Week 6Geosphere · Term 1

Earthquakes

Faults, waves, magnitude & damage factors

Reading: Earthquakes

Earthquakes

The ground feels completely solid and permanent, but it is not. Stress is building in rocks miles below us right now. When those rocks finally slip, sometimes just a few centimeters, that energy travels through the entire planet in minutes.

How Earthquakes Form

Earthquakes are the sudden release of energy stored in stressed rock along a fault: a fracture in Earth's crust where blocks of rock move relative to each other.

The process works like this:

1. Tectonic plates move, but rocks along faults get stuck and locked.

2. Stress builds up over decades or centuries as the plates continue trying to move.

3. When stress exceeds the strength of the rock, it suddenly slips, releasing enormous energy as seismic waves.

4. This is called elastic rebound: rock bends like a compressed spring, then snaps back when it breaks.

5. Aftershocks: smaller earthquakes, follow as the rock re-adjusts along the fault.

Types of Faults

Fault Type

How Blocks Move

Caused By

Example Location

Normal fault

Hanging wall drops down

Tension (plates pulling apart)

Divergent boundaries, rift valleys

Reverse fault

Hanging wall moves up

Compression (plates colliding)

Convergent boundaries, mountain ranges

Thrust fault

Low-angle reverse, one block rides far over another

Strong compression

Base of mountain ranges

Strike-slip fault

Blocks slide horizontally past each other

Shear stress

Transform boundaries (e.g., San Andreas)

Seismic Waves

An earthquake sends out three types of waves:

P-waves (Primary waves): Fastest. Compress and expand rock in the direction of travel. Travel through both solids AND liquids. Felt as a sudden jolt or thud. Always arrive first at a seismograph.

S-waves (Secondary waves): Slower. Shake rock perpendicular to the direction of travel (side-to-side). Travel through solids only, they cannot pass through liquids. This fact proved Earth's outer core is liquid. Arrive second at a seismograph.

Surface waves (Love and Rayleigh waves): Slowest, but largest amplitude. Travel along Earth's surface. Love waves shake side-to-side; Rayleigh waves roll like ocean waves. These cause the most damage to buildings.

The time gap between P-wave and S-wave arrival at a seismograph tells scientists how far away the earthquake was. Using three stations, scientists can triangulate to find the exact epicenter location.

Key Earthquake Terms

Focus (hypocenter): the point underground where the earthquake actually starts.

Epicenter: the point on Earth's surface directly above the focus. Shallow earthquakes (close to the surface) generally cause more damage.

Measuring Earthquakes

Richter scale: measures the amplitude of seismic waves on a logarithmic scale. Each whole number step = 10 times greater amplitude and about 32 times more energy released. Easy to use but limited for very large earthquakes.

Moment magnitude (Mw): measures the total energy released. More accurate for large earthquakes. This is what scientists use today, though news media still often say "Richter."

Modified Mercalli Intensity (MMI): measures how strongly shaking was felt at a specific location, from I (not felt) to XII (total destruction). Different from magnitude: two cities can feel the same earthquake very differently depending on their distance and local geology.

Key Vocabulary

Fault: A fracture in rock where blocks move relative to each other

Stress: Force applied to rock that can cause deformation or fracture

Focus: Underground point where an earthquake originates

Epicenter: Surface point directly above the focus

Seismograph: Instrument that detects and records seismic waves

Seismogram: The record produced by a seismograph

Aftershock: Smaller earthquake following the main shock

Elastic rebound: The snapping-back of stressed rock that releases seismic energy