Earthquakes

What causes Earthquakes?

• Earthquakes are sudden movements or vibrations in the earth’s crust.

• They are caused by faulting and folding activity along converging plate boundaries, as well as with landslides and volcanic activity.

Where do they occur?

• The most and the strongest are located along subduction and transform plate boundaries.

• Minor earthquakes can occur with volcanic activity or landslides.

• http://video.nationalgeographic.com/video/environment/environment-natural-disasters/earthquakes/earthquake-101/

• Focus: The actual location in the earth where the earthquake occurs.

• Epicenter: the point on the surface directly above the earthquake focus.

• Tectonic creep: rock masses sliding past each other at a fairly regular rate releasing strain energy. At times these rock masses lock up and strain builds up. They snap back into unstrained position. This releases energy outward as a seismic shock producing an earthquake.

• Example: San Andreas fault in California.

How are Earthquakes Measured?

• Mercali scale: the intensity of the earthquake is measured. This is a measure of the human experience, and the impact of the earthquake. Values are determined by observing the damage done to structures.

• Richter Scale: The most common scale used to measure Earthquakes.

• It is based on intensity over a given distance.

• The largest Earthquakes record have measured just over 9.0 on the Richter scale.

• Earthquakes over 9.0 on the Richter scale are very rare.

• Example: – 2004 Indian Ocean earthquake of

magnitude 9.3 (caused a devastating tsunami)

– 1960 Chilean earthquake of a magnitude of 9.5.

• http://www.britannica.com/blogs/2008/12/the-indian-ocean-earthquake-of-2004-will-history-repeat-itself/

• A megathrust earthquake is an earthquake that can exceed 9.0 on the Richter scale.

• These generally occur where one plate subducts (slips below) another plate.

• Most occur in the Pacific or Indian Oceans, and because of the fact that they deform the ocean floor, tsunamis are usually associated with them.

Secondary Effects of Earthquakes

• There can be several secondary effects of earthquakes: – Tsunamis– fire and gas lines damaged or broken– flooding, – landslides, – famine / disease, – land altered (ex: shoreline raised 30

feet in Alaska, 1964).

• Earthquakes can cause a lot of damage and deaths in a matter of seconds.

• Examples: – China 1976: whole city destroyed,

deaths around 600,000– China 1556: 830,000 people killed

Questions

• Explain how Earthquakes are measured using Seismometers.

• Read the “Tragedy in Turkey” on page 56/57 in your text book and answer questions 1, 2, and 3.

• Answer question 8 and 13 on page 61.