geology earthquakes lecture

fault motion

how blocks move relative to each other accross a fractrue in the Earth’s crust

why is the term fault motion a bit misleading

because it isn’t the fault that’s moving, but rather the lithosphere on either side

what was something that happened in the lead up to the 1906 earthquake

fences/survey line/figures along the san andreas fault were beginning to bend

why did the fences bend?

elastic deformation of the underlying rocks 

what happened to the fence after the 1906 earthquake

it separated by up to 7 meters (avg. offset was 11-15 ft)

elastic strain

(or deformation) the principle that as long as stress is applied, the object on which stress is applied will strain

what happens when stretch is removed in elastic strain

the object will snap back into its original position

elastic limit 

point of elastic strain that an object reaches where the stress is too much, and strain that has been stored is released, typically resulting in the object breaking (or an earthquake!) 

elastic rebound model

model for the generation of earthquakes that is in terms of elastic strain and stress (as defined earlier) and the principle that rocks act elastically 

why is the common misconception about little earthquakes preventing a big one wrong

because it operates on the belief that little releases of pressure would prevent bigger ones, but earthquakes don’t have anything to do with “pressure", it’s about directed forces in the crust of the earth 

continuous slip

a characteristic of fault creep, a slow, steady, and aseismic (non-earthquake) movement along a fault surface that lacks strain buildup

how does fault creep prevent earthquakes

it creeps slowly, and does not generate strain that requires sudden release which would result in an earthquake 

what is the reality if many little earthquakes are occuring 

they are more likely to be foreshocks and a harbinger of the big one rather than “little releases of energy” (which doesn’t exist!)

fault trace

the surface line where a geological fault intersects the Earth's surface (most often a visible crack)

aftershocks

smaller earthquakes that occur after a big earthquake; can last for days, weeks, months, years

why do aftershocks happen

since a lot of strain is released during an earthquake, the rocks in and around the area of rapture need to catch up to the slip to redistribute strain 

what is being looked at when studying earthquakes 

slip along the area of rupture along the plane of a fault 

focus

the midpoint of an area of rupture; is in the middle of the propagation of seismic waves

epicenter

the place directly above the focus of an earthquake

fault

a break in the earth’s crust in which there’s been appreciable motion

what kind of fault is this 

dip-slip fault

geometry of a dip-slip fault

hanging wall moves down relative to the foot wall

crust of a dip-slip fault

becomes longer across the falt

what creates a normal fault on a large scale

the earth’s crust gradually moving apart

how does stress work in a normal fault

the crust of the earth has more pressure that is compressional, but this is less than the push inward by gravity and what results is a normal fault 

the gravity and stress relationship in a normal fault 

the dominant stress is vertical stress from gravity which means that the horizontal stresses are insufficient to prevent the rock from extending horizontally and sliding down 

why crust extends in a normal fault

Because the fault plane is inclined, the downward vertical movement also creates a horizontal component of separation called heave. This heave results in a net increase in the horizontal length of the crust across the fault l

what kind of fault is this 

reverse fault/thrust fault 

reverse faults, historically 

very steep faults 

thrust faults, historically

low angle or more complex reverse faults

geometry of a reverse fault

hanging wall moves up relative to the foot wall

crust of a reverse fault

becomes shorter across the fault

thrust fault is a result of

horizontal compression

how compression works in a reverse fault 

the major component of stress is coming in perpendicular to the fault — and outweighs the minimum principle stress

the gravity and stress relationship in a reverse fault

compression is the maximum principal stress and gravity is the minimal, which causes the reverse movement 

what type of fault is this 

left-lateral strike-slip fault

geometry of a strike-slip fault

either left or right lateral slip

“looking across a fault”

always the fault further from where you stand. on either side, no matter which you’re standing on, movement will be in the same direction 

how to tell lateral motion

look at the apparent change in direction looking across a fault line

crust activity of a strike-slip fault

no change in length

strike-slip faults are a product of

lateral differential movement of the crust

what kind of fault is this 

right-lateral strike-slip 

stress in a strike-slip fault

acts somewhat differently; the stress applied comes from all directions, but at an angle to the fault itself which results in a strike-slip fault

shear stress

a force acting parallel to a material's surface, causing internal planes to slide past each other; found in strike-slip faults

lateral motion

found in strike-slip faults

vertical motion

found in dip-slip faults

oblique faults

faults with both lateral and vertical motion

where most earthquakes occur in california

right-lateral strike-slip faults 

vertical motion in strike-slip faults

does exist—during an earthquake there is differential motion and sudden slip —however it is much fewer than the lateral motion 

body waves

the seismic waves that emanate from an area of rupture, they propagate through the body of the earth (hence the name)

why surface waves occur 

because the epicenter receives the brunt of the energy of an earthquake due to its location above the focus 

surface waves

when the epicenter of an earthquake is lifted up and dropped suddenly, causing undulations to radiate on the surface 

types of body waves

p-waves and s-waves

p-wave

primary wave, travels faster than an s-wave but carries less energy 

order of wave propagation in an earthquake

p-wave comes first (primary) and s-wave comes second (secondary)

s-wave

secondary wave, travels slower than a p-wave but carries more energy and more shaking 

what is this 

a p-wave 

how p-waves travel 

particles of what they travel through are compressed then stretched, like a slinky. energy is transferred by particles bumping into adjacent molecules in the same direction of the wave 

compressional waves

alternate name for p-waves due to the fact that their motion is dependent on compression within the direction the wave travels

what surfaces p-waves can travel through

any—solid, liquid, or gas; most notably through the center of the earth

what is this

an s-wave

shear wave

alternate name for s-wave; an elastic body wave where particles move perpendicular to the wave's direction of travel

how do p-waves move

parallel

what can s-waves move through

solids and gases, no liquids; cannot move through molten material in earth

2 types of surface waves

rayleigh wave and love wave 

rayleigh wave

what causes swaying, a motion back and forth within the surface of the earth

love wave

retrograde undulations that cause the earth to move ocean-like

what kind of wave is this 

rayleigh wave

what kind of wave is this 

love wave 

what is this

a seismogram

seismometer

instrument that measures and records details of earthquakes; a triaxial device that allows us to see components of motion found in earthquakes—in 3 directions

what is plotted on a seismogram

ground motion against time

what components of motion a seismogram records

east-west, north-south, and vertical

why are more focal radii better for determining an earthquake’s epicenter

there are small disturbances within measurements, so more makes it more accurate 

what is a magnitude about

ground motion in the area of rupture

moment magnitude scale

magnitude scale used by scientists; more accurate for large earthquakes—takes into account factors other than amplitude

richter magnitude scale

what the public uses for magnitude scale; weaker because its accuracy declines as earthquakes get bigger due to its reliance on the amplitude of an earthquake as shown on a seismogram

what factors moment magnitude takes into account

calculates the seismic moment, which is proportional to the fault's area, the amount of slip on the fault, and the rigidity of the rock. 

ground motion and energy relationship for magnitude

for each increase in magnitude on ground motion, 10x (logarithmic); for each increase in magnitude on energy, 31.5x 

what is this called

a nomogram

nomogram

scale used for calculating richter magnitude; peak amplitude of s wave and s-p time/distance on either side, the middle point on the line is richter magnitude

magnitude

a single number that refers to the GROUND MOTION and energy released at the area of rupture 

intensity

what is experienced at a certain area when an earthquake occurs

magnitude vs intensity

there is one magnitude for an earthquake, but intensity can vary for multiple diff. factors 

modified mercalli scale

scale which describes the intensity of an earthquake; is observation-based and has 12 levels, using roman numerals and with higher numbers being higher intensity

ground rupture

visible cracking along a fault line that can be due to the main trace of the fault moving (if it ruptures all the way to the earth’s surface)

why does ground rupture occur

in large earthquake events with so much deformation, a number of ruptures occur (some with appreciable motion) in order to accomodate deformation

why is ground rupture not as important

it accounts for less than 1% of earthquake damage, and in california is strictly regulated to make sure structures aren’t built on faults to avoid this

seismic shaking

the cause of most earthquake damage around the world; dependent upon earthquake’s magnitude, distance from epicenter, and amplification of shaking (jello + matthew and lisa) 

magnitude in seismic shaking 

the larger the quake, the more seismic shaking 

distance from epicenter in seismic shaking

the closer to the quake, the more shaking

the jello effect

the principle that less rigid items or surfaces are subject to more seismic shaking

jello effect example

a house of cards is built on both a slab of granite and a jello mold, then hit with a rubber mallet. the house of cards on the jello mold will shake much more and collapse, while the one on the granite slab will remain fine. the jello is non-rigid

jello effect explanation

a seismic wave’s velocity is related to density (almost negligible effect) and rigidity. the more rigid, the faster the waves travel. when a seismic wave passes through a non-rigid object (jello), the waves slow down and are compressed, which increases the amplitude of the wave and thus the shaking AND it makes the wave take more time to travel through the area, giving it more time to shake and inflict damage

where does the jello effect happen IRL

unconsolidated sediment, bayfill, soft mud and slits, surficial deposits (things that gather on the earth’s surface)

matthew and lisa sitting in a swing

the principle that if a seismic wave is at the peak of its amplitude and an object is at its resonance frequency, they line up and create a constructive interference where the seismic wave will resonate (vibrate with increasingly large amplitude)

resonance frequency

the natural frequency at which a system vibrates with maximum amplitude when subjected to a driving force at that same frequency — the object’s peak

constructive interference

the phenomenon where two waves combine to form a new wave with a larger amplitude than the original waves

matthew and lisa & jello effect relationship

non-rigid surfaces/items are more susceptible to the matthew and lisa effect, meaning it can combine with the jello effect and makes non-rigid items even more susceptible to seismic shaking

ground acceleration

a vertical component of the seismic waves coming in; if there is a structure that holds weight (a force), a moment can occur where the ground and structure pulls away, then the item will accelerate down due to gravity, and if the structure accelerates up, the two meet and the force can double which leads to catastrophic collapse