Hazards exam 2

earthquake definition

actual motion/slip across a fault when stress overcomes friction

where to earthquakes occur

• convergent boundaries

• transform boundaries

• divergent boundaries at the transform faults

• fracking

• extension zones

• intraplate fault zones

• shallow and deep crust

• upper mantle (deep as 800 km)

• intraplate and transform faults only active in shallow crust (20 km deep)

seismology definition

study of earthquakes and the interior of the earth through seismic waves

• info comes from the time it takes waves to travel through earth

types of seismic waves

surface waves

body waves

• p waves

• s waves

what are seismic waves

a propagation of deformation or strain through the material

p-waves

primary waves

SOUND waves

longitudinal waves (compression and dilation)

propagate parallel to the direction of travel

change shape and volume of material

s-waves

secondary waves

transverse waves (shearing)

the shearing does not permanently deform the material, only changes shape not volume

propagate parallel to the direction of motion

velocity of p wave equation

k (kappa) = compressibility (resistance to volume reduction) (bulk modulus)

μ (mu) = shear modulus (resistance to shear) (rigidity modulus)

ρ (rho) = density

velocity of s wave equation

μ (mu) = shear modulus (resistance to shear)

ρ (rho) = density

which waves can travel through fluids

p waves (not s waves) (mu = 0 in liquids)

snells law

tells you when a wave hits a surface of different density whether it will be refracted, reflected, or absorbed

how does wave velocity change with depth in the earth

velocity increases with depth in the earth

density also increases with depth, and since it is the denominator, that means mu (shear modulus) must increase faster

following this, p wave velocity must increase faster than s wave velocity

total internal reflection

when the waves make it back to the surface because they hit the critical angle and reflected in the earth

velocity density paradox

seismic waves travel faster in denser rocks even though density is on the denominator. this is because the elastic moduli (kappa and mu) increase with density as well but at a faster rate

shadow zones

locations where the primary p and s waves don’t reach

p waves: 104 - 145 degrees

s waves: 100 - 100

what do seismograph stations produce

3 seismograms: up-down, N-S, E-W

p waves = mostly up-down

s waves = mostly N-S, E-W

surface waves = all 3

types of surface waves

rayleigh = rolling waves, long period

love = snake like waves

Richter scale

• logarithmic, 1-9

• calculates energy of earthquake based on amplitude of waves

  • high proportion of low frequency waves = more energy

the seismic moment (Mo)

more consistent measurement of energy released in an earthquake than richter scale

fundamental measure of eq size

classes of earthquakes

convergent = major - great

transform = moderate - major

divergent = minor - light

fault surface

whole plane of the fault

fault trace

fault intersection with earth’s surface (can see fault scarp)

hypocenter

focus - origin of earthquakes in 3D space

epicenter

location on surface directly above the origin of earthquake

types of faults and their stress

normal = extensional

reverse = compressional

strike-slip = shear

Modified Mercalli Scale

non-scientific scale of earthquake intensity that measures shaking and destruction (impact on people)

how we measured eqs through time

• very first seismometers were balls balanced on pedestals - could see which direction was initial jolt

• Kiyoo Wadati - waves diminish with distance

• Charles F Richter - amplitude on seismograph can be adjusted for epicentral distance, measured surface waves also

• Keiiti Aki - seismic moment (Mo), dislocation theory

Gutenberg and Richter energy equation

log₁₀Es ~~ 4.8 + 1.5Ms

Ms = richter scale

problem: saturation. did not work for high values (greater than 6)

moment magnitude (Mw)

Hiroo Kanamori: 𝑀w= 2/3 log₁₀⁡(𝑀o)−6.1 — defined magnitude based on energy (opp of richter)

scaled final equation: 𝑀w= 2/3 log₁₀⁡(𝑀o)−10.7

for eqs >3.5 Mw

fault creep

• motion between major earthquakes

• gradual, slow slip

• more or less aseismic

• due to rock type (at surface but also because of changes at depth), pressure conditions, post-eq creep, location of stress

• associated with structure damage, mass wasting

deep slip

Fault slip that occurs below where most large earthquakes occur. This deep portion of the two sides of the fault may be sliding aseismically (creeping)

slow earthquakes

• hours-months long and recurrence interval of months-years

• can accompany megathrusts

  • and maybe predict megathrusts? (post or pre)

careers in volcanology

• geochemist - gas

• Geophysicist- seismic

• Volcanologist – Ash/tephra

• Emergency management – coordinate evacuation, public information,