General Information on Earthquakes
               Earthquakes are seismic events caused by the sudden release of energy built up in the Earth's crust in order to maintain balance between the tectonic plates.

           Causes of earthquakes
               There are two major causes of earthquakes. The first is human action, such as atomic bomb tests, the storage of large amounts of water in a dam, or explosions in a mining operation. The second and most frequent cause of earthquakes is the natural movement of tectonic plates or magma within volcanoes. Additionally, there are two other widely accepted theories regarding the geological mechanics of earthquakes: Dilation source theory proposes that an earthquake is caused by the sudden build-up of stress in the Earth's crust, leading to an equally sudden release of energy as seismic waves. Elastic rebound theory proposes that earthquakes are caused by movements of faults in the surface of the Earth, which gradually build up stress. Once the rocks accumulate energy to the breaking point, they are suddenly and drastically deformed, leading to a release of seismic waves before returning to their original shape.

           Severity of earthquakes
               The power of an earthquake can be measured in magnitude and intensity - two entirely different measurements that are often mistaken for one another. Magnitude is the measurement of energy released from the earthquake's hypocenter. This value varies directly with the quake's amplitude, which can be measured using a seismometer and multiplied with a distance correction factor to obtain the magnitude. Thus, each earthquake will only have one magnitude value. The magnitude is measured using the Richter scale, which assigns numbers to quantify the amount of released energy to allow for easy comparison and understanding of the scale of each tremor. It must be noted that the magnitude does not indicate degree of damage caused by the quake. Intensity is the measurement of the earthquake’s impact on the senses and damage caused to various structures and the natural environment. Intensity naturally varies in each location and is determined by factors like distance from the epicenter and geological features. Intensity can be gauged by observing the response of people, the movement of household furniture and various items, damage done to chimneys, or even downright devastation of large buildings. The Modified Mercalli Intensity (MSI) scale is used to classify intensity into 12 separate levels designated by Roman numerals, ranging from minuscule tremors that have to be detected using specialized equipment to destruction of everything in the vicinity.

 

Intensity	Effects
I - Instrumental	Not felt by people, but detectable with instruments
II - Weak	Felt by some sensitive people
III - Slight	People standing still may feel the ground shake
IV - Moderate	Felt by people walking/commuting
V – Rather strong	People sleeping may be jolted awake
VI - Strong	Trees and houses shake, some weak structures collapse
VII – Very strong	Cracks appear in walls, dust falls from ceilings
VIII - Destructive	Vehicles are forced to stop, buildings damaged, chimneys destroyed
IX - Violent	Buildings on cracks in the Earth’s surface collapse, water and gas pipes broken up
X - Intense	Cracks appear in the ground, strong buildings collapse, railroads bent out of shape, landslides on hills and mountains
XI - Extreme	Buildings and bridges collapse, railroads and underground power cables damaged, major landslides, floods
XII - Cataclysmic	Total devastation in affected area, ground moves in waves

 

 

           Source: Documents on ecological disasters, Department of Mineral Resources (http://www.dmr.go.th/ewt_news.php?nid=6814)