Geology 101 Exam 3

fuckass class

Earthquake Distribution

earthquakes cluster along tectonic plate boundaries. Can also happen within plates

Why do earthquakes cluster in particular regions

plate movement builds stress in rocks, which can cause faults to slip and release energy

Ring of fire

region around pacific ocean where earthquakes occur

Elastic rebound

rocks gradually deform from stress in a locked fault, elastic strain energy makes the fault slip, causing an earthquake.

How does fault slip lead to release of stored elastic energy

stress, strain, fault slip, strain energy release.

Ductile deformation

rock bends or flows without breaking

Brittle

rock fractures or breaks under stress

Richter Magnitude

measures intensity based on amplitude of largest seismic wave on seismogram (energy release)

Modified Mercalli Intensity

measures intensity based on observed damage or personal recounts.

Ground shaking

movement and vibration from seismic waves

Permanent surface displacement

lasting movement of ground from fault slip

Offset road demonstrates

permanent surface rupture

Collapsed highway demonstrates

structural damage from earthquake

Tilted buildings demonstrates

liquefaction and loss of ground strength

Liquefaction

when water saturated, loosely packed sediment temporarily loses strength during earthquake shaking. Increases pressure in water between sediment grains. Causes buildings to sink or tilt

What sediments are most susceptible to liquefaction

loose, water saturated sediment, especially sand

Normal fault

hanging wall moves DOWN relative to footwall. Caused by tensional stress

Reverse fault

hanging wall moves UP relative to foot wall. Caused by compressional stress

Strike-slip fault

rock blocks move SIDEWAYS past each other. Caused by shear stress (sliding motion)

Seismic energy

energy released during an earthquake, travels outward as seismic waves

Wave propagation

direction that seismic wave energy travels

Particle motion

the movement of rock particles as seismic wave passes

How does energy travel outward from earthquake

seismic energy travels outward in all direction recorded by seismograms. But individual rock particles vibrate as the wave passes

Seismometer

instrument that detects and measures ground motion

Seismogram

recorded graph of ground motion overtime, records data

Seismograph

recording system or instrument used to produce a seismogram

What does a seismograph identify

P waves first, S waves, and surface waves

P waves

primary waves are the fastest, arrive first, particle motion is parallel to wave propagation, compressional, and travel through solids, liquids, and gas.

S waves

secondary waves arrive second, particle motion is perpendicular to wave propagation, transverse motion, travel through only solids

Why can P waves travel through liquids

liquids transmit compression but can't sustain shear forces required to transmit S waves

Surface Wave - Rayleigh Waves

rolling elliptical particle motion, ground moves vertically and horizontally

Surface Waves - Love Waves

horizontal, side to side particle motion, perpendicular to wave travel

Why do seismic waves that travel along Earth's surface (seismic waves) cause damage to the surface

surface waves can produce large ground movements that damage buildings, roads, and other structures.

P-S Time Difference formula

time of s - time of p

What does a larger P-S interval signify

a higher time means the earthquake is further away from the epicenter.

Two rock layers experience compression. One bends into a fold while the other breaks along a fault. Which condition most likely favored folding rather than breaking

Higher temperature and higher confining pressure

Tectonic plates move slowly, often only a few centimeters per year. Why can an earthquake produce sudden, rapid ground motion

Stress builds along a locked fault until the rocks suddenly slip

What is a plate boundary?

The region where two tectonic plates meet and interact; earthquakes cluster here as plates build stress.

What happens while a fault is locked?

Plate motion continues, surrounding rocks deform, and elastic strain energy builds until sudden fault slip.

What is the hanging wall?

The rock block above an inclined fault plane.

What is the footwall?

The rock block below an inclined fault plane.

What types of stress produce normal, reverse, and strike-slip faults?

Normal: tension/extension. Reverse: compression. Strike-slip: shear.

How do you identify P, S, and surface waves on a seismogram?

P waves arrive first, S waves second, and surface waves generally later. Read their arrival times along the time axis.

How do you calculate the P-S interval?

Subtract P-wave arrival time from S-wave arrival time: tS - tP. A greater interval generally means a more distant epicenter.

How do you use P- and S-wave travel times to locate an epicenter?

Find the P-S arrival-time difference at each station; use a travel-time graph to estimate the distance; draw a distance circle around each station. Three circles approximately intersect at the epicenter.

What is an epicenter?

The point on Earth's surface directly above the earthquake focus.

What is the focus (hypocenter)?

The underground point where earthquake rupture begins.

Why are three stations usually used to locate an epicenter?

One station gives distance but not direction; three stations provide intersecting distance circles to locate the epicenter.

Moho

the boundary separating earth's crust from the mantle .

How do faster seismic velocities reveal the Moho

seismic waves travel faster in the mantle in the crust because rocks have different elastic properties and compositions. Faster seismic waves below the boundary help reveal its location

What is the role of stiffness on seismic wave speed

stiffer material generally transmits waves faster

What is the role of density on seismic wave speed

affects speed together with stiffness. Denser doesn't automatically mean faster

What is the role of pressure on seismic wave speed

can increase rock stiffness, making seismic waves travel faster

What is the role of temperature on seismic wave speed

higher temperatures often reduce stiffness and slow waves

What is the role of composition on seismic wave speed

different minerals and rock types have different seismic speeds

What is the role of partial melt on seismic wave speed

small amounts of molten material can reduce wave speed by weakening the rock.

Reflection

some wave energy bounces back

Refraction

wave energy enters another material and changes speed, often changing direction

Normal ray path

imaginary line perpendicular to the boundary

Curved ray paths

seismic waves bend continuously when wave speed changes gradually with depth, deeper route can take less time than a shallower route.

S-wave shadow zone

direct mantle S waves disappear beyond about 103 degrees. This is because the liquid outer core cannot transmit a shear wave

P wave shadow zone

P waves pass through the liquid outer core, sharp speed drop strongly refracts them so ordinary direct P leave a shadow at roughly 103-140 degrees. This is due to ray geometry.

What is the evidence for a solid inner core

additional P arrivals require a boundary within a core, some P paths reflect that, showing evidence of shear rigidity.

P-S conversion

when some compressional P wave energy changes into shear S wave energy at a boundary

How do P-waves generate shear waves at solid inner core

P waves can travel through the liquid outer core, and when reaching the solid inner core at an angle, some can convert to S waves due to having the shear rigidity required.

P wave

compressional wave through the mantle

PP Wave

P path with one reflection at Earth's surface

PKP wave

enters and crosses the liquid outer core as a compressional wave (K)

PKIKP wave

travels through the mantle, outer core, and inner core as a compressional wave (I)

SKS wave

S in mantle, P in outer core, then S in mantle

Seismic tomography

technique for imaging earth's interior using seismic wave measurements along many different paths

How many crossing paths reveal faster and slower regions relative to a reference model

a predicted model of seismic wave speeds can have velocity anomalies where seismic waves travel faster of slower than predicted by the reference model

Earlier arrival / faster region in reference model

cooler, stronger material, such as a subducting slab

Later arrival / lower region

warmer material, sometimes with partial melt

Limitations of seismic tomography

sparse seismic station coverage and measurement noise limit resolution, different materials produce similar seismic properties, velocity anomalies don't directly measure temperature, the image Is a data based model, not a photo of the interior.

Why can both the crust and mantle be solid across the Moho?

The wave-speed jump reflects changes in rock composition and elastic properties; a change in speed does not require liquid on either side.

When do refracted waves bend away from the normal?

When transmitted into a faster material, except at normal incidence.

When do refracted waves bend toward the normal?

When transmitted into a slower material, except at normal incidence.

What is a transmitted seismic wave?

Wave energy that continues into the second material rather than reflecting back into the first.

How do additional P-wave arrivals reveal an inner-core boundary?

Some P-wave paths reflect from or travel through a boundary within the core, producing arrivals that a single uniform core cannot explain.

What additional evidence supports a solid inner core?

Evidence that the inner core supports shear rigidity, which a liquid would not support for propagating shear waves.

Does P-to-S conversion mean an S wave passed through the liquid outer core?

No. A P wave crossed the liquid outer core; some energy converted to an S wave after reaching the solid inner-core boundary.

What is a reference model in seismic tomography?

A model that predicts seismic-wave arrival times and speeds; actual recordings are compared with these predictions.

What is a seismic velocity anomaly?

A region where observed waves travel faster or slower than predicted by a reference model.

Seismic methods

investigates subsurface using mechanical waves that travel through Earth's materials

Electromagnetic methods

investigate materials using electromagnetic signals rather than mechanical vibrations

Ground Penetrating Rader (GPR)

a geophysical method that sends electromagnetic pulses into the ground and records returning reflections to investigate subsurface features

Surface imaging

shows features visible at ground surface

Subsurface investigation

detects features or changes beneath the ground surface

why is GPR is useful for investigating possible unmarked graves and protecting historical cemeteries

helps locate unmarked graves, can detect disturbed soil and buried objects, all without disturbing the site and document historical cemeteries. Cannot automatically determine whether an anomaly is grave.

Interpretation thresholds

the amount or quality of evidence required before classifying a radar anomaly as a possible grave. A radar anomaly alone is not a confirmation of a burial

How does strict criteria change the balance between identifying non-graves as graves and missing possible graves

requires strong evidence before identifying, risking false negatives

How does moderate criteria change the balance between identifying non-graves as graves and missing possible graves

balances evidence requirements, balancing both errors

How does lax criteria change the balance between identifying non-graves as graves and missing possible graves

accepting weaker evidence risks more false positives

What is the difference between mechanical seismic waves and GPR signals?

Seismic waves are mechanical vibrations in a material. Ground Penetrating Radar uses electromagnetic pulses and reflections.

False positive in a cemetery GPR survey

An anomaly is classified as a possible grave even though it is not a grave.

False negative in a cemetery GPR survey

A grave is present but not identified as a possible grave.

What is a radar anomaly?

An unusual return or pattern in GPR data; it may warrant investigation but does not confirm a burial.

Distribution of earth's water

oceans hold about 97%, glaciers hold most freshwater, groundwater is major source of available freshwater

Why do rivers only hold a small fraction of stored freshwater

water flows through them rather than remaining stored for long periods