Earthquakes by definition are a shaking or trembling of the earth caused by underground volcanic forces or by breaking and shifting of rock beneath the surface of earth. Earthquakes are caused by movements of unstable rocks. Earthquakes slip in any direction along a fault. A fault is where rocks break and slide. Energy is built up and lets out when the fault weakens and breaks.

Earthquakes can be felt miles away. The earthquake can go in a radius of miles and miles. An earthquake is very powerful force and can have about 10,000 times the force of the first atomic bomb. Most earthquakes occur about 25 miles below the surface of the Earth. Some earthquakes occur at the Earth’s surface. Some earthquakes occur for as much as 500 miles.

When large earthquakes occur in the sea, they can cause large waves in the ocean called tidal waves. The Japanese called these waves tsunami. As many as one million earthquakes occur around the world each year. Many people are killed each year by earthquakes and earthquakes cause millions of dollars in damage.

1. What Is Fault?

Earthquakes occur when stressed rocks get greater pressure that what is possible to withstand. The rocks break away and cause an earthquake. The rocks that break are called fault lines. Most fault lines are beneath the Earth’s surface but some are visible on the surface. The most famous fault line is the San Andreas fault in California.

On a normal fault line a hanging wall move up and down approximately one foot. On an upward movement it increases in height and a downward movement it decreases in height. Another type of fault is a reverse fault, which does the exact opposite of a normal fault.

Several factors can affect the shaking. Earthquake waves do not travel evenly in all directions from the rupture surface. The orientation of the fault and the direction of slip can change the characteristics of the waves in different directions. This is called the radiation pattern. When the earthquake rupture moves along the fault, it focuses energy in the direction it is moving so that a site in that direction will receive more shaking than a site at the same distance from the fault but in the opposite direction. This is called directivity.

2. How Are Earthquakes Measured?

Earthquakes are recorded by a seismographic system. Each seismic position in the system measures the movement of the ground at that site. The ground can move because of an earthquake, the wind, or a passing truck. The slip of a rock over another in an earthquake releases energy that makes the ground tremble. That trembling pushes the adjoining piece of ground, causing it to vibrate, and thus the energy travels out from the earthquake in a wave. As the wave passes by a seismic station, that piece of ground vibrates and a signal is recorded. Earthquakes produce two kinds of waves, the P-wave, a compressional wave that travels fast but is not as large, and the S-wave, a shear wave that is slower but larger and does most of the destruction.

Seismologists analyze what time and what location of the earthquake would give the pattern of shaking recorded by the seismographic system by knowing how fast the waves go through the earth. They calculate the time of the wave that arrives first the wave traveling from the hypocenter, the first part of the fault to slip. The process of measuring the entrance times of the waves and calculating the location used to take almost an hour. A computer now determines arrival times and locations within minutes. Determining the location of the rest of the fault plane, beyond the hypocenter, requires more complex procedures and can take several more hours.

Magnitude is the most common measure of an earthquake's size. In the 1930s, Beno Gutenberg and Charles Richter borrowed the idea of a magnitude scale from astronomers and defined it in terms of how fast the ground moved, recorded on a particular seismograph, at an exacting distance from the earthquake. Each time the magnitude increased by one unit, 4 to 5 or 5 to 6, it meant the ground moved 10 times faster.

More recently, seismologists have shown that magnitude is proportional to the energy released in the earthquake. A magnitude 6.0 earthquake has about 32 times more energy than a magnitude 5.0 and almost 1,000 times more energy than a magnitude 4.0 earthquake. This does not mean there will be 1,000 times more shaking at your home. A bigger earthquake will last longer and release its energy over a much larger area. In recent years, seismologists have developed a new scale, called moment magnitude, which avoids many of these limitations. Moment is a physical quantity related to the total energy released in the earthquake. It can be estimated by geologists examining the geometry of a fault in the field or by seismologists analyzing a seismogram. Several recent earthquakes have confirmed that these two estimates agree. Because the units of moment are cumbersome, it has been converted to the more familiar magnitude scale for communication to the public.

When scientists refer to a "great" earthquake, they do not mean the earthquake was great, they mean it was huge. Informally, earthquakes are classified according to their magnitude size.

Earthquake Magnitude

under 5 small

5 – 6 moderate

6 – 7 large

7 – 7.8 major

7.8 or above great

3. How Do Soil Conditions Affect Earthquakes?

Soil is a sacrificial material formed by element, physical, and natural weathering. Soils vary according to grain size and hardness (3 basic particle sizes create 3 basic soil types: sand, silt, and clay), color, grain size and shape, element composition, amount of pore spaces (open spaces filled with air), quantity of moisture, and permeability. Certain soils greatly amplify the shaking in an earthquake. Just as sound carries differently in water and in air, seismic waves travel at different speeds in different types of rock. Passing from rock to soil, the waves slow down but get bigger. A soft, loose soil will shake more intensely than hard rock at the same distance from the same earthquake. The looser and thicker the soil is, the greater the amplification will be. The same factors apply to areas covered by recent sediment. Shaking there could be five or more times greater than it would be on a nearby hard rock site.

Soil is essential to consider in an earthquake because structures are supported on soil and, therefore, has a result on structures in an earthquake. Some soil is hard, like rock, and can support over 40 tons per square foot whereas other soil is weak, like loose sand. Different soil properties can affect seismic waves as they pass from beginning to end of a soil layer. In some areas, there may be many different types of soils layered one upon another before hard rock is encountered. Sometimes, ground shaking will be amplified. This will influence what needs to be done to structures to help them fare better in an earthquake. Also, a phenomenon known as liquefaction or ground failure can occur in moderate to major earthquakes.

Liquefaction occurs when there is ground water less than 30 feet from the surface in soils that contain layers of sand. The pressures generated by repetitive squeezing of the earth by several seconds of seismic wave vibrations will cause the ground water to flow up and out. When this occurs, the sand grains, which have no strength except when touching each other, are forced apart. The ground then takes on the properties of a semi-solid. When it happens over a large area, houses and buildings with inadequate foundations may actually sink slightly. When liquefaction happens in a small area, liquefied sand can be ejected to the surface through fissures in the overlying layers. Soil failure, as described earlier, will have a larger impact on pipelines and pile foundations, and other structures below the surface of the earth.

Liquefaction occurs only under ideal conditions as a result of an earthshaking event and is controlled by several variables including grain size of the soil, duration of the earthquake and amplitude and frequency of shaking, distance from the epicenter, location of the water table, cohesiveness of the soil, and permeability of the soil layers.

Liquefaction is a physical process that takes place during some earthquakes that may lead to ground failure. As a consequence of liquefaction, soft, young, water-saturated, well-sorted, fine grain sands and silts behave as viscous fluids rather than solids. Liquefaction takes place when seismic shear waves pass through a saturated granular soil layer, distort its granular structure, and cause some of its pore spaces to collapse. The collapse of the granular structure increases pore space water pressure, and decreases the soil's shear strength. If pore space water pressure increases to the point where the soil's shear strength can no longer support the weight of the overlying soil, buildings, roads, houses, etc., then the soil will flow like a liquid and cause extensive surface damage.

Fortunately, areas in danger of liquefaction can be readily identified and the hazard can often be decreased. Because of the ease of identifying hazardous areas, numerous liquefaction maps have been made by government agencies. Liquefiable sediments are young, loose, water saturated, well-sorted and are either fine sands or silts.

References

[1]www.parsons.lsi.ku.edu/t3/ Downloads/Wp/Ch3EarthquakeReport