Eathquakees.docx

**CHAPTER 5: EARTHQUAKES** -Sudden and rapid movement of the earth's crust. -areas prone to them are called **seismic zones** and those not prone are called **aseismic zones**. It's caused by shock waves. There are 3 types of earthquake waves namely: 1. Primary waves-which travel fastest and c...

**CHAPTER 5: EARTHQUAKES** -Sudden and rapid movement of the earth's crust. -areas prone to them are called **seismic zones** and those not prone are called **aseismic zones**. It's caused by shock waves. There are 3 types of earthquake waves namely: 1. Primary waves-which travel fastest and cause the rock particles to vibrate in a push and pull manner and can pass through gases, liquids and solids.  2. Secondary waves-which cause rock particles to vibrate at right angles to the direction of wave movement. 3. Surface longitudinal waves-which cause surface rocks to shake sometimes causing buildings to collapse. i. Rayleigh waves-Which cause surface rocks to move in elliptical orbits.  ii. Love waves-which cause rock particles to move in a horizontal manner at right angle to the direction of wave. Earth quake originates from a point known as **seismic focus/origin**. The part of the earth vertically above the seismic focus and where the shock waves are first experienced is called **epicentre**. **Causes of Earthquakes** **Natural Causes** a. Tectonic movements e.g. movement of tectonic plates. They cause tectonic earthquakes. b. Vulcanicity when magma movement displaces rocks suddenly shaking and shuttering them. c. Gravitative force when crustal rocks collapse into cauldron due to gravity. d. Energy release in the mantle when radioactivity takes place in mantle releasing explosive energy which sends shock waves outwards. e. Isostatic adjustment when the continental masses rise to restore the upset state of balance between sial and sima layers. **Human Causes** a. Exploding nuclear bombs underground which causes shock waves which spread outwards and are felt in the neighbourhood. b. When a train rolls on its rails causing the ground to vibrate. c. Explosion of explosives used in mining and quarrying which cause vibrations to be felt in the neighbourhood. d. When large reservoirs are constructed and the heavy weight of water reactivate dormant faults causing tremors. **Measurement of Earthquakes** Seismograph is a pendulum based instrument used to measure earthquakes. It records seismic impulses on a graph-like record called seismogram mounted on it. Earthquakes are measured by their intensity and magnitude. **Intensity** -Measure of how strong/hard the quake shakes the ground. It's seen from the effects the earthquake has on people, buildings and other structures. It's measured on the **Mercalli Scale** which uses a scale running from Roman i-xiii e.g. - I- description -imperceptible - V-rather strong-sleepers are awakened and there is swinging of objects. - VIII-destructive-gaping cracks in walls some brought down. - XII- major catastrophe-every building destroyed. **Magnitude** -Measure of amount of energy given off by an earthquake. It's measured on Ritcher Scale which ranges from 0-8.9. Intensity values depend on how far a place is from epicentre. The higher the scale the more severe the earthquake is. - Intensity I-magnitude 2 - Intensity VIII-magnitude 6 - Intensity XII-magnitude 8.5. **World Distribution of Earthquakes** a. Within the zones of major faulting e.g. Rift Valley. b. In areas of Vulcanicity e.g. Oldonyo Lengai in Tanzania. c. Along boundaries of tectonic plates e.g. Japan, Philippines, East Indies and west coast of north and South America. **Effects of Earthquakes** a. Can cause loss of life and property when buildings collapse burying people. b. Disrupt transport and communication by vertically and laterally displacing land which disconnects pipelines, electricity lines, roads and railways. c. Causes landslides which also cause loss of life and property and disrupts communication. d. Causes raising and lowering of the sea floor and the coastal regions. e. Cause huge sea waves called Tsunami which may flood the neighbouring coastal areas. f. Trigger folding, Vulcanicity and fires. g. Give off a lot of explosive energy more than an atomic bomb. h. Cause fear and panic. i. Hinder settlement as it is restricted to aseismic areas. j. Cause violent motions of the earth's surface.