Earthquakes:

How to find where they originate…

Terminology:

• Epicenter vs. Focus

• Focus is the point beneath Earth's surface where rock breaks under stress (or plates shift) and causes an earthquake.

• Epicenter is the point on land, directly above the focus

Graph vs Gram

• Seismograph are instruments that measure and record motions of the ground, including seismic waves generated by earthquakes, nuclear explosions, and other seismic sources

• Seismogram is a graph output by a seismograph. It is a record of the ground motion at a measuring station as a function of time.

• How Seismograph Works

S waves vs P waves• P waves

– Primary Waves– Named that because they leave the focus first – Travel faster than S waves– Compression Waves

• S waves– Secondary Waves– Named that because they leave the focus second– Travel slower than P waves– Transverse Waves

• S wave P wave Animation• (slinky)

How do we locate earthquakes?

• Seismograph data (seismograms) will tell us distance from an earthquake epicenter

• We need 3 seismograph readings to find an epicenter

1400 km from Denver, 2600 km from Houston, 4000 km from Miami

How do we determine distance?

• Lag time between P wave and S wave tells us distance

• Fast Runners and Slow Runners...

How to read a Seismogram

Use lag time to figure out distance traveled

Try the activity:

• http://www.sciencecourseware.com/VirtualEarthquake/VQuakeExecute.html