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Modified Mercalli Intensity ScaleThe problem is this is an indirect subjective measurement- what happens if the earthquake is in a non-populated area? What happens if the earthquake is offshore? Not useful to geologists that what to get at “the science” of an earthquake!Magnitude – estimates the amount of energy released at the source of the earthquake
The Richter scale of magnitude concept, introduced by Charles Richter in 1935, is based on the amplitude of the largest seismic wave recorded which accounts for the decrease in wave amplitude with increased distanceSome Richter scale fun facts:-Largest magnitude recorded on a Wood-Anderson seismograph was 8.9 -Magnitudes less than 2.0 are not felt by humans -Each unit of Richter magnitude increase corresponds to a tenfold increase in wave amplitude and a 32-fold energy increase-A magnitude 8 earthquake releases 1000 times more energy than a magnitude 6.-A great earthquake occurs every 5 years; In California there has been 27 5.7-7.0 since 1979
Moment MagnitudeMoment magnitude(in California local magnitude Richter magnitudes)-Measurement of area of rupture and the amount of rock displaced-Amount of energy released-Richter and Moment are the same up to magnitude of 7.2 but Moment Magnitude is better for larger earthquakesEarthquake Destruction Controlled By:The amount of structural damage attributable to earthquake vibrations depends on:1) Intensity and duration of the vibrationsstructures progressively lose their ability to withstand the motionexample: Loma Prieta- 15 seconds; a few more and the Embarcadero Freeway would have completely collapsed, 1964 Alaska –ground shaking for 3 minutes
2) Nature of the material upon which the structure rests 3) Design of the structure Destruction from Seismic Vibrations:Ground shaking: Regions within 20 to 50 kilometers (12-30 miles) of the epicenter will experience about the same intensity of ground shaking. However, destruction varies considerably, mainly due to the nature of the ground on which the structures are built and the building design.If the area within 20-50 km experiences the same amount of ground shaking, what would you consider to be the safest distance for an active fault trace to build a home, school or office building?What do you think the current legislation in California is regarding the safe distance to build a home, school or office building from an active fault trace?This is a photograph of Hayward California; the Hayward fault runs through the picture at about the location of the guard rail in the photograph.
Estimate the distance from the approximate fault trace and the corner of the building.The distance is 50 ft almost exactly what the current legislation is in California for a “safe distance” to build a home from an active fault trace.