Causes and locations of earthquakes

Earthquakes are sudden releases of energy in the Earth's crust, often linked to the movement of tectonic plates. Earthquakes primarily occur where these plates interact, but they can also happen within plates under specific conditions.

Main locations of earthquakes

Earthquakes happen most frequently at plate boundaries, which are the edges where tectonic plates meet.

There are four main types of plate boundaries:

• Destructive boundaries - Where one plate is forced under another in a process called subduction, building up tension that releases as earthquakes.
• Constructive boundaries - Where plates move apart, often causing less intense but frequent earthquakes.
• Conservative boundaries - Where plates slide past each other horizontally, generating friction and sudden jolts.
• Collision boundaries - Where two continental plates push together, crumpling the crust and triggering earthquakes.

Many earthquakes cluster around the Pacific plate's edges, especially near countries like Indonesia, Japan, and the Philippines, forming part of the 'Ring of Fire'. This area experiences frequent seismic activity due to multiple plate interactions.

Intraplate earthquakes

Although rare, earthquakes can occur within the middle of tectonic plates, away from boundaries. These intraplate earthquakes result from old fault lines – cracks in the crust – adjusting to their natural positions, or new faults forming as plates stretch during movement. An example is the East African Rift Valley, where the African plate is slowly splitting apart.

Causes and locations of tsunamis

Tsunamis are large ocean waves triggered by underwater disturbances, most commonly submarine earthquakes at subduction zones. Subduction zones are areas where one tectonic plate slides beneath another, often at destructive boundaries. These events displace massive volumes of water, generating waves that radiate outwards.

High-risk areas for tsunamis

Tsunamis are most likely along coastlines near subduction zones, particularly around the Pacific Rim. This includes regions like Japan, the Philippines, Indonesia, and the western coast of South America, where frequent earthquakes increase the hazard. The risk is higher when the earthquake's epicentre – the point on the Earth's surface directly above the earthquake's origin – is close to the shore or in deep water, allowing waves to build power before hitting land.

Factors influencing earthquake strength

The strength, or magnitude, of an earthquake depends on physical processes at plate boundaries. When plates move past each other, they create tension along fault lines, which are weak points in the Earth's crust. This tension builds until it releases suddenly, sending out seismic waves – vibrations that carry energy from the point of release.

Key processes during an earthquake

• Focus and epicentre - The focus is the underground point where the movement begins, either as a single spot or along a fault line section. The epicentre lies directly above it on the surface, where shaking is first felt.
• Seismic waves and ground effects - Waves spread from the focus, causing the crust to fracture deep below (crustal fracturing) or buckle and crack on the surface. The crust's elasticity allows it to rebound, creating the shaking sensation.
• Factors affecting magnitude - Two main elements determine how strong an earthquake feels: the type of plate boundary and the depth of the focus.

Influence of plate boundary type on magnitude

• Destructive boundaries - These produce the highest magnitude earthquakes due to massive pressure in subduction zones. When released, the energy is immense.
• Constructive boundaries - Earthquakes here are common and frequent but usually lower in magnitude.

Influence of focus depth on magnitude and damage

The focus can be shallow (near the surface) or deep (more than 300 km deep). Shallow-focus earthquakes can reach the highest magnitudes and cause the most surface damage because their energy reaches the surface quickly. Deep-focus earthquakes, while they can still be powerful, cause less surface damage as waves travel farther and lose energy before reaching the surface.

At destructive boundaries, deep-focus earthquakes occur along the Wadati-Benioff Zone, a sloping area mapping the subducting plate's path. This zone often generates the strongest events. The rate of plate movement does not directly affect magnitude.

Types of seismic waves

Earthquakes generate different seismic waves, each with unique characteristics that influence the damage caused. The severity relates to a wave's amplitude – the size of its disturbance from a straight line. Primary waves have the smallest amplitude, while secondary and love waves have larger ones, increasing their destructive potential.

Primary waves

Primary waves, or P waves, are fast-moving compressional vibrations originating from the focus. They push and pull the ground, creating alternating expansion and compression. These waves travel through solids, liquids, and gases but cause minimal damage due to their low amplitude.

Secondary waves

Secondary waves, or S waves, are slower transverse vibrations that travel through the Earth's interior. They can only pass through solids and cause more damage than P waves by shaking the ground up and down, though they are typically less destructive than love waves.

Love waves

Love waves, or L waves, move horizontally at right angles to P waves but are surface waves that follow S waves. They travel only through solids along the crust's surface from the epicentre. With the highest amplitude, love waves often cause the most damage, such as ground fracturing.

Secondary hazards from earthquakes

Earthquakes trigger secondary hazards – effects resulting from the primary ground shaking. These include soil liquefaction, landslides, and tsunamis, which can amplify overall impacts depending on local geography and conditions.

Soil liquefaction

Soil liquefaction occurs when earthquake vibrations pressurise groundwater in loose, waterlogged soils. This forces water between soil particles, turning the ground into a liquid-like state. The weakened soil leads to subsidence, where buildings sink, or heavy objects like vehicles partially submerge. Areas with saturated soils, such as floodplains or those with small particles like sand, are most vulnerable.

Case study – 2011 Christchurch earthquake:

• The 2011 earthquake in Christchurch, New Zealand, caused widespread liquefaction due to the city's sand-based soils, which have small particles prone to this hazard.
• Underground pipelines broke, roads buckled, and vehicles sank.
• Some areas were declared 'red zones' – unsafe for rebuilding – to prevent future risks.

Landslides

Landslides involve rocks and soil dislodging and sliding downslope due to shaking. This can happen immediately or later if water infiltrates loosened material, adding weight and triggering movement. They are more common on young, steep slopes that have been deforested, reducing stability.

Case study – 2015 Nepal earthquake:

• The 2015 Nepal earthquake, centred near Gorkha, weakened Himalayan slopes, leading to numerous landslides and avalanches.
• Around two dozen climbers died in a Mount Everest avalanche.
• The subsequent monsoon rains from June to September exacerbated slips, with mass movements continuing for months.

Tsunamis

Tsunamis form when submarine earthquakes displace the seafloor, pushing up or down large water volumes (water column displacement). Waves start small and fast in deep ocean but slow and grow taller near coasts due to friction with the rising seabed. They arrive in series, often minutes apart, with greater power if the epicentre is coastal or in deep water.

Waves radiate from the epicentre in all directions, posing little open-sea threat but building height as they approach land. This backing-up effect makes tsunamis devastating on shorelines near subduction zones.