Earthquakes: Waves, Faults and Plate Boundaries

Earthquakes

• Earthquakes are vibrations in the Earth due to plate movement at fault lines.
• Most earthquakes occur along plate boundaries.
• A fault is a break in Earth's lithosphere where rock blocks move.
• Strike-slip faults occur at transform plate boundaries.
• Normal faults occur at divergent plate boundaries.
• Reverse faults occur at convergent plate boundaries.
• Seismic waves are the energy that travels as vibrations on and in Earth.
• The focus is the point inside Earth where the earthquake first starts.
• The epicenter is the location on Earth's surface directly above the focus.
• Primary waves (P-waves) travel in a push-pull motion and are the fastest seismic waves.
• P-waves can travel through solids and liquids.
• Secondary waves (S-waves) travel only through solids with an up and down motion, and are slower than P-waves.
• Surface waves move in a rolling motion and are the slowest, the most damaging seismic wave on the Earth's surface.
• Scientists discovered that the Earth's outer core is liquid because S-waves cannot travel through liquids, but P-waves can.
• Finding an epicenter involves triangulation, using the difference in arrival times between P-waves and S-waves to determine the distance from the epicenter.
• The Richter Scale measures the amount of ground motion at a given distance.
• The Moment Magnitude Scale measures the total amount of energy released by an earthquake.
• The Modified Mercalli Scale measures the intensity of an earthquake based on the amount of damage, rated from I to XII.
• Indicators to assess earthquake risk include past earthquakes, probability, population density, geology around a fault, and building design.

Volcanoes

• A volcano is a vent in Earth's crust through which molten rock flows.
• Volcanoes form where two plates collide and one subducts under another, at convergent boundaries.
• Volcanoes form also where two plates separate and magma comes out, at divergent boundaries.
• Volcanoes also form at hot spots not associated with plate boundaries, often creating chains of islands like Hawaii.
• Shield volcanoes are large shield-shaped with gentle slopes and gentle eruptions.
• Composite volcanoes are large and steep-sided, resulting from explosive eruptions.
• Cinder cone volcanoes are small and steep-sided and produces moderately explosive eruptions of gas-rich, basaltic lavas
• A caldera is a large volcanic depression created when the summit of the volcano collapses during violent eruption.
• Violent eruptions involve lava with high gas content and high viscosity (thick lava).
• Quiet eruptions involve lava with low gas content and low viscosity (thin lava).
• Lava flows move slowly and can destroy towns but are rarely deadly.
• Ash fall can cause breathing problems, cool Earth's atmosphere, and disrupt air traffic.
• Mudflows can occur when snow and ice melt and mix with mud/ash.
• Pyroclastic flows are deadly and produced from violent eruptions, throwing ash and rock into the air.
• Predicting volcanoes involves observing ground deformation, increases in earthquakes and volcanic gas, and water near the volcano becoming more acidic.
• Volcanic ash can block the sun, decreasing global temperatures, and potentially causing acid rain, therefore volcanic activity can affect climate.

This lesson explains earthquakes, their causes and different types of faults. It describes seismic waves, including primary (P-waves), secondary (S-waves) and surface waves. It also defines focus and epicenter.