1 Summary Geography Class 16 ## Last class revision (5:16 PM). ## Earthquakes (continued) (5:28 PM): - ***Causes of earthquake:*** - ==Natural causes:== - Plate movements - Volcanic eruption. - Landsides. - Land Subsidence. - Meteoritic impact. - ==Anthropogenic causes:== - Mining and blasting. - Nuclear explosion. - Reservoir-induced seismicity(RIS). - The scientific study of earthquakes is called seismology. ## Earthquake waves(5.35 PM): - Diagrammatic representations of Earthquake waves: - ![](https://qph.cf2.quoracdn.net/main-qimg-61dbe7e20ba5d0ba96c05794edaa4f36-pjlq) - During an earthquake, the point where the energy is released below the surface is called the focus/hypocentre. - The point on the surface nearest to the focus is called the epicentre. - An earthquake generates two types of waves, body and surface waves. - ==Body waves:== - These earthquake waves are generated at the focus and move through the earth's interior. - There are two types of body waves, P-wave and S-wave. - ==P-waves(Primary waves):== - These are the first to be detected during an earthquake. - They are also called compressional waves and longitudinal waves. - They oscillate back and forth in the direction of propagation. - P-Waves depend on the density and rigidity of the material. - P- waves can travel through solid, liquid, and gas. - ==S-waves(secondary waves):== - S waves oscillate perpendicularly to the direction of propagation. - They are also called transverse waves. - The S-wave movement depends on the material's density and rigidity. - Therefore S waves cannot pass through liquid and gas. - **==Surface waves:==** - These are the earthquake waves confined to the surface of the earth. - They are generated due to the interaction of body waves with the surface and generally at the epicentre. - Surface waves are much slower than body waves but cause more damage. - Examples are Love waves and Raleigh waves. ## Shadow zones(6.18 PM): - Diagrammatic representation of shadow zones: - ***![](https://img.homeworklib.com/questions/bda1f1f0-73c4-11ea-ae17-87d719c7696e.png?x-oss-process=image/resize,w_560)*** - The seismic waves undergo reflection, bending, or changes in direction while passing from one medium to another. - This results in the creation of shadow zones. - The Shadow zone is the zone in the earth's interior from where earthquake waves are not reported. - ==P-wave shadow zone:== - P-waves are not detected between 105-145 degrees from the focus resulting in the P-wave shadow zone. - It is due to a change in direction of P-waves as it enters the outer core. - ==S-wave shadow zone:== - S-waves cannot pass through the liquid outer core. - Therefore beyond 105 degrees, we observe S-wave shadow zones. - By studying the different earthquake waves, the extent of their shadow zone, and other characteristics we can conclude about the earth's interior. ## Type of earthquake (6:32 PM) - Shallow focus earthquake occurs at the depth of  0-70 km. - Intermediate focus earthquake occurs at the depth of 70-350 km. - Deep-focus earthquake occurs at the depth of more than 350 km. ## Distribution of earthquakes (6:35 PM): - ***Circum-pacific belt:*** - Around the pacific ocean. - Along the west due to ocean-ocean convergence. - Along the east due to ocean-continent convergence. - ***Mid-oceanic ridges:*** - Around the regions of ocean-ocean divergence - ***Meditarian and trans-asiatic belt:*** - Continent-continent convergence. - ***East African rift valley:*** - Continent-continent divergence. - Diagramatic representation of the distribution of earthquakes: - ![](https://static.educalingo.com/img/zh/800/de-zhen-1.jpg) ## Measurement of earthquakes (7:03 PM) - The earthquake waves can be recorded by a seismograph. - The record produced by the seismograph is called a seismogram. - ***Magnitude and intensity of the earthquakes:*** - | | | | --- | --- | | **Magnitude:** | **Intensity:** | | It is the measure of the amount of energy released during an earthquake. | Intensity is the assessment of visible damage caused by an earthquake. | | The magnitude remains constant for earthquakes. | Intensity decreases away from the epicentre. | | It is an objective measure. | It is a subjective measure. | | The magnitude is measured by the Richter scale. There is no upper limit for the Richter scale. Richter scale is a logarithm scale that is 1 unit change means a 10 times stronger earthquake. | Intensity is measured by a Modified Mercalli scale. The range of the Mercalli scale is 1-12. | - The highest recorded earthquake is a 9.5 Richter scale in Chile in 1960. - 2004 Indian ocean(Indonesia) earthquake was  9.1 magnitudes. - 2011 japan earthquake was 9.1 magnitudes. - ***Earthquake in India:*** - 2001 Bhuj earthquake was 7.7 magnitudes. - The 2005 earthquake in Kasmir was of 7.6 magnitudes. - 2011 Sikkim-Nepal earthquake was of 6.9 magnitudes. - The 1993 Latur earthquake was of 6.2 magnitudes. - 1991 Uttarkashi earthquake was of 6.1 magnitudes. ## T-sunami (7:31 PM) - A tsunami is a Japanese word for harbour waves. - They are also called seismic sea waves. - They are four stages in the occurrence of a tsunami: - ***Generation:*** - Tsunamis are generated due to vertical displacement of the column of seawater which causes by: - Earthquakes, normal and reverse fault earthquakes along the oceanic floor. - Underwater Volcanic eruptions. - Underwater landslide and subsidence. - Meteorites impact. - Underwater nuclear test. - ***Propagation in deep water:*** - Along deeper waters, the speed of waves is very high and the amplitude will is low. - Therefore tsunamis will not be visible in the open ocean nor will cause any damage. - ***Propagation in shallow water:*** - As depth decreases, the amplitude increases, and the speed of waves decreases. - This causes a pilling up of water near the coast, generating very high waves, this is called the shoaling effect. - ***Landfall:*** - Where the waves break and submerge the coastal regions. ## The topic of the next class: Exogenetic movements.