When two blocks of the earth slip past one another, an earthquake occurs. Earthquakes are also called temblors. The earthquake starts at the hypocenter. Hypocenter is the area below the earth’s surface. The area directly above the hypocenter is the epicenter. Before the earthquakes occur, sometimes there occur foreshocks. These are smaller earthquakes. When the larger earthquakes occur, one might tell there is a foreshock. Main shock is the largest main earthquake. Aftershock occurs after the main shocks. Smaller earth quakes occur after the main shock. The aftershock can continue for weeks, depending on the magnitude of the main shock. In plate tectonics theory of the earth crust is a set of large plates that slowly grind and bump into each other. Those are giant rocks slabs that collide against each other. The earthquakes result when faults release large amounts of energy. Then, the energy builds’ up over a long time. Earthquakes can also technically be defined as a vibration travelling through the earth’s crust. The impact of this vibration is gradual. It is unnoticeable on the surface of the earth. The stress builds up between the tectonic plates. When this stress is released, it sends enormous vibrations that are called seismic waves. These waves go hundreds of miles to the surface through the rocks. As the waves move through the rocks, they shake the earth and on reaching the earth surface, they shake the ground and everything on it. Some quakes occur far away from the faults, when the plates are squeezed or stretched.
Earthquakes are caused by several reasons. They can be caused by meteor impacts or volcanic eruptions. Man-made events also cause earthquakes. Underground nuclear tests and mine collapses can cause earthquakes. Every part of the earth surface can be faced with earthquakes. However, some regions more often experience earth quakes than others. It is common at tectonic plates boundaries. That is the places were different boundaries meet. Mostly, it would happen where two plates are colliding. The most common place is like the edges of the pacific plate in New Zealand, Japan, Papua New Guinea and in America. There are other earthquakes that occur away from the plate boundaries. This is the intraplate earthquakes which occur in the stable interior parts of the continent. They are not quite common. They do not have a pattern that can be observed. They mostly originate at shallow depths (Bolt, 2005).
Seismographs are the instruments used to record earthquakes. The recording itself is called a seismogram. The seismograph is an instrument that is set firmly on the ground. It has a weight that hangs freely on it. The earthquake causes the ground to shake, and then the base of seismograph shakes, as well. The hanging weight on it does not shake. The string that the hanging weight hangs on absorbs all the movement, and the difference in the position of the seismograph and the part does not move like it is recorded.
The size of an earthquake is not easy to measure. It is determined by the size of the fault. The magnitude of slip is on the given fault. The seismograph can assist in determining how large an earthquake can be. Magnitude is the size of an earthquake. For each earthquake, there is one magnitude. The intensity of an earthquake depends on where an individual is during the earthquake.
Seismograms can also be used to locate earthquakes. The p and s waves shake the earth differently as they travel through it. During an earthquake, seismic waves are generated by the release of energy. The seismic waves travel through the earth like the normal waves travel through the water. The place where the rocks break within the surface of the earth is called the focus of the earthquake. At the focus of the earthquake, two types of seismic waves occur. The body waves and the surface waves. The body waves spread from the focus to all directions. The surface waves spread from the epicenter to the earth’s surface. The way the waves move is that they move rock particles in a rolling in a way that a few structures can withstand. Body waves move faster than surface waves. The body waves are of two types; the primary wave (p wave) and secondary wave (s wave). The primary wave is compressional and travels in the same direction as the wave. They arrive at the recording point fast as they are extremely fast. They can also pass through fluid. On the other hand, the secondary waves are transverse waves. They travel in a perpendicular way to the wave movement. They are slow and are caused by a shearing motion. They cannot pass through any fluid or gas. The p and s waves shake the earth differently. The fact that p waves are faster than s waves allows one to tell where an earthquake was (Branley, 2005).
To understand this more, one can compare p waves and s waves to lightning and thunder. When there is a lightening, one will first see lightening and then hear the thunderstorm. This is because light travels faster than sound. If one is not close to the lightning, he/she will stay for a few seconds before one hears the thunder. At the same time, if the person is near the lightning, he will hear the thunder right after the lightening. The further away from the storm, the longer it will take for the lightning and thunder. P waves are similar to lightening, and s waves are similar to thunder. P waves move faster and shake the ground first. Then s waves follow afterwards and also shake the ground. When an earthquake is close, P and S wave will just follow each other. If one is far, there will be time between the two waves. The scientists by looking at the time between P and S waves on a seismograph can tell how far an earthquake was. The only thing scientists can tell is how far the earthquake was, but they cannot tell in what direction the earthquake was. Scientists use triangulation method to determine the exact location of an earthquake.
The seismogram recordings on a seismograph are used to determine how large an earthquake was. Scientists have tried to predict earthquakes, but none of the trials have been successful. They can only state that an earthquake is going to happen, but they cannot specify when it will happen. Earthquakes are extremely destructive on the earth’s surface. The magnitude of an earthquake measures on how destructive it is. It shows how much energy is released. The Richter scale shows the earthquake’s magnitude on the basis of the amplitude of p and s waves on a seismogram. This Richter’s scale has an ability to account for decreased amplitude with an increase in distance from the main epicenter. It is also a logarithmic scale. The seismic moment is a standard magnitude scale. It accurately represents the energy that is released during an earthquake. On the Richter’s scale, the minor earthquakes have magnitudes of 3-4.9. Most earthquakes have this measure. This earthquake can have a small impact. Other small earthquakes with magnitude less than 3.0 are never felt. The metrical scale measures the intensity of how people and structures are affected by the seismic event. It uses numbers from 1 to 12. 1 shows no damage and 12 shows destruction (Kusky, 2008).
It is impossible to predict the occurrence of earthquakes. However, measures can be taken to reduce devastation by construction of buildings that are earthquake resistant. Earthquakes cause considerable havoc. They destroy bridges, roads, buildings and breaks underground water lines. In case of buildings that are not constructed well, during an earthquake the loss will be serious. The buildings should be constructed with moorings that are filled with alternating layers of rubber and steel. These are base isolators, the rubber acts as the absorber for earthquakes. This should be done in areas that are prone to earthquakes. These moorings can withstand magnitude of 8.3 of earthquake. Engineers try to reduce the damage of structures. They try to increase the natural period of the structure. They can also use energy dissipating devices to dampen the system. There are also clear reinforcement methods used by engineers who include use of large bolts to secure buildings to their foundation, as well as provide support to walls. The rocking effect of a building during a seismic event can be reduced by this reinforcement method.
A tsunami is a form of an earthquake. It is an underwater earthquake. On a Richter scale, it has magnitude of 8 and above. It mostly affects the earth’s ocean bodies. Tsunami’s can also be caused by volcanic explosions or submarine landslides. Tsunamis are sometimes called the seismic sea waves. They always destroy everything along the coastal line. They can look like small unnoticeable waves at the sea. The earthquake generates wave that moves out in the open water. As the waves come close to the shore, they start feeling the bottom of the sea floor. The waves on the top still move at the similar speed, although the waves at the bottom slow down and cause an immense wave to build up on the top. The tsunami can move at 450 miles per hour and attain heights of hundreds of feet. Tsunamis have created more damage than earthquakes. An example of a tsunami that occurred recently is the catastrophic tsunami in the Indian Ocean on the 26th December, 2004. The epicenter of the earthquake was located in Indonesia. It was at the off coast of Sumatra. It affected 11 countries. More than 225,000 people perished. The waves were 100 feet high. The earthquake causes the entire planet to vibrate. It also caused smaller earthquakes in some other parts of the world. On the Richter scale, it had magnitude of 9.2. It took between 15 minutes to 7 hours to affect various coastlines and cause serious damages. The sea withdrew from the shores attracting people, causing fatal consequences.
Basically, aseiche or a slosh is also an earthquake. It occurs mainly in enclosed water bodies. It can occur on a lake. They are small rhythmic waves. The wave has a long wavelength. The speed of the wave is affected by how deep the water body is. Seiches can also be caused by the meteorological phenomena. The effects are similar to a storm surge. The wave has extreme wave heights. The waves oscillate from one side of the lake to the other for some time.
Liquefaction also is resulted by earthquakes. It is the point where water saturated soil turns into a liquid from solid form. This is due to lack of cohesion between the soil particles. When soil becomes saturated with water and undergoes some external pressure, it will lose its strength and liquefy. Liquefaction can occur some minutes after the earthquake. It may lead to sinking buildings and collapsing dams, when the soil flows like water. The sand on a beach sinks when an individual walks on it. The sand is being liquefied by stepping on it. The sand is saturated and the stepping on it just raises pore water pressure, resulting to sand flowing.
An earthquake occurred in Montana in the year 1959. Magnitude of the earthquake was 7.3. The faulting which occurred near Hebgen Lake, beneath the lake the bedrock warped, causing the floor of the lake to generate a seiche. Turbidity currents are also caused by earthquakes. They are large masses of sediment-laden water, flowing down the continental slope. This is usually in a submarine canon and the difference in gravity and density of the ocean water, and the dense sediment-laden water. It is also called the density currents. They are as a result of underwater earthquakes. Floods from sediment rich rivers and strong storms can also cause the turbidity currents. In the 1929, an earthquake in the Grand Banks region took place; the underwater cable on the sea of the Atlantic floor started breaking, and did so for more hours after the start of the earthquake. Turbidities are the sediments that are deposited by turbidity currents. The major earthquakes in the world are: the Charleston earthquake which struck in South Carolina. It occurred on the 31 August 1886. The earthquakes that occur on the East coast are felt over a much a larger parts than West coast earthquakes. The older rocks were mainly on the eastside. The rocks are usually fractured and cracked by frequent earthquake activity. The rocks then absorb more seismic energy. The Charleston earthquake had magnitude of 7.0 and was felt 1300 km to the Northwest, in Messina, which is located between the island of Sicily and mainland. Italy experienced an earthquake of magnitude between 6.7 and 7.2. It was known as Messina-Reggio. This triggered a tsunami in the area, which happened within minutes of the earthquake. Cities within this region were destroyed, as many buildings collapsed. It remains (up to now) the most deadly of all catastrophes in Europe with at least 60000 to 120000 fatalities. This could have been as a result of faulting, which could be considered normal, along the straits of Messina. This happened on the morning of a 28th December 1908, and it went on for a period of 30 seconds. A century later, risk management solutions have been put in place, and if the event was to recur again, recovery efforts would be put in place, such as earthquake insurances (Lassieur, 2000).
Annually, earthquakes occur in different places. The quakes with magnitude 8 and above also occurs causing significant damages. Collapsing buildings claim majority of lives. The destruction is usually compounded by mud slides, floods and tsunamis. Smaller temblors that occur in days after the earthquake can complicate rescue efforts. This leads to further death and destruction. Emergency planning, education and putting all the measures described above for buildings can help reduce the loss of lives. The minor quakes seldom cause any direct effect of injury and death. The collapsing walls, falling object caused by shaking are what causes the most danger. Personal safety during an earthquake is very important. The damages can be predictable and preventable. Knowledge gained through awareness is for eliminating all hazardous places.