Earthquakes are sudden, violent shakings of the ground that result from the rapid release of energy within the Earth’s crust. This energy propagates outward in the form of seismic waves. They are natural phenomena primarily caused by the movement of tectonic plates. While ground vibration is the most recognized aspect, earthquakes can trigger various other distinct and dangerous hazards that extend beyond direct shaking, contributing significantly to the overall impact and destruction.
Ground Shaking
Ground shaking is the most immediate and widespread hazard during an earthquake. It is the vibration of the ground surface caused by seismic waves radiating from the earthquake’s source. These include primary (P-waves) and secondary (S-waves) body waves, which travel through the Earth’s interior, and surface waves, which cause most of the shaking experienced on the ground.
The intensity of ground shaking at a particular location depends on several factors, including the earthquake’s magnitude, the distance from the epicenter, and the local geological conditions. For instance, soft, unconsolidated sediments can amplify ground motion, leading to more severe shaking than in areas with solid bedrock. The duration of shaking also plays a role, as longer periods of vibration can cause greater fatigue and damage to structures.
The primary consequence of intense ground shaking is widespread damage or collapse of buildings, bridges, and other infrastructure. This structural failure can lead to numerous injuries and fatalities from falling debris and crushing. Even in well-constructed areas, severe shaking can disrupt essential services by damaging roads, utility lines, and communication networks, complicating rescue and relief efforts.
Liquefaction
Liquefaction occurs when saturated, loose, granular soils temporarily lose strength and stiffness, behaving like a liquid. This phenomenon affects sandy or silty soils that are water-saturated and lack cohesive properties. Strong ground shaking increases water pressure within the soil, reducing contact between individual soil particles.
When the inter-particle contact is lost, the soil can no longer support structures built upon it, leading to significant ground deformation. Visible effects of liquefaction include buildings tilting or sinking into the softened ground, even if their structural integrity remains intact. Buried objects, such as underground storage tanks, can float to the surface due to buoyancy in the liquefied soil.
Another common manifestation is the appearance of “sand boils” or “sand volcanoes” on the ground surface. These occur when pressurized water and sand erupt from below, forming small cone-shaped deposits. This hazard is common in coastal areas, river floodplains, or reclaimed land where susceptible soil conditions are prevalent.
Landslides
Landslides are the mass movement of rock, debris, or earth down a slope, often triggered by intense ground shaking. Earthquakes can destabilize slopes prone to failure, leading to various landslide types. These range from rapid rockfalls and debris flows to slower-moving slumps, which involve rotational movement of a coherent mass of earth.
Earthquake-induced landslides have considerable destructive potential. Large volumes of moving earth can bury homes, infrastructure, and entire communities, causing widespread destruction and loss of life. Landslides can also block transportation routes, isolating affected areas and hindering emergency response efforts.
Beyond immediate destruction, landslides can alter the landscape, creating new hazards or changing drainage patterns. This hazard involves the large-scale displacement of geological material, distinct from ground vibration. The stability of slopes is compromised, leading to secondary events.
Tsunamis
Tsunamis are powerful ocean waves caused by large-scale displacement of the seafloor, often due to vertical movement along a fault during an undersea earthquake. Such earthquakes occur in subduction zones, where one tectonic plate slides beneath another. The sudden uplift or subsidence of the seabed displaces a massive volume of water, generating the initial wave.
These waves can travel across entire ocean basins at speeds comparable to a jet airplane, often unnoticed in deep water due to their long wavelengths and low amplitudes. As tsunamis approach coastlines and enter shallower water, their speed decreases, but their height dramatically increases, sometimes reaching tens of meters. This transformation concentrates their immense energy into a destructive wall of water.
The impact on coastal communities can lead to extensive flooding, severe erosion, and the complete destruction of buildings and infrastructure. Powerful currents can sweep away people, vehicles, and debris, causing widespread fatalities and injuries. This hazard affects distant coastal regions hours after the initial earthquake.
Earthquake-Induced Fires
Earthquake-induced fires are a secondary hazard that can cause widespread devastation after seismic activity. Earthquakes frequently trigger fires by damaging infrastructure such as natural gas lines, electrical wiring, and fuel storage tanks. Ruptured gas pipes release flammable gas, while damaged electrical systems can create sparks, igniting the gas or other combustible materials.
Fighting these fires becomes challenging due to the chaos and damage immediately following an earthquake. Water mains often rupture during severe shaking, leaving firefighters without adequate water pressure. Additionally, roads can become blocked by debris from collapsed buildings, hindering the movement of emergency vehicles and personnel.
These fires can spread rapidly, especially in urban areas with high building density, causing destruction that often surpasses the initial ground shaking damage. The inability to contain and extinguish these blazes quickly can lead to widespread conflagrations, posing a prolonged threat to lives and property.