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Lecture

Earthquakes.docx

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Department
Earth Sciences
Course
EARTHSC 2GG3
Professor
Sergei Basik
Semester
Winter

Description
Earthquakes  Where do earthquakes occur?  Mostly around convergent plate boundaries  Grenville (S. Ontario) – faults in this region are somewhat active and cause minor earthquakes  Most earthquakes (3 plates) coast of B.C.  After affects can be worse than earthquakes itself (ex. Tsunami caused by earthquake) Waves  Period – the time between seismic waves  Wavelength – the length from crest to crest of a wave  Frequency – the number peaks per second Seismograph  Tool to measure and record earthquake waves  Old fashioned seismographs also used today  Body waves occur FIRST  After S wave – when energy hits earth’s surface  Types of Seismograph:  Short-period seismographs: record local earthquakes (high freq)  Long-period seismographs: record distant earthquake (low freq)  Broadband seismographs: record both local and distant earthquakes; but cannot accurately measure strong earthquakes in the direct vicinity  Strong-motion Seismographs: record local and very strong earthquakes Earthquake Wave Types  Body Waves (go through earth’s surface)  P waves (primary or compressional): wave shakes back and forth along the direction of wave travel  8 km/s upper mantle  5-6 km/s continental crust  S waves (secondary or shear): shakes back and forth perpendicular to the direction of wave travel (cannot travel through a liquid)  4.5 km/s upper mantle  3.5 km/s crust  Surface Waves (slower)  2 – 4.5 km/s  Love Waves : move side to side  Rayleigh Waves: move up and down Basic Fault Types  Normal Fault  Lower wall will fall relative to upper wall  Reverse Fault  Lower wall will rise relative to upper wall  Thrust Fault  Move over each other  Strike-Slip Fault  Back and forth (slipping Elastic Rebound Theory  The theory applied to most earthquakes in which movement on two sides of a fault leads to bending of the rocks until they slip and snap to release the bending strain  Stress: the force on a body  Strain: change in size or shape of a body in response to an imposed stress (how much is body going to deform)  Faults changes, rock goes back to normal shape Earthquake Epicentre  Earthquake occurs and energy radiates  Focus is where rupture occurs  Epicentre is origin of Earthquake  Take reading from different seismographs and measure  You don’t have to be right next to epicenter to determine where it is  Determined by a logarithmic scale – measure difference between p and s waves (nomograph chart) and compare to amplitude of waves Intensity Scale  Intensity scale – indicates the severity of an earthquake in terms of the damage that it inflicts on structures and people (can be qualitative or quantitative)  Mercalli Intensity Scale – developed in 1902 by Italian seismologist Giuseppe Mercalli; Modified Mercalli Intensity scales account for various construction practices used in different cultures and countries  Roman Numerals 1 – XII  Mercalli Intensity scale maps  Based in observations of people who felt the earthquake  qualitative!!!!!  Objective descriptions of the level of damage  Intensity pattern not circular; intensity pattern elongated along fault lines  Larger intensity over areas with loose soft sediment (e.g Marina district and Oakland)  Tougher building codes and new developments in construction should decrease future Mercalli intensity scores ; intensity can be serious even away from epicenter due to loose sediments  Magnitude Scale: measure earthquakes quantitatively, independent of location, and assigns an magnitude value based on energy released  Richter Magnitude Scale (M ) :Ldevelopled in 1935 by Charles Richter; based on maximum amplitude of earthquake waves recorded on a Wood Anderson seismograph  Logorithmic scale  energy level between Richter scale units differs by 31.5 times (e.g. a magnitude 6 released 31.5 times more energy than a magnutide 5 earthquake
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