Quiz 3 Topic 3 Flashcards
What causes earthquakes?
Rocks have rough surfaces so they stick, when you add pressure you will get some slip and this will induce an earthquake.
Where do earthquakes happen?
Plate boundaries and faults
What is a seismograph?
Is a recording of measuring ground shaking
Do earthquakes always happen at plate boundaries?
No, they can also occur at faults due to fracking
What causes a plate boundary?
Movement in different directions and at different speeds produces plate boundaries.
What is the diff between all 3 types of plate boundaries? Name em
Transform plate boundary- moves past each other
Divergent- away
Convergent- together
What is a fault?
fault plane- surface of two sides of fault
hanging wall is on top, foot wall is on bottom
Faults are located at plate boundaries but also other places of the earth where stress builds and is released by faults
Faults are planar fractures in rocks where the rocks on either side have moved
What are the three types of faults? Describe them
Strike-slip, normal and reverse faults (dip-slip faults) .
Strike slip- moving past eachother horizontally (happen when you have sheer)
normal fault- hanging wall goes down vertically and move outward
thrust fault- hanging wall goes up and onto foot wall (same as reverse fault but shallower angle)
reverse fault- blocks move together hanging wall climbs up foot wall
Why do faults occur?
Faults occur because of directed stress which builds up in rocks.
What are the three different types of stress?
compression, tension, and shear (slipping along a plane parallel to the stress)
What does stress lead to?
strain (deformation)
IF you keep stress what does that lead to in rocks?
Breakage, (fracture)
What are the three stages strain takes place in?
Elastic Deformation
Fully reversible – stretch the spring a little and it springs back
Ductile Deformation
Irreversible – stretch the spring too much
and it won’t go back to its original shape
Brittle Deformation (Fracture)
Stretch the spring so much that it breaks.
What are the scales that fractures can occur at?
can be at the scale of a plate boundayr, can be in a moutain, or can even be microscopic
What is elastic rebound theory?
friction exists along the fault plane, so rocks on either side of the fault resist movement until strain overcomes the friction- resulting in an earthquake
name four steps in an earthquake
stress is added (via sheer), rocks bend (ductile deformation) and store energy, rocks fracture and release energy (get earthquake) then rocks rebound to their deformed shape.
Where does slip occur in a fault?
On the rupture zone
what is the focus?
where the rupture starts.
What is the epicentre
is on the surface above the focus
Where do the biggest earthquakes occur?
At subduction zones
Can shallow and intermediate earthquakes also occur at subduction zones?
yes, earthquakes happen at clusters at specific depths, this is because of mineralogical changes due to metamorphism of the rock. This is because water in the minerals gets released as they melt which weakens rock and earthquakes occur. The shallow earthquakes occur due to energy released from the rocks snagging on eachother.
The pattern of the clusters of earthquakes is called the Wadati-Benioff Zone. Can use these zones to see which way the subduction zone is dipping and angle of the dip.
the shallower the angle of the bed the stronger the earthquake!
What type of plate boundaries usually have shallow earthquakes?
Transform and divergent boundaries are associated with shallow earthquakes.
What are body waves?
Some seismic waves are generated at the focus and travel through the interior of Earth = body waves
What are the two types of body waves?
p waves and s waves
Describe p waves
stand for primary waves
Is like a slinky, you move the slinky and all the rings move with energy.
P waves are like that but they move in circles from the epicentre.
P waves do not cause much damage
describe s waves
S waves stand for shear waves or secondary waves
Are analogous to a rope wiggling, move out in a sphere except instead of a pressure wave they shear all the rock around them.
slower then P waves,
S-wave ground motion is in both horizontal and vertical direction, can cause a lot of damage. More than P waves as they move in multiple dimensions.
What are surface waves?
Is the arrival of body waves at the surface of the earth
Are surface waves more or less destructive than body waves?
Surface waves tend to have the greatest amplitudes in near- surface layers of sediment and are the most destructive of the earthquake waves.
What are the two type of surface waves?
rayleigh and love waves
How do rayleigh waves move?
rotating waves along surfaces, create vertical displacement, are like ripples on water.
Energy moves in a rolling way through rock at the surface of earth.
How do love waves move?
Are horizontal moving not up and down like rayleigh waves. Are side to side shearing.
Recite all of table properties of seismic waves
How are seismographs built?
Have ground movement, base, frame, and fix mass
What does attenuate mean?
Earthquakes attenuate as they travel through Earth (the amplitude of the wave gets smaller
If earth was completely homogenous how would waves travel?
If Earth was completely homogeneous, waves would travel along essentially straight paths.
What do we actually see in the way waves travel?
We see P shadow zones and well as S shadow zones
What do the shadow zones tell us about the interior of the earth?
It tells us that the interior of the earth must have layers of materials with different properties
Why do we get a P wave shadow zone?
Velovity of p waves change as it interacts with rocks of different properties, travels slowly through solid inner core, and then travel faster in liquid outer core.
P waves can travel through liquids and solids and velocities of the wave will change based on the density of the rock as well as depth as density of rock increases as the depth increases, s wave is a shear wave, p waves are compressional waves.
The change in p waves as it travels through diff materials results in an area with no p waves, when it hits the solid inner core it speeds up again and then will wave again.
Why do we get an s wave shadow?
As liquid has no sheer strength they cannot transmit S waves
What is the modified mercalli scale?
IS a qualitative description of what earthquake feels like.
This is an intensity scale, how it feels and the impacts of the earthquake.
The further up you go in this scale the worse the destruction is.
Goes upto 12
What is the richter magnitude scale?
Consists of an amplitude scale, measures time between wave arrivals, measures distance from the epicentre to get magnitude of earthquake.
What are the limitations of the richter magnitude scale?
For amplitude we’re measuring biggest peak on seismogram, when waves travel further from epicenter they attenuate (get smaller) so we apply a correction factor to the wave. On the richter scale 0 is calibrated at 100 km from the epicenter. This scale does not take inot account how long the ground shakes for. Is bad at measuring big earthquakes, this scale only takes into account the amplitude of the wave not the shaking of the ground. It also doesn’t work well for deep earthquakes as as waves trave up surface they attenuate, so the surface waves end up not being that big. Also really bad at measuring earthquakes with an epicenter that’s very far away from the seismonitor. Everyscale we use is generally imperfect bcuz it only focuses on the amplitude of the wave.
It doesn’t take into account differences in rock types, you can see changes in lithology over a space of a few metres, changes in lithology affect you waves velocity, for ex surface waves get amplified in loose sediments. And this is not included in the richter scale as the richter scale was developed for California and it’s lithology,so therefore it can’t be used everywhere even though certain countries have diff rock types so the scale won’t be exact place to place. They adjust the richter scale according to it’s local area to make up for this error when using it in diff places and call it a local magnitude scale.
Frequency is also missing as well as time from the richter scale.
What is each level in the richter scale represent?
Each level in the richter scale is an order of magnitude, this is because amplitude varies extremely differently, so instead of expressing the actual numbers we use magnitudes in order for ease. The richter scale is also logarithmic, every time you increase the magnitude by 1, a 10X greater amount of amplitude occurs and 33X more energy occurs.
Why do we convert amplitude into magnitude?
Do this to compare simple numbers
What are the energy changes between each step in the richter scale?
Minor earthquakes happen all the time
One step up is a light earthquake, going to the feeling of tornado, from 5-6 feel an atomic bomb, upper than that is nuclear energy feeling
What is the moment magnitude?
is MW equal to Average slip of fault (= amnt distance rock has moved) times the ruptured area on the fault, times the rigidity of the faulted rock (how spongy rock on either side of fault is) the larger the area of slip the bigger the earthquake, the greater the amount of distance rock has moved bigger the earthquake, the less spongy the rock- the bigger the earthquake.
What number gets recorded in earthquake history?
moment magnitude
Can we predict where earthquakes will happen? When they will happen?
Can predict where earthquakes will happen (ex plate boundaries/faults), but cannot predict when they will happen due to not being able to predict changes in rock in subsurface, any computer that could do this would take a really long time to happen and by that time earthquake will happen
What are secondary hazards associated with earthquakes?
Changes in ground level (due to fault ruptures that come to the surface), landslides (due to rumble from earthquakes moving metastable rocks), fires (can be caused by ruptured gas lines, fallen power lines), liquefaction, and tsunamis
How does liquifaction work?
Need loose sediment for this (has no cohesion, for ex no clay) need water saturation (all pores filled with water) and then need ground shaking.
When you have a loose sediment and shake it it will compress, the grains will support it, in an earthquake the squeezing motion of sediment put pressure on water in pores in respponse the water produces pore pressure equal to the pressure of the shaking of the grains this will push grains apart and the whole thing behaves like a fluid (quick sand).
What causes tsunamis?
These happen when you have displacement in the oceans, some type of movement in sea floor which causes waves (can be earthquake, nucelar blast or underwater land slide)
Wave in the middle of the ocean will not be big at all, but as the tsunami heads towards land it will ride up the ramp towards land, due to friction water slows down and wave sstart to pile upm on eachother which causes height of wave tp be much much higher as frequency increases (waves are much closer to each other). Often called tidal waves, since they look like tides but don’t have to do anything with tides and are much much bigger.
Describe some of the diff ways engineers make earthquake resistant buildings?
Braces stop deformation in buildings, braces are designed to be ductile without fraction, earthquakes hit weakest link (breaks). Good as it avoids links which will fracture.
Sheer walls, strongest and stiffest of all buildings
Dampers- is not a brute fort system, has water here to reduce earthquake vibrations via sloshing.
Seismis proofing- is best move for earthquakes- puts base isolation system in it, stop building from moving.
