Earthquakes: Definition, Faults, and Seismic Waves

• Earthquakes and volcanoes are caused by specific geological activities.

Earthquakes Definition and Location

• An earthquake is a vibration in the Earth due to the movement of plates at fault lines.
• Most earthquakes occur along plate boundaries.
• A fault references a break in Earth’s lithosphere where a block of rock moves.
• A strike-slip fault occurs at transform plate boundaries.
• Normal faults occur at divergent plate boundaries.
• Reverse faults occur at convergent plate boundaries.
• Seismic waves are energy that travels as vibrations on and in Earth.
• The focus is the point inside Earth where an earthquake starts.
• The epicenter is the location on Earth’s surface directly above the focus.
• Primary waves (P-waves) are the fastest moving seismic waves, which travel in a push-pull motion and through solids and liquids.
• Secondary waves (S-waves) are slower than P-waves, travel only through solids, and move particles in an up-and-down motion.
• Surface waves move in a rolling motion, are the slowest seismic waves, and cause the most damage.
• Scientists discovered that Earth’s outer core is liquid because S-waves cannot travel through liquids, but P-waves can.
• The difference (lag time) between the arrival time of P-waves and S-waves is used to determine the distance from the epicenter.

Measurement and Risks

• 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 the earthquake based on the amount of damage.
• Seismologists use past earthquakes, probability, population density, geology around a fault, and building designs, to assess earthquake risk.

Volcanoes Definition and Formation

• A volcano is a vent in the Earth's crust molten rock flows through.
• Volcanoes form where two plates collide (convergent boundaries) and one subducts under the other causing volcanoes to form.
• Volcanoes occur where two plates separate and magma rises (divergent boundaries) such as mid-ocean ridges and rift valleys.
• Volcanoes that are not associated with plate boundaries often form chains of islands like Hawaii (hot spots).
• Shield volcanoes are large, shield-shaped volcanoes with gentle slopes and gentle 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 collapsed during a violent eruption.

Eruptions

• Violent eruptions consist of lava with a high viscosity and high gas.
• Quiet eruptions consist of lava with a low viscosity and gas.
• Lava flows move slowly, can destroy towns, and are rarely deadly.
• Ash fall can cause breathing problems, can cool the Earth's atmosphere, and can disrupt air traffic.
• Mudflows can happen when snow and ice melts which results in mixes with mud/ash.
• Pyroclastic flows are from violent eruptions, and throw gas, ash, and rock into the air and can be deadly.
• Signs of ground deformation, increase in earthquakes, increased volcanic gas, and acidic water near the volcano can indicate a volcano eruption.
• Volcanic ash blocks the sun, causes a decrease in global temperatures, and can cause acid rain.

Explore the causes and locations of earthquakes, focusing on their relationship to plate tectonics and fault lines. Understand the different types of faults—strike-slip, normal, and reverse—and their association with plate boundaries. Learn about seismic waves, including P-waves and S-waves, and their properties.