Chapter 13 - Earthquakes and Earth's Interior

Causes of tremors

sudden release of energy (seismic waves) at faults

Elastic rebound theory

a sudden release of strain progressively stored in rocks that bend until they finally break and move along a fault

Epicenter

point on Earth’s surface directly above the focus

Focus

(or hypocenter) the point of initial breakage and movement along a fault, where seismic waves originate

body waves

travel outward from the focus in all directions through Earth’s interior

surface waves

travel along Earth’s surface away from the epicenter

body wave types

P waves, S waves

P wave speed

(4-7km/s), arrive 1st at recording station

what materials do P waves pass through

solids and fluids

P wave motion

compressional (longitudinal), vibrates back and forth parallel to the direction of wave propagation

S wave speed

(2-5km/s), arrive 2nd at recording station

S wave motion

shearing (transverse), perpendicular to the direction of wave propagation

surface wave types

Love waves, Rayleigh waves

Love wave speed

3rd to reach recording station

Rayleigh wave speed

slowest, 4th to reach recording station

What category of waves do actual structural damage?

Surface waves

Love waves motion

side-to-side motion of the ground surface

Rayleigh waves motion

ground moves in an elliptical path opposite the direction of wave motion; most destructive to buildings

Seismometer

used to measure seismic waves

Seismogram

permanent paper (or digital) records of the earthquake vibrations, used to measure the earthquake strengths

Travel-time curve

used to determine distance to the focus point. based on time between first P-wave and S-wave arrivals

Location of epicenter

Plotting distances from 3 stations on a map

Depth of focus

Distance beneath earth’s surface at which the focus lies

MMS (Modified Mercalli Scale)

a measure of the effects (I-XII) an earthquake produces (on both structures and people)

amplification of shaking in rocks ranked from low to high

Hard igneous rock, Sedimentary rock, Alluvium, Silt, Mud

Richter Scale

(1 - 10) the amount of energy released by an earthquake by measuring the amplitude of seismic waves on a seismogram

Magnitude/earthquake frequency relationship

as magnitude/severity goes up, frequency/amount of earthquakes goes down.

Magnitude rating power

each increase by 1 is 10 times more energy

How much more energy does a 6 quake have than a 3 quake?

1000x

How can earthquakes happen away from plate boundaries

Magma movement pushing up through the earth

New Madrid, MO

High seismic risk area in the middle of the US, 250ma

Where do almost all intermediate to deep focus earthquake occur?

Benioff zones

Common place for shallow focus earthquakes

along the crests of mid-oceanic ridges

Benioff zone

planar earthquake zone within a subduction zone.

Which depth of focus causes the most damage?

Shallow. Seismic waves are stronger the closer they are to the focus point

Earthquake hazards

ground motion, fires, mass wasting, liquefaction, fractures, scarps, aftershocks, tsunamis

liquefaction

solid ground acts like liquid (when looser sediments is more saturated).

Cause of aftershock

little snags in the fault line that release after main quake. Can occur at any time up to years after main shock

requirements for tsunamis

minimum magnitude of 7.5 and a subduction zone. Cont-Ocean or Ocean-Ocean.

warning sign of a tsunami

retreating water on shore

Intraplate earthquakes

Rare earthquakes that occur in the middle of plates, can occur from ancient faults that have been buried over time.

Earthquake Precursors and Prediction

(NOT DEPENDABLE)
Microseisms – small tremors caused by small breaks in the rocks

Water level in wells

Animal behavior

Foreshocks

Long term prediction example

earthquake will happen in the next 25 years (much more accurate)

Blind fault

Faults that do not reach the surface

how do we know about the deep earth?

Using seismic waves to create an image (like an X-RAY)

Geophysics

Branch of Geology that studies Earth’s interior using concepts of physics.

Seismic reflection

the return of some waves to the surface after bouncing off a rock layer boundary (based on density)

Seismic refraction

bending of seismic waves as they pass from one material to another having different seismic wave velocities

How do we know the earth’s core is liquid?

S-wave shadow zone

Magnetic field origin

a liquid outer core, as the movement of molten iron within it generates electrical currents which create the magnetic field.