Seismology Flashcards
1
Q
what is an earthquake?
A
- Sudden release of elastic energy in response to build-up of stress
- This energy is released when stress is greater than strength of the fault
- The energy is released as:
- Seismic waves
- Displacement along fault
- Heat and other energy
2
Q
elastic rebound theory
A
- elastic stress build up (energy is stored) as rock deforms slowly over time
- Rupture occurs when elastic stresses exceed what the fault can bear (friction) -> rocks along fault spring back to undeformed state (elastic rebound)
3
Q
types of faults (and hanging wall/footwall)
A
- Hanging wall: one that wouldn’t be supported without the footwall
- Foot wall: one that would stay the same without hanging wall
- Reverse: block move up; shortening/compression
- Compressional regimes like these store more energy
- Normal: block moves down; extension
- Strike slip: dextral if motion is to the right, sinistral if motion is to the left
- Oblique slip: combination of movement
4
Q
convergent boundary/subduction zone earthquakes
A
- Subduction zones move shallow -> intermediate -> deep – look for this to determine direction of subduction
- More active subduction zones = more active volcanoes
- can have small quakes - quakes up to 9.4
5
Q
divergent zone earthquakes
A
- Shallow earthquakes and normal faulting
- Small to moderate earthquakes
- Ex. East African rift, Mid-Atlantic range
6
Q
transform boundaries (and oceanic vs. continental)
A
- Plates move horizontally past each other
- Strike-slip parallel to boundary
- Oceanic (majority): transform faults
– Shallow earthquakes (thin oceanic crust)
– Small to moderate (warm, weak crust) - Continental
– Shallow (<20km)
– Small to very large (up to M8)
Ex. Queen Charlotte and San Andreas (continental transform)
7
Q
intraplate earthquakes
A
- Intraplate earthquakes not associated with plate boundaries
- Occur at great distances from active plate margins
- Rupture is due to gradual accumulation of strain within the plate
- Rupture tends to occur in weak zones within the plate
- Ex. Eastern Canada seismic zones, Western Quebec seismic zone
8
Q
seismic waves
A
- Body waves travel inside materials (the earth)
- Surface waves travel along boundaries between materials
- speed order: p-wave, s-wave, surface wave
9
Q
p-wave (primary wave)
A
- Compression and extension of the solid (or fluid), like a sound wave
- Particles move in same direction wave propagates
- Fastest type of seismic wave (6km/sec in continental crust)
10
Q
s-wave (secondary wave)
A
- Shearing distortion of solid
- Particles move perpendicular to direction wave propagates
- Slower than P wave: about 3.5km/second in continental crust
- Cannot pass through fluids
11
Q
surface waves (and its 2 sub-types)
A
- Require interface: ground-air, water-air, mantle-liquid outer core
- Slower than body waves
- Rayleigh wave: vertical and horizontal motion parallel to wave travel direction (like an ocean wave)
- Love wave: horizontal movement perpendicular to wave travel direction
12
Q
what do we use seismometers for?
A
- Locate earthquakes and find magnitude
- Maybe provide early warning
13
Q
frequency vs. wavelength vs. velocity
A
- Frequency: number of waves that pass per second
- Wavelength: length of a wave in meters (trough to trough or peak to peak)
- Velocity = frequency x wavelength
14
Q
analyzing seismogram results
A
- P waves arrive first, change in amplitude indicates S waves, change in amplitude indicates surface waves
- Ts – Tp can tell us distance to earthquake (“s-p lag time”) -> on graph, draw line from time or distance to the s-p lag time line
- Calculate distance to quake using 3 seismograms: the epicenter is where the 3 circles intersect
15
Q
magnitude
A
- how much energy is released
- Quantitative measure of maximum ground motion produced by earthquake
- describes strain energy along rupture surface
- measured by Moment magnitude scale (Mw)
- used to be measured by Richter scale
