earthquakes and seismicity

pnsn interview prep

where and why do earthquakes happen?

• occur along active faults where energy is stored and suddenly released

• stresses along the fault exceed the frictional forces resisting motion

what is the focus?

• initiation point of fault motion within the Earth

• seismic waves propagate concentrically from the focus

• seismic waves weaken the more it travels through the Earth

what is the epicenter?

• point on the surface of the Earth directly above the focus

• with all things being equal, the epicenter is where you tend to have the most damage because it’s closest to the energy release

longer time interval between seismic events on an active fault mean…

• greater magnitude earthquakes

• if faults are more active (ex: daily fault creeps), it reduces the energy/damage of an earthquake

what are body waves?

• seismic waves that propagate concentrically outward within the Earth

• include P and S waves

what is a P-wave?

• primary wave

• compression waves (ex: how a slinky pushes and pulls)

• propagate through all phases of matter but move slower in liquid

• travel the fastest (~6 - 7 km/second)

• measured first by seismograph

what is a S-wave?

• secondary wave

• sinusoidal waves (ex: how a jump rope moves when you whip it)

• propagate only through solids

• disappear in liquids and gas (can’t be sheared)

• travel at ~3 km/second

• measured second by seismograph

what are surface waves?

• created from residual energy as the body waves reach and interact with the surface of the Earth

• like a rolling ocean wave

• most damaging to buildings and structures close to the earthquake source

• travel with the slowest velocity

• measured last by seismograph

how can you find the focus or epicenter of an earthquake?

• if you know the velocity of seismic waves, you can determine the focus or epicenter using the difference in arrival time between P-waves and S-waves at a given seismic station

• difference in arrival time increases with greater distance

• need three seismic stations (or more) to pinpoint the focus of the earthquake

how are amplitude and frequency related?

• inversely related

• large amplitude waves have a lower number of waves per unit time

• multiply amplitude and frequency to find the total energy of the seismic wave

difference in energy release between each unit magnitude scale is…

• 30 times

divergent and transform boundaries are characterized by…

• shallow-focused earthquakes

• ocean lithosphere is relatively thin

• earthquake is going to occur in the brittle fraction of that lithosphere

convergent boundaries tend to have…

• deep-focused and shallow-focused earthquakes

• shallow-focused earthquakes happen because there is an interface between the overriding plate and the subducting plate that starts at the surface

• deep-focused earthquakes happen inside the subducting plate as it sinks because it’s cooler and stronger than the surrounding mantle

where do megathrust earthquakes happen?

• at the inflection point of the subducting plate where it meets the overriding plate

what factors influence the amount of damage from an earthquake or its intensity?

• magnitude of the earthquake (energy release)

• proximity to epicenter: earthquake waves slow as they propagate through Earth rock

• local substrate conditions: rocks that are more brittle are going to have higher velocity seismic waves than rocks that are less brittle; when seismic waves decelerate, the ground shakes more

• duration of earthquake: the longer you shake, you might reach a critical stress where something breaks

• local construction techniques and engineering design: can engineer to protect against earthquake damage

what are the tectonic settings where earthquakes occur in washington?

• along shallow faults within the north american plate

  • recurrence of ~ hundreds of years

  • potential for magnitude 7+

• deep faults within the juan de fuca plate

  • most happen here

  • recurrence of 30-50 years

  • potential for magnitude 7+

• along large sections of the cascadia subduction zone

  • recurrence of 500-600 years

  • potential for magnitude 8 or 9