Earthquakes Earth Science

What is an Earthquake?  A vibration of the Earth produced by a rapid energy release  The point within the earth where an earthquake starts is called the focus  Energy travels from the focus in all directions- think of a stone being tossed into a pond  Stone sets waves of motion in water just as seismic waves move through earth The point on the surface of the Earth above the focus is called the epicenter

The Epicenter  The epicenter is the location directly above the focus

Earthquakes over a 10 yr. period  Pacific Ring of fire

Faults  Faults are fractures in the Earth where plate movement has occurred  There are three types of forces that create faults.  Compression - squeezes  Tension - stretches  Shear – causes slippage  Each of these three forces can create a different kind of fault.

Earthquake causes  Elastic Rebound hypothesis  Rock is bending causing elastic stress  Eventually the rock will slip and break at the weakest point (the focus)  The vibrations we feel occur as the rock returns to its original shape

Types of faults  Normal Faults- caused by tension forces  Reverse Faults- compression  Strike-Slip Faults- shear forces

Normal Faults  Tension causes rocks to be pulled apart.  Rock above the fault surface moves downward in relation to rock below the fault surface.  Can help to form mountains such as the Sierra Nevada Mountain Range

Reverse Fault  Compression causes rock to break.  Rocks above the fault surface are forced up and over the rocks below the faults.  Can form mountains such as the Rockies.

Strike-Slip Fault  Two of Earth’s plates move past each other due to shear forces.  There is very little vertical movement  Ex: San Andreas Fault

Earthquake Info  Seismic waves – waves that move through the Earth’s crust  Seismic waves travel outward from the focus of an earthquake  Surface Waves- these travel along Earth’s outer layer  Body waves- travel through Earth’s interior  Primary Waves (P) – waves that travel in the same direction the wave is moving  Secondary Waves (S) – move through the Earth causing particles to move at right angles to the direction of the wave.

Wave Facts  Surface waves (S) are the slowest waves, primary (P Waves) are the fastest. P Waves can only travel through solids.  P waves travel 1.7 times faster than S waves  Surface waves cause the most destruction during an earthquake.

Measuring Earthquakes  Seismologists use seismographs to measure earthquakes.  The more energy that an earthquake releases, the greater its magnitude.  One of the largest earthquakes occurred in Haiti very recently and killed over 200,000 people

Finding the Epicenter- Distance  Remember P Waves travel faster than S waves  The greater the distance between the P wave and S wave the farther the source  1) find the time interval between the first P wave and first S wave using the seismogram  2) use the graph to find distance from the seismogram

Finding the Epicenter- Direction  Can use 3 or more seismograms to do this  Create a circle using direction found on graph  Where circles intersect is the exact location of the epicenter

Richter  Earthquakes are ranked on the Richter Scale – measures amount of energy released  1-10  For each increase of 1.0, amplitude of the largest surface wave is 10 times greater  About 32 times as much energy is released

Tsunami  Means “Harbor Wave”  If an earthquake happens underwater, it can create a huge wave.  This wave travels to the surface  After it reaches the surface, it can spread out for thousands of kilometers in every direction  As the wave approaches the shore, it can form a towering crest that can reach 30m high.

Earthquake Safety  Be prepared – know if an area has had earthquakes in the past or if it is near a pate boundary.  Make your home earthquake safe  Take heavy objects down from high shelves  Hot water heaters and gas appliances should be securely in place  During earthquakes, stay away from windows or anything that could fall on you.

Living on a Fault  Seismic Safe structures  Resistant to vibrations  Buildings with bases of alternating rubber and steel can withstand quakes of up to 8.3 on the Richter scale without major damage  Buildings and highways can be made so they will bend but not break  Seismic safe structures are more expensive to build than regular structures.

More Earthquake Damage Anchorage, Alaska 1964 – Magnitude 9.2

Oakland, CA 1989

Northridge, CA 1994 – 62 killed

Former hospital in Mexico City 1985 – Magnitude 8.1 with 7.5 aftershock 18 hours later

Turkey 1999