Earthquake Basics and Faults

Earthquake Basics

• An earthquake is the sudden release of strain energy in the Earth's crust, causing shaking that radiates outwards from the source.
• A fault is a fracture or zone of fractures between two blocks of rock, allowing them to move relative to each other.
• Not every movement of tectonic plates causes an earthquake, and not every region with faults will experience earthquakes.

Earthquake Magnitude and Intensity

• Magnitude measures the amount of energy released by an earthquake, expressed as numbers.
• Intensity measures the severity of ground shaking at a location, based on its effects on people, structures, and the landscape; expressed using Roman numerals.
• Moderate earthquake: Magnitude 5.3
• Strong earthquake: Magnitude 6.3

Types of Faults

• Normal fault: Plates pull apart, hanging wall moves down relative the footwall, typically producing shallow, less powerful earthquakes.
• Reverse fault: Plates push together, hanging wall moves up relative to the footwall, often producing stronger earthquakes.
• Strike-slip fault: Plates move horizontally past each other, also capable of producing powerful earthquakes.

Earthquake Focus and Epicenter

• The focus is the point inside the Earth where the earthquake originates.
• The epicenter is the location directly above the focus on the Earth's surface.
• Epicenter is where the first shaking is felt.

Earthquake Effects & Processes

• Stress can deform rocks. Massive earthquakes are linked to high temperatures deep below the Earth's surface. Man-made earthquakes can be caused by explosions, nuclear tests, etc.
• Compression effect: Plates moving toward each other, forming land.
• Shearing effect: Plates sliding past each other. Decompression can deform previously formed land.
• Footwall: Narrow Top, wide bottom
• Hanging wall: Wide top, narrow bottom

Foreshocks, Mainshocks, Aftershocks

• Foreshocks: Smaller earthquakes preceding a major earthquake
• Mainshocks: Main, largest earthquake event
• Aftershocks: Smaller earthquakes that follow a major earthquake

Seismic Waves

• Seismic waves are waves that travel through and on the Earth's surface, causing shaking and vibrations.
• Two main types of seismic waves are body waves and surface waves.

Body Waves

• Travel through the Earth's interior.
• Arrive before surface waves.
• Higher frequency waves.
• Primary (P) waves: Fastest waves, push-pull motion through solids and liquids.
• Secondary (S) waves: Slower than P waves, move through solids only; this property helps determine the Earth's liquid outer core.

Surface Waves

• Travel only through the Earth's surface.
• Arrive after body waves.
• Lower frequency waves.
• More damaging than body waves due to the larger amplitude of ground motion.
• Love waves: Fastest surface waves, side-to-side motion.
• Rayleigh waves: Move the ground up and down and side-to-side in a rolling circular motion.

Seismograms

• A seismogram is a record of ground motion produced by earthquakes, explosions, or other sources, recorded by a seismograph.