Earthquakes

Earthquakes When plates move very slowly and over a long period of time Eventually a sudden shaking of the crust occurs at a particular place: Earthquake

Earthquakes The majority of earthquakes occur at the boundaries of tectonic plates. Individual earthquakes also occur where there is a fault. A fault is a zone of weakness in the crust along which some movement of rock occurs.

Three Types of Faults Reverse – compression causes horizontal and vertical movement Normal: Tension causes horizontal and vertical movement Strike-Slip – shear causes horizontal and vertical movement

Earthquake Terms Focus: the area in the crust (below surface) where energy is released during an earthquake (where the earthquake begins) Shallow: 0-70 km Intermediate: 70-300 km Deep: over 300 km Epicenter: The point on the surface directly above the focus of an earthquake

Process As tectonic plates move, friction causes the rocks at plate boundaries to stretch or compress. Like a stretched rubber band or a compressed spring, these rocks store energy called the Elastic Rebound Theory. When the pressure builds up within the crust, the stored energy is suddenly released as the rocks slip along the fault, and an earthquake occurs.

Process More than ½ of all earthquakes occur along the edges of plates Subduction zones Mid-ocean ridge Collision zones (continent-continent convergence)

Seismology The study of earthquakes Scientists cannot predict the exact date and time of an earthquake But… they can identify areas likely to have an earthquake in the next 10 years.

Seismology A seismograph measures earthquakes, and seismologists use seismic waves to study The energy released by an earthquake travels away from the focus in waves Magnitude: relative size of an earthquake depends on amount energy released

How Seismographs Work the pendulum remains fixed as the ground moves beneath it http://www.uwgb.edu/dutchs/EarthSC-102VisualsIndex.HTM

Seismology Seismographs show the kinds of waves, their amplitude, and the timing of the waves Foreshocks are small tremors that precede an earthquake Aftershocks are small tremors that follow after an earthquake

Scales Magnitude is expressed on the Richter Scale Study the waves to determine the magnitude and location of the earthquake Uses number 1 & up Each # indicates ten times stronger than the # below Ex: A rating of 8 has a magnitude ten times as great as an earthquake with a magnitude of 7 7 or higher indicates a major earthquake ( 10-felt all over the earth)

How are Earthquakes Measured? Richter Scale

Scales Modified Mercalli Scale: use to rate intensity Roman numerals I to XII describes damage done by the quake XII : total destruction An earthquake with one magnitude, damage can vary depending on the location

How are Earthquakes Measured? Mercalli Intensity Scale Click Link for Interactive Demo http://elearning.niu.edu/simulations/images/S_portfolio/Mercalli/Mercalli_Scale.swf

Seismic Waves The conversion of potential to kinetic energy results in seismic waves Travel through the ground 20 times faster than the speed of sound Seismic waves reach the ground at the epicenter

Seismic Waves Body waves – originate from the focus of the earthquake P waves (primary waves) are compression waves that push and pull rock as they travel Cause particles to move back & forth Move through solids, liquids and gases 1st waves to arrive

Seismic Waves S waves – (shear/ secondary waves) Body waves that have reached the surface Cause particles to move side to side Do not travel through liquids

Comparing Seismic Waves

Seismic Waves Surface Waves: Third & Fourth waves Love waves & Rayleigh waves Travel along the surface & more slowly Cause the most damage Love: no vertical movement of the surface Rayleigh: both horizontal and vertical movement

How to determine the distance to the epicenter Need data from three seismographic stations including the spread of S and P waves and the time between the arrival of the s and p waves. Forecasting Based on calculating the probability 2 factors History of area Rate at which strain builds up

Locating Earthquakes http://www.uwgb.edu/dutchs/EarthSC-102VisualsIndex.HTM

Locating Earthquakes http://www.uwgb.edu/dutchs/EarthSC-102VisualsIndex.HTM

Locating Earthquakes http://www.uwgb.edu/dutchs/EarthSC-102VisualsIndex.HTM

Earthquake Hazards Factors that determine the severity of damage Design of buildings Wooden structure = less damage 2) Ground shaking - tall building – vibrations too rapid -short building – vibrations too slow - 5-15 stories collapsed

Earthquake Hazards 3) Soil Fracture Sloping area = landslides Liquefication = sand becomes liquidy due to vibrations Soft materials amplify the motion Hard/resistant materials lessen the motion Causes sinking, falling over, buildings Underground pipes & tanks rise to the surface

Earthquake Hazards 4) Tsunami: A huge wave generated by an underwater earthquake or landslide Caused by vertical motion on the ocean floor Open ocean Height less than 1 m Speed 500-800 km/h

Earthquake Hazards Shallow water Height increases due to breakers/shallow water Height may exceed 30 m

Tsunami Warning System http://isu.indstate.edu/jspeer/Earth&Sky/EarthCh11.ppt