GEOL 200 - Midterm 2 - Earthquakes and Tsunamis

What is the definition of a fault?

A fracture or zone of fractures in the Earth's crust.

What is a fault scarp?

step in the ground caused by an earthquake one side rises more than the other

What is stress?

force per unit area that pushes, pulls, or shears the rock.

What are the two main types of stress?

Compressional stress — pushes rocks together

Tensional stress — pulls rocks apart

What is compressional stress?

Rocks are pushed together from opposite sides.

What effect does compressional stress have on rocks?

Causes rocks to shorten and thicken.

Where is compressional stress commonly found?

At convergent boundaries (where plates collide).

What types of faults are often produced by compressional stress?

Reverse and thrust faults.

What is tensional stress?

Rocks are pulled apart like stretching a rubber band.

What effect does tensional stress have on rocks?

Causes rocks to thin and lengthen.

Where is tensional stress commonly found?

At divergent boundaries (where plates move apart).

What type of fault is produced by tensional stress?

Normal faults, where the hanging wall drops down.

What is strain?

the change in shape or size of a rock that happens because of stress

What is ductile deformation?

Rocks deform by folding and reshaping.

§ Occurs in deeper crust.

What is brittle deformation?

when rocks fracture or break in response to stress

Ductile vs Brittle deformation

Ductile bends; brittle breaks

How does the mode of deformation vary with depth?

at the surface, most rocks break

while the lower they go they flow

What are the strike of a planar fault?

the direction of the fault line on the surface

What is the dip of a planar fault?

the angle the fault slopes into the ground

What are the two types of strike-slip faults?

right lateral and left lateral

Right Lateral Strike-Slip Fault

the opposite side of the fault moves to the right from your perspective

Left Lateral Strike-Slip Fault

the opposite side of the fault moves to the left from your perspective.

What are the two types of dip-slip faults?

Normal fault and Reverse (or thrust) fault

Normal Fault (Dip Slip)

the hanging wall moves down due to tension pulling the crust apart.

Reverse (or thrust) fault (Dip Slip)

he hanging wall moves up due to compression pushing the crust together

What is the hanging wall?

the block of rock above the fault plane

What is the foot wall?

the block below the fault plane

What is a blind thrust fault? Why are they dangerous?

a thrust fault that does not break the surface

it is dangerous because it is hidden beneath cities.

Name an example of a deadly earthquake on a blind thrust fault.

The 1994 Northridge, California earthquake.

How do faults manifest in surface topography?

They can create linear valleys, offset streams, ridges, sag ponds, and fault scarps depending on the type of movement.

Which types of faults are found at divergent, convergent, and transform boundaries?

Divergent boundaries have normal faults, convergent boundaries have reverse or thrust faults, and transform boundaries have strike-slip faults.

Divergent Boundaries

places where plates are pulling apart (normal faults)

Convergant boundaries

two tectonic plates are moving towards each other (reverse or thrust faults)

Transform Boundaries

places where tectonic plates slide along beside one another as they move (strike-slip faults)

What kind of faults created the Basin and Range?

normal faults created by crustal extension.

What is a megathrust earthquake?

large earthquake that occurs on the shallow part of a subduction-zone thrust fault between the subducting and overriding plates

What happens to the overriding and subducting plates during a megathrust earthquake?

overriding plate which has been locked and bent down, snaps upward

Two famous strike-slip faults

San Andreas Fault

(right-lateral)

North Anatolian Fault (right-lateral)

Garlock Fault (left-lateral)

What are the ways that earthquakes can affect the chance of quakes occurring on other faults?

Changing stress on nearby faults

What is the pattern of displacement (motion) across a fault in a strike-slip earthquake (coseismic)

The ground on each side of the fault slides sideways past each other, creating a clear horizontal offset right along the fault.

What is the pattern of displacement between earthquakes (inter-seismic)

ground slowly stretches and bends near the fault because it's stuck, building stress until it finally breaks in an earthquake

How do we measure displacements that result from earthquakes?

GPS and satellites track how the ground shifts before and after a quake to show how much it moved

How do we measure displacements that result between earthquakes?

High-precision GPS that show small ground movements over time

What is the characteristic earthquake model?

a fault tends to produce earthquakes of similar size at fairly regular intervals because it builds up stress in a repeatable cycle

How do stress and displacement vary with time?

Stress slowly builds as the fault stays stuck, then drops suddenly during an earthquake

displacement jumps all at once.

How would you use the characteristic earthquake model to predict earthquake occurrence rate given fault slip rate and the size of the last major earthquake?

Divide the average slip per major earthquake by the fault's slip rate. This gives the estimated recurrence interval (time between similar large earthquakes).

What does the characteristic earthquake model predict for when the 1906 San Francisco and 1857 Fort Tejon quakes should repeat?

Both should occur roughly every few hundred years based on slip amounts and slip rates

Typically

What is the Parkfield experiment?

project on the San Andreas Fault designed to capture detailed data on an expected moderate earthquake.

Were precursors observed at Parkfield?

No. No consistent or reliable data were detected before the earthquake.

Was the Parkfield prediction successful?

No it was not. Predicted earthquake around 1988 but actually happened in 2004

What is the meaning of the term "recurrence interval"?

average time between large earthquakes on a specific fault or fault segment

What is a time-predictable earthquake model?

earthquakes occur when stress reaches a certain level.

tress builds over time, then drops during an earthquake. amount of slip varies depending on how long the stress had been building

How does variable loading affect the evolution of stress and displacement with time?

If the rate of tectonic loading changes, stress builds at different speeds

Faster loading; shorten time to failure

slower loading; longer time to failure

make intervals irregular

What are seismic gaps and what might they tell us about a fault?

a section of a fault that has not ruptured recently compared

may indicate a location where failure is likely

Was the Tohoku earthquake predicted? What, if any, aspect was surprising?

No. It was not predicted.

The magnitude. did not expect a M 9 in that segmant

Amplitude

Height of a wave

Wavelength

the distance between two identical points on the wave

Period

time it takes one full wave cycle to pass

Frequency

number of waves per cycle

What is carried by seismic waves

energy

What are seismic body waves?

Waves that travel through the interior of Earth. both P and S

What are seismic surface waves?

Waves that travel along Earth's surface.

Slower but more damaging (love and rayleigh)

What are the two types of body waves

P waves and S waves

P waves: how fast do they travel, and what kind of particle motions do they have?

fastest; particals move back and forth in direction of travel (compressional)

S waves: how fast do they travel, and what kind of particle motions do they have?

slower; move perpendicular to direction of travel (shearing)

What are the two types of surface waves?

Love waves and Rayleigh waves

Love Waves: how fast do they travel, and what kind of particle motions do they have?

slower than body waves; particles move side to side

Rayleigh Waves: how fast do they travel, and what kind of particle motions do they have?

slower than body waves; move in a rolling motion

Which waves are least destructive?

P waves

Which waves are the most destructive close to an earthquake?

s waves and surface waves

Which waves are the most destructive far away from an earthquake?

surface waves (love and rayleigh)

What is a seismometer?

the instrument that detects ground motion

What is a seismograph?

the system that records the ground motion (seismometer+recording device)

What is a seismogram?

the actual record or graph of the seismic waves.

On what principle do seismometers function?

rely on the motion of mass that remains stationa

What is the seismoscope? What was its limitation?

an early device that indicated an earthquake occured but didnt record. showed direction but not timing and motion

What is EarthScope USArray?

network of movable seismic stations across the US

What is an earthquake hypocenter (focus)?

The point inside the Earth where the quake starts.

What is an earthquake Epicenter?

The point on the surface directly above where the quake starts.