Earthquakes Most earthquakes occur at Plate Boundaries The
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.
�Seismographs are used to record the various types of seismic waves that are generated during an eathquake. �(see picture above right in packet to see how one looks/works)
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 through solids
Types of Seismic Waves �L-waves (Rayleigh/Love waves)- rolling surface waves; arrive last • Only travel through solids
Recording Earthquakes �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 �The further away the seismograph station, the greater the lag time.
Discussion/Answer in packet �Which city is located farthest from the epicenter? (use picture at left in packet) �What is the difference between arrival times of the first P and S waves in the seismogram above (in the packet)?
Travel-Time Graphs �Used to record the distance (measured in km) from the recording center to the epicenter of an earthquake
�The difference in arrival times of p and s waves= lag time
Discussion/Answer in Packet �When the first P-wave arrives 4 minutes before the first S-wave, the earthquake occurred _____ km from the recording station.
Locating an Epicenter �Using seismograms from 3 record stations and a travel-time graph, we can establish the location of the earthquake ____________.
Discussion/answer in packet �How far from the epicenter is each of these three cities?
Discuss/Answer in packets �Why do seismologists need to plot data from three recording centers?
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.
- Slides: 20