Earthquakes important notes, Study Guides, Projects, Research of Geology

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EarthquakeS

Earthquakes are the shaking of ground surfaces generally due to natural causes.

• (^) faulting. The energy accumulated during faulting is released. This release of energy gives rise to mighty waves. These waves originate from a point called Focus in the interior of the earth and then spread out in all directions. On the surface whatever comes into their contact begins to vibrate. The chief cause of earthquakes felt often in California in USA is often the San Andreas Fault found there.

• (^) Volcanic Eruptions: The volcanic eruptions are often very violent and cause vibrations in the earth crust. Sometimes the vent of a volcano is blocked temporarily and explosive eruption takes place suddenly causing tremors in the earth crust. The Krakatoa that erupted in 1883 became the cause of a violent earthquake there.
• (^) Other Reasons: The roofs of underground caves sometimes give way and release great force to cause minor tremors in the earth crust. Nuclear explosions also release massive energy to cause tremors in the earth crust.

Intensity Scale

• (^) Intensity Scale- Modified Mercalli Scale
• (^) Uses Roman Numerals example: VI or X
• (^) The effects of an earthquake are an indication of an earthquake intensity..
• (^) Examples: people awaking, damage to brick and stone structures, and movement of furniture
• (^) The scale ranges from I, which corresponds to imperceptible events, up to XII which corresponds to total destruction.

• (^) No mathematical basis for the scale
• (^) Ranking based on observed effects
• (^) Measure of the actual effects at a certain location
• (^) Subjective measure, perceived, based on qualitative descriptions.

Worldwide Distribution of Earthquakes Earthquakes have a definite distribution pattern. There are three major belts in the world which are frequented by earthquakes of varying intensities. These belts are as under:

1. The Circum-Pacific Belt
2. The Mid-Atlantic Belt
3. The Mid-Continental Belt

This belt is located around the coast of the Pacific Ocean. In this belt the earthquakes originate mostly beneath the ocean floor near the coast. The Circum- Pacific Belt represents the convergent plate boundaries where the most widespread and intense earthquakes occur.

• (^) This belt runs from Alaska to Kurile, Japan, Mariana and the Philippine trenches. Beyond this, it bifurcates into 1. The Circum-Pacific Belt:

• (^) On the eastern side of the Pacific Ocean, the earthquake belt runs parallel to the west coast of North America and moves on towards the South along the Peru and Chile trench lying on the west coast of South America.
• (^) This belt has about 66 percent of the total earthquake that are recorded in the world. Most of the earthquakes occurring in this belt are shallow ones with their focus about 25 km deep.

2. The Mid-Atlantic Belt

This belt is characterized by the sea floor spreading which is the main cause of the occurrence of earthquakes in it. This earthquake belt runs along the mid- oceanic ridges and the other ridges in the Atlantic Ocean.

• (^) In this belt most of the earthquakes are of moderate to mild intensity. Their foci are generally less than 70 km deep. Since the divergent plates in this belt move in opposite directions and there is splitting as well,