EARTHQUAKES: WHY? AND HOW?

EARTHQUAKES Caused by plate tectonic stresses sudden movement or shaking of the Earth Located at plate boundaries Resulting in breakage of the Earth’s brittle crust

PLATE TECTONIC STRESSES faults (= cracks where plate sections are moving in different directions)Plate boundaries and faults (= cracks where plate sections are moving in different directions) cause friction as plates move Plates in a fault zone have STICK-SLIP motion –Periods of no movement (stick) and fast movement (slip) –Energy stored as plates stick, –Energy released as plates slip

EARTHQUAKE DAMAGE LandsidesLandsides Building damageBuilding damage LiquefactionLiquefactionLiquefaction

LIQUEFACTION Results in a loss of soil strength & the ability of the soil to support weight when a solid (sand and soil) becomes saturated with water and acts like a heavy liquid

EARTHQUAKE DAMAGE Most caused by SURFACE waves (arrive last)

EARTHQUAKE INTENSITY Modified Mercalli scaleModified Mercalli scale= measurement of damage to structures Modified Mercalli scale From I to XII (Roman numerals)From I to XII (Roman numerals) Descriptive, changes with distance from epicenterDescriptive, changes with distance from epicenter Can change from location to locationCan change from location to locationlocation What you need: Your senses!Your senses! measures damage to man-made structures at certain location

ISOSEISMIC MAPS Loma Prieta Earthquake 1989 Connects areas of with the same Modified Mercalli numberConnects areas of with the same Modified Mercalli number Areas are colored according to Modified Mercalli numberAreas are colored according to Modified Mercalli number show the distribution of intensities show the distribution of intensities

EARTHQUAKE WAVES FOCUS = place deep within the Earth and along the fault where rupture occursFOCUS = place deep within the Earth and along the fault where rupture occurs EPICENTER = geographic point on surface directly above focusEPICENTER = geographic point on surface directly above focus SEISMIC WAVESSEISMIC WAVES produced by the release of energySEISMIC WAVES SEISMIC WAVES –move out in circles from the point of rupture (focus) –2 types: surface & body (travel inside & through earth’s layers) P waves: back and forth movement of rock; travel thru solid, liquid, gas S waves: sideways movement of rock; travel thru solids only

EARTHQUAKE WAVES Seismographs record earthquake waves Seismograms show: Amplitude of seismic waves (how much rock moves or vibrates)Amplitude of seismic waves (how much rock moves or vibrates) Distance to the epicenterDistance to the epicenter Earthquake directionEarthquake direction

EARTHQUAKE WAVES 3 types of seismic waves show up on seismogram3 types of seismic waves show up on seismogram –P waves: shake earth in same direction as wave; travel thru solid, liquid, gas –S waves: Shake earth sideways to wave direction; travel thru solids only –Surface waves: circular movement of rock; travel on surface – cause most damage!!

EARTHQUAKE WAVES P waves move through solids & liquids S waves move through solids only!!!

EARTHQUAKE WAVES Body P wavesS waves waves AKA Moves through Movement of rock Primary (1st to arrive) Longitudinal, Compression all states of matter (solid, liquid, gas) back and forth movement of rock push/pull or compression/stretch out Like slinky down stairs Vibration is same as the direction of travel Secondary (2nd to arrive - larger) Transverse, Shear Can go through solids only Move sideways perpendicular to direction of wave travel Like snake

EARTHQUAKE WAVES Lets test your understanding!! Is this a P or an S wave? P wave! S Wave

EARTHQUAKE MAGNITUDE measures the size of seismic waves  the energy released by the earthquake measures the size of seismic waves  the energy released by the earthquake Richter scale=measurement of energy released based upon wave amplitude (size of vibration) <2 to ~10<2 to ~10 Amplitude of wave goes up by 10 (Logarithmic scale)Amplitude of wave goes up by 10 (Logarithmic scale) What you need: Amplitude (size of vibration = wave height)Amplitude (size of vibration = wave height) Time between arrival of 1 st P and 1 st S wavesTime between arrival of 1 st P and 1 st S waves

HOW TO READ SEISMOGRAMS P & S (body waves) move through earth & arrive first P & S waves used to calculate magnitude of earthquake Amplitude = height of wave (how much the rock moves; size of vibration)

MERCALLI VS. RICHTER

Let’s try a simulation Simplified Epicenter Triangulation P waves generally travel between 5.95 and 6.75 kilometers per second in the crust, depending on compressibility, rigidity, uniformity, and density of the materials traversed. S waves tend to move at velocities between 2.9 and 4.0 km/sec in the crust. Rayleigh waves travel somewhere between 2.7 and 3.7 km/sec. Time and distance can be graphed for each of the wave types. By comparing the time between pairs of wave types on the graph, a rough estimate of the distance to the earthquake focus can be made. In each of the following sets of problems, you need to convert the time difference given between the arrivals of each of the two wave types into kilometers. To do this, measure the difference in time on the Y axis, marking it on a ruler or piece of paper. Then, find the distance on the X axis where the two relevant curves diverge by just that time difference on your marked paper or ruler. Give your answer rounded to the nearest 250 km.

In each of the following sets of problems, you need to convert the time difference given between the arrivals of each of the two wave types into kilometers. To do this, measure the difference in time on the Y axis, marking it on a ruler or piece of paper. Then, find the distance on the X axis where the two relevant curves diverge by just that time difference on your marked paper or ruler. Give your answer rounded to the nearest 250 km.

Simplified Graph of P, S, and R Waves through Time

Calculate together Notebooks…… see handout