Plate Tectonics and Seismic Activity

Types of Seismic Waves

Vibration caused by plate slippage along fault blocks, transmitted from the origin.

• P/Primary waves: Longitudinal, quick (~5 km/s), transmitted through solids, liquids, and gases.
• S/Secondary waves: Transverse, slower, do not pass through liquids.
• L/Surface waves: Slower than P and S waves, cause most surface destruction.
  • Rayleigh waves: Elliptical motion.
  • Love waves: Horizontal motion, nearly perpendicular to the direction of travel.

Earthquake Distribution at Plate Boundaries

Earthquake activity varies depending on the type of plate boundary:

• Oceanic ridges: Zones of lithosphere expansion, moderately active surface earthquakes.
• Subduction zones: Zones of lithosphere destruction, strong seismic activity, where one plate bends and plunges beneath another.
• Slippage between plates: Frequent surface earthquakes along transform faults.
• Alpine ranges: Areas of collision between two continental plates, moderately active.
• Intraplate areas: Less frequent, often related to movement along former faults.

Earthquake Intensity

Effects observed on the ground. Determined using the Mercalli scale.

Earthquake Magnitude

Measures the amplitude of seismic waves recorded at 100 km from the epicenter. Determined using the Richter scale.

Types of Plate Margins

Plate tectonics involves three main types of plate margins:

• Constructive (Divergent): Areas of tension where plates separate due to ascending and divergent convection currents. Correspond to oceanic ridges. Moderately active.
• Destructive (Convergent): Limits where plates collide under compression, causing intense seismic and volcanic activity.
  • Oceanic-Oceanic: One oceanic plate subducts under another, forming trenches, earthquakes, and volcanoes.
  • Oceanic-Continental: Oceanic plate subducts under continental plate, forming trenches, earthquakes, and volcanoes.
  • Continental-Continental: Collision forms mountain ranges (like the Alps) when both continental sides are consumed; little subduction occurs.
• Transform (Conservative): Slippage occurs between plates along transform faults. Neither creates nor destroys lithosphere. Intense seismic activity (e.g., San Andreas Fault, California).