Theory of plate tectonics
Earth’s surface is divided into seven major plates ( plate tectonics ) plus a few more minor ones. These plates move a few centimeters every year constantly changing the shape of earth’s surface. Movement is made possible due to the semi-molten layers or rock that they rest upon underneath the crust of the earth.Moving plates either pull apart from each other or collide causing powerful shock waves known as earthquakes.On the image below, you can see the plate boundaries of the seven major plates, the plate movement direction and the type of boundaries they create.
Types of Plate Boundaries
Different types of plate boundaries exist, depending on the movement of two neighboring plates and the way they collide or move apart.The boundaries between two plates are also called fault lines. As plates collide or move apart, earth’s crust deforms along these fault lines creating the following types of plate boundaries :
- Rift Valley
- Mountain Building
- Subduction zone
- Sea floor spreading
- Strike-slip faults
Plate movement is not constant but happens unexpectedly releasing huge amounts of pressure.
Basically, two plates are held together by friction. The more time this goes on the more pressure is built up. At some point friction cannot hold up the amount of pressure that has been built up over time, resulting in violent movement of the crust until all the energy is dissipated.
The RICHTER scale
The Richter scale was invented by Charles F. Richter in 1934, in order to measure an earthquake’s magnitude ( force ), calculated form the amplitude of the strongest seismic wave recorded for a specific earthquake.The amplitudes of seismic waves of an earthquake are recorded using a Seismograph ( or seismometer ).The Richter scale is calculated by measuring the time between the Primary ( P ) and Secondary ( S ) waves of the earthquake – also giving us an approximate distance to the focus of the quake – which is then plotted on a graph similar to the one below against the largest amplitude of the seismograph thus giving us the earthquake magnitude. ( see graph below ).In the example below, the time between P and S was 24seconds thus giving the focus distance at approximately a little more than 200 km against the largest amplitude ( 23mm ), thus showing an earthquake of magnitude 5 on the Richter scale.
Earthquake Facts and Statistics
Thousands of earthquakes occur around the world every year but most of them are only picked up by seismographs and cannot be felt by people.
An estimated average ( based on statistics from 1900 ) for the number of quakes per year is :– 1,300,000 quakes with magnitude lower than 2,5 Richter
– 130,000 quakes with magnitude 2,5 – 4 Richter
– 13,000 quakes with magnitude 4 – 5 Richter
– 1,319 quakes with magnitude 5 – 6 Richter
– 134 quakes with magnitude 6 – 7 Richter
– 15 quakes with magnitude 7-8 Richter, and
– 1 quake with magnitude 8 or higher on the Richter scaleEarthquake effects, in correlation to the magnitude, are shown on the table below :
An earthquake and its’ variables are at the present time unpredictable. There is no way of knowing beforehand when an earthquake is going to strike, where, its’ magnitude, focus distance and the way earth will respond and propagate as well as how the waves will reach the surface.Designing a structure to resist such unknown forces is therefore based on statistical data from previous earthquakes for a specific area as well as probability analysis and assumptions of how a structure will be designed. It would be extremely uneconomical to design a structure to withstand the largest magnitude earthquake ever and yet it would not be accurate since no earthquake is identical to another, depending on distance, ground etc.Assumptions depending on the importance of each building, its’ life span and many more are calculated for each individual building in order to determine the type, magnitude and other characteristics of an earthquake in order to calculate applied forces on a structure and proceed with the design.