Hazards Associated With Earthquakes
Possible hazards from earthquakes can be classified as follows: Ground Motion: Shaking of the ground caused by the passage of seismic waves, especially surface waves, near the epicenter of the earthquake are responsible for the most damage during an earthquake. The intensity of ground shaking depends on:
Local geologic conditions in the area. In general, loose unconsolidated sediment is subject to more intense shaking than solid bedrock.
Size of the Earthquake. In general, the larger the earthquake, the more intense is the shaking and the duration of the shaking.
Distance from the Epicenter. Shaking is most severe near the epicenter and drops off away from the epicenter. The distance factor depends on the type of material underlying the area.
Damage to structures from shaking depends on the type of construction. o Concrete and masonry structures are brittle and thus more susceptible to damage. o Wood and steel structures are more flexible and thus less susceptible to damage.
Faulting and Ground Rupture
Ground rupture generally occurs only along the faulty zone that moves during the earthquake. Thus, structures that are built across faulty zones may collapse, whereas structures built adjacent to, but not crossing the fault may survive.
Earthquake Prediction And Control
Long-Term Forecasting: Long-term forecasting is based mainly on the knowledge of when and where earthquakes have occurred in the past. Thus, knowledge of present tectonic setting, historical records, and geological records are studied to determine locations and recurrence intervals of earthquakes. Two aspects of this are important.
Paleoseismology-The study of prehistoric earthquakes. Through study of the offsets in sedimentary layers near fault zones, it is often possible to determine recurrence intervals of major earthquakes prior to historical records. If it is determined that earthquakes have recurrence intervals of say 1 every 100 years, and there are no records of earthquakes in the last 100 years, then a long-term forecast can be made and efforts can be undertaken to reduce seismic risk.
Short-term prediction involves monitoring of processes that occur in the vicinity of earthquake prone faults for activity that signify a coming earthquake. Anomalous events or processes that may precede an earthquake are called precursor events and might signal a coming earthquake. Despite the array of possible precursor events that are possible to monitor, successful short-term earthquake prediction has so far been difficult to obtain. This is likely because: The processes that cause earthquakes occur deep beneath the surface and are difficult to monitor. Earthquakes in different regions or along different faults all behave differently, thus no consistent patterns have so far been recognized. Among the precursor events that may be important are the following:
- Ground Uplift and Tilting: Measurements taken in the vicinity of active faults sometimes show that prior to an earthquake the ground is uplifted or tilts due to the swelling of rocks caused by strain building on the fault. This may lead to the formation of numerous small cracks (called microcracks). This cracking in the rocks may lead to small earthquakes called foreshocks. Foreshocks: Prior to a 1975 earthquake in China, the observation of numerous foreshocks led to successful prediction of an earthquake and evacuation of the city of the Haiphong. The magnitude 7.3 earthquake that occurred, destroyed half of the city of about 100 million inhabitants, but resulted in only a few hundred deaths because of the successful evacuation.
- Water Level in Wells: As rocks become strained in the vicinity of a fault, changes in pressure of the groundwater (water existing in the pore spaces and fractures. In rocks) occur. This may force the groundwater to move to higher or lower elevations, causing changes in the water levels in wells.
- Emission of Radon Gas: Radon is an inert gas that is produced by the radioactive decay of uranium and other elements in rocks. Because Radon is inert, it does not combine with other elements to form compounds, and thus remains in a crystal structure until some event forces it out. Deformation resulting from stress may force the Radon out and lead to emissions of Radon that show up in well water. The newly formed microcracks discussed above could serve as pathways for the Radon to escape into groundwater. Increases in the amount of radon emissions have been reported prior to some earthquakes.
- Changes in the Electrical Resistivity of Rocks: Electrical resistivity is the resistance to the flow of electric current. In general rocks are poor conductors of electricity, but water is more efficient than conducting electricity. If microcracks develop and groundwater is forced into the cracks, this may cause the electrical resistivity to decrease (causing the electrical conductivity to increase). In some cases a 5 − 10% drop in electrical resistivity has been observed prior to an earthquake. Unusual Radio Waves: Just prior to the Lorna Pieta earthquake or 1989. Where these were generated and why, is not yet known, but research is continuing.
- Strange Animal Behavior: Prior to a magnitude 7.1 earthquake in Tianjin, China, zookeepers reported unusual animal behavior. Snakes refusing to go into their holes, swans refusing to go near water, pandas screaming, etc. This was the first systematic study of this phenomenon prior to an earthquake. Although other attempts have been made to repeat a prediction based on animal behavior, there have been no other successful predictions.
Although no attempts have yet been made to control earthquakes, earthquakes have been known to be induced by human interaction with the Earth. This suggests that in the future earthquake control may be possible.
These are usually smaller earthquakes that occur after a main earthquake, and in most cases there are many of these aftershocks occur because the main earthquake changes the stress pattern in areas around the epicenter, and the crust must adjust to these changes. Aftershocks are very dangerous because they cause further collapse of structures damaged by the main shock.
- Fire-Fire is a secondary effect of earthquakes. Because power lines may be knocked down and because natural gas lines may rupture due to an earthquake, fires are often started closely following an earthquake. The problem is compounded if water lines are also broken during the earthquake since there will not be a supply of water to extinguish the fires once they have started.
- Landslides-In mountainous regions subjected to earthquakes ground shaking may trigger landslides, rock and debris falls, rock and debris slides, slumps. And debris avalanches. Liquefaction-Liquefaction is a process that occurs in water-saturated unconsolidated sediment due to shaking. In areas underlain by such material, the ground shaking causes the grains to lose grain to grain contact, and thus the material tends to flow.
- Changes in Ground Level-A secondary or tertiary effect that is caused by faulting. Earthquakes may cause both uplift and subsidence of the land surface.
- Tsunamis-Tsunamis are giant ocean waves that can rapidly travel across oceans. Earthquakes that occur beneath sea level and along coastal areas can generate tsunamis, which can cause damage Thousands of kilometers away on the other side of the ocean.
- Flooding-Flooding is a secondary effect that may due to rupture of human made dams, due to tsunamis, and as a result of ground subsidence after an earthquake.