Anatomy of an Earthquake

Seismic waves and the shadow zone

What are Earthquakes? Sudden release of energy in the

Earth’s crust and creates seismic waves. • Occurs naturally or human induced.

What is the anatomy of an earthquake?

Faulting: most common cause of earthquakes Focus: Point below the surface where rocks

break and move (underground point of origin) Epicenter: Directly above the focus at surface

(most violent shaking occurs here)

What are the 3 types of Seismic Waves?

Three types of seismic waves• 1. Primary (P waves)• 2. Secondary (S waves)• 3. Surface (L waves)

What are Body waves? P waves

• Fastest• Travel through solid, liquid, and gas• Push pull waves• Moves through the core and faster through

dense material S waves

• Arrive after P waves (slower)• Travel only through solids• Particles move side to side• Shake waves

What are Surface waves? L waves

• Slowest• Originate at the surface (epicenter)• Move up and down like ocean waves• Cause the most damage

What is a Shadow Zone? A shadow zone is an area of the earth that is

"shadowed" by the planet's internal structure, in that it doesn't receive seismic waves.

Seismic waves will bend and reflect at the interfaces between different materials• P waves will travel and refract through both fluid and

solid materials. S waves, however, cannot travel through fluids like air or water.

What do seismographs measure?

Seismographs record earthquake waves

Seismograms show:• Amplitude of seismic waves (how

much rock moves or vibrates)

• Distance to the epicenter

• Earthquake direction

How do I read a seismograph?

• P & S waves used to calculate magnitude of earthquake

• Amplitude = height of wave (how much the rock moves; size of vibration)

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

How do we find the epicenter of an earthquake?

Exact location can be known by using three seismic stations or triangulation.

The time interval between the arrival of the first P wave and S wave determines distance to epicenter. • The longer the “lag” the farther away the epicenter is.

What is the difference between an intensity scale and

magnitude scale? Intensity Scales: measure the degree of

shaking and amount of DAMAGE done• Modified Mercalli

Magnitude scale: relies on calculations of estimated energy released by seismic waves. • Richter Scale• Moment Magnitude scale

Increases by a factor of 10

Mercalli vs Richter