Interpreting Seismograms

Wave Types

P waves (compression waves)Fastest

S waves (transverse waves)Mid Speed

Surface waves Slowest, may not reach far from epicenter

Review

• Do we feel every earthquake that happens in the world?

• Do mechanical waves generated by earthquakes reach our location?

How do geologists know that a major earthquake occurred on the other side

of the world?

DETECTING EARTHQUAKES

Seismometer

RECORDING EARTHQUAKE EVENTS

Seismograph

Seismogragh reading

Measuring an earthquake at a distance

Seismographs-sensitive machines that record the different types of seismic waves generated by an earthquake

Measuring the difference between the arrival times of different types of waves indicates how far away the epicenter is from the seismograph machine

What information can be gathered from a seismograph reading?

ARRIVAL TIMES FOR DIFFERENT WAVES

Determine distance to epicenter

• The difference in time between arrival of the P and S wave can be used to determine distance between epicenter and seismograph

• The bigger the difference, the greater the distance

Seismograph reading

Why can’t the difference between waves give the distance but not

direction to epicenter?

WHAT IS NEEDED TO DETERMINE THE EPICENTER OF AN EARTHQUAKE?

• Seismograph readings from instruments in 3 different locations

Finding the epicenter

Other uses for Seismic waves

• Used to gather data on the interior structure of the earth

• S-waves do not go through __________ material.

A cross sectional map using info from seismic waves

Locations of different earthquakes over the last 20 years

• Usgs program

• Show large quakes• Show shallow quakes

• Look for patterns

World Earthquakes

Volcanoes and Earthquakes

Ring of Fire

• Location

• Significance?

• Atlantic ocean• Mediteraanen• East Africa