Earthquakes Flashcards
What is an Earthquake?
The sudden release of stored energy due to slippage along a fault in Earth’s crust
- energy is released as heat + seismic waves
Where and how does seismic waves radiate from?
Radiate outwards from source, the focus
What is the epicentre?
the point on Earth’s surface directly above the focus
What is the Elastic Rebound Theory?
- rocks on both sides of existing fault is deformed by tectonic forces
- rocks bend + store elastic energy
- once frictional forces along fault are overcome, slip occurs at weakest point (the focus)
- earthquake occurs when deformed rock “springs back” to its original shape (elastic rebound)
What are the types of seismic body waves?
- Primary (P) waves
- Secondary (S) waves
where do body waves travel through?
Through Earth’s Interior
What are primary waves?
- body wave
- compressional
- push-pull (compress + expand) motion, changing volume of intervening material
- travel through solids, liquids, + gases
(faster in solids than liquids)
What are secondary (S) waves?
- body wave
- shear motion at right angles to their direction of travel
- take longer than P waves
- travel only through solids
What are the types of seismic surface waves?
- Reyleigh (LR) waves: ground roll (up + down mvmt)
- Love (LQ) waves (side-to-side shaking)
Where do surface waves travel?
surface waves travel at Earth’s surface
In order, what waves are faster to slow?
body waves (faster) -> surface waves(slower)
P waves , S waves, surface waves
how does the difference btwn the arrival times of waves hold importance?
diff of arrival times of waves at diff recording stations helps one determine dist btwn recording station + earthquakes epicentre
____ the time difference between arrival of first P waves + arrival of S waves, the _____ the dist btwn the recording station + the earthquake epicentre
Longer, greater
What is Magnitude
quantitative measure of amt of energy released during an earthquake (based on characteristics of seismic waves)
what is intensity
semi-quantitative measure of observed effects of an earthquake on the natural + human - built environment (based on human eyewitness accounts)
What is the Richter Magnitude?
a concept introduced by Charles Richter
- “Ritcher Scale” (aka Local Magnitude Scale)
-based on amplitude (wave height) of largest seismic waves recorded
- accounts for decrease in wave amplitude w/ increase distance
- Ritcher scale is logarithmic
What is the Modified Mercalli Index?
MMI scale is used to describe earthquake intensity
- ranked I-XII (in roman numerals)
- lower intensity: felt observations
- higher intensities: damage observation
What is the relationship of the magnitude of earthquake to epicentre?
Magnitude of earthquake is constant, the intensity decreases w/ increasing distance from epicentre
Can intensity vary?
intensity varies from place to place according to local characteristics of geologic materials
- particles in unlithified (loose) sediment move during shaking
Why is the level of destruction from earthquakes high in urban areas?
-poor integrity of human-built structures (buildings, bridges, roads, etc.)
- esp in developing countries where earthquake-proof building design is not high priority in city planning
What are tsunamis?
-destructive waves that are often inappropriately called ‘tidal waves’
how do tsunami occur
result from vertical displacement along a fault located on ocean floor, a large undersea landslide triggered by an earthquake, or other disturbances
How high are tsunamis
-in open ocean, tsunami wave height is less than 1m
- in shallower coastal waters, water piles up to over 30m (due to friction generated on seafloor)
- wave breaks when it can no longer support itself
- can be very destructive
When body waves encounter a boundary (change in density), what does the wave energy do?
wave energy is reflected (bounce off) and some is refracted (bends as it transmits through)
In solids, the ____ the material, the ____ the wave travels through it
Denser
faster
What is the crust-mantle boundary?
- studies of P-waves indicated outer rocky part of Earth (lithosphere) were not uniform
- observations of seismic recording stations, P waves arrived sooner than expected
- led to realization that the crust was underlain by denser material (ultramafic mantle rock (peridotite))
What is the Core-Mantle Boundary
-Determined a (slower) outer core based on its P-wave shadow zone
-found that S waves do not pass through outer core (therefore must be liquid)
How was the solid inner core discovered
discovered using faint observations of P-waves that bounced off of , or were sped up by, solid inner core + emerged in unexpected locations
