Earthquakes How and Where Earthquakes Occur

Is there such thing as “earthquake weather?” Absolutely NOT!!! Geologists believe that there is no connection between weather and earthquakes. They are the result of geologic processes within the earth and can happen in any weather and at any time during the year. Earthquakes originate miles underground. Wind, precipitation, temperature, and barometric pressure changes affect only the surface and shallow subsurface of the Earth. Earthquakes are focused at depths well out of the reach of weather, and the forces that cause earthquakes are much larger than the weather forces. Earthquakes occur in all types of weather, in all climate zones, in all seasons of the year, and at any time of day.

When do earthquakes occur? Earthquakes happen once every 10 seconds on Earth. About 3 million earthquakes happen per year. On average there is one 8 magnitude, 10 7 magnitudes, magnitudes and so on per year. –Most earthquakes are of very low magnitudes

What is an earthquake? They are caused by the release of energy They cause the ground to shake Reasons: –Plates Moving –Faults (cracks in Earth’s surface) –Caverns collapsing –Meteor Impacts –Volcanic Eruptions

In California earthquakes are caused by The San Andreas Fault System –San Andreas + Faults = EARTHQUAKES

The San Andreas is a Strike Slip (Transform) Fault –One piece of Earth slides past another –The fault in the picture on page 214 is a left lateral fault –What kind of fault is the San Andreas? Click here to see the faults in action

San Andreas Fault In Action

Draw California and the San Andreas

Terms used to describe an earthquake Epicenter Focus Fault Seismic Waves Focus Vs. Epicenter

How does energy travel in an earthquake Energy travels in waves from the focus The focus is the part of the fault that moved where energy was released The epicenter is located directly above the focus on the surface

Elastic Rebound Theory The elastic rebound theory is an explanation for how energy is spread during earthquakes. As plates on opposite sides of a fault are subjected to force and shift, they accumulate energy and slowly deform until their internal strength is exceeded. At that time, a sudden movement occurs along the fault, releasing the accumulated energy, and the rocks snap back to their original undeformed shape.

How does energy travel in an earthquake? When P and S waves reach the surface they make surface waves Body WavesSurface Waves Travel Through the Earth P Waves S Waves Raleigh Waves Love Waves

Travel Through the Earth P Waves Primary Waves Reach an area first Can travel through solid, liquid and air Compress and Expand Body Waves

Travel Through the Earth S Waves Shear Waves Reach an area second Can only travel through solid Move at right angles to the direction they are traveling Body Waves

P-Waves Vs. S-Waves P-Waves (Compression) S-Waves (Right angles) Interior Of Earth

Travel Along the Surface

Love Waves Vs. Rayleigh Waves Surface

Locating and Measuring Earthquakes Loma Prieta, Ca 1989 Kobe, Japan 1995

How are earthquakes detected? Earthquakes are detected by seismographs Seismographs record the information on a seismogram

P-Waves and S-Waves Travel At Different Speed!

The difference in arrival times can tell us the distance from the earthquake The amplitude of the s-wave can be compared with the time to determine the magnitude

How do you find the epicenter of an earthquake? You must have at least 3 seismograms from different stations You determine the distance each station in by determining the lag time between the p wave and the s wave –The farther away the station the greater lag You draw circles out at those distances on a map Where the circles intersect is the location of the epicenter

** Three stations does not provide exact data the data from many stations is compiled to determine the exact epicenter

How is the strength of the earthquake determined? Intensity –The effect of the Earthquake on the area –changes with distance and type of ground the location is on –Measured by the Mercalli Scale Magnitude –The actual energy released by the Earthquake –This number does not change with distance –Measured by the Richter Scale

Differences In Intensity Low IntensityHigh Intensity Intensity = The effect of the Earthquake on the area Rayleigh Waves

How does the Richter Scale show magnitude? Every number on the Richter scale shows an increase of 31 times in energy released What is the difference in energy between a 3 and a 4? –31 times What is the difference between a 7 and a 9? – x’s 31 = 961 times more powerful! 31x31

Bigger Faults Make Bigger Earthquakes