Earthquakes and Seismic Waves

• Earthquakes and volcanoes are caused by geological processes.

Earthquakes

• An earthquake is the vibration of the Earth's ground.
• This is due to the movement of plates at fault lines.
• Most earthquakes occur along plate boundaries.
• A fault is a break in Earth's lithosphere.
• It is where one block of rock moves toward, away from, or past another block of rock.
• A strike-slip fault occurs at transform plate boundaries.
• A normal fault occurs at divergent plate boundaries.
• A reverse fault occurs at convergent plate boundaries.
• Seismic waves are energy that travels as vibrations on and in Earth.
• The focus is a point inside Earth where the earthquakes first start.
• The epicenter is the location on Earth's surface directly above the focus.
• P-waves travel in a push-pull motion and are the fastest-moving seismic waves that can travel through solids and liquids.
• S-waves are slower than p-waves, but faster than surface waves and only travel through solids.
• Surface waves move in a rolling motion and are the slowest seismic wave, and cause the most damage to Earth's surface.
• Scientists determined that Earth's outer core is liquid, because the s-wave cannot travel through liquids, but p-waves can travel through both solid and liquids.
• Scientists use triangulation to find an earthquake's epicenter.
• This involves finding the time difference between P and S wave arrival, determine the distance from the epicenter using an earthquake distance graph, and locating where circles intersect on a map.
• 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, and is measured on a scale of I-XII.
• Seismologists use 5 indicators to assess earthquake risk, including 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 at convergent plate boundaries.
• Volcanoes form at divergent boundaries, such as mid-ocean ridges and rift valleys.
• Volcanoes form at hot spots, like Hawaii, that are not associated with plate boundaries.
• The three types of volcanoes are shield volcano, composite volcano, and cinder cone volcano.
• Shield volcanoes are large, shield-shaped volcanoes with gentle slopes and eruptions.
• Composite volcanoes are large, steep-sided volcanoes that result from explosive eruptions.
• Cinder cone volcanoes are small, steep-sided volcanoes that erupt gas-rich, basaltic lavas and have moderately explosive eruptions.
• A caldera is a large volcanic depression created when the summit of the volcano collapses during a violent eruption.
• There are two types of eruptions.
• Violent eruptions occur when lava has high viscosity and gas content.
• Quiet eruptions occur when lava has low viscosity and gas content.
• The effects of volcanic eruptions include lava flows, ash fall, mudflows, and pyroclastic flows.
• Lava flows move slowly, can destroy towns, and are rarely deadly.
• Ash fall can cause breathing problems, cool Earth's atmosphere, and disrupt air traffic.
• Mudflows can cause snow and ice to melt and mix with mud/ash.
• Pyroclastic flows can be deadly and throw gas, ash, and rock into the air.
• Scientists predict volcanoes by observing ground deformation, monitoring increases in earthquakes and volcanic gases, and tracking the acidity of water near the volcano.
• Volcanic eruptions affect climate, because volcanic ash can block the sun and cause a decrease in global temperatures, and cause acid rain.

Learn about earthquakes with this lesson. It covers causes of earthquakes, the different types of faults, and seismic waves. It also differentiates between focus and epicenter.