Earthquakes Presented By L.V. Sandaru Denuwan Somachandra 189 Locknath Saha 190 Srigunesan Sriragavarajan 193

What are Earthquakes? Earthquakes are the vibrations in the ground that result from movement along breaks in Earth’s lithosphere These breaks are called faults The forces that move tectonic plates also push and pull on rocks along the fault and if these become big enough the blocks of rock on either side of the fault can move horizontally or vertically The greater the force the larger and more disastrous the earthquake

Where do Earthquakes Occur?

Earthquakes and Plate Boundaries Earthquakes result from the build up and release of stress along active plate boundaries Some earthquakes occur more than 100km below Earth’s surface The deepest earthquakes occur at convergent plate boundaries –Here the denser oceanic plate subducts into the mantle –These earthquakes release tremendous amounts of energy

Earthquakes and Plate Boundaries Shallow earthquakes occur along divergent plate boundaries, like mid-ocean ridges Shallow earthquakes also occur along transform boundaries Along continental convergent boundaries, earthquakes of varying depths occur

Rock Deformation When a force such as pressure is applied to rock along plate boundaries, the rock can change shape. This is called rock deformation. Eventually rocks can be deformed so much that they break and move. This is a lot like bending a stick until it breaks

Faults When stress builds in places like a plate boundary, rock can form faults. A fault is a break in Earth’s lithosphere where one block of rock moves toward, away from, or past another When rocks move in any direction along a fault, an earthquake occurs The direction depends on the force applied

Types of Faults

Reverse Fault Forces push two blocks of rock together. The rock above the fault moves up relative to the block of rock below the fault Occurs at convergent boundaries

Normal Fault Forces pull two blocks of rock apart. The rock above the fault moves down relative to the rock below the fault Occurs at divergent plate boundaries

Strike-Slip Two blocks of rock slide horizontally past each other in opposite directions Occurs at transform plate boundaries

Earthquake Focus and Epicenter When rock moves along a fault, they release energy that travels as vibrations on and in Earth called seismic waves These waves originate where rocks first move along the fault, at a location inside Earth called the focus – An earthquakes focus can occur anywhere between Earth’s surface and depths of greater than 600km Earthquakes are often referred to by their epicenter – The epicenter is the location on Earth’s surface directly above the earthquake’s focus

Seismic Waves During an earthquake, a rapid release of energy along a fault produces seismic waves Seismic waves travel outward in all directions through rock –Similar to a stone being dropped in water, seismic waves move outward in circles –Seismic waves transfer energy through the ground and produce the motion that you feel during an earthquake –The energy released is stronger near the epicenter and decrease in energy and intensity as you move outward

Types of Seismic Waves

Primary Waves (P-Waves) Causes rock particles to vibrate in the same direction that waves travel Fastest seismic wave First to be detected and recorded Travels through solids and liquids

Secondary Waves (S-Waves) Causes rock particles to vibrate perpendicular to the direction that waves travel Slower than P-waves, but faster than surface waves Detected and recorded after P-waves Only travels through solids

Surface Waves Cause rock particles to move in a rolling or elliptical motion in the same direction that waves travel Slowest seismic wave Generally causes the most damage

Mapping Earth’s Interior Scientists that study earthquakes are called seismologists They use the properties of seismic waves to map Earth’s interior P-waves and S-waves change speed and direction depending on the material they travel through

Inner and Outer Core Through extensive earthquake studies, seismologists have discovered that S-waves cannot travel through the outer core This discovery proved that Earth’s outer core is liquid unlike the solid inner core By analyzing speed of P-waves traveling through the core, seismologists also discovered that the inner and outer cores are composed of mostly iron and nickel

The Mantle Seismologists also have used seismic waves to model convection currents in the mantle The speeds of seismic waves depend on the temperature, pressure, and chemistry of the rocks that the seismic waves travel through. Seismic waves tend to slow down as they travel through hot material –For example, seismic waves are slower in areas of the mantle beneath mid-ocean ridges or near hotspots –Seismic waves are faster in cool areas of the mantle near subduction zones