An earthquake is the sudden release of stored energy within the Earth’s crust, caused by the shifting of tectonic plates along a fault line. This energy radiates outward as seismic waves, causing the ground to vibrate. The threat to human life and infrastructure comes from a chain reaction of physical phenomena triggered by the initial ground shaking.
Understanding the Five Primary Earthquake Hazards
The first primary hazard is Ground Shaking, which describes the disruptive movement of the ground. This vibration is directly responsible for the collapse of buildings and the failure of infrastructure like bridges and roads. High-frequency shaking tends to damage shorter, stiffer structures, while lower-frequency shaking causes more stress on taller buildings.
Liquefaction occurs when water-saturated, loose soil temporarily loses its strength and stiffness, causing it to behave like a liquid. Seismic waves increase the water pressure in the soil’s pore spaces, forcing the grains apart. When the ground becomes fluid, structures resting on it can sink, tilt, or shift laterally, even if they remain structurally intact.
Seismic shaking can trigger mass movements of rock and soil known as Landslides. These events are prevalent in areas with steep slopes or where the soil is saturated with water. The shaking destabilizes the slope, leading to rockfalls, deep slides, or debris flows that can bury communities or block transportation routes.
Surface Fault Rupture is the physical breaking and permanent displacement of the ground surface along the fault line. This occurs when the fault movement extends to the Earth’s surface, resulting in a visible offset that can be vertical, horizontal, or both. Any structure built directly over this rupture zone will be torn apart or severely deformed by the differential movement.
Large earthquakes can generate a Tsunami, a series of powerful waves caused by the rapid vertical displacement of a large volume of ocean water. This happens when a shallow, high-magnitude earthquake occurs beneath the ocean floor, causing the seafloor to uplift or subside. The resulting waves travel across the ocean at high speed and can inundate coastal areas, leading to flooding and erosion.
Variables That Influence Hazard Severity
The severity of these hazards is modified by several local and regional variables. A primary factor is the geological makeup beneath the surface, specifically the local soil composition. Soft sediments, such as those found in old riverbeds or reclaimed land, amplify seismic waves, causing more intense ground shaking than areas built on solid bedrock.
The distance from the earthquake’s epicenter and the depth of the rupture influence the intensity of shaking felt at the surface. Shallower earthquakes produce more concentrated and damaging shaking than deeper ones of the same magnitude. The quality of a region’s seismic building codes and their enforcement also affect the outcome of a hazard event.
Modern building codes require structures to incorporate reinforced concrete or steel frames designed to withstand seismic loads without collapsing. Communities with outdated or unenforced building standards are more susceptible to structural failure. These codes aim to prevent total collapse, which is where the majority of fatalities occur.
Immediate Actions During and After an Earthquake
The immediate protocol during ground shaking is to “Drop, Cover, and Hold On.” Drop to your hands and knees immediately to prevent being knocked down by the tremor. Take cover under a sturdy desk or table, protecting your head and neck with your arms. Hold onto your cover until the shaking completely stops, moving with the furniture if it shifts.
Once the shaking ceases, the danger continues, as aftershocks can strike moments later, potentially damaging weakened structures. Wear sturdy shoes and protective clothing to avoid injury from broken glass and debris. Check for fire hazards, especially gas leaks, and only shut off the main gas valve if a leak is suspected due to a smell or sound.
If you are near a coast and the earthquake made standing difficult, immediately move inland or to higher ground, as this warns of a potential tsunami. Use text messages instead of phone calls to keep lines clear for emergency services. If trapped, make noise by banging on a pipe or wall rather than shouting to conserve energy.