The Geology Behind Earth’s Features Natural Disasters

Essential Questions  What is an earthquake?  What is the way to detect an earthquake?  What is a Tsunami?  What are the ways to prepare for a natural disaster?

What is an earthquake?

Bill Nye - Earthquakes  asp?ProgID=DEP_BN027 asp?ProgID=DEP_BN027 asp?ProgID=DEP_BN027  Complete the Question sheet provided.  24mins

 Earthquake: –Occur at the boundaries between tectonic plates –Occur as a result of the forces of stress, strain, and strength

 Stress –Is the local force per unit area that causes rocks to deform  Strain –Is the relative amount of deformation, expressed as the percentage of distortion (eg. compression of a rock by 1% of its length)  Strength –Rocks fail that is they lose cohesion and break into two or more parts when they ae stressed beyond a critical value

Seismic Waves  There are three types of seismic waves –P-waves –S-waves –Surface waves

P-waves  Primary waves, also called P waves or compressional waves  P waves arrive first at any surface location  can travel through solid, liquid and gas  Can travel through rock at 6km/s  are waves that have the same direction of vibration along their direction of travel, which means that the vibration of the medium (particle) is in the same direction or opposite direction as the motion of the wave  As they travel through rock, the waves move tiny rock particles back and forth -- pushing them apart and then back together 

S-waves  Secondary waves, also called S waves or shear waves  As these waves move, they displace rock particles outward  S waves don't move straight through the earth  only travel through solid material  the ground is displaced perpendicularly to the moves alternately to one side and then the other  wave_animation.gif wave_animation.gif wave_animation.gif

Surface Waves  sometimes called long waves, or simply L wave  are responsible for most of the damage associated with earthquakes, because they cause the most intense vibrations  stem from body waves that reach the surface  are something like the waves in a body of water -- they move the surface of the earth up and down  L waves are the slowest moving of all waves

 Both P and S waves refract or reflect at points where layers of differing physical properties meet. They also reduce speed when moving through hotter material. These changes in direction and velocity are the means of locating discontinuities.  Seismic discontinuities (a surface at which velocities of seismic waves change abruptly) aid in distinguishing divisions of the Earth into inner core, outer core, D", lower mantle, transition region, upper mantle, and crust (oceanic and continental).

Earthquakes  Did you know… –According to the United States Geological Survey, more than three million earthquakes occur every year. That's about 8,000 a day, or one every 11 seconds!

How does an Earthquake occur?

How do Earthquakes Occur?  When brittle rocks being stressed suddenly fail along a geologic fault  Most large earthquakes are caused by ruptures of pre-existing faults, where past earthquakes have already weakened the rocks on the fault surface  The two blocks of rock on either side of the fault slip suddenly, releasing energy in the form of seismic waves –When the fault slips, the stress is reduced, dropping to a level below he rock strength

 A fault rupture does not happen all at once –It begins at the focus and expands outward along the fault surface ~2-3km/s –It stops where the stresses become insufficient to continue breaking the fault or where the rupture enters ductile material in which it can no longer propagate as a fracture  Fault ruptures in the largest earthquakes can extend for more than 1000km and the fault slip can be as large as 20m

Earthquake Terminology  Focus –The point at which fault slipping begins  Epicenter –The geographic point on Earth’s surface directly above the focus  Focal depth –In continental crust is ~2-20km –Below 20km is rare because under the high temperatures and pressures the crust behaves as a ductile material  However in subduction zones where cold oceanic crust plunges into the mantle earthquakes can originate at depths as great as 690km

Earthquake Terminology continued  Foreshock –A small earthquake that occurs near, but before, a mainshock  Aftershock –Large earthquakes trigger smaller earthquakes –Follow the mainshock  Their foci are distributed in and around the rupture plane of the mainshock –Happen where that stress exceeds he rock strength –The number and sizes depend on the magnitude of the mainshock –P 348 fig 13.6

The Elastic Rebound Theory  Proposed by Henry Fielding Reid of John Hopkins University in 1910  Explains why earthquakes recur on active faults  P345 Fig 13.1

 P 347 fig 13.5

What is the way to detect an earthquake?

Detection  Detecting an earthquake is much easier than predicting one. –a powerful earthquake can be felt by people in the area, and the damage it causes can be seen.  Seismograph –An instrument that records the seismic waves generated by earthquakes  Seismogram –A record, graphed or digital, of the seismic activities of an area

Ideal Seismographs  It would be a device affixed to a stationary frame not attached to Earth when the ground shook, the seismograph would measure the changing distance between the frame which did not move and the vibrating ground

Current Seismographs  Attach a dense mass, such as a piece of steel to Earth so loosely that the ground can vibrate up and down or side to side without causing much motion of the mass  Attachment is usually a spring (for vertical movement) or hinge (for horizontal movement)  When seismic waves move the ground, the mass tends to remain stationary because of its inertia, but the mass and the ground move relative to each other because the spring compresses or stretches or the hinge swings left and right  Record bthe movements automatically

Reading a Seismogram  When you look at a seismogram, there will be wiggly lines all across it. These are all the seismic waves that the seismograph has recorded.  Most of these waves were so small that nobody felt them.

Reading a Seismogram continued  The P wave will be the first wiggle that is bigger than the rest of the little ones (the microseisms). –Because P waves are the fastest seismic waves, they will usually be the first ones that your seismograph records.  The next set of seismic waves on your seismogram will be the S waves. –These are usually bigger than the P waves. –If there aren't any S waves marked on your seismogram, it probably means the earthquake happened on the other side of the planet. S waves can't travel through the liquid layers of the earth so these waves never made it to your seismograph.  The surface waves are the other, often larger, waves marked on the seismogram. –They have a lower frequency. –Surface waves travel a little slower than S waves.

Finding the Epicenter  Measure the distance between the first P wave and the first S wave. In this case, the first P and S waves are 24 seconds apart.  Find the point for 24 seconds on the left side of the chart below and mark that point. According to the chart, this earthquake's epicenter was 215 kilometers away.  Measure the amplitude of the strongest wave. The amplitude is the height (on paper) of the strongest wave. On this seismogram, the amplitude is 23 millimeters. Find 23 millimeters on the right side of the chart and mark that point.  Place a ruler (or straight edge) on the chart between the points you marked for the distance to the epicenter and the amplitude. The point where your ruler crosses the middle line on the chart marks the magnitude (strength) of the earthquake. This earthquake had a magnitude of 5.0.

Magnitude  Is related to the total area of the fault rupture –Most earthquakes are very small and the rupture never breaks the ground surface –In large earthquakes surface breaks are commonb

The Richter Scale  The Richter Scale is the best known scale for measuring the magnitude of earthquakes.  The energy released by an earthquake increases by a factor of 30 for every unit increase in the Richter scale.  An earthquake that measures 4.0 on the Richter scale is 10 times larger than one that measures 3.0

Richter scale no. No. of earthquakes per year Typical effects of this magnitude < Detected only by seismometers Just about noticeable indoors Most people notice them, windows rattle Everyone notices them, dishes may break, open doors swing Slight damage to buildings, plaster cracks, bricks fall Much damage to buildings: chimneys fall, houses move on foundations Serious damage: bridges twist, walls fracture, buildings may collapse Great damage, most buildings collapse. > 8.0 One every 5 to 10 years Total damage, surface waves seen, objects thrown in the air.

What is a Tsunami?

What are the ways to prepare for a natural disaster?