A vibration of the Earth produced by a rapidrelease of energy
Often occur along faults – breaks in theEarths crust and mantle (plate boundaries)
1. Focus – Point within the Earth where theearthquake starts
◦Energy moves in all directions from this point
2. Epicenter – Point on the Earth’s surfacedirectly above the focus
◦Most intense movement during the earthquake
The Elastic Rebound Hypothesis
◦Stress builds along an existing fault
◦Deformation of crustal rock – bent features ofthe rocks caused by increasing stress – elasticenergy
◦Slippage (earthquake) – energy is stored inthe rock overcomes frictional forces keepingcrust in place
◦Energy released – rock returns to originalshape
At spreading ridges, or similar extensionalboundaries, earthquakes are shallow, andtend to be smaller than magnitude 8.
At compressional boundaries, earthquakesare found from the very near surface toseveral hundred kilometers depth,
◦since the coldness of the subducting plate permitsbrittle failure down to as much as 700 km.
Compressional boundaries host Earth'slargest quakes, with some events onsubduction zones in Alaska and Chile havingexceeded magnitude 9.
At transforms, earthquakes are shallow, as deep as25 km; mechanisms indicate strike-slip motion.
◦Tend to have earthquakes smaller than magnitude 8.5.
The San Andreas fault in California is an exampleof a transform, separating the Pacific from theNorth American plate.
At transforms the plates mostly slide past eachother laterally, producing less sinking or lifting ofthe ground than extensional or compressionalenvironments.
◦The yellow dots below locate
earthquakes along strands of
this fault system in the
San Francisco Bay area.
Measured by a seismograph (instrument)
Seismographs produce seismograms – thewritten record of the movement
Surface Waves – earthquake wavesthat travel along the Earth’s outerlayer
◦Up and down, side to side, twistingmotion
◦Most destructive of the waves
◦Slowest wave
P waves: (Body waves)
Compression waves
Alternately expandand compress materialthey pass through
Travel the fastest ofall three waves
Travels parallel to itsmovement
S Waves (Body waves)
•Transverse wave
•Travel slower than Pwaves
•Travels at a rightangle to the directionof its movement
•Will not travelthrough liquids andgases
Duration
Intensity
Building Design – reinforced/flexible buildingsbest
Materials built on – hard, dense material thebest
Liquefaction - when loose sediments aresaturated with water, during an earthquake,shaking creates a liquid like material not ableto support structure – structure sinks
Richter scale: This scale was based on theground motion measured
◦there is an upper limit on the highest measurablemagnitude, and all large earthquakes will tend to havea local magnitude of around 7.
◦the magnitude becomes unreliable at a distance ofmore than about 600 km (370 mi) from the epicenter
Moment Magnitude Scale (MW or M) is used tomeasure the size of earthqukes in terms of theenergy released
◦Rigidity of Earth x Amount of Slip x Area that slipped
caused by an earthquake that displaces theocean floor vertically
◦Convergent and strike-slip boundaries
water is pushed upwards and toward the land
speeds of 500/9000 km/hr
low waves in open water – as wave nears shorewater builds upwards
Fires - from broken gas pipes and fallingpower lines
Landslides – rock and soil slide downhill fromshaking
No predicting devices
Only measurements – stress along faults,water level in wells, gas emissions fromfractures
Seismic gap – time between earthquakes
Foreshocks-small quakes preceeding big one
◦Could be hours, days, even years before
Aftershocks-small quakes following big one
Knowledge of Earth’s interior comes fromEarthquake waves
By measuring the speed of a wave - able todetermine the composition of the Earth
P waves travel through liquids and solids
S waves travel only through solids
As P waves travel they bend as they enternew material - bending shows change ofmaterial
S waves not traveling through the outercore also shows change in material
Conclusion – Earth is made up of differentstates of material
