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Earth Quakes
Natural Disasters
| What is an earthquake? | Definition: seismic (shock) waves within the Earth Triggered by sudden slippage of rock along fault planes in the crust or mantle Release of accumulated strain energy at rough spots on plate |
| What is a foreshock? | small, early EQs as rocks begin to fracture |
| What is aftershock? | rocks along fault adjust and transfer strain |
| What is a hypocentre? | the location where the actual slippage occurs and where the EQ originates from |
| What is the Epicenter: | the on-ground location that is directly above the hypocenter |
| Where body waves (primary and secondary) travel? | travel through the earth |
| Primary (P) Waves: | Compressional - shakes back and forth along the direction of wave travel Fastest waves |
| Secondary (S) Waves: | Shear - back and forth perpendicular to the direction of wave travel Cannot travel through a liquid! This is why we do not observe S-waves on the other side of the Earth (since the outer core is liquid) |
| Where do surface waves (love and rayleigh) travel? | travel near Earth’s surface and cause the most damage - slowest waves |
| How do love (L) waves travel? | Move side to side |
| How do Rayleigh (R) waves travel? | Move up and down |
| Speed of Seismic Waves | Depends on the elastic properties and densities of the rocks P waves travel fast through rocks and liquids S waves don't travel through liquids at all (cant support shear stress) |
| Recording Seismic Waves | Seismograph records seismograms which shows the activity of seismic waves We can see several things Waves weaken with increasing distance from the focus point Some rocks absorb energy As energy spreads out, it covers more area Waves slow down as they |
| Mercalli Intensity Scale | No longer used Would rank based on observations of people who felt EQ and objective descriptions on the level of damage Used Roman Numerals up to XII |
| Richter Scale | Measured the magnitude of the EQ which is proportional to the amplitudes of seismograms The scale is logarithmic Also not used as frequently anymore |
| Moment Magnitude (MW) | Measure of total energy expended during EQ Depends on rock shear strength, area of rock broken, average slip distance (offset) across the fault |
| Depth and Intensity | EQs that happen closer to the surface are usually more disastrous It is important to note however that magnitude does NOT equate to destructiveness. Several factors including building codes, type of structures, time of day, proximity to population center |
| What is a fault? | Definition: Fractures in the earth’s crust along which rocks on one side of the break move past those on the other |
| Normal fault | Extension causes leaning face to fall down |
| Reverse fault | compression causes leaning to go up |
| Thrust fault | (less steep version of reverse fault) - compression causes leaning to go up |
| Strike-slip fault | right lateral) - shear force causes plates to slide past each other |
| Convergent margins | Increasing focus depth inland We can have shallow (<60km), intermediate (60-300km) and deep (300-600km) EQs Shallow-focus and large slip areas generate megathrust EQs (this can lead to tsunamis when continental plate snaps back up!) |
| Divergent margins | Shallow MOR quakes shallow = tensile strength Transform boundary (sliding past) quakes shallow = shear stress |
| Liquefaction | Soil particles are held together by friction (space inbetween is filled with water) but can be destabilized by vigorous shaking from EQ. This can cause buildings, cars, roads, etc., to |
| What are some other effects of EQs? | Rockslide - single block of rock hurtling downslope Sturzstrom - rock avalanches; masses of broken rock and other debris moving downslope Tsunami - long, high wave (not all EQs produce tsunamis) Landslide - rapid downslope movement of soil and rock. Th |
| Case study: San Andreas Fault (1989) | Transform boundary EQ A seismic gap (long period of time when an EQ does NOT happen) was filled in Magnitude 7.8 → lots of building collapse, fires, buildings sinking (liquefaction) |
| Case study: Haiti (2010) | Transform boundary EQ 200 years of stress was released Liquefact, building collapse, lack of building codes and poor construction |
| Case study: Tohoku, Japan (2011) | Subduction zone EQ Magnitude 9.0 → Triggered a tsunami, nuclear accident |
| Bam, Iran (2003) | Continent-Continent collision zone EQ Magnitude 6.3 → poor construction materials, building collapses |
| Valdivia, Chile (1960) | Magnitude 9.5 Subduction zone EQ Tsunami and lots of destruction of land and buildings |
| EQ Forecasts | identifies that a future event will occur in a certain area in a given span of time with a particular probability |
| Recurrence Interval | the average number of years between an event |
| EQ Predictions | identifies that a future event will occur at a certain time at a certain place |
| Indicators of an Impending EQ | Foreshocks Changes in ground level Seismic gaps and migrating EQs EQ regularity Animal behaviour |
| Ground Measurements | Measures crustal movement on either side of a fault over time in millimeters. InSAR - measures vertical ground movement (also in mm) This allows the recording of deformation - an increase in deformation may preclude a major EQ |
| Paleoseismology | looks at geologic sediments and rocks, for signs of ancient earthquakes. It is used to supplement seismic monitoring, for the calculation of seismic hazard |
Created by:
SAdriana
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