Earthquakes: Tectonic Plates and Seismic Waves

• Earthquakes involve a sudden, violent shaking of the ground due to movement within the Earth's crust.
• This movement stems from energy release, often near a fault or crack where movement occurs.
• Mechanical waves propagate this energy through rock layers, radiating outward.
• Seismographs, delicate instruments, measure and record the motion of these seismic waves.

Earthquake Location

• Most earthquakes occur on faults, specifically those along tectonic plate boundaries.
• Tectonic plates comprise the Earth's crust and uppermost mantle (lithosphere).
• Convection within the mantle primarily drives their movement and interaction.
• Earthquakes can happen anywhere on the Earth's surface, including underwater.
• Underwater earthquakes can cause tsunamis, large water waves, which can cause surges of water inland.
• Some earthquakes occur on intraplate faults, located within tectonic plates, like the New Madrid Fault.

Earthquake Components

• Focus: The origin point of the earthquake, defined by geographic location and depth, marking the start of the event.
• Seismic Waves: Mechanical energy propagating through Earth's layers, categorized into Body (primary/secondary) and Surface (Rayleigh/Love) waves.
• Fault: A line where rock sections can move against each other, releasing mechanical energy through slip.

Focus vs Epicenter

• Epicenter: It is the surface location directly above the focus.
• Focus: The location of the earthquake at depth.
• Hypocenter: A synonym for the focus.

Seismic Waves

• Primary (P-) waves: They are compressional/longitudinal waves, the fastest, arriving first at seismographs.
• Secondary (S-) waves: They are transverse waves, arriving second at seismographs, and are body waves.
• Rayleigh waves: Longitudinal waves causing elliptical shaking, diminishing with depth, and are surface waves.
• Love Waves: Transverse waves causing shear shaking, and are surface waves.

Fault Types

• Thrust or Reverse Faults: Vertical slip with one block moving upwards relative to the other.
• Normal Faults: Experience vertical slip with one block moving downwards relative to the other.
• Strike-slip Faults: Horizontal movement where blocks slide past each other.

Earthquake Causes

• Caused by tectonic plates moving; pressure builds near the surface.
• Rock breakage releases stress, converting it into mechanical energy that propagates as seismic waves.
• Compressional stress: Objects colliding (reverse fault).
• Tensional Stress: Rock pulling apart (normal fault).
• Shear Stress: Objects sliding past each other (strike-slip fault).

Earthquake Types

• Tectonic: Occur due to sliding along a fault or tectonic boundary.
• Volcanic: Triggered by nearby volcanic activity.
• Collapse: Small earthquakes in caves or mines caused by surface explosions.
• Explosion: Result from nuclear or chemical bomb detonations.

Earthquake Stages

• Elastic Build-up: Elastic strain accumulates as faults move, compressing rocks over time.
• Dilatancy: Rocks crack to create space, expelling water and intaking air, increasing elastic strain capacity.
• Influx of Water: Water re-enters cracks, weakening the rock and preventing further cracking.
• Earthquake: The fault ruptures, releasing stored energy and generating seismic waves.
• Aftershocks: Smaller ruptures occur due to stress reduction, releasing remaining elastic strain unpredictably.
• Foreshock: Small release of elastic strain before the main earthquake, which isn't always present.

Earthquakes are sudden ground shaking events caused by energy release in the Earth's crust, often along faults. Seismic waves propagate this energy, measured by seismographs. Most earthquakes occur at tectonic plate boundaries, driven by mantle convection.