Earthquakes How and Where Earthquakes Occur

Transcription

1 Earthquakes How and Where Earthquakes Occur PPT Modified from Troy HS

2 Is there such thing as earthquake weather? Absolutely NOT!!! Geologists believe that there is no connection between weather and earthquakes. They are the result of geologic processes within the earth and can happen in any weather and at any time during the year. Earthquakes originate miles underground. Wind, precipitation, temperature, and barometric pressure changes affect only the surface and shallow subsurface of the Earth. Earthquakes are focused at depths well out of the reach of weather, and the forces that cause earthquakes are much larger than the weather forces. Earthquakes occur in all types of weather, in all climate zones, in all seasons of the year, and at any time of day.

3 When do earthquakes occur? Earthquakes happen once every 10 seconds on Earth. About 3 million earthquakes happen per year. On average there is one 8 magnitude, 10 7 magnitudes, magnitudes and so on per year. Most earthquakes are of very low magnitudes

4 What is an earthquake? Movement or trembling of the ground that is caused by a sudden release of energy when rocks along faults move Reasons Earthquakes happen: Plates Moving Faults (cracks in Earth s surface) Caverns collapsing Meteor Impacts Volcanic Eruptions

5 Where do most earthquakes occur? Plate boundaries or Faults

6 What is a Fault? a fracture along which the blocks of crust on either side have moved. Plate boundaries are always faults, but not all faults are plate boundaries. Fault zones or systems region of numerous, closely spaced faults

7 In California earthquakes are caused by The San Andreas Fault System San Andreas + Faults = EARTHQUAKES

8 Three Types of Faults Strike-slip Dip-slip Normal Reverse (Thrust is a reverse fault with a small dip angle) Click here to see the faults in action

9 Strike-Slip Fault Also called transform faults or boundaries are vertical (or nearly vertical) fractures where the blocks have mostly moved horizontally. If the block opposite an observer looking across the fault moves to the right, the slip style is termed right lateral; if the block moves to the left, the motion is termed left lateral.

10 Dip-Slip Faults Dip-slip faults are inclined fractures where the blocks have mostly shifted vertically. If the rock mass above an inclined fault moves down, the fault is termed normal if the rock above the fault moves up, the fault is termed reverse. A thrust fault is a reverse fault with a dip of 45 degrees or less.

11 Dip-slip Faults Animations

12 San Andreas Fault In Action

13 Draw California and the San Andreas

14 Terms used to describe an Fault Epicenter Focus earthquake Seismic Waves Focus the location within Earth along a fault at which the first motion of the earthquake occurs Epicenter- point on Earth s surface directly above an earthquake s starting point, or focus Focus Vs. Epicenter

15 How does energy travel in an earthquake? Energy travels in waves from the focus The focus is the part of the fault that moved where energy was released The epicenter is located directly above the focus on the surface

16 Elastic Rebound Theory The elastic rebound theory is an explanation for how energy is spread during earthquakes. As plates on opposite sides of a fault are subjected to force and shift, they accumulate energy and slowly deform until their internal strength is exceeded. At that time, a sudden movement occurs along the fault, releasing the accumulated energy, and the rocks snap back to their original undeformed shape.

17 How does energy travel in an earthquake? Waves Body Waves Surface Waves Travel Through the Earth When P and S waves reach the surface they make surface waves P Waves Raleigh Waves S Waves Love Waves

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19 Body Waves Travel Through the Earth P Waves Primary Waves Reach an area first Can travel through solid, liquid and air Compress and Expand

20 Body Waves Travel Through the Earth S Waves Shear Waves Reach an area second Can only travel through solid Move at right angles to the direction they are traveling

21 P-Waves Vs. S-Waves P-Waves (Compression) Interior Of Earth S-Waves (Right angles) Interior Of Earth

22 Surface Waves Travel Along the Surface Raleigh Love Rolling Waves Jolting Waves Like a ripple in a pond Move at right angles

23 Love Waves Vs. Rayleigh Waves Surface Surface

24 MythBusters 1zH_Js

25 Loma Prieta, Ca 1989 Locating and Measuring Earthquakes Kobe, Japan 1995

26 How are earthquakes detected? Earthquakes are detected by seismographs (detects and records EQ) Seismographs record the information on a seismogram (the EQ recording) Seismology is the study of earthquakes and their seismic waves

27 Locating the Epicenter In order to determine the location of an earthquake, the earthquake needs to be recorded on three different seismographs that are at significantly different locations. The other piece of information needed is the time it takes for P-waves and S- waves to travel through the Earth and arrive at a seismographic station.

28 The Triangulation Method Triangulation A mathematical method for locating the epicenter of an earthquake using three or more data sets from seismic stations. Data is collected using earthquake monitoring instruments called seismographs which record the seismic waves of the earthquake.

29 Triangulation If three arrival times are available at three different seismic stations then triangulation can be used to find the location of the focus or epicenter and the time of occurrence of the earthquake. The distance between the beginning of the first P wave and the first S wave tells you how many seconds the waves are apart.

30 Triangulation Since the P waves travel faster than the S waves, P waves will arrive at a given seismograph station sooner than S waves. In other words, the S waves lag behind the P waves. In fact, the time difference between when the P waves arrive at a seismograph station and when the S waves arrive at the same station is called Time Lag.

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32 The difference in arrival times can tell us the distance from the earthquake The amplitude of the s-wave can be compared with the time to determine the magnitude

33 How do you find the epicenter of an earthquake? You must have at least 3 seismograms from different stations You determine the distance each station is by determining the lag time between the p wave and the s wave The farther away the station the greater lag You draw circles out at those distances on a map Where the circles intersect is the location of the epicenter

34 ** Three stations does not provide exact data the data from many stations is compiled to determine the exact epicenter

35 How is the strength of the earthquake determined? Intensity The effect of the Earthquake on the area or strength of shaking changes with distance and type of ground the location is on Measured by the Mercalli Scale Magnitude The actual energy released by the Earthquake This number does not change with distance Measured by the Richter Scale

36 How does the Richter Scale show magnitude? Every number on the Richter scale shows an increase of 31 times in energy released What is the difference in energy between a 3 and a 4? 31 times What is the difference between a 7 and a 9? x s 31 = 961 times more powerful! 31x 31

37 Kilometers Bigger Faults Make Bigger Earthquakes Magnitude 8