Chapter 13 Earthquakes and Earth s Interior The crust of the Earth is made up of floating tectonic plates- huge continent-sized chunks of solid rock floating on molten rock. Rock masses along the boundaries of these plates are constantly exerting huge forces as they butt up against each other. These pressures can be in any combination of horizontal and vertical directions. Although the pressure is constantly being exerted, once 1
the pressure gets too great; a breaking point is reached and the plates actually move. This happens along fault lines. The potential energy (stored) that had been building up for some time finally erupts into kinetic energy (motion) the quake. Cause- The Elastic Rebound Theory 2
Definitions seismologist a scientist who studies earthquakes. seismograph a device that measures quakes. seismogram the paper tracing produced by the device. epicenter the place on the surface above the focus of the earthquake. magnitude- the strength of an earthquake. focus the location on the fault where there is the greatest amount of movement. (Normally located many kilometers beneath the surface.) Effects Outward ripple effect Groundwaves up to 2 m high Buildings and bridges collapse Landslides 3
Fissures Tsunamis up to 70 m high Aftershocks sometimes greater than the initial quake Ruptured gas lines and fire Ruptured water mains Contaminated water supply disease Measuring earthquakes Mercalli Scale a classification based on observable phenomena and damage caused by the quake. (I to XII) Very inaccurate because it depends upon the observer and quality of building construction. Richter Scale a classification based on seismograph readings where every whole number increase reflects a ten-fold increase in ground motion and a thirty-fold increase in vibration energy. Richter Ground Vibration Number Vibration Energy 0 background 0 1 10 30 2 100 900 3 1000 27000 If the earth s crust was of uniform density, the ripple effect would decrease uniformly as you moved away from the epicenter. However, 4
solid bedrock shifts very little compared to loose soil. Because of this, quake intensities can vary widely from region to region. P-waves these waves move quite quickly and so are the first felt ones felt. This is considered the quake. (Push-pull pressure waves) S-waves these waves move slowly, and although they originated at the focus at the same time as the P-waves, they arrive later. These are what cause aftershock. (Secondary, Side-to-side or shear waves) 5
The above diagram is meant to show the effects of Primary and Secondary waves on the Earth s crust. Using P & S Waves to Find Epicenters Because P and S-waves travel at different speeds, the difference in their arrival time can be used to estimate the distance the epicenter of 6
a quake is from the recording device (seismograph). If three seismographs at remote locations record the same quake, the three estimated distances can be used to pinpoint the epicenter B A C Where all the radii of the estimated distances cross, determines the epicenter Activity 13D (II) and Discussion 7
The Earth s Interior / Plate Tectonics and Earthquakes How do the tectonic plates move? As magma is heated near the core of the Earth, it expands and rises toward the surface, where it cools and sinks back toward the center of the earth. This produces an up and down cycling of magma called convection current. As the magma reaches the top. It cycles past the crust where it grabs the tectonic plate and pushes it along. Since this cycling is continuous, so is the pressure applied by the magma. 8
13.4 Earthquakes in BC High risk earthquake zones have three characteristics: The west coast of BC showing the Juan de Fuca subduction plate (shaded) and other rifting faults. Located near a plate boundary There is a history of earthquakes in the vicinity There are numerous active faults The majour fault-lines are in the Pacific Ocean off the coast of BC (See above map). These faults are part of the San Andreas fault system which runs through California and is responsible for their lively earthquake history. Movement in off-shore fault could produce upheavals or drops in the ocean floor. This would most likely 9
generate tsunami waves which would devastate the coastline, causing huge geographic and environmental changes. 10
13.5 Predicting Quakes There are a number of indicators that a quake may be near. These include changes in rock a) density b) electrical conductivity c) gas content It appears that just before the fault slips, very tiny cracks appear in the rock surrounding the focus, these fill with water and are responsible for the above hints. Unfortunately it is not quite as simple as that and so far no real success. Using Earthquakes to Map the Earth s Interior 11
Similar to how whales, bats and other animals use sound to echolocate prey, seismologists can use waves generated by earthquakes to tell them about the shape and composition of the Earth s interior. Similar to sound waves, P and S-waves will change speed and direction as they go from one area of density to another. Seismologists have used these differences to map Earth s interior and to conclude that the Earth has a solid core of nickel and iron surrounded by an outer core of liquid nickel and iron with a thick mantle and a very thin crust (sorta sounds like a pizza!) Activity 13E (Part II) How many Sphere s Does the Earth Have? Three. (Okay, I m lying there are many subdivisions for each of the following, but for the time being lets leave it at: atmosphere air. 100 km thick hydrosphere water. up to 11 km thick lithosphere ground. about 6,400 km thick The lithosphere is (again, for the time being) divided into four subdivisions a) crust thin rock layer. 2-8 km under the oceans and 20 50 km thick on land. b) mantle mixed solid/liquid in constant motion, cycling through convection currents. ~2,900 thick. c) outer core liquid iron and nickel at about 3,000 o c. ~ 2,200 km thick 12
d) inner core solid iron and nickel at about 6,000 o c. the tremendous overlying pressure keeps the inner core solid. ~1,300 km in radius. Activity 13E (II) and Discussion 13