IB Geography D.1.3 Characteristics of Earthquakes and Secondary Hazards Notes

Earthquakes Release Strain Stored Around Faults

1. Elastic rebound describes how rocks bend gradually as stress accumulates and then spring toward a less strained shape when the fault ruptures.

1. Rupture propagation spreads from the focus across part of a fault plane, so rupture length and slip help determine earthquake magnitude.
2. Shaking duration increases when a large rupture continues for longer, raising the chance of structural fatigue and slope failure.
3. Plate motion loads a fault with stress while friction temporarily prevents the rocks from slipping.
4. When stress exceeds friction and rock strength, sudden fault movement releases stored elastic strain as seismic waves.
5. Aftershocks occur as the crust adjusts to the new stress pattern around the ruptured fault.
6. Focus or hypocentre means the point within Earth where rupture begins.

1. Epicentre means the point on Earth's surface directly above the focus.
2. Magnitude measures energy released at the source, while intensity describes shaking and damage at a particular place.

Focal Depth Changes the Pattern of Shaking

1. Attenuation reduces wave amplitude as seismic energy spreads and is absorbed while travelling away from the source.
2. Deep-source reach can make an earthquake perceptible across a broad area even though greater source distance usually reduces damaging high-frequency shaking at the surface.
3. Depth comparison is meaningful only when magnitude, distance, rupture direction and ground conditions are also considered.
4. Shallow-focus earthquakes begin less than about 70 kilometres deep and occur at every major plate-margin type.
5. A shallow source places the rupture close to the surface, but damage still depends on magnitude, distance, duration and local conditions.
6. Intermediate-focus earthquakes begin about 70 to 300 kilometres deep within descending slabs.
7. Deep-focus earthquakes begin about 300 to 700 kilometres deep and are restricted to subduction zones.
8. Deep events may be felt across a wide area because their waves travel long distances through the mantle, but surface shaking near the epicentre is often weaker than for an equivalent shallow event.

P, S and Surface Waves Move Energy Differently

1. Wave conversion occurs where seismic waves meet boundaries between rock layers, creating reflected and refracted arrivals on a seismogram.
2. Site resonance can amplify shaking where soft sediment oscillates at frequencies close to those of buildings above it.
3. Early warning uses the faster arrival of P waves to provide seconds of notice before stronger S and surface waves reach places farther from the focus.
4. P waves are compressional body waves that arrive first and can travel through solids and liquids.
5. S waves are shear body waves that arrive after P waves and travel only through solids.
6. Surface waves travel along Earth's surface and can produce large horizontal or rolling motions with long durations.
7. The time gap between P-wave and S-wave arrivals increases with distance from the earthquake.
8. Seismologists combine arrival times from several stations to locate the epicentre and use wave records to estimate magnitude and depth.

Plate Margins Produce Distinct Earthquake Patterns

1. Megathrust earthquakes occur where a large locked section of a subduction interface ruptures and displaces the overriding plate over a wide area.
2. Transform segmentation creates uneven risk because bends and stepovers can lock some fault sections while adjacent sections creep or rupture separately.
3. Intraplate rarity can increase consequences where long recurrence intervals reduce public memory and building standards are not designed for strong shaking.

1. Divergent margins usually produce frequent shallow earthquakes as crust stretches and breaks along normal faults.
2. Transform margins produce shallow earthquakes when locked plates suddenly overcome friction and slip sideways.
3. Subduction margins produce earthquakes from shallow to deep focus along the descending slab and include the largest recorded events.
4. Collision zones produce shallow and intermediate earthquakes as continental crust shortens, thickens and faults.
5. Intraplate earthquakes occur when stress reactivates weaknesses within a plate, so the global pattern is concentrated at margins rather than confined to them.
6. Human activities can also trigger earthquakes through reservoir loading, mining, fluid withdrawal or fluid injection.
7. Injected fluid may raise pore pressure on a fault and reduce the effective friction that resists slip.

Local Conditions Convert Shaking into Secondary Hazards

1. Tsunami shoaling raises wave height as a tsunami enters shallow water and slows, concentrating energy into destructive run-up and currents.
2. Liquefaction susceptibility is highest in loose, young and water-saturated sediment such as reclaimed land, river deposits and coastal plains.
3. Landslide dams can create delayed flooding when earthquake-triggered debris blocks a river and the temporary lake later overtops or breaches.

1. Tsunami can form when sudden vertical displacement of the seabed moves a large volume of water.
2. Most underwater earthquakes do not generate damaging tsunamis because the rupture must displace the seabed enough and in the right direction.
3. Liquefaction occurs when shaking raises pore-water pressure and loose saturated sediment temporarily loses strength.
4. Liquefaction can tilt buildings, rupture pipes and spread the ground laterally even away from the surface fault.
5. Earthquake shaking can trigger landslides where steep slopes contain weak, weathered or water-saturated material.
6. Surface rupture and transverse faulting can offset roads, railways, rivers and pipelines where fault movement reaches the ground surface.

The 2004 Indian Ocean earthquake became a transboundary disaster because seabed displacement transferred energy across the ocean as a tsunami.

Hazard Becomes Disaster Through Exposure and Vulnerability

1. Lifeline failure can cascade when damaged electricity, water, transport and communications systems delay rescue and interrupt hospitals or businesses.
2. Differential vulnerability means renters, informal settlers, older people and households without savings may experience longer displacement from the same event.
3. Recovery capacity depends on governance, finance, insurance and access because reconstruction must restore services as well as replace damaged buildings.
4. Ground motion is the physical event, while disaster losses depend on who and what occupy the affected area.
5. Soft sediment can amplify shaking, while steep slopes and saturated coastal deposits increase secondary-hazard potential.
6. Building quality, emergency access, governance and poverty affect whether similar shaking produces limited damage or mass casualties.
7. The 2010 Haiti earthquake combined shallow rupture near a dense urban area with highly vulnerable buildings and limited response capacity.

Collapsed buildings in Haiti show that seismic energy becomes a human disaster through vulnerable construction and concentrated exposure.