Earthquakes and Seismic Waves

Earthquakes

• Earthquakes are vibrations in the Earth's ground caused by the movement of plates along fault lines.
• Most earthquakes occur along plate boundaries.

Faults

• Faults are breaks in the Earth's lithosphere where one block of rock moves toward, away, from, or past another block of rock.
• Strike-slip faults occur at transform plate boundaries; normal faults occur at divergent plate boundaries; and revers faults occur at convergent plate boundaries.

Earthquake Focus and Epicenter

• Seismic waves are energy that travels as vibrations on and inside the Earth.
• The focus is a point inside the Earth where the earthquake first starts.
• The epicenter is the location on the Earth's surface directly above the focus.

Types of Seismic Waves

• Primary Waves (P-waves) travel in a push-pull motion, are the fastest seismic waves, and can travel through solids and liquids.
• Secondary waves (S-waves) are slower than P-waves but faster than surface waves, only travel 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 to the Earth's surface.

Earth's Interior

• Scientists learned that Earth's outer core is liquid because S-waves cannot travel through liquids, but P-waves can travel through both solid and liquid.

Finding the Epicenter Using Triangulation

• Find the difference between the arrival time of the P-wave and the arrival time of the S-wave to find the lag time.
• The epicenter distance is determined using an earthquake distance graph.
• Draw a circle using the correct distance around the station, repeating for two more stations.
• The epicenter is located where the circles intersect.

Measuring Earthquakes

• 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 on a I-XII scale.

Earthquake Risks

• Seismologists use 5 indicators to assess earthquake risk: past earthquakes, probability, population density, geology around a fault, and building design.

Volcanoes

• A volcano is a vent in the Earth's crust through which molten rock flows.
• Volcanoes form at convergent boundaries when plates collide and one plate subducts under another plate, at divergent boundaries where plates separate and magma comes out, and at hot spots unrelated to plate boundaries.

3 Types of Volcanoes

• 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.

Caldera

• A caldera is a large volcanic depression created when the summit of the volcano collapses during a violent eruption.

2 Types of Volcanic Eruptions

• Violent eruptions are characterized by lava with a high viscosity (thick lava) and high gas content.
• Quiet Eruptions are characterized by lava with a low viscosity (thin lava) and a low gas content.

Effects of Volcanic Eruptions

• Lava flows move slowly, can destroy towns, and are rarely deadly.
• Ash fall causes breathing problems, can cool Earth's atmosphere, and can disrupt air traffic.
• Mud flows can cause snow and ice to melt, mixing with mud/ash and causing mudflows.
• Pyroclastic flows can be deadly, are produced from violent eruptions, and throw ash and rock into the air.

Predicting Volcanoes

• Volcanoes can be predicted by ground deformation, increase in earthquakes, volcanic gas increases, and water near the volcano becoming more acidic.

Climate

• Volcanic ash blocks the sun and causes a decrease in global temperatures, and can also cause acid rain.

Learn about earthquakes, their causes related to plate movement along fault lines, and the different types of seismic waves including P-waves and S-waves. Understand the concepts of focus and epicenter in relation to earthquake occurrences.