An earthquake is a sudden, rapid shaking of the Earth caused by the release of energy in the Earth’s crust. While the initial ground motion is the primary hazard, the resulting seismic waves can trigger a cascade of subsequent events known as secondary hazards. These follow-on disasters often cause widespread damage and loss distinct from the immediate effects of the initial ground shaking, transforming a singular geological event into a multi-hazard catastrophe.
How Earthquakes Generate Tsunamis
The most destructive secondary hazard originating from an earthquake is the tsunami, a series of powerful ocean waves. A tsunami is typically generated by large, shallow earthquakes that occur beneath the ocean floor, usually in subduction zones where one tectonic plate is forced under another. This type of fault movement, known as thrust faulting, causes an abrupt, massive vertical displacement of the seafloor.
This vertical shift displaces the entire overlying water column. Gravity then acts on this displaced mass of water, creating the initial tsunami wave that propagates outward from the source region. A significant earthquake, generally exceeding magnitude 7.5, is necessary to generate a destructive tsunami.
In the deep ocean, the tsunami travels extremely fast—sometimes over 500 miles per hour—but the wave height is often less than three feet. This allows the wave to travel across entire oceans while losing very little energy.
As the wave approaches the shoreline and moves into shallower water, friction with the seabed causes the wave to slow down considerably. The slowing forces the water to pile up, dramatically increasing the wave’s height upon landfall.
Triggers for Landslides and Rockfalls
Seismic shaking can destabilize slopes, leading to the rapid downslope movement of soil and rock masses, known as landslides and rockfalls. The intense vibration from an earthquake temporarily reduces the internal strength and stability of a slope already under stress from gravity. This dynamic loading introduces powerful inertial forces that push the material outward and downward.
The shaking effectively reduces the friction and cohesion that hold soil and rock particles together. For rockfalls, the seismic inertia force can cause fractured or jointed rock on steep slopes to topple and fall abruptly.
The severity of the resulting landslide or rockfall is highly dependent on local conditions, including the steepness of the slope and the composition of the material. Slopes composed of weakly cemented rock or highly fractured bedrock are particularly susceptible to this mechanical failure. The resulting movement can block transportation routes and bury structures.
Ground Failure Through Liquefaction
A damaging form of ground failure triggered by earthquakes is liquefaction. This process occurs when strong ground shaking affects saturated, loose, granular soils, typically sand or silt, causing them to temporarily lose their strength and stiffness. The cyclic stress from the seismic waves rapidly increases the pore water pressure within the soil.
The rise in water pressure causes the soil particles to lose contact with one another, transforming the solid ground into a material that behaves like a fluid. The liquefied ground can no longer support the weight of structures built upon it.
Liquefaction leads to severe consequences such as ground subsidence, where structures sink or tilt, and lateral spreading, where large blocks of surface soil slide on the liquefied layer beneath. Another common sign of this failure is the formation of sand boils, which are small volcano-like cones of sand and water ejected onto the ground surface. These effects cause extensive damage to buildings, roads, and buried utilities.
Indirect Hazards from Infrastructure Damage
Beyond the direct geological and hydrological disasters, earthquakes also create indirect hazards through the failure of human-built infrastructure. Fire is one of the most common consequences in urban areas, often initiated by ruptured natural gas lines, broken fuel tanks, or electrical shorts caused by severe shaking. Flammable materials released from these breaches quickly encounter ignition sources, leading to widespread blazes.
The ability to control post-earthquake fires is compromised when the infrastructure needed to fight them is damaged. Broken water mains and distribution pipes can deplete the public water supply, leaving fire departments without the necessary pressure or volume to suppress the flames. This lack of water can allow individual fires to merge into massive conflagrations that burn for days.
Flooding represents another major indirect hazard, particularly in regions near large bodies of water. The strong ground motion can compromise the structural integrity of water retention systems, such as dams and levees. A breach in these structures can unleash a sudden surge of water, flooding surrounding lowland areas and causing additional destruction and fatalities.