earthquakes

geography chapter 8

define an earthquake.

an earthquake is defined as a tremor below the surface of the earth which causes shaking of the crust.

how are earthquakes caused?

• they are caused by the strain in the earth’s crust which in turn is a result of a number of factors.

• when the earth’s crust is unable to accommodate itself to the strain, it results in a release of tremendous energy in the form of a sudden violent shock.

• a series of these shocks are known as seismic waves.

• the waves spread out from the seismic focus of the earthquake and cause widespread destruction.

• severity of earthquakes - measured on richter scale.

what are the main causes of earthquakes?

volcanoes, plate tectonics, folding and faulting, man-made causes.

how do volcanoes cause earthquakes?

• during eruptions, hot gases are pushed upward and they push violently against the earth’s surface causing earthquakes.

• volcanic eruptions and earthquakes are inter-related.

how do plate tectonics cause earthquakes?

• acc. to plate tectonic theory - earth is made of series of movable plates. plates move and slide over each other.

• when 2 plates slip past each other or collide against each other, their edges produce faults along the lines of weakness.

• tectonic earthquakes are most frequent and immensely powerful.

how do folding and faulting cause earthquakes?

• internal horizontal and vertical movements due to compressional and tensional forces cause folding and faulting in the earth’s crust.

• this causes displacement of rocks in the crust - this imbalance causes earthquakes.

what are the man-made causes of earthquakes?

• human activity like construction of huge dams, nuclear explosions, blasting of rocks, mining etc. near fault zoned causes earthquakes.

what is used for the measurement of earthquakes?

• instrument - seismograph.

• richter scale and mercalli scale.

what is a seismograph?

• the direction of movement of waves and their passage at a particular point is recorded by an instrument called seismograph.

• has a pen attached to it which vibrates with the earthquake waves, recording their movements on a traveling strip of paper.

• also calculates difference in arrival of P and S waves.

describe the richter scale.

• measures the intensity of an earthquake.

• measures the power of an earthquake on scale of 1 to 9.

• measures absolute intensity with mathematical precision.

describe the mercalli scale.

• measures intensity of an earthquake.

• grades earthquakes on basis of observed effects on a 12-point scale.

what are some constructive effects of earthquakes?

1. energy release: earthquakes help the earth release its stored up energy. majority earthquakes occur around the plate margins.

2. landforms: on acc of both vertical and lateral displacement of crust, earthquakes may raise or lower parts of earth especially near the seashore. landslides triggered by earthquakes cause formation of lakes.

what are some destructive effects of earthquakes?

1. collapse of structures: human beings have settlements in earthquake-prone zones. these settlements often collapse and cause great loss of property.

2. submergence: force of uplift and subsidence also cause submergence of coastal parts.

3. course of rivers: earthquakes change the course of rivers, rendering many areas unsuitable for irrigation and agriculture.

4. danger to human life: severe earthquakes cause widespread loss of life. they die on account of falling trees, poles, bridges or other structures.

explain the distribution of earthquakes.

• most earthquakes originate from plate boundaries.

• in regions where an ocean plate and land plate converge, submarine ridges are formed.

• the underlying rocks are unstable and subjected to movement - any such movement triggers earthquakes in the continental areas.

what are the main earthquake belts of the world?

1. the circum pacific mountain belt: this belt has 70% of all earthquakes. part of san andreas fault in USA lies in this belt. this fault zone makes california in USA highly prone to earthquakes. area of japan, phillippines and indonesia are all parts of this belt.

2. mid-world mountain belt: stretches from eastern europe covering alpine-himalayan ranges in europe and asia. about 20% of all earthquakes occur in this zone.

3. the mid-atlantic ridge: this belt comprises areas along the mid-oceanic ridges as well as many islands near the ridges of the atlantic ocean. the remaining 10% earthquakes occur in this belt. in this ridge, 2 tectonic plates move in opposite direction.