MYP Science 7: Comprehensive Notes on Earthquakes and Seismology

Definition of Earthquakes: Earthquakes are the vibrations in the ground that result from movement along breaks in Earth’s lithosphere.
Faults: These are the specific breaks in the lithosphere where movement occurs.
Mechanics of Movement:
- Tectonic forces push and pull rocks along a fault.
- When these forces become large enough, the blocks of rock on either side of the fault move past each other.
- Motion can be vertical (up or down) or horizontal (sideways).
Size Factors: The size of an earthquake is determined by the amount of force applied to the fault.

Distribution:
- Relatively few earthquakes occur in the middle of a continent.
- The majority occur in the oceans and along the edges of continents where tectonic plates meet.
Stress Accumulation: Stress builds up specifically along plate boundaries. Earthquakes are caused by the buildup and subsequent release of this stress along active boundaries.
Convergent Plate Boundaries:
- Associated with the deepest and strongest earthquakes.
- Characterized by subduction, where a denser oceanic plate drops down into the mantle.
- These events release massive amounts of energy.
Divergent and Transform Plate Boundaries:
- Commonly characterized by shallow earthquakes.
Continental Collisions:
- Occur where continents collide, forming large, deformed mountain ranges.
- Earthquakes in these regions occur at varying depths.

Rock Deformation: This is the process where force or pressure applied along plate boundaries causes a body of rock to bend and change shape.
Fault Definition: A fault is a break in Earth's lithosphere where one block of rock moves toward, away from, or past another. An earthquake occurs whenever rocks move in any direction along these faults.
Classification of Faults:
- Strike-slip Fault:
  - Location: Transform plate boundaries.
  - Movement: Two blocks of rock slide horizontally past each other in opposite directions.
- Normal Fault:
  - Location: Divergent plate boundaries.
  - Movement: Forces pull two blocks of rock apart. One block drops down relative to the other.
- Reverse Fault:
  - Location: Convergent plate boundaries.
  - Movement: Forces push two blocks of rock together. One block moves up relative to the other.

Seismic Waves: Energy that travels as vibrations on and in Earth.
Focus (Hypocenter): The location inside Earth where rocks first move along the fault and where seismic waves originate.
Epicenter: The location on Earth’s surface directly above the earthquake’s focus.
Energy Propagation:
- Seismic waves travel outward in all directions through rock, similar to ripples in water.
- Energy is strongest near the epicenter.
- Energy and intensity decrease as waves move away from the epicenter; greater distance results in less ground movement.

Primary Waves (P-waves):
- Description: Cause rock particles to vibrate in the same direction as the waves travel.
- Speed: Fastest seismic waves.
- Detection: The first waves that seismometers detect.
- Medium: Can travel through both solids and liquids.
Secondary Waves (S-waves):
- Description: Cause rock particles to vibrate perpendicular to the direction the waves travel.
- Speed: Slower than P-waves but faster than surface waves.
- Detection: The second waves that seismometers detect.
- Medium: Travel only through solids.
Surface Waves:
- Description: Cause rock particles to move in a rolling or elliptical motion in the same direction as wave travel.
- Speed: Slowest seismic waves.
- Impact: Cause the most damage at Earth’s surface.

Seismologists: Scientists who study earthquakes.
Wave Analysis: Seismologists use the properties of P-waves and S-waves (speed and direction changes through different materials) to map the interior.
The Outer and Inner Core:
- State of Matter: The discovery that S-waves cannot travel through the outer core proved that the outer core is liquid, while the inner core is solid.
- Composition: Speed analysis of P-waves reveals the core is primarily iron and nickel.
The Mantle: Seismic waves are used to model convection currents. Wave speed depends on the temperature, pressure, and chemistry of the rocks.

Seismometer: An instrument that measures and records ground motion and determines the distance seismic waves travel.
Seismogram: A graphical illustration of seismic waves recorded by a seismometer.
Triangulation Method: Requires data from at least three different seismometer locations.
1. Find the Arrival Time Difference: Calculate the "lag time," which is the number of seconds between the arrival of the first P-wave and the first S-wave on a seismogram.
2. Find the Distance: Plot the lag time against distance on a graph to reveal how far the epicenter is from the seismograph station.
3. Plot on a Map: Draw a circle around each of the three stations where the radius equals the calculated distance to the epicenter. The point where all three circles intersect is the epicenter.

Richter Magnitude Scale:
- Uses the amount of ground motion at a given distance to determine magnitude.
- Logarithmic scale: Each increase of one unit on the Richter scale represents $10 \times$ the amount of ground motion.
Modified Mercalli Scale (Intensity):
- I: Not felt except under unusual conditions.
- II: Felt by few people; suspended objects swing.
- III: Most noticeable indoors; strong vibrations.
- IV: Felt by many indoors but few outdoors; dishes rattle.
- V: Felt by nearly everyone; dishes break.
- VI: Felt by all; furniture shifts.
- VII: Everyone runs outdoors; some chimneys break.
- VIII: Chimneys, smokestacks, and walls fall.
- IX: Great damage; buildings shift off foundations.
- X: Most ordinary structures destroyed; landslides occur.
- XI: Few structures remain standing; bridges destroyed.
- XII: Damage is total.