A hazard is defined as a process, phenomenon, or human activity that holds the potential to cause harm, including loss of life, property damage, or environmental degradation. This potential source of harm can originate from natural processes, such as geological shifts, or from human-induced activities like industrial operations. Understanding these threats is fundamental to safety planning and mitigation efforts.
A secondary hazard represents a subsequent threat that arises solely as a consequence of an initial, or primary, hazard. This distinction is significant because the secondary event often generates a greater overall impact and may occur hours, days, or even weeks after the first event. Identifying and preparing for these cascading consequences is a major focus of modern disaster risk reduction.
Distinguishing Primary and Secondary Hazards
The difference between a primary and a secondary hazard rests on their causal and temporal relationship. A primary hazard is the direct, immediate result of a major event, representing the initial trigger. For example, in an earthquake, the intense ground shaking is the primary hazard.
The secondary hazard, by contrast, is an indirect effect caused only because the primary effect has already occurred. It is a resultant effect, often delayed from moments to extended periods. For example, the building collapse caused by an earthquake’s shaking is a secondary effect of the ground movement itself.
This sequential relationship means that without the primary event, the secondary hazard would not exist. For instance, a tsunami is a secondary hazard triggered by the primary event of underwater seismic activity or ground displacement. The primary hazard sets off a chain reaction, which can be unidirectional or, in some cases, bidirectional, exacerbating the initial conditions.
The Causal Mechanisms of Secondary Hazards
The physical and chemical processes leading to secondary hazards often involve a transformation of energy or material state following the initial disruption. One common mechanism is ground instability, where the intense vibratory energy from an earthquake causes liquefaction. In saturated, non-cohesive soil, the shaking increases pore water pressure, causing the soil matrix to temporarily lose strength and behave like a dense liquid.
Another mechanism involves structural failure leading to technological hazards, often termed a “Na-Tech” event. Physical stress from a primary event, such as a hurricane’s high winds or an earthquake, can rupture industrial pipelines or storage containers. This structural breach releases hazardous materials, like toxic chemicals or flammable gas, which become a secondary hazard through fire or environmental contamination.
These events are often described as cascade effects, where the impact propagates through a series of interdependent systems. For example, heavy rain may saturate a slope, triggering a landslide. That landslide could then block a river, creating an upstream flood hazard, which is a subsequent event in the chain.
Common Categories of Secondary Hazards
Secondary hazards can be broadly categorized based on the environment they impact and the type of event that triggers them. Geological secondary hazards frequently manifest as mass movements in the aftermath of a primary seismic event. Earthquake ground shaking often destabilizes slopes, leading to widespread landslides, rockfalls, or debris flows.
Hydrological secondary hazards involve the movement and distribution of water, frequently following severe weather or coastal events. A hurricane’s intense rainfall, for instance, can saturate the ground, leading to riverine or flash flooding as a secondary consequence. Similarly, a volcanic eruption can melt glacial ice and snow, generating devastating mudflows known as lahars.
Technological and infrastructure secondary hazards arise when primary events cause failure in human-made systems. The collapse of buildings and utility poles during a disaster can sever gas lines and electrical wires, sparking widespread fires in urban areas. Furthermore, the disruption of water and sewer services can lead to secondary public health hazards, such as outbreaks of waterborne diseases.
Forecasting and Risk Assessment
Identifying and predicting the potential for secondary hazards is an integral part of modern disaster planning. Risk assessment must incorporate the likelihood of multi-hazard and cascading scenarios, rather than focusing only on the initial event. This requires a detailed analysis of the physical dependencies between different hazard types within a specific geographic area.
Planners use models to forecast the potential scale and location of these secondary threats, allowing for proactive measures. By understanding causal mechanisms, such as which geological conditions are prone to liquefaction after a seismic event, authorities can anticipate consequences and prioritize resource allocation. This analytical approach helps reduce overall risk exposure by managing the vulnerability of infrastructure and populations to the full sequence of potential impacts.