Earthquake

 EARTHQUAKE

"An earthquake is a sudden motion or trembling of the ground produced by the abrupt displacement of rock masses".

• Most earthquakes result from the movement of one rock mass past another in response to tectonic forces.
• The focus is the point where the earthquake's motion starts.
• The epicenter is the point on the earth's surface that is directly above the focus.
• Earthquake Magnitude is a measure of the strength of an earthquake as calculated from records of the event made on a calibrated seismograph.
• In 1935, Charles Richter first defined local magnitude, and the Richter scale is commonly used today to describe an earthquake's magnitude.
• Earthquake Intensity.  is a measure of the effects of an earthquake at a particular place. It is determined from observations of the earthquake's effects on people, structures and the earth's surface.
• Among the many existing scales, the Modified Mercalli Intensity Scale of 12 degrees, symbolized as MM, is frequently used
  

Earthquake Hazards
Earthquake hazards can be categorized as either direct hazards or indirect hazards.
Direct Hazards

• Ground shaking
• Differential ground settlement
• Soil liquefaction
• Immediate landslides or mud slides, ground lurching and avalanches
• Permanent ground displacement along faults
• Floods from tidal waves, Sea Surges & Tsunamis
  

Indirect Hazards

• Dam failures
• Pollution from damage to industrial plants
• Delayed landslides.

Most of the damage due to earthquakes is the result of strong ground shaking. For large magnitude events, trembling has been felt over more than 5 million sq. km.

Site Risks
Some common site risks are:

• Slope Risks - Slope instability, triggered by strong shaking may cause landslides. Rocks or boulders can roll considerable distances.
• Natural Dams - Landslides in irregular topographic areas may create natural dams which may collapse when they are filled. This can lead to potentially catastrophic avalanches after strong seismic shaking.
• Volcanic Activity - Earthquakes may be associated with potential volcanic activity and may occasionally be considered as precursory phenomena.
• Explosive eruptions are normally followed by ash falls and/or pyroclastic flows, volcanic lava or mud flows, and volcanic gases.