Table of Contents • What is it • Measuring and Predicting • Where is it • Waves • Scales • Damage • Tsunamis
What is it? • Result of plate movements • Rock on sides of a fault (a crack or a plate boundary) become gradually bent. • Elastic strain energy builds up until it reaches the breaking point. • The rock snaps back (elastic rebound) into its unstrained position by slipping along the fault. • The strain of this energy is released and the energy moves outward as seismic shock waves, producing an earthquake.
Measuring and Predicting • Seismometer: measures intensity • Rod is anchored deep in the ground and vibrates when a quake occurs. • Joined to the rod is a pendulum with a pen. • Seismograph: graph created from seismometer.
Where? Focus: actual location in the earth where earthquake occurs- depth is important. Epicentre: point on the surface directly above the earthquake's focus
Where do the deepest earthquakes occur? In Tranform zones? In Convergent zones? In Divergent zones?
Waves • The seismic waves produced by an earthquake radiate out from its focus. • These waves cause the earth to vibrate in response to the pushing and pulling forces that are applied to them. • Two category of waves (each with subsets): • Body waves: Primary(P), Secondary(S) • Surface waves: Love(L), Rayleigh (R)
SEISMIC WAVES: Body Waves • Body waves arrive first since they are the fastest. Body waves are broken down into two types: • Primary (P) waves, which cause the rock particles through which they pass to shake back and forth (compression effect) and can result in a noise similar to a sonic boom. • Secondary (S) waves make the particles vibrate both vertically and horizontally (shearing effect). • Note that Body waves travel mostly downwards into the interior of the earth and are rarely felt by humans. They disappear as soon as they hit a gas (atmosphere).You can hear Body waves, but it is the next set of waves that actually move outwards along the ground – causing damage.
SEISMIC WAVES: Surface Waves • Surface waves arrive last and as their name implies they travel along the surface of the earth. It is these waves that cause damage. They begin with: • Love/Long (L)waves that move the ground from side to side, again, in a shearing effect. These are followed by: • Rayleigh (R) waves which aresimilar to ocean waves. These cause surface materials to move in a vertical circle just as a floating object would move as a sea wave passes under it. These waves are responsible for most of the damage to buildings. • The greatest shaking occurs near the centre of large earthquakes. It is here that all the waves originate.
MEASURING THE ENERGY RELEASED The energy released by an earthquake is measured by a Seismograph!
MEASURING THE DAMAGE The violence of an earthquake can be measured using two different scales: The Richter scale - devised in 1932 by Charles RichterThe Richter scale actually uses a numerical value. It is important to note that this scale is logarithmic in nature meaning that the numbers measure in factors of 10. In other words, an earthquake measuring a 4 on the richter scale is 10x as damaging as an earthquake that measured 3 on the richter scale. The Mercalli scale - devised in 1905 by a gentlemen named Mercalli. The Mercalli scale is a visual scale based on the destruction that occurs.
WHAT IS THE EARTH MADE OF? • The study of earthquakes has helped us determine what the earth is made up of. • Geophysicists have calculated that the earth has a • mass of 5876 million million million tonnes. (A Zillion) • In addition, it has been determined that the earth • has an average density of 5.5, based on water having • a density of 1. • But the rocks that we can examine near the surface • of the earth have an average density of 2.7. This suggests that the density of the earth is higher in some parts and lower in others.Assumption: density increases as depth into the earth increases.
WHAT IS THE EARTH MADE OF? • Can it be proven? • In an attempt to prove this assumption , investigations have been made by analysing earthquake waves – the ones that go down into the earth – Body waves. • The study of earthquakes (which is caused by a • shifting of the earth’s crust ) is referred to as Seismology. • As mentioned earlier (a quick review follows) when a earthquake occurs, it gives off two types of energy (shock-vibration) waves: • 1. Body waves - which travel through the earth • 2. Surface waves - which travel along the earth’s surface
Short Review and some repetition: • Body waves (travel through the interior of the earth) and they are also divided into two types: • 1. P waves - (Primary waves) • They are compressional (squeeze) waves, they compress then stretch. • They travel very quickly through the earth’s surface. • They will go through anything: liquid, solid, gas. Their speed increases with density. • 2. S waves - (Secondary waves) • They are slower than a P wave • They produce a side to side movement, shaking action. • Their speed increases with density and will only go through solids
Surface wavesare confined to the outer crust of the earth. • These are the ones we feel when there is an earthquake and the ones that causes all the destruction. • There are two type of Surface waves: • 1. L waves - (Love waves) they have a side to side motion. • 2. R waves - (Raleigh waves) they have an up and down motion. • You can actually see these waves go through the earth during an earthquake. These waves move the surface of the earth like waves moving through water.
SO WHAT? • By studying the movement of these waves , Seismologists have determined the characteristics of the inside of the earth. For instance: • The S and P waves move with a constant speed in the Lithosphere, but slow down in the asthenosphere. This backs up the notion that the asthenosphere is semi-solid in nature. • Both waves pick up speed in the Mantle and increase their speed as their depth in the Mantle increases. • When these waves reach the core the S waves stop and the P waves slow down, which suggests that the outer core is liquid.
SO WHAT? Cont’d • When the P waves reach the inner core they pick up speed tremendously, which again suggests that the inner core is solid and quite dense. • In theory the core of the earth should have a density of about 15 and the material in the inner core has been called NIFE - a combination of 10% Nickel and 90% Iron.
Tsunamis • Giant waves generated by an underwater disturbance that cause destruction to coastal areas
Tsunamis • Waves will travel outward in all directions • Time between wave crests may be from 5 to 90 minutes • Wave speed in the open ocean will average 450 miles per hour. • Heights of over 100 feet recorded
Tsunamis • When created, it has a very long wave length and short wave height. • As it approaches shallow coastal waters, length is compressed and heights are increased • Wave becomes unbalanced and topples
Tsunamis • Rapid changes in the water level are an indication of an approaching tsunami. • Arrive as a series of successive crests (high water) and troughs (low water) • After a severe earthquake on November 18, 1929, in the Grand Banks of Newfoundland • generated a tsunami that caused considerable damage and loss of life at Placentia Bay, Newfoundland.
The following animations were captured from the following site: www.pbs.org/savageearth/animations/tsunami/index.html
The Tsunami of Dec. 26, 2004 The Toronto Star