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The Shaking Earth: Earthquakes (Definitions (Describing Waves (Amplitude:…
The Shaking Earth: Earthquakes
Definitions
Normal Fault: One slab drops down (divergent)
Reverse Fault: One slab forced up (convergent)
Faulting: used to describe fault rupture, faulting causes earthquakes
Focus/Hypocentre: point where fault rupture first occurs
Epicentre: geographic point directly above the hypocentre on the Earths surface
Mainshock: earthquake, largest event; strong shaking from a major shock lasts 30-60 seconds
Aftershocks: small events after the mainshock
Foreshock: smaller events preceding the mainshock, but can only be identified after the mainshock occurs
Seismic Waves: Energy released by faulting, which moves outwards from the hypocentre
Describing Waves
Amplitude: corresponds to displacement
Wavelength: distance between successive waves
Period: time between waves (1/Frequency)
Frequency: number of waves in one second (1/Period)
Types of Seismic Waves
Body Waves: Pass through the entire Earth
Primary Waves
Fastest Waves(short period), first to reach recording centres
Push-pull motion (Spring)
Move through liquids, solids, gases
Sudden audible jolt at epicentre
Secondary Waves
Second to reach recording station
Transverse in vertical plane motion (Shaking a rope)
Move through solids
Up & down, side to side shaking, that does damage to buildings
Surface Waves: Only travel on Earth's outer layer
Love Waves
Travel only on surface of the Earth
Longer periods than body waves, meaning they can carry energy farther
Transverse on horizontal plane motion (Snake)
Can only move through solids
Long periods cause swaying of buildings, wave motion in bodies of water
Rayleigh Waves
Travel on surface of Earth, arrive last at recording stations
Backwards rotating motion (rolling boat at sea)
Moves through solids and liquids
More energy is released in the form of Rayleigh waves when the hypocentre is close to the surface
Produces vertical and horizontal shaking
How Seismic Waves Damage Buildings
Natural Frequency: Frequency at which a system vibrates naturally once it has been set into motion
Resonance: Tendency of a system to oscillate with a larger amplitude (sway more) when it is subjected to vibrations with a frequency that matches its natural frequency
All buildings have resonant frequencies depending on their height. When the frequency of seismic waves matches the resonant frequency of a building, the building resonates, and sways more violently
Short Buildings: short periods = high resonant frequencies = resonate with high frequency waves
Tall buildings: Long periods = low resonant frequencies = resonate with low frequency waves
Measuring Earthquakes
Seismographs(seismometers): instruments that record earthquake waves
Seismograms: traces(diagrams) of amplified, electronically recorded ground motion made by seismographs
Recorded amplitudes of seismic waves indicate the amount of energy released by the quake
Seismic monitoring: recording drum moves with the shaking ground, recording a pattern, called a seismogram(P waves recorded first)
High-Frequency Seismic Waves: caused by short rupture, and do more damage at the epicentre, then die out with distance
Low Frequency Seismic Waves: caused by a long rupture, and do more damage farther away, as they can travel far from epicentre
Locating the Epicentre
Since P waves arrive first and S waves second, there two separate shocks. The farther from the epicentre, the longer the time delay between the two shocks(called the P-S interval)
Travel-Time graph plots the P-S gap vs. the distance from the epicentre, helping determine how far each station is from the epicentre
At least 3 seismograms from 3 different recording stations should be used to locate the epicentre
The 3 seismograms will produce three circles and the epicentre will be where the circles intersect
Classifying Earthquakes according to magnitude
Magnitude: refers to size; I.e the amount of energy stored in the fault before it ruptured
With increasing magnitude, shaking will last longer and spread outer a bigger area
Richter Scale
Measures the maximum amplitude on a seismogram and adjusts for distance from epicentre
Ranges from 1-10 and functions as a logarithmic scale(magnitude 6 is 1000 times bigger than magnitude 3)
Every one increase means energy release increases by 45 times, & spreads out over a larger area and longer time
Only for small to moderate, shallow quakes
Moment Magnitude (Mw)
Used to rank big quakes, because it takes into account:
The size of the ruptured area ion the fault plane
Amount of offset or slip movement along the fault plane
Strength of rocks on either side of fault plane
The longer the fault rupture, the greater the magnitude
Classifying Earthquakes according to Intensity
Intensity: effects on people and buildings
Mercalli Scale: Ranges from 1-12, quantifies what people feel during an earthquake
Earthquakes only have one magnitude, but can have several intensities depending on where and when it strikes
Building in Earthquake Country
Damage to buildings occurs at magnitude 5
Soft Stories: large openings(garages, storefronts, unbraced windows) which are weak spots, because they are flexible
Fixes
Cross-braicng
Plywood reinforcement
Diagonal braces(for soft stories)
Steel jackets(for highway support columns)
Alternating layers of steel and concrete(for bridges)
Base Isolation(absorbs shaking, e.g wheels, ball bearings, rubber doughnuts)
Shear walls: designed to receive horizontal forces from floors, roofs, and trusses, and transmit them to the ground (parking garages)
Eliminating resonance: change height/shape, moving weight to lower floors, building tall flexible structures on hard foundations, or short stiff structures on soft foundations
Defining Hazard Risk
Earthquakes don't kill people, buildings do
Risk is higher in countries where no earthquake proof building codes exist
Hazard depends on
Population density
Construction Standards
Emergency preparedness
Primary vs. Secondary Effects
Primary
Ground Shaking: initial effect of earthquake
Ground Rupturing : permanent fissures/cracks 10-15m
Liquefaction: when water moves upwards, out of loose sandy soils with high moisture contents
Secondary
Landslides
Tsunamis
Fires
Droughts
Earthquake Forecasting: Precursor Events
Ground uplift/tilting:due to swelling of rocks from building pressure
Higher or lower than normal water levels in wells
Emission of Radon Gas: emissions show up in well water
Unusual Radio waves
Strange animal behaviour