GEOL 101 1nd Edition Lecture 28 Outline of Last Lecture I Earthquakes II Seismology III Seismic Waves Outline of Current Lecture I Locating earthquakes II Measuring earthquakes III Earthquake Hazards IV Earthquake Destruction Current Lecture Locating Earthquakes In locating an earthquake we want to find the epicenter The point on the surface directly above the focus The difference in p wave and s wave travel times is the key The greater the time difference between p and s waves the farther away the epicenter This relationship can be displayed graphically The p wave arrives first at the station These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Five minutes later the s wave arrives at the station Using the graph we estimate that the epicenter is 3400 km away from the seismic station We now know that our epicenter is 3400 km away from the seismic station Our problem is we don t know what direction Station 2 data The s wave arrived 4 minutes after the p wave Using the travel time graph we can estimate that the epicenter is 2700 km from seismic station 2 Station 3 data The s wave arrived 3 minutes after the p wave From the travel time graph the epicenter is about 1800 km from the seismic station We can now see that it is necessary to have at least 3 seismic stations to triangulate the epicenter of the earthquake It is important to note that the circles may not always perfectly overlap Therefore the more seismic stations the more accurate the location of the epicenter Measuring Earthquakes The scale for measuring earthquakes that most people are familiar with is the Richter scale This is a magnitude scale It measures the amount of energy that is released at the focus o The Good Friday Earthquake in Anchorage 1964 Epicenter was of the coast between Anchorage and Valdez Greatest magnitude earthquake ever recorded in North America 8 4 Richter 9 2 Mw Within 24 hours there were 28 recorded aftershocks above a 6 0 The Richter scale is a logarithmic scale that is based on the amplitude of the largest seismic wave recorded by the seismic station o The amount of energy released is roughly 32 fold for each number increase a magnitude 6 4 release 32 times as much energy as a 5 4 1960 Valdivia Earthquake the largest earthquake ever recorded 9 5 Mw epicenter located along the coast of Chile affecting the town of Valdivia Two days after the earthquake Cord n Cualle erupted possibly due in part to the earthquake Another common scale that is used is the Modified Mercalli scale This is an intensity scale rarely used Measures the degree of Earth shaking at a given location based on amount of damage This scale ranges from I to XII The Mercalli scale has its uses in areas where there are very few seismic stations however there are several drawbacks Damage may be more severe in densely populated areas areas with poor construction standards soft sediment rather than bedrock etc Many seismologists are shifting away from both the Mercalli and Richter scales and adopting another magnitude scale again measuring energy released referred to as Moment Magnitude Mw Moment Magnitude is better at gauging the size of very large earthquakes It can be derived mathematically from the size of the surface rupture and displacement along a fault It can be verified by both field studies and by seismograph data Earthquake Hazards Destruction from earthquakes comes in several forms Destruction from seismic vibrations Tsunamis Landscapes o Secondary hazards Fires San Francisco Disease Haiti Nuclear Hazards Fukushima Damage done by seismic vibrations The amount of damage that a structure sustains from the vibrations depends on 1 the intensity of the earthquake a The larger the magnitude more energy released 2 the duration of the earthquake a The longer the shaking the more damage the structure 3 the material that the structures sit upon loose sediment vs bedrock a Softer loose sediment is less stable than bedrock 4 the design of the structure a Poorly built structures v earthquakes resistant engineering Earthquake Destruction Damage done by seismic vibrations Amplification of seismic waves Soft sediments will amplify waves and increase the damage to structures Bedrock tends to dampen and seismic waves and decrease the amount of damage o 1985 earthquake in Mexico Magnitude 8 0 epicenter along the coast Significant damage was sustained in Mexico City 400 km away from epicenter Mexico City sits on top of soft soil which amputated waves Damage done by seismic vibrations Liquefaction earthquake vibrations cause loose water saturated sediment to turn from relatively stable ground into mobile fluid Buildings and other structures collapse and or sink into the ground o 1989 Loma Prieta World Series Earthquake in San Francisco Bay 6 9 Mw epicenter in Aptos California along San Andres coastline Game 3 Oakland A s vs SF Giants Tsunamis massive waves set in motion by seismic activity Earthquakes may cause oceanic crust to be displaced along a fault or may trigger underwater landslides Tsunamis waves are unrecognizable in the open ocean It is when the water shallows that the wave begins to drag along the ocean bottom and build in height Heights may reach 100 feet or more o Dec 26 2004 Indonesian Earthquake Tsunami 9 1 Mw centered west of Sumatra Waves reached heights of 30 feet flooding many Indonesian areas claiming over 200 000 lives