Earthquake Disasters

Definition

Earthquake is a term used to describe the radiated seismic energy and ground shaking caused by the sudden slip of a geological fault, volcanic activity, or other sudden stress changes in the earth.

A fault is a fracture of the earth's crust along which separate blocks move relative to each other parallel to the fracture. A slip is the relative displacement of formerly adjacent points on opposite sides of a fault, measured on the fault surface.

In addition to slips, earthquakes can result from volcanic eruptions or the subterranean movement of magma. Small human-generated earthquakes have also been reported to be caused by massive explosions, the release of a large body of water striking a dam, and underground injection associated with fracking.

Most earthquake disasters occur along the boundaries of tectonic plates that actively move against each other to create, capture and release seismic energy.

The majority of the world's earthquakes occur along the Pacific plate. The plate has been called the Ring of Fire because of the frequent seismic and volcanic activity along its boundary.

A significant exception is a border between the Pacific and North American plates. In this area, plates do not subduct but rather move sideways past each other. This type of boundary generates earthquakes as tension in the earth's crust builds up and is released.

Most of the world's earthquakes, volcanic eruptions, and tsunamis originate along the boundary of the Pacific plate. A similar band of relatively high seismic activity, known as the Belt of Fire, is located in the Mediterranean region of the northern African plate. Along these areas plates overlap at convergent boundaries called subduction zones. Earthquakes occur when these boundaries suddenly slip against each other. That is, one tectonic plate is pushed down (i.e., subducted) by another above. As rock is further subducted, it also melts and becomes magma, giving rise to volcanoes.

Speed of Onset and Duration

Earthquakes are the quintessential rapid-onset disaster that often occurs with little or no warning. Strong ground shaking typically lasts less than a minute. Readjustments after a fault slip cause aftershocks that can occur intermittently for weeks or months.

Consequences

Earthquakes represent one of the most violent natural disasters in terms of the severity and scale of destruction. Earthquakes can result in very severe and widespread damage to the built environment and critical infrastructure of affected populations. This critical infrastructure often includes extensive damage to crucial systems like communications, transportation, energy, commerce, and health care.

Earthquakes typically cause many injuries and extensive population displacement (and its associated loss of shelter, water, food, sanitation, hygiene, and personal safety). Earthquakes disrupt ground transportation, damage airports, and render significant transportation routes (e.g., roads, rail, subways, ports) impassable. Earthquakes also create secondary disaster hazards like fires and explosions, hazardous material releases, and landslides. One of the deadliest secondary disasters associated with an earthquake is the tsunami generated by a submarine earthquake.

?Scale

During the past 60 years there were 1,153 earthquake disasters reported worldwide that caused an average of 1,362 deaths per incident.3

Earthquakes can immobilize an entire city or region, disrupting finance, power, water, commerce, transportation, communications, health care, and even emergency services. The 2008 Sichuan earthquake, for example, is estimated to have affected 46 million people.

There are multiple scales available to measure earthquake activity. Earthquake severity is often reported according to two scales. The Modified Mercalli Intensity (MMI) Scale is a 12-point seismic scale used to estimate potential damage. It does not have a mathematical basis; instead, it is a ranking based on observed effects. Compared with measuring the effects of the earthquake, the Richter scale is logarithmic and represents the total amount of energy released by the earthquake. Thus, while the Mercalli scale describes the intensity of an earthquake based on its observed effects, the 10-point Richter scale (depicted in the article photo above) describes the earthquake's overall magnitude by measuring the seismic waves that cause the earthquake. The two scales have different applications and measurement techniques.

From: Keim M. Emergency Health: Practical application of public health principles. APHA Press. Washington, DC. 2023?