Earthquakes

Summary

An earthquake is the sudden release of elastic strain energy along a fault, producing seismic waves.

Core concept

Understand the system

An earthquake is the sudden release of elastic strain energy along a fault, producing seismic waves.

Why / how it happens

Causes and context

Earthquakes occur when tectonic stress overcomes frictional resistance on a fault. Elastic strain accumulates during locked motion, rupture releases that strain suddenly, and the resulting seismic waves transmit energy through the crust and across the surface.

Mechanism / process

First, stress deforms rocks around a locked or slipping fault. When resistance is exceeded, rupture starts at the focus and propagates. P, S and surface waves then travel outward; shaking, rupture, liquefaction and secondary hazards vary with distance and ground conditions.

Key facts

• Focus is the underground origin; epicentre is its surface projection.
• P-waves are fastest; S-waves cannot pass through liquids.
• Magnitude measures released energy, whereas intensity describes observed effects at a place.

Important terminology

• focus
• epicentre
• fault
• magnitude
• intensity

Named examples / case studies

• The 2015 Nepal earthquake illustrates how Himalayan convergence creates seismic risk.

Causes / components

• Elastic strain accumulates as fault-block motion is temporarily locked by friction.
• Rupture begins when shear stress exceeds fault resistance.
• Tectonic loading, volcanic activity, collapse or human activity can trigger seismic events.

Effects / significance

• Seismic shaking damages buildings, roads, bridges and lifelines.
• Surface rupture, liquefaction, landslides and fires create secondary losses.
• Undersea displacement can generate a tsunami.

Geography connection

The 2015 Nepal earthquake illustrates how Himalayan convergence creates seismic risk. This illustrates how earthquakes varies by location, scale or environmental context.

Bangladesh connection

Bangladesh faces earthquake risk from the Dauki Fault, Madhupur Fault and active tectonic systems toward the Indo-Burma ranges.

BCS preliminary facts

• Focus is the underground origin; epicentre is its surface projection.
• P-waves are fastest; S-waves cannot pass through liquids.
• Magnitude measures released energy, whereas intensity describes observed effects at a place.

BCS written analysis

Build an analytical answer

• Explain elastic rebound from stress accumulation to fault rupture and wave propagation.
• Distinguish magnitude as source energy from intensity as observed effect at a site.
• Discuss Bangladesh preparedness through seismic codes, land-use planning, public awareness and resilient lifelines.

Common misconceptions

• Magnitude and intensity are not interchangeable.
• The epicentre is not the underground point where rupture begins.

Quick revision

• Earthquakes
• Focus is the underground origin; epicentre is its surface projection.
• P-waves are fastest; S-waves cannot pass through liquids.
• focus / epicentre
• Definition → process → spatial example