EQ Preparedness Presentation

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EARTHQUAKE Preparedness Presentation


By: anbrcama (70 month(s) ago)

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By: mkmarwal (70 month(s) ago)

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Earthquake Awareness Training Presented by: Anbr Cama for This presentation is copyrighted to Marikina 2 nd Ward and copying and duplication is prohibited.


What is an earthquake ? Movements within the Earth’s crust cause stress to build up at points of weakness, and rocks to deform . Stored energy builds up in the same way as energy builds up in the spring of a watch when it is wound. When the stress finally exceeds the strength of the rock, the rock fractures along a fault, often at a zone of existing weakness within the rock.


The stored energy is suddenly released as an earthquake. What is an earthquake ? Intense vibrations, or seismic waves, spread out from the initial point of rupture, the focus, like ripples on a pond . These waves are what makes the ground shake and can travel large distances in all directions. Near the focus, the waves can be very large, making them extremely destructive .


The Crust This brittle outermost layer varies in thickness from 25 to 60 km under continents, and from 4 to 6 km under the oceans. Continental crust is quite complex in structure and is made from many different kinds of rocks. The structure of the Earth The Mantle Below the crust lies the dense mantle, extending to a depth of 2890 km. It consists of dense silicate rocks. Both P- and S-waves from earthquakes travel through the mantle, demonstrating that it is solid . The Core At a depth of 2890 km is the boundary between the mantle and the Earth's core. The core is composed of iron and we know that it exists because it refracts seismic waves creating a 'shadow zone' at distances between 103º and 143º (see above diagram). We also know that the outer part of the core is liquid, because S-waves do not pass through it.


What Is Seismology ? Seismology  is the study of earthquakes and seismic waves that move through and around the earth. A  seismologist  is a scientist who studies earthquakes and seismic waves .


What Are Seismic Waves? Seismic waves  are the waves of energy caused by the sudden breaking of rock within the earth or an explosion. They are the energy that travels through the earth and is recorded on seismographs.


Types of Seismic Waves There are several different kinds of seismic waves, and they all move in different ways. The two main types of waves are  body waves  and  surface waves . Body waves can travel through the earth's inner layers, but surface waves can only move along the surface of the planet like ripples on water. Earthquakes radiate seismic energy as both body and surface waves .


BODY WAVES Traveling through the interior of the earth,  body waves  arrive before the surface waves emitted by an earthquake. These waves are of a higher frequency than surface waves .


The first kind of body wave is the  P wave  or  primary wave . P WAVES This is the fastest kind of seismic wave, and, consequently, the first to 'arrive' at a seismic station. The P wave can move through solid rock and fluids, like water or the liquid layers of the earth. It pushes and pulls the rock it moves through just like sound waves push and pull the air.


Have you ever heard a big clap of thunder and heard the windows rattle at the same time? P WAVES CONTINUED… The windows rattle because the sound waves were pushing and pulling on the window glass much like P waves push and pull on rock. Sometimes animals can hear the P waves of an earthquake. Dogs, for instance, commonly begin barking hysterically just before an earthquake 'hits' (or more specifically, before the surface waves arrive). Usually people can only feel the bump and rattle of these waves.


P waves are also known as  compressional waves , because of the pushing and pulling they do. Subjected to a P wave , particles move in the same direction that the the wave is moving in, which is the direction that the energy is traveling in, and is sometimes called the 'direction of wave propagation'. P WAVES CONTINUED…




S WAVES The second type of body wave is the  S wave  or  secondary wave , which is the second wave you feel in an earthquake. An S wave is slower than a P wave and can only move through solid rock, not through any liquid medium.


S WAVES CONTINUED… It is this property of S waves that led seismologists to conclude that the Earth's  outer core  is a liquid. S waves move rock particles up and down, or side-to-side-- perpindicular to the direction that the wave is traveling in (the direction of wave propagation). 




SURFACE WAVES Travelling only through the crust,  surface waves  are of a lower frequency than body waves , and are easily distinguished on a seismogram as a result. Though they arrive after body waves , it is surface waves that are almost enitrely responsible for the damage and destruction associated with earthquakes. This damage and the strength of the surface waves are reduced in deeper earthquakes.


LOVE WAVES The first kind of surface wave is called a  Love wave , named after A.E.H. Love, a British mathematician who worked out the mathematical model for this kind of wave in 1911. It's the fastest surface wave and moves the ground from side-to-side. Confined to the surface of the crust, Love waves produce entirely horizontal motion.




RAYLEIGH WAVES A Rayleigh wave rolls along the ground just like a wave rolls across a lake or an ocean. Because it rolls, it moves the ground up and down, and side-to-side in the same direction that the wave is moving. Most of the shaking felt from an earthquake is due to the Rayleigh wave, which can be much larger than the other waves. 


RAYLEIGH WAVES The other kind of surface wave is the  Rayleigh wave , named for John William Strutt , Lord Rayleigh, who mathematically predicted the existence of this kind of wave in 1885.




Plate Tectonics Hot magma rises from the mantle at mid-ocean ridges pushing the plates apart.   Divergent Boundary   Earthquakes occur along the fractures that appear as the plates move apart


Plate Tectonics At subduction zones, the oceanic plate is pushed down, or subducted , below the continental lithosphere. As the oceanic slab decends , earthquakes are generated within the slab and at the interface between the plates.   Destructive Boundary  


Plate Tectonics Continental collisions result in the creation of mountains and fold belts as the rocks are forced upwards.   Convergent Boundary  


Plate Tectonics Where two plates slide past each other, earthquakes originate at shallow depths. California is a good example of this type of boundary.   Transcurrent Boundary  


Tectonic Plates There are about 20 plates along the surface of the earth that move continuously and slowly past each other. When the plates squeeze or stretch, huge rocks form at their edges and the rocks shift with great force, causing an earthquake.


Earthquake intensity Intensity is a measure of an earthquake determined from the observed effects, especially damage. For a given earthquake, intensity normally decreases with distance from the epicentre . The observations can then be compiled to make macroseismic maps showing lines of equal intensity.


Earthquake intensity I. Not felt except by a very few under especially favorable conditions. II . Felt only by a few persons at rest, especially on upper floors of buildings. III . Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated. IV. Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably. V . Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop . Abbreviated Modified Mercalli Intensity Scale


Earthquake intensity VI . Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight. VII . Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken. VIII . Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. IX . Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations .. Abbreviated Modified Mercalli Intensity Scale


Earthquake intensity XII . Damage total. Lines of sight and level are distorted. Objects thrown into the air. Abbreviated Modified Mercalli Intensity Scale


Magnitude / Intensity Comparison Magnitude and Intensity measure different characteristics of earthquakes. Magnitude measures the energy released at the source of the earthquake. Magnitude is determined from measurements on seismographs. Intensity measures the strength of shaking produced by the earthquake at a certain location. Intensity is determined from effects on people, human structures, and the natural environment .


The size or magnitude of earthquakes is determined by measuring the amplitude of the seismic waves recorded on a seismograph and the distance of the seismograph from the earthquake. These are put into a formula which converts them to a magnitude, which is a measure of the energy released by the earthquake. For every unit increase in magnitude, there is roughly a thirty-fold increase in the energy released. For instance, a magnitude 6.0 earthquake releases approximately 30 times more energy than a magnitude 5.0 earthquake, while a magnitude 7.0 earthquake releases approximately 900 times (30x30) more energy than a magnitude 5.0.


The following table gives intensities that are typically observed at locations near the epicenter of earthquakes of different magnitudes. Magnitude Typical Maximum Modified Mercalli Intensity 1.0 - 3.0 I 3.0 - 3.9 II - III 4.0 - 4.9 IV - V 5.0 - 5.9 VI - VII 6.0 - 6.9 VII - IX 7.0  and higher VIII  or higher



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