Earthquakes Flashcards
On Exam 2 (Apr 1)
Divergent Boundary Earthquakes
The least damaging
Caused by tension
Transform Boundary Earthquakes
Intermediate danger
Caused by shear stress (acting in parallel but opposite directions)
Convergent Boundary Earthquakes
The most damaging
Caused by compression
Do all earthquakes happen at plate boundaries?
No, there are intraplate earthquakes caused by ancient plate boundaries or weaknesses in the crust
What are the two types of motion along faults?
Creep: slow, continuous, gradual, and imperceptible motion that rarely causes damage (besides a city planner’s nightmare)
Elastic Rebound: a buildup of stress that causes faults to deform and quickly snap, releasing energy as they return to their original shape (like a rubber band)
Hypocenter vs Epicenter
Hypo = the point on the fault where the motion first occurs (usually underground)
Epi = the location on the surface above the hypocenter
What are the two types of Seismic waves?
Body Waves: travel inside the earth, which is faster because of fewer changes in density
Surface Waves: travel on top of the earth, which is slower because of obstacles
However, they are larger and more destructive
What are the two types of Body waves? What are their characteristics? What materials can they travel through?
Primary (P): travels parallel to the hypocenter (in other words, it causes the ground to compress and expand, or move back and forth, in the direction of travel)
Move through all materials (solid, liquid, and gas)
Secondary (S): travels perpendicular to the hypocenter (in a shaking motion that causes the ground to move up/down or side/side)
Moves only through solids
What are seismic shadow zones?
Areas where certain seismic waves cannot get to
S-waves leave a big shadow zone on the opposite side of the earth from the hypocenter because they get deflected by the liquid outer core
P-waves can travel through the core, but get bent in the process, leaving some small gaps
What does the travel of seismic waves tell us about the inside of the earth?
We know the core is liquid because of the deflection of S-waves, leaving shadow zones
We know that there are three layers because P-waves will change in velocity as they transition between them (waves travel differently through different density materials)
What are the two types of Surface waves?
Rayleigh: travels like a ripple in water (causing the ground to shake in an elliptical pattern)
Usually lasts the longest
Love: travels horizontally from side-to-side like a slithering snake (shears the surface)
Is slightly faster than Rayleigh
How do we measure seismic waves?
With a seismometer, which detects waves in 3 different directions: X and Y (horizontal) and Z (vertical)
Measures both amplitude and arrival time
What is Arrival Time? What can it be used to determine?
How long it takes for the wave to get to a specific location (which will vary based on the speed)
The difference in arrival time between the seismic wave types can be used to determine the distance traveled
The distance traveled can then be used to find the epicenter using triangulation (requires 3 readings from different seismic stations)
What is Amplitude? What does it show us?
The height, which determines how strong it is
How do we measure seismic amplitude?
Richter Magnitude Scale: based on the amount of energy released
Mercalli Intensity Scale: based on damage and human perception
How can we find the Richter magnitude?
Using amplitude and distance: put a dot on each scale, then connect them with a line that overlaps the magnitude scale
What is Site Amplification?
An increase of ground shaking at a specific location based on the material present
Seismic waves will travel faster and farther in denser (older) rock
However, soft soil sites or less dense rock will lead to higher wave amplitude and more ground shaking
What is liquefaction?
Loosely-packed, saturated sediments will lose their integrity when the ground shakes, causing less dense materials (including water) to rise and more dense materials to sink
This will lead to flooding and sinking of heavy buildings
Which type of seismic wave is the most damaging? Which is the least?
Surface Waves are the most damaging because they have the highest amplitudes and shear the surface
By contrast, P-waves are the least damaging because they have the smallest amplitudes and mainly cause the ground to move up and down
What seismic waves are buildings built to withstand?
Body waves, particularly S-waves, because they move the ground back-and-forth and shake buildings at the base
We know that, because of inertia, things will move opposite to the force of acceleration (ex: a bus speeds up unexpectedly = you jerk backwards)
Inertia affects taller things more
How do earthquakes cause damage?
Building Collapse, Fire Damage (downed power lines), Landslides, Floods, Tsunamis
Similar to Volcanoes, most people die in the aftermath, not the actual event
What is a tsunami? How is it formed as it approaches the shore?
A surge of water caused by movement of the seafloor
As the water approaches the coast, the waves will compress (as most waves do when they move towards something) and get higher
The energy of the approach will lead to a retreat of water from the shoreline (big bad sign, run away)
What are some mitigation techniques for earthquakes (how can we protect people from them)?
Hazard Maps, Insurance, Building Locations (do not build on a fault for the love of god), Building Materials (flexible) and Codes (base isolation), Landslide Retention (terraces/trees), Tsunami Warnings
