Earthquakes On Shaky Ground

Earthquakes The shaking of the Earth’s surface Caused by faulting beneath the surface –Faults are when the rock splits and slips along that split causing vibration The vibration causes shock waves called seismic waves which move outward from the fault

Seismographs Instruments that measure the movement of the Earth’s surface They measure the seismic waves of earthquakes and even the waves created by large explosions They are sensitive enough to pick up even minor earthquakes that we can’t feel

Seismographs

Seismograms The written record produced by a seismograph It looks like an EKG when the doctor checks your heart rhythm

Seismograms

Focus The location beneath the Earth’s surface where the faulting of the Earthquake occurred –It is beneath the Earth’s surface at some depth (focal depth) –Where the faulting occurs

Epicenter The location on the Earth’s Surface directly above the focus The location shown on a map Where the circles of three seismic station intersect

Seismic Waves The shock waves that move outward from the focus (the fault) They speed up as the rock gets denser This increase in velocity causes them to be refracted (bent) back toward the surface

Types of Seismic Waves There are many types of seismic waves Some are important to us and some are important to seismologists –P-Waves (important to us) –S-Waves (important to us) –Love Waves –Rayleigh Waves –Oscillations

P-Waves Primary Waves (they arrive first) They travel with the fastest speed Compressional Waves (Push-Pull Waves) –Wave motion is the same as direction of travel Travel through solids, liquids, and gases

S-Waves Secondary Waves (arrive second) They travel second fastest (slower than P-Waves) They are shear waves (Side-to-Side waves) –move 90 o to the direction of wave travel Travel through solids only

Where do the waves go? The refraction (bending) of the waves as they enter different layers with different densities creates a shadow zone where no seismic waves are detected S-Waves are absorbed when they reach the liquid outer core so there is an S-Wave shadow where only P-Waves are detected

How Bad Was It? The Richter Scale is used to describe the amount of shaking –Based on the magnitude of the waves The Mercalli Scale is used to describe the amount of damage the earthquake causes –Depends upon the location of the earthquake –Depends upon the population

Determining Travel Time … Find the distance along the bottom Go up to the appropriate curve Go over and read the time off the side axis

What is the Travel time for a P-Wave that travels 2,800 km? How far does an S-Wave travel in 0:05:20?

Distance to epicenter Use scrap paper and mark off 0 min and the difference in arrival times Slide the paper so that… –The left edge of paper is straight up and down –The 0 min mark hits the P-Wave curve –The other mark hits the S-Wave curve Go straight down and read the distance off the bottom axis

What can be done in areas where earthquakes are common? Let’s make a list – What do you think?