Is Montana at Risk?
Identified Hazards for the State of Montana
Basic Disaster Information:
EARTHQUAKES
An earthquake is a 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 event natural hazard faced by Montana. 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. Western Montana contains a zone of high seismicity, the Intermountain Seismic Belt, which also covers parts of Nevada, Arizona, Utah, Wyoming and Idaho. In Montana, this seismic belt trends north from Yellowstone National Park to Helena, then heads northwest, terminating beyond Flathead Lake. Most of the earthquake activity in the state occurs within this zone.
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 ricks slide past on another, and ground shaking, by seismic waves. Secondary earthquake hazards result from distortion of the surface materials such as water, soil, or structures. The hazard of ground breaking is confined to a single fault or a narrow zone of multiple faults. Within the fault zone, which is generally less than .5 miles wide, most structures will be destroyed and utilities will be cut. In the case of a moderate, small or deep earthquake, ground breaking may not occur at all.
In contrast, 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 also 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 major form of damage from most earthquakes is damage to construction. Bridges are particularly vulnerable to collapse, and dam failure may generate major downstream flooding. Buildings vary in susceptibility, dependent upon construction and the types of soils on which they are built. Fires caused by ruptured gas mains may also destroy structures.
The damage caused by both ground breaking and ground shaking can lead to the paralysis of the local infrastructure: police, fire, medical and governmental services. As with many catastrophes, the worst hazard to the survivors is their own shock and inability to respond to the necessity for prompt, effective action.
Earthquakes are measured according to their intensity (observed effect) and magnitude (energy released). Intensity is an indication of an earthquake’s apparent severity at a specified location, as determined by experienced observers. For seismologists and emergency workers, intensity becomes an efficient, though subjective, shorthand for describing the effects of an earthquake in a given area. Magnitude expresses the amount of energy released by an earthquake as determined by standardized recording instruments.
The Modified Mercalli Scale is the method most commonly used in the United States for measuring earthquake intensity. This twelve tier scale ranks observed effects from 1, felt only under especially favorable circumstances to XII, damage total.
The magnitude of an earthquake is most commonly measured through the use of the Richter Scale. Earthquake magnitudes describe the subject on an absolute, not an arithmetic, scale. An earthquake of magnitude 8, for example, is ten times stronger than a magnitude 7 earthquake, 100 times stronger than a magnitude 6 earthquake, and so on. There si no highest or lowest value, and it is possible here, as with temperature, to record negative values. The largest earthquakes of record were rated at magnitude 8.9; the smallest, about minus 3. The historic earthquakes of Montana were among the largest recorded on the continental United States. Measured on the Richter Scale, the 1925 Clarkston Valley earthquake was recorded at 6.75, the largest of the earthquakes located in the Helena area in 1935 was recorded at 6.25, and the legendary Hebgen Lake Earthquake of 1959 was recorded at a magnitude of 7.5.
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 inexplicable, earthquakes also strike within zones that do not contain faults, and, because the community is unaware of the potential hazard, extensive damage often occurs.
Instead of predicting when an earthquake will strike, an estimate of their likelihood of recurring within a given time frame is given. Some thoughts:
- In all of western Montana an event of magnitude greater than 5.0 can be expected every 1.5 years, a magnitude of 6.0 or greater should occur ever ten years, and a magnitude 7.0 or greater should occur every 77 years.
- The highest recurrence rate of large earthquakes in Montana occurs in the Hebgen Lake-Yellowstone Region, followed by Helena and Three Forks.
- In the Three Forks and Helena-Ovando regions the return time for a magnitude 6+ event is about 70 years, and that of a magnitude 7+ si 360 to 470 years.
- The number of large earthquakes in the Flathead Lake region is abnormally small compared to the number of small events. The recent discovery of an active f ault system in that area identifies it as a potential location for a large magnitude (6.0 to 7.5) seismic event.
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, and are prominent in the Madison and Yellowstone Valleys. Interestingly, neither of these valleys has recorded and earthquake in historical times.
As the stress builds, an impending earthquake may be signaled by precursors: 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 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.
All of these phenomena may occur prior to a major earthquake, yet be unobserved or unrecognized due to the lack of a monitoring system. Unlike volcanos, where only about 25 sites nationwide require monitoring, earthquakes may occur at thousands of localities. Because of the cost of monitoring facilities, it is unlikely that such warning signs will be observed outside of major populations centers and university cities.
Montana has experienced many major earthquakes in the past. There is every reason to believe that similar events will occur in the future. Future earthquakes will, in general, occur where they have been recorded or where evidence is preserved of their prehistoric occurrence. Western Montana is more susceptible to a large earthquake than the eastern part of the state, however, a significant seismic event in eastern Montana is possible.
Damage from earthquakes includes the direct and indirect effects of ground breaking and ground shaking. The former are confined to fault zones, and can be reduced by mapping of fault zones and mandating their open-space use. Ground shaking and attendant hazards can be minimized by recognition of geological factors which may amplify shaking (primarily the presence of thick, unconsolidated, water-saturated sediment), and by design and construction of earthquake-resistant structures. The implementation of these mitigative actions requires education of the people and their government as to the risks and the alternatives.
