central and southern regions. The southeastern most corner of Stevens County has between a 7% and 8% chance of experiencing a magnitude 5 or higher earthquake in next 10 years. The probability of an earthquake in Stevens County is relatively low compared to that of western Washington State.

Past events suggest that an earthquake in the Stevens County area would cause little to no damage. Most crustal earthquakes are in 5.0 to 5.5 magnitude range, and do not have a history of occurrence in the County. Nonetheless, severity can increase in areas that have softer soils, such as unconsolidated sediments. Damage would be negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; and considerable in poorly built or badly designed structures.

6.3.2.1.1 Liquefaction Susceptibility

The Washington State Department of Natural Resources, Division of Geology and Earth Resources received grant funding through the Hazard Mitigation Grant Program (HMGP) following the Nisqually earthquake of February 2001 (FEMA-1361-DRWA). This grant required the Division of Geology and Earth Resources to develop statewide liquefaction susceptibility and NEHRP (National Earthquake Hazards Reduction Program) site class maps.

Liquefaction is a phenomenon in which strong earthquake shaking causes a soil to rapidly lose its strength and behave like quicksand. Liquefaction typically occurs in artificial fills and in areas of loose sandy soils that are saturated with water, such as low-lying coastal areas, lakeshores, and river valleys. When soil strength is lost during liquefaction, the consequences can be catastrophic. Movement of liquefied soils can rupture pipelines, move bridge abutments and road and railway alignments, and pull apart the foundations and walls of buildings. Ground movement resulting from liquefaction caused massive damage to highways and railways throughout southern Alaska during the 1964 Good Friday earthquake. During the 1989 Loma Prieta earthquake, liquefaction was a contributing factor to severe building damage in the Marina District of San Francisco. Liquefaction-induced ground movements also broke water lines, severely hampering control of the ensuing fires. Damage caused by liquefaction to the port area of Kobe, Japan, during the 1995 earthquake resulted in billions of dollars in reconstruction costs and lost business (Division of Geology and Earth Resources 2004).

A liquefaction susceptibility map provides an estimate of the likelihood that soil will liquefy as a result of earthquake shaking. The susceptibility is a measure of the physical characteristics of a soil deposit, such as grain texture, compaction, and depth of groundwater, that determine the propensity of the soil to liquefy during earthquake shaking. A liquefaction susceptibility map depicts the relative hazard in terms of high, moderate, low, or very low liquefaction susceptibility, and cannot be used to directly predict the severity of permanent ground deformation resulting from liquefaction. Assessment of ground failure effects depends on local site conditions, such as the configuration of the ground slope. A geotechnical evaluation is necessary for a detailed localized assessment of ground failure effects (Division of Geology and Earth Resources 2004).

Figure 6.6. is based solely on published geologic correlations and similarity of the geologic units in the map area to units that have been subjected to a quantitative susceptibility analysis (Palmer 2004).


Figure 6.6. Probability of Soil Liquefaction in Stevens County.


Earthquakes can cause several secondary effects. They can cause large and sometimes disastrous landslides and rock or mud slides. River valleys are vulnerable to slope failure, often as a result of loss of cohesion in clay-rich soils. Soil liquefaction occurs when water saturated sands, silts or gravelly soils are shaken so violently that the individual grains lose contact with one another and “float” freely in the water, turning the ground into a pudding-like liquid. Building and road foundations lose load-bearing strength and may sink into what was previously solid ground. Unless properly secured, hazardous materials can be released causing significant damage to the environment and people. In Stevens County there are 1,190 parcels (683 structures) completely within the moderate to high liquefaction susceptibility category according to data developed by the Division of Geology and Earth Resources and used to create Figure 6.6. These parcels have a total improvement value of $59,911,738.

6.3.2.1.2 Value of Resources at Risk

Unreinforced masonry structures and unreinforced chimneys of homes will likely be damaged during an earthquake. There are several unreinforced masonry structures in the County in addition to the numerous homes and other buildings throughout the County with unreinforced chimneys. Damaged or collapsed chimneys could result in the secondary hazard of fire. Nonstructural damage caused by falling and swinging objects may be considerable after any magnitude earthquake. Damage to some older, more fragile bridges and land failure causing minor slides along roadways may isolate some residents. Mobile homes and/or manufactured homes, particularly those not attached to a foundation, are also at higher risk.

Table 6.2. Estimate of Unreinforced Masonry Buildings by Jurisdiction.

Jurisdiction Estimate of Unreinforced Masonry Buildings

Value of Buildings

Unincorporated County 100 - 200 $7.4 to $14.8 million Colville 100 - 200 $7.4 to $14.8 million Chewelah 50 - 75 $3.7 to $5.6 million Kettle Falls 30 - 50 $2.2 to $3.7 million Marcus 0 $0 Northport 5 - 10 $370,385 to $740,770 Springdale 5 - 10 $370,385 to $740,770 Spokane Indian Reservation 5 - 10 $370,385 to $740,770

Critical facilities include: medical and health services, governmental functions (including executive, legislative, and judicial offices); protective functions (including police and fire stations), community shelters, water supplies, wastewater treatment, and schools (including pre-school, primary, and secondary schools). Loss or damage to critical facilities would result in severe disruption in the daily functioning of Stevens County. Incapacitation of one or more of these critical facilities could restrict emergency response and medical care, prevent children from going to school, stop normal governmental functions, contaminate water supplies, and potentially lead to chaos and looting.

Water, sewer, and irrigation infrastructure would likely suffer considerable damage in the event of an earthquake. Several communities in Stevens County, both incorporated and unincorporated, have water storage tanks that could be damaged during an earthquake; potentially cutting off access to clean drinking water for some residents. In addition, personal well systems could also collapse or become damaged. All or part of the municipal sewer systems in the County could also be damaged causing backups and/or detriment to the surrounding ecosystem. Without further analysis of the individual components of this type of


infrastructure, all of these systems are exposed to potential breakage and failure as a result of earthquake.

No specific jurisdictions or special districts were identified as having differing issues or levels of risk associated with this hazard.

6.3.2.2 Countywide Potential Mitigation Activities

Many researchers have unsuccessfully tried to forecast earthquake occurrence. Even guessing that an event will occur within six months cannot be done with any degree of accuracy. Predicting the area where an earthquake will happen is an easier, more reliable task. Since earthquakes are usually associated with faulting, any region containing active faults is potentially dangerous. Unfortunately and inexplicably, earthquakes also strike within zones that do not contain faults, and, because the community is unaware of the potential hazard, extensive damage often occurs.

Although earthquake prediction is difficult at best, there are warning signs which can be interpreted to indicate both the place and the time of an impending event. Earthquakes most commonly occur in the same place as prior earthquakes, that is, along active faults. The term active is often interpreted by non-scientists as meaning active during historical time (the last 100 years). Active faults are most commonly indicated by micro-seismicity (earthquakes so small they can only be detected by instruments) and by the presence of scarps. Scarps are steep, linear slopes, up to 65 feet high, showing offset of the ground surface. They are commonly found along the base of mountain ranges.

As the stress builds, an impending earthquake may be signaled by precursors or phenomena which occur in a characteristic way prior to an earthquake. Precursors include an increase in micro-seismicity, which has been credited with causing unusual animal behavior. Dogs have howled and cattle have left an area hours before an earthquake. Instruments, however, may be more reliable. The velocities of seismic waves through stressed rocks may decrease immediately prior to an event. Well water quality may change, as well as spring discharge. The ground surface may also be slightly deformed. Earthquake lightning has been observed just prior to an earthquake, and is believed to be due to the development of an electrical charge on stressed quartz grains. The Stevens County strategy for preparing for earthquakes should include:

• Assessment of seismic hazards to quantify and understand the threat;

• Adoption and enforcement of seismic building code provisions;

• Implementation of land-use and development policy to reduce exposure to hazards;

• Implementation of retrofit, redevelopment, and abatement programs to strengthen existing structures;

• Support of ongoing public-education efforts to raise awareness and build constituent support; and

• Development and continuation of collaborative public/private partnerships to build a prepared and resilient community.

There are several earthquake-related mitigation activities outlined in the Washington State Hazard Mitigation Plan that pertain to Stevens County including:

• Develop a plan to install satellite-based, realtime earthquake information systems in County emergency operation center.


• Develop a pilot project that analyzes vulnerability of various school construction types to earthquake damage and recommend mitigation measures for each construction type.

• Develop a pilot project that provides funding or incentives for non-structural seismic mitigation in facilities that serve vulnerable populations (e.g., children, elderly, low income).

• Develop a real-time monitoring program (SHAKECAST) for critical bridges and make the data available for use in regional shake maps.

• Seek additional funding for the state’s geologic survey for research to improve understanding of the threats posed by earthquakes, landslides, and other geologic hazards in Washington.

• Seek additional funding for maintenance and expansion of the Pacific Northwest Seismic Network, and for deploying the Advanced National Seismic System.

• Expand the concept of the disaster information clearinghouse (e.g., Nisqually Earthquake Clearinghouse) into a multihazard information center.

The media can raise awareness about earthquakes by providing important information to the community. Here are some suggestions:

• Publish a special section in your local newspaper with emergency information on earthquakes. Localize the information by printing the phone numbers of local emergency services offices, the American Red Cross, and hospitals.

• Conduct a week-long series on locating hazards in the home.

• Work with local emergency services and American Red Cross officials to prepare special reports for people with mobility impairments on what to do during an earthquake.

• Provide tips on conducting earthquake drills in the home, schools and public buildings.

• Interview representatives of the gas, electric, and water companies about shutting off utilities.

6.3.3 Individual Jurisdiction Risk Assessments Due to the nature of the earthquake hazard, all jurisdictions and special districts may be at risk in Stevens County. Overall, the County has a 5-8% chance of experiencing an earthquake in the next 50 years. Currently, there is no basis to assume that any of the adopting jurisdictions or special districts has a differing level of risk or vulnerability than Stevens County based on the location of the hazard or potential extent of the hazard’s impacts. There are currently no measurable differences in the location, extent, occurrence of past events, or probability of future occurrence specifically affecting properties within the jurisdictions of any of the adopting cities, fire districts or departments, Conservation District, Public Utilities District, or the Spokane Indian Reservation. These jurisdictions, including all of their assets and/or critical facilities, have the same level of vulnerability and risk to earthquakes as does Stevens County.

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