Earthquakes

What Causes Earthquakes? The sudden formation of a new fault Sudden slip on an already existing fault (#1 cause of) Movement of magma in, or explosion of, a volcano Plus more! – giant landslides Meteorite impact Underground nuclear-bomb test!

The place within the Earth where rock ruptures and slips, is the focus of the earthquake. The point on the surface of the Earth that lies directly above the focus is the epicenter.

Seismic Waves Seismologists distinguish different types of seismic waves: 1. Compressional waves (P-waves): waves that cause particles of material to move back and forth parallel to the direction of the wave itself – push and pull

2. Shear waves (S-waves): waves that cause particles of material to move back and forth perpendicular to the direction of the wave itself

Different waves travel at different velocities: P-waves travel the fastest (arriving first) S-waves travel 60% the speed of P-waves Friction absorbs energy as waves pass through a material, so the amount of energy carried by seismic waves decreases the farther they travel

How to measure Earthquakes Researchers use an instrument, called a seismometer, to systemically measure the ground motion from an earthquake They measure the vertical and horizontal ground motion

An earthquake record produced is called a seismogram The horizontal axis represents time and the vertical axis represents the amplitude of the seismic waves.

To determine the location of a epicenter, seismologists calculate the distance between the epicenter and the seismometer station – using the difference in time between the P and S waves (using time vs. distance graphs!! Yay Grade 10 Science!) Vertical axis time – horizontal axis distance

Using triangulation, seismologists take the distance from THREE stations! The epicenter lies at the intersection of the three circles!

Earthquake Magnitude Scales The magnitude of an earthquake is a number that represents the maximum amplitude of ground motion that is measured by a seismometer. The Richter scale (1935!) is based on the maximum amplitude of motion that is recorded at a station about 100 km from the epicenter. These days, seismologists actually use several different scales! (moment magnitude scale) Because the original scale only works well for shallow quakes

All magnitude scales are logarithmic, meaning that an increase of one unit of magnitude represents a tenfold increase in max. amplitude! Example: an 8 magnitude earthquake results in ground motion that is 10 times greater than a 7 magnitude Largest recorded quake: 9.5 Mw In 1960 Chile

Hyperlink to US geological survey! Maps! Latest Earthquakes