1 Earthquake Impact
Emmanuel Des Meaux
Most importantly, there are going to be people who are injured or killed during the earthquake. The shaking itself does not usually cause injuries, but rather falling objects, shattering glass, and collapsing structures are the culprit.
When a large earthquake occurs, usually it is after the shaking stops that the major problems occur. Earthquakes are a type of disaster that can cause what are known as cascading events. Examples of cascading events would be broken gas lines fueling fires, loss of electricity, disrupted routes of transportation, landslides, dam ruptures, or any other secondary disaster caused initially by the earthquake.
Now, in addition to the damage caused by the earthquake, emergency services have to deal with these cascading events as well. With limited resources and damaged roadways, you can see how these cascading events can compound an already difficult situation.
In Emergency Management, we refer to this as Teton County's "islands". An earthquake can collapse bridges, cause both landslides and avalanches, and damage roads making them impassable. This can easily isolate communities within Teton County not allowing the people already there to leave, and more importantly blocking emergency services from reaching them. Teton County has taken steps to alleviate the "island effect" by spreading out our emergency apparatus such as ambulances and fire trucks throughout the county. Emergency Management and Jackson Hole Fire/EMS have also placed mass casualty equipment caches in all of the fire stations located throughout the county so that major first aid supplies are available to communities even if routes are shut down to Jackson.
Due to Teton County's remote location, and the fact that an earthquake of the 7 magnitude range would be a regional event, there may be limited outside resources initially available to assist us. Major emergency response teams from Idaho Falls, Salt Lake City, or Denver can generally mobilize within half a day to a day. They may not be able to reach Teton County for days, however, due to collapsed bridges, damaged airstrips, or ruptured roadways. That is why Teton County Emergency Management stresses that every family has at least a 72 hour preparedness kit, and preferably one that will last a week.
These damaged roads and bridges will also make it difficult for local emergency responders to report for duty. During a widespread disaster such as a major earthquake, you will need to band together with your neighbors to do the most good for the most people, all the while keeping your personal safety in mind.
According to a 2006 Small Business Administration study, up to 25% of small businesses fail to reopen following a disaster. Most of this is due to lack of planning, but some disasters are just too great to overcome. In our community we depend on small businesses, and if 25% of them failed to reopen it would be catastrophic for our economy.
Ground shaking is a term used to describe the vibration of the ground during an earthquake. Ground shaking is caused by body waves and surface waves. As a generalization, the severity of ground shaking increases as magnitude increases and decreases as distance from the causative fault increases. Although the physics of seismic waves is complex, ground shaking can be explained in terms of body waves, compressional, or P, and shear, or S, and surface waves, Rayleigh and Love.
P waves propagate through the Earth with a speed of about 15,000 miles per hour and are the first waves to cause vibration of a building. S waves arrive next and cause a structure to vibrate from side to side. They are the most damaging waves, because buildings are more easily damaged from horizontal motion than from vertical motion. The P and S waves mainly cause high-frequency vibrations; whereas, Rayleigh waves and Love waves, which arrive last, mainly cause low-frequency vibrations. Body and surface waves cause the ground, and consequently a building, to vibrate in a complex manner. The objective of earthquake resistant design is to construct a building so that it can withstand the ground shaking caused by body and surface waves.
In land-use zoning and earthquake resistant design, knowledge of the amplitude, frequency composition, and the time duration of ground shaking is needed. These quantities can be determined from empirical (observed) data correlating them with the magnitude and the distribution of Modified Mercalli intensity of the earthquake, distance of the building from the causative fault, and the physical properties of the soil and rock underlying the building. The subjective numerical value of the Modified Mercalli Intensity Scale indicates the effects of ground shaking on man, buildings, and the surface of the Earth.
When a fault ruptures, seismic waves are propagated in all directions, causing the ground to vibrate at frequencies ranging from about 0.1 to 30 Hertz. Buildings vibrate as a consequence of the ground shaking; damage takes place if the building cannot withstand these vibrations. Compressional waves and shear waves mainly cause high-frequency (greater than 1 Hertz) vibrations which are more efficient than low-frequency waves in causing low buildings to vibrate. Rayleigh and Love waves mainly cause low-frequency vibrations which are more efficient than high-frequency waves in causing tall buildings to vibrate. Because amplitudes of low-frequency vibrations decay less rapidly than high-frequency vibrations as distance from the fault increases, tall buildings located at relatively great distances (60 miles) from a fault are sometimes damaged.
Surface faulting is the differential movement of the two sides of a fracture at the Earth's surface and can be strike-slip, normal, and reverse (or thrust). Combinations of the strike-slip type and the other two types of faulting can be found. Although displacements of these kinds can result from landslides and other shallow processes, surface faulting, as the term is used here, applies to differential movements caused by deep-seated forces in the Earth, the slow movement of sedimentary deposits toward the Gulf of Mexico, and faulting associated with salt domes.
Death and injuries from surface faulting are very unlikely, but casualties can occur indirectly through fault damage to structures. Surface faulting, in the case of a strike-slip fault, generally affects a long narrow zone whose total area is small compared with the total area affected by ground shaking. Nevertheless, the damage to structures located in the fault zone can be very high, especially where the land use is intensive. A variety of structures have been damaged by surface faulting, including houses, apartments, commercial buildings, nursing homes, railroads, highways, tunnels, bridges, canals, storm drains, water wells, and water, gas, and sewer lines. Damage to these types of structures has ranged from minor to very severe. An example of severe damage occurred in 1952 when three railroad tunnels were so badly damaged by faulting that traffic on a major rail linking northern and southern California was stopped for 25 days despite an around-the-clock repair schedule.
The displacements, lengths, and widths of surface fault ruptures show a wide range. Fault displacements in the United States have ranged from a fraction of an inch to more than 20 feet of differential movement. As expected, the severity of potential damage increases as the size of the displacement increases. The lengths of the surface fault ruptures on land have ranged from less than 1 mile to more than 200 miles. Most fault displacement is confined to a narrow zone ranging from 6 to 1,000 feet in width, but separate subsidiary fault ruptures may occur 2 to 3 miles from the main fault. The area subject to disruption by surface faulting varies with the length and width of the rupture zone.
Liquefaction is not a type of ground failure; it is a physical process that takes place during some earthquakes that may lead to ground failure. As a consequence of liquefaction, clay-free soil deposits, primarily sands and silts, temporarily lose strength and behave as viscous fluids rather than as solids. Liquefaction takes place when seismic shear waves pass through a saturated granular soil layer, distort its granular structure, and cause some of the void spaces to collapse. Disruptions to the soil generated by these collapses cause transfer of the ground-shaking load from grain-to-grain contacts in the soil layer to the pore water. This transfer of load increases pressure in the pore water, either causing drainage to occur or, if drainage is restricted, a sudden buildup of pore-water pressure. When the pore-water pressure rises to about the pressure caused by the weight of the column of soil, the granular soil layer behaves like a fluid rather than like a solid for a short period. In this condition, deformations can occur easily.
Liquefaction is restricted to certain geologic and hydrologic environments, mainly areas where sands and silts were deposited in the last 10,000 years and where ground water is within 30 feet of the surface. Generally, the younger and looser the sediment and the higher the water table, the more susceptible a soil is to liquefaction.
Liquefaction causes three types of ground failure: lateral spreads, flow failures, and loss of bearing strength. In addition, liquefaction enhances ground settlement and sometimes generates sand boils (fountains of water and sediment emanating from the pressurized liquefied zone). Sand boils can cause local flooding and the deposition or accumulation of silt.
Lateral Spreads - Lateral spreads involve the lateral movement of large blocks of soil as a result of liquefaction in a subsurface layer. Movement takes place in response to the ground shaking generated by an earthquake. Lateral spreads generally develop on gentle slopes, most commonly on those between 0.3 and 3 degrees. Horizontal movements on lateral spreads commonly are as much as 10 to 15 feet, but, where slopes are particularly favorable and the duration of ground shaking is long, lateral movement may be as much as 100 to 150 feet. Lateral spreads usually break up internally, forming numerous fissures and scarps.