EARTHQUAKES Click on picture for video Earthquakes are

EARTHQUAKES Click on picture for video

Earthquakes are the shaking, rolling or sudden shock of the earth’s surface. Earthquakes happen along "fault lines" in the earth’s crust. Earthquakes can be felt over large areas although they usually last less than one minute. Earthquakes cannot be predicted -- although scientists are working on it!

Most of the time, you will notice an earthquake by the gentle shaking of the ground. You may notice hanging plants swaying or objects wobbling on shelves. Sometimes you may hear a low rumbling noise or feel a sharp jolt. A survivor of the 1906 earthquake in San Francisco said the sensation was like riding a bicycle down a long flight of stairs.

RICHTER SCALE Small: 5. 0 to 5. 9 Moderate: 6. 0 to 6. 9 Major: 7. 0 to 7. 9 Great: 8. 0 or greater

SAN FRANCISCO EARTHQUAKE April 18, 1906 3000 deaths 28, 000 buildings destroyed (most by fire) $10 B damage “The whole street was undulating as if the waves of the ocean were coming toward me. ” “I saw the whole city enveloped in a pile of dust caused by falling buildings. ” “Inside of twelve hours half the heart of the city was gone”

Convergent Boundaries What happens when plates collide? It depends how the plates are moving when they meet: When two plates collide head-on, they push each other up and form mountains. That's how the Himalayas and other great mountain ranges (including the Rockies, long ago) were created.

Divergent Boundary Another boundary is the Divergent boundary: this is where the plates separate and move apart. They often form a rift zone. Most are located on the oceanic floor where new seafloor is created at the separating edges. One example is the Mid-Atlantic Ridge.

Transform Boundary When two plates slide past each other, they create a transform fault, like the San Andreas fault.

ELASTIC REBOUND

ELASTIC REBOUND Over many years, rocks on opposite sides of the fault move, but friction on fault "locks" it and prevents slip Eventually strain accumulated overcomes friction, and fault slips in earthquake

WHAT IS THE ELASTIC REBOUND THEORY? Explains how energy is stored in rocks Rocks bend until the strength of the rock is exceeded Rupture occurs and the rocks quickly rebound to an undeformed shape Energy is released in waves that radiate outward from the fault

STRESS AND STRAIN: ROCK BEHAVIOR Strain - the result of stress or deformation elastic deformation - when stresses are removed, rock returns to original shape plastic deformation - permanent deformation. when stresses are removed, rock stays bent rupture - breakage and fracturing of the rock, causing an earthquake. Brittle materials break during elastic deformation.

The Focus and Epicenter of an Earthquake • The point within Earth where faulting begins is the focus, or hypocenter • The point directly above the focus on the surface is the epicenter

WHERE DO EARTHQUAKES OCCUR AND HOW OFTEN? ~80% of all earthquakes occur in the circum-Pacific belt most of these result from convergent margin activity ~15% occur in the Mediterranean-Asiatic belt remaining 5% occur in the interiors of plates and on spreading ridge centers more than 150, 000 quakes strong enough to be felt are recorded each year

THE ECONOMICS AND SOCIETAL IMPACTS OF EQS • • Building collapse Fire Tsunami Ground failure Damage in Oakland, CA, 1989

WHAT ARE SEISMIC WAVES? Seismic Waves : Response of material to the arrival of energy fronts released by rupture Two types: Body waves P and S Surface waves R and L

BODY WAVES: P AND S WAVES Body waves P or primary waves fastest waves travel through solids, liquids, or gases compressional wave, material movement is in the same direction as wave movement S or secondary waves slower than P waves travel through solids only shear waves - move material perpendicular to wave movement

SURFACE WAVES: R AND L WAVES Surface Waves Travel just below or along the ground’s surface Slower than body waves; rolling and side-to-side movement Especially damaging to buildings

Click on picture for video This is an image of a seismograph, an instrument used to record the energy released by an earthquake. When the needle is moved by the motion of the earth, it leaves a wavy line.

HOW IS AN EARTHQUAKE’S EPICENTER LOCATED? Three seismograph stations are needed to locate the epicenter of an earthquake A circle where the radius equals the distance to the epicenter is drawn The intersection of the circles locates the epicenter