Earthquakes

Most earthquakes occur at Plate Boundaries

The deepest earthquakes occur at subduction boundaries

The focus of an earthquake is the source, or where it originates.

The epicenter is the point on the Earth’s surface directly above the focus.

Types of Seismic Waves P- Waves (elastic) – push and pull the

ground• Travel through solids,liquids & gases

S-Waves (shear) – move the ground up and down. • Only travel throughsolids

Types of Seismic Waves L-waves (rolling surface waves)-

arrive last; move ground up and down and to the side• Only travel through solids

Seismographs record results on seismograms

The difference in arrival times of p and s waves= lag time• Using lag time we can find the distance to

the epicenter

Recording Earthquakes

Recording Earthquakes The further away the seismograph

station, the greater the lag time.

Recording Earthquakes We need 3 seismograph stations to

find the epicenter of an earthquake. • With one, it could be anywhere on the

circle.• With two, it could be either two areas they

touch.• With three, its where they all meet!

Why plate boundaries?? This is where the crust is under stress

and potential energy is stored.

Magnitude Measure of how much energy is

released.

Richter scale is used to plot, from 1 – 10, how strong an earthquake is.

Each increase in 1 equals 10 times more energy released.

Travel-Time Graphs Used to record the distance

(measured in km) from the recording center to the epicenter of an earthquake