EARTHQUAKES:�WHY? AND HOW?

EARTHQUAKES

• Caused by plate tectonic stresses

sudden movement or shaking of the Earth

• Located at plate boundaries
• Resulting in breakage of the Earth’s brittle crust

PLATE TECTONIC STRESSES

• Plate boundaries and faults (= cracks where plate sections are moving in different directions) cause friction as plates move�

• Plates in a fault zone have STICK-SLIP motion
• Periods of no movement (stick) �and fast movement (slip)
• Energy stored as plates stick,
• Energy released as plates slip

EARTHQUAKE DAMAGE

• Landsides �
• Building damage�
• Liquefaction

LIQUEFACTION

• Results in a loss of soil strength & the ability of the soil to support weight

when a solid (sand and soil) becomes saturated with water and acts like a heavy liquid

EARTHQUAKE DAMAGE

Most caused by SURFACE waves (arrive last)

EARTHQUAKE INTENSITY

Modified Mercalli scale= measurement of damage to structures

• From I to XII �(Roman numerals)
• Descriptive, changes with�distance from epicenter
• Can change from location �to location�

What you need:

• Your senses!

measures damage to man-made structures at certain location

ISOSEISMIC MAPS

show the distribution of intensities

EARTHQUAKE WAVES

• FOCUS = place deep within the Earth and along the fault where rupture occurs�
• EPICENTER = geographic point� on surface directly above focus�
• SEISMIC WAVES produced by the release of energy
• move out in circles from the point of rupture (focus)
• 2 types: surface & body (travel inside & through earth’s layers)
• P waves: back and forth movement of rock; travel thru solid, liquid, gas
• S waves: sideways movement of rock; travel thru solids only

EARTHQUAKE WAVES

Seismographs record earthquake waves�

Seismograms show:

• Amplitude of seismic waves (how much rock moves or vibrates)
• Distance to the epicenter
• Earthquake direction

EARTHQUAKE WAVES

• 3 types of seismic waves show up on seismogram
• P waves: shake earth in same direction as wave; �travel thru solid, liquid, gas
• S waves: Shake earth sideways to wave direction; �travel thru solids only
• Surface waves: circular movement of rock; �travel on surface – cause most damage!!

EARTHQUAKE WAVES

P waves move through solids & liquids

S waves move through solids only!!!

EARTHQUAKE WAVES

Primary (1st to arrive)��Longitudinal, Compression

​

​

all states of matter �(solid, liquid, gas)

back and forth movement of rock

• push/pull or compression/stretch out
• Like slinky down stairs�

Vibration is same as the direction of travel

Secondary (2nd to arrive - larger)

�Transverse, Shear

Can go through solids only

Move sideways

• perpendicular to direction of wave travel
• Like snake

EARTHQUAKE WAVES

Lets test your understanding!!

​

Is this a P or an S wave?

​

P wave!

S Wave

EARTHQUAKE MAGNITUDE

measures the size of seismic waves 🡪 �the energy released by the earthquake

Richter scale=measurement of energy released based upon wave amplitude (size of vibration)

• <2 to ~10
• Amplitude of wave goes up �by 10 (Logarithmic scale)�

What you need:

• Amplitude (size of vibration = wave height)
• Time between arrival of 1^st P and 1^st S waves

HOW TO READ SEISMOGRAMS

P & S (body waves) move through earth & arrive first

• P & S waves used to calculate magnitude of earthquake
• Amplitude = height of wave (how much the rock moves; size of vibration)

MERCALLI VS. RICHTER