Overview Of The Main Earthquake Hazards
Earthquakes can be defined as the shaking of earth caused by seismic waves of moving on and below the earth’s surface creating hazardous conditions that can be detrimental to all surrounding life including humans and property in a variety of ways. Earthquakes really pose little direct danger to a person. People can't be shaken to death by an earthquake. Some movies show scenes with the ground suddenly opening up and people falling into fiery pits, but this just doesn't happen in real life. Earthquake hazards arise from a combination of factors such as the size of the earthquake, distance to the epicenter, the underlying material and geologic structures, and building construction.
Let’s start with ground shaking. The shaking your house experiences depends on how far it is from the fault and the soil under and around it. Seismic waves travel faster through hard rock than through softer rock and sediments like soil and sand. But as the waves pass from harder to softer rocks, they slow and their strength increases, so shaking is more intense where the ground is softer. Low-frequency waves diminish less rapidly with distance than do high-frequency waves. If you are near an earthquake, you will experience all the shaking produced by the earthquake and feel "jolted." Farther away, the higher frequencies will have died away and you will feel a rolling motion.
The amount of damage to a building does not depend solely on how hard it is shaken. In general, smaller buildings such as houses are damaged more by higher frequencies, so usually houses must be relatively close to the hypocenter to be severely damaged. Larger structures such as high-rises and bridges are damaged more by lower frequencies and will be more noticeably affected by the largest earthquakes, even at considerable distances.
Earthquakes can turn the ground into soup through a hazard called liquefaction. This is the process by which loose sand is converted into a suspension during an earthquake. The effect on structures and buildings can be devastating, and is a major contributor to urban seismic risk. Earthquake waves cause water pressures to increase in the sediment and the sand grains to lose contact with each other, leading the sediment to lose strength and behave like a liquid. Resulting in soil that loses its ability to support structures. If the pressure of the water in the pores is great enough to carry the entire load, it will have the effect of holding the particles apart and of producing a condition that is practically equivalent to that of quicksand.
Now let’s talk about landslides, yet another earthquake hazard. Landslides occur during an earthquake when masses of rock or debris flows move down a slope due to gravity. During a landslide, earthquakes create stress that can make weak slopes fail. If an earthquake reaches a magnitude over 4.0 or greater it can trigger a landslide almost immediately. Landslides can occur on any terrain if the conditions are right and cause extensive building damage, which often results in a total loss as well as casualties to people. According to the Pacific Northwest Seismic Network, landslides result in $1-2 billion in damages and cause more than 25 fatalities each year. Earthquakes are commonly associated with another hazard called subsidence. Land subsidence can occur in various ways during an earthquake. Movement that occurs along faults can be horizontal or vertical or have a component of both. As a result, a large area of land can subside drastically during an earthquake. Another way that earthquakes can cause subsidence is by rapidly decreasing the strength of loose earth sediments, due to liquefaction. The weight of the overlying materials is supported only by the water between the grains. When the grains begin to settle, the weight of the overlying sediment causes the grains to be forced closer together and the land to subside.
The fifth earthquake hazard we will discuss is the tsunami. A tsunami is a series of waves or surges most commonly caused by an earthquake beneath the sea floor. Tsunamis can cause great loss of life and property damage in coastal areas. Very large tsunamis can cause damage to coastal regions thousands of miles away from the earthquake that caused them.
If a tsunami-causing disturbance occurs close to the coastline, a resulting tsunami can reach coastal communities within minutes. Although many people think of a tsunami as a single, breaking wave, it typically consists of multiple waves that rush ashore like a fast-rising tide with powerful currents. Tsunamis can travel much farther inland than normal waves.
The US Geological Survey website conclude that the 2004 Indian Ocean tsunami reached heights of 65 to 100 feet in Sumatra, caused more than 200,000 deaths from Indonesia to East Africa, and registered on tide gauges throughout the world.
The final most common earthquake hazard worth mentioning is fire. Fires caused by an earthquake can be started by broken gas lines and power lines, or tipped over wood or coal stoves. They can be a serious problem, especially if the water lines that feed the fire hydrants are broken, too. For example, after the Great San Francisco Earthquake in 1906, the city burned for three days. Most of the city was destroyed and 250,000 people were left homeless.
Most of the hazards to people come from man-made structures themselves and the shaking they receive from the earthquake. The real dangers to people are being crushed in a collapsing building, drowning in a flood caused by a broken dam or levee, getting buried under a landslide, or being burned in a fire.
