Objective: To define and discuss earthquakes and its effects.
Earthquakes are a major catastrophe and can be a big threat to human lives. To understand earthquakes you must have a clear definition, know its mechanisms, be able to recognize the size and dynamics and understand its effects:
I.Definition of Earthquake
b.Classification of Seismic Waves
b.Identify plate boundaries
In order to define earthquake properly many things are taken into consideration. First of all an earthquake is basically defined as a naturally induced shaking of the ground, caused by fracture of rock within the Earth. The results of an earthquake are sliding rocks past one another along a geological fault. Earthquakes are one of the deadliest of natural catastrophes. In the 20th century alone the average annual death toll is about 20,000 people.
As a result of earthquakes tsunamis, landslides and volcanoes are formed. Tsunamis are characterized as large waves in oceans as well as seiches, which are similar waves in lakes. Landslides are simply a downward movement of rocks due to gravity. Lastly volcanoes are hills or mountains formed from rock fragments ejected through a vent. Earthquakes sometimes cause this.
Next lets explore the mechanisms of an earthquake. Stress builds up in a specific area on the Earths crust to a level sufficient to overcome frictional forces resisting the sliding of a preexisting fault or to break rock and create a new fault. The slipping fault then creates seismic waves in the surrounding rock. Seismic waves create the sound waves in the air. It also carries energy away from the fault, first to nearby rock. In hours to follow or even days many small earthquakes occur. These are referred to as aftershocks. Aftershocks are a result of rocks near the fault adjusting to accommodate the new stress levels.
In order to understand the size of an earthquake certain items must be taken into consideration. Strength, classification of seismic waves, logarithm and intensity are some of the main areas to focus on. The strength depends upon the area of the fault length times width on which the slip occurs, the amount of the slip and the inherent stiffness of the nearby rock. An earthquake occurring a 100-km (62-mi) fault is about one billion times stronger than a quake on a 100-m (328-ft) fault. Seismic waves are classified in two areas P-waves, which are your compressional waves, and S-waves, which are shear waves. These waves are used in detection and location of earthquakes. Several thousand seismometers are in continuous operation worldwide. These stations record ground motions, detect waves even at great distances from the fault as well as measure arrival time of P and S waves. Logarithm of moment also referred to as magnitude characterizes the earthquakes strength. According to the use of logarithm earthquakes are factored as one billion 10 raised to the power of 9. This theory is used when working with small numbers. An earthquake that has a magnitude of M-1 is barely felt, and a great quake is recorded as M-9. Most reading are referred to as being on the Richter scale, which got its name from the American seismologist Charles Richter, who popularized their use. Intensity is judges by its effects on buildings and other structures. The Modified Mercalli Scale ranks damage at a specific site on an I-XII scale with XII being complete destruction. All these things are taken into consideration when determining the size of an earthquake.
Along with size, dynamics or an earthquake is measured. During the 1980s 15,436 earthquakes with a magnitude of 5 or greater were detected. However, smaller quakes are much more common. The largest quakes occur on subduction zones. These are normally located between to convergent tectonic plates. This happens when one plate is thrust beneath the other, sometimes sinking up to 800 km beneath the surface of the earth before being destroyed. Smaller earthquakes occur within both plates in a roughly 200-km wide zone surrounding the plate boundary. The very damaging 1995 Kobe, Japan earthquake measures at M-6.9 occurred in a zone of crustal deformation near the Japan subduction zone.
There has been a lot of destruction caused by earthquakes in the past. The effects have been enormous throughout the world.