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Natural Hazards and Mitigation Earthquakes

The sinking of ground is known as earthquake. Earthquakes occur when energy stored in elastically strained rocks is suddenly released. This release of energy causes intense ground shaking in the area near the source of the earthquake and sends waves of elastic energy, called seismic waves, throughout the Earth. Earthquakes can be generated by bomb blasts, volcanic eruptions, and sudden slippage along faults. Earthquakes are definitely a geologic hazard for those living in earthquake prone are, but the seismic waves generated by earthquakes are invaluable for studying the interior of the Earth.

Origin of Earthquakes

Most natural earthquakes are caused by sudden slippage a long a fault zone. The elastic rebound theory suggests that if slippage along a fault is hindered such that elastic strain energy builds up in the deforming rocks on either side of the fault, when the slippage does occur, the energy released causes an earthquake. This theory was discovered by making measurements at a number of points across 8 fault. Prior to an earthquake it was noted that the rocks adjacent to the fault were bending. These bends disappeared after an earthquake suggesting that the energy stored in bending the rocks was suddenly released during the earthquake.

Seismology, the Study of Earthquakes

When an earthquake occurs, the elastic energy is released sending out vibrations that travel throughout the Earth. These vibrations are called seismic waves. The study of how seismic waves behave in the Earth is called seismology. Seismograms - Seismic waves travel through the Earth as vibrations. A seismometer is an instrument used to record these vibrations, and the resulting graph that shows the vibrations is called a seismogram. The seismometer must be able to move with the vibrations, yet part of it must remain nearly stationary. This is accomplished by isolating the recording device (like a pen) from the rest of the Earth using the principal of inertia. The source of an earthquake is called the focus, which is an exact location within the Earth where seismic waves are generated by sudden release of stored elastic energy. The epicenter is the point on the surface of the Earth directly above the focus. Seismic waves emanating from the focus can travel in several ways, and thus there are several different kinds of seismic waves. Body Waves - Emanate from the focus and travel in all directions through the body of the Earth. There are two types of body waves: P-waves and S-waves: P-Waves - Primary waves, travel with a velocity that depends on the elastic properties of the rock through which they travel. P-waves are the same thing as sound waves. They move through the material by compressing it, but after it has been compressed it expands, so that the wave moves by compressing and expanding the material as it travels. Thus the velocity of the P wave depends on how easily the material can be compressed (the incompressibility), how rigid the material is (the rigidity), and the density of the material P-waves have the highest velocity of all seismic waves and thus will reach all seismographs first. S-Wave B - Secondary waves, also called shear waves. Surface Waves - Surface waves differ from body waves in that they do not travel through the Earth, but instead travel along paths nearly parallel to the surface of the Earth. Surface waves behave like S-waves in that they cause up and down and side to side movement as they pass, but they travel slower than S-wavcs and do not travel through the body of the Earth. Surface waves are often the cause of the most intense ground motion during an earthquake. These are known as L-waves (Longitudinal Waves). The record of an earthquake, a seismogram, as recorded by a seismometer, will be a plot of vibrations versus time. On the seismograph, time is marked at regular intervals, so that we can determine the time of arrival of the first P-wave and the time of arrival of the first S-wave. Since P-waves have a higher velocity than S-waves, the P-waves arrive at the seismographic station before the S-waves.

  • Magnitude of Earthquakes -The size of an earthquake is usually given in terms of a scale called the Richter Magnitude. Richter Magnitude is a scale of earthquake size developed by a seismologist named Charles Richter. The Richter Magnitude involves measuring the amplitude (height) of the largest recorded wave at 8 specific distance from the earthquake. While it is correct to say that for each increase in I in the Richter Magnitude, there is a tenfold increase in amplitude of the wave, it is incorrect to say that each increase of 1 in Richter Magnitude represent B a tenfold increase in the size of the Earthquake.

  • The Richter scale is an open ended scale with no maximum or minimum. The largest earthquakes are probably limited by rock strength. although meteorite impacts could cause even larger earthquakes. The largest earthquakes so Scale is shown in the table below. Note that far recorded are the Chile earthquake in correspondence between maximum intensity 1960 with a Richter Magnitude 8.5, and Richter Scale magnitude only applies in the Alaska (Good Friday) earthquake of 1964 with a Richter Magnitude of 8.6. area around the epicenter.

  • It usually takes more than one

  • Thus, a given earthquake will have zones seismographic station to calculate the of different intensity all surrounding a zone of magnitude of an earthquake. Thus you maximum intensity. will hear initial estimates of earthquake The Modified Mercalli Scale is shown in the magnitude immediately after an table below. Note that correspondence between earthquake and a final assigned maximum intensity and Richter Scale magnitude for the same earthquake that magnitude only applies in the area around the may differ from initial estimates, but is epicenter. assigned after seismologists have had time to evaluate the data from numerous Characteristic Effects Richter Scale seismographic stations Equivalent I People do not feel any <3.4 Frequency of Earthquakes of Different Earth movement Magnitude Worldwide IT A few people notice movement Magnitude Number of Description if at rest and/or on upper floor Earthquakes of tall buildings per Year 111 People indoors feel movement. 4.2 > 8.5 0.3 Great Hanging object swing back 8.0. 8.4 1 and forth. Sleeping people are awakened. Doors swing Modified Mercalli Intensity Scale open/close. Dishes break. Small The Richter magnitude scale results in one object a move or are turned over. number for the Size of the earthquake. Trees ahake. Liquids spill from Maximum ground shaking will occur only in open containers the area of the epicenter of the earthquake, but VI Everyone fee is movement. 5.5 . 6.1 the earthquake may be felt over a much larger People have trouble walking. area. The Modified Merca1li Scale was developed Objects fall from shelves. in the late 18005 to assess the intensity of ground pictures of fallen walls. Furniture shaking and building damage over large areas moves. Plaster in walls may

  • The scale is applied after the earthquake crack. Trees and bushes shake. Damage alight in poorly built by conducting surveys of peoples response to building the intensity of ground shaking and destruction. People have difficulty in standing. 6.5

  • Thus, a given earthquake will have zones Drivers feel cars shaking of different intensity all surrounding a zone of Furniture breaks. Loose bricks fall from buildings. maximum intensity.

  • The Mercalli Scale is very useful on examining the effects of an earthquake over a large area, because it is responsive not only to the size of the earthquake as measured by the Richter scale for areas near the epicenter, but will also show the effects of the efficiency that seismic waves are transmitted through different types of material near the Earths surface.

  • The Mercalli Scale is also useful for determining the size of earthquakes that occurred before the modern seismographic network was available (before there were seismographic stations, it was not possible to assign a Richter Magnitude).

Earthquake Risk

  • Many seismologists have said that "earthquakes dont kill people, buildings do", This is because most deaths from earthquakes are caused by buildings or other human construction falling down during an earthquake,

  • Earthquakes located in isolated areas far from human population rarely cause any deaths.

  • Thus, earthquake hazard risk depends on 1, Population density 2. Construction standards (building codes)

Emergency Preparedness

  • Worst earthquake in recorded history occurred in 1556 in Shaanxi, China, killed 830,000 people, most Having in caves excavated in poorly consolidated loess (wind deposited silt and clay).

  • Worst earthquake in the last century also occurred in China (Tang Shan Province), killed 240,000 in 1976. Occurred at 3:42 AM, magnitude 7.8 earthquake and magnitude 7.1 aftershock. Deaths were due to collapse of masonry (brick) buildings. Contrast- In earthquake prone areas like California, in order to reduce earthquake risk, there &restrict building codes requiring the design and construction of buildings and other structures that will withstand a large earthquake. While this program is not always completely successful, one fact stands out to prove its effectiveness. In 1989 an earthquake near San Francisco, California (The Lorna Pieta, or World Series Earthquake) with a Richter Magnitude of 7, 1 killed about 62 people. Most were killed when a double decked freeway in Oakland collapsed. About 10 months, an earthquake with magnitude 6.9 occurred in Armenia, where no earthquake- proof building codes existed. The death toll in the earthquake was about 25,0001

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