Earthquakes: Causes, Measurement, and Effects

• An earthquake is a sudden, violent shaking of Earth's crust, caused by the release of energy from tectonic movement.
• The focus is the underground point of energy release that initiates an earthquake.
• The epicenter is the surface point directly above the earthquake's focus.
• Seismic waves are energy waves that travel through the Earth, originating at the focus and causing the shaking.
• The Moment Magnitude Scale measures the strength of earthquakes, enabling global comparisons.

Earthquake Magnitude and Effects

• Magnitude 2.5 or less: Usually not felt, but recorded by seismographs; approximately 900,000 occur annually.
• Magnitude 2.5 - 5.4: Often felt, may cause minor damage; approximately 30,000 occur annually.
• Magnitude 5.5 - 6.0: Can cause slight damage to buildings; approximately 500 occur annually.
• Magnitude 6.1 - 6.9: May cause significant damage in populated areas; approximately 100 occur annually.
• Magnitude 7.0 - 7.9: Major earthquake, causing serious damage; approximately 20 occur annually.
• Magnitude 8.0 or greater: Great earthquake, capable of destroying communities near the epicenter; occurs about once every 5-10 years.

Earthquake Impacts

• Primary impacts are damages or hazards occurring immediately due to the earthquake.
• Secondary impacts are subsequent damages or hazards occurring hours or days after the earthquake.

Destructive Plate Boundaries

• Plates move towards each other.
• When a denser oceanic plate meets a continental plate, the oceanic plate is forced into the mantle.
• Friction causes the oceanic plate to melt, forming magma.
• Magma rises through gaps in the continental plate, creating volcanoes.
• Friction generates earthquakes.
• An example is the collision between the Eurasian Plate and the Philippine Plate

Constructive Plate Boundaries

• Plates move away from each other slowly.
• Magma rises from the mantle to fill the gap, cools, and forms new crust.
• This process contributes to continental drift.
• An example is the Eurasian Plate and the North American Plate at the Mid-Atlantic Ridge.

Conservative Plate Boundaries

• Plates move sideways past each other, in the same direction but at different speeds.
• Crust is neither created nor destroyed.
• Earthquakes occur when friction is overcome, causing the plates to slip past each other.
• An example is the Pacific Plate moving past the North American Plate at the San Andreas Fault.

Collision Boundaries

• These are special destructive boundaries with colliding plates of the same type.
• Material between the plates buckles and rises, forming fold mountains.
• The Himalayas formed by the collision of the Indo-Australian and Eurasian plates.