Earthquakes

Earthquakes

A natural vibration of the ground caused when the stress built up between tectonic plates is suddenly released.

There are three types of stress:◦Compression – caused by two plates being push

together.◦Tension – Pulls plates apart.◦Shear – causes plates to twist.

Types of faults

There are three types of Faults1. Normal fault

caused by tension.

2. Reverse fault caused by compression.

3. Strike-slip fault caused by shearing.

Three Types of Seismic Waves

1. Primary waves (p-waves)◦ Longitudinal waves – these vibrate by the

compression of material in the direction of the wave’s motion. These can travel through the core.

2. Secondary waves (s-waves)◦ Transverse waves (Like in water) – these vibrate by

moving the particles perpendicular to the direction of wave’s motion. These cannot travel through the core.

3. Surface waves◦ Vibrate in all directions.

Measuring Earthquakes

Richter Scale◦Based on size of the largest seismic waves

generated by the Earthquake.◦Each level increase represents an increase of

energy by a factor of 32 and a wave size of 10 times.

Measuring Earthquakes

Moment of Magnitude Scale◦Based size of fault rupture, amount of

movement, and rock characteristics.◦More accurate than Richter scale.

Modified Mercalli Scale◦Measures the amount of damage caused by the

Earthquake.

Strongest Earthquakes of 2010

1. Chile (Feb 27th) Mag. 8.82. Indonesia (April 6th) Mag. 7.73. Vanuatu (May 27th) Mag. 7.74. Indonesia (May 9th) Mag. 7.25. Mexcali Valley, California (April4th) Mag. 7.26. Solomon Islands (Jan 3rd) Mag. 7.17. Haiti (Jan 12th) Mag. 7.08. Ryukyu, Japan (Feb 27th) Mag. 7.0

There have been 7460 earthquakes so far this year. Most are below a magnitude of 4.9.

Chili

7th Strongest in History130 after shocks over the next

week.The focus ground at epicenter

moved 3.0 up and 10 m west. 500 times more powerful than

HaitiCalled a megathrust quake

because it is in a subduction zone

Location of the Earthquake

Focus ◦The location in the Earth’s crust where the

earthquake occurs.◦It can occur anywhere between the surface and

a depth of 700 km. The closer to the surface it occurs, the more powerful the earthquake is.

Epicenter ◦The location on the Earth’s surface above the

focus.

Finding the Epicenter

Epicenters are found using triangulation from three or more seismograph stations.

P-waves travel a lot faster than s-waves. The difference between the arrival time of each type of wave tells you the distance to the epicenter.

The speed of each wave is a constant factor.

Steps for finding the Epicenter

1. Determine the difference in time between the arrival of the s & p waves.

Steps for finding the Epicenter

1. Use the travel-time graph at which the p-curve and s- curve have the same separation.

2. Record this distance.

Steps for finding the Epicenter

1. Repeat the steps #1-3 for two other stations

Steps for finding the Epicenter

1. Use a compass to draw a circle around each station with the radius in step #3.

2. The epicenter is located where all three circles intersect.

Homework

P. 499 #1-4p. 504 #2, 4P. 510 #1-4