The Category of Geophysical Disasters in Disaster Risk Reduction

There are several categories of disasters that DRR is often applied to. Each one involves different disasters, risk factors, and outcomes that need to be addressed in DRR and in any kind of preparedness or recovery plan. The majority of these disaster categories involve naturally occurring phenomena. They include geophysical, hydrological, meteorological, climatological, biological, and man-made.

This article will look at geophysical disasters, discussing what qualities such disasters have. Several main geophysical disasters and the potential dangers they pose will be discussed, along with the risks and the actions that need to be taken with them.

What Is A Geophysical Disaster?

Geophysical disasters are disasters that are brought about by tectonic and seismic activity below the Earth's surface.1 Any kind of geological disturbance can cause a geophysical event to occur. Many of these kinds of disasters have similar signs that they are about to occur, such as shaking and unstable ground. Geophysical disasters can also trigger any other geophysical disaster because of their similarities, as well as several other disasters from different categories.

For most geophysical disasters, there is a degree of unpredictability that makes it hard to properly forecast the onset of one of these events. Geologists who study geophysical disasters are able to identify areas that have a higher risk of experiencing a disaster like an earthquake, but they cannot identify if one is about to occur or the specific timeframe in which it will happen.2 Patterns of seismic activity can help in determining if one is going to happen in the near future, although it still isn't an exact diagnostic identifier for most geophysical disasters.

Risks Involved With Geophysical Disasters

The risks associated with geophysical disasters are often dependent on factors of location. Geophysical disasters often occur along tectonic plates, which move and collide with each other and release energy from under the Earth's crust to produce these events. One of the areas that is best known for this kind of activity and for producing geophysical disasters is the "Ring of Fire" or the Pacific ring. The Pacific Plate sits underneath the entirety of the Pacific Ocean and is one of the most active and largest tectonic plates in the world. Its 25,000 mile edge is littered with 452 volcanoes--including roughly 75% of the Earth's active volcanoes-and it produces about 90% of the earthquakes that occur each year.3

Of course, the communities along the edges of the Ring of Fire are going to have a considerably higher risk of experiencing a geophysical disaster as a result of its activity. This includes the western coasts of North and South America, the eastern-most countries of Asia, New Zealand, Australia, and other Pacific islands. Thus, these are all areas that are commonly associated with geophysical events like earthquakes and volcanic eruptions. However, these are not the only areas that can be affected by geophysical disasters. There is significant geophysical activity in the Mediterranean that regularly causes these disasters, which puts those that live there at risk. Mountain ranges are often evidence of convergent plate boundaries, where tectonic plates have pushed up against each other and forced the ground upwards, and are also high-risk areas.4 Historically, the eastern and central parts of North America have faced geophysical threats before and some areas are well-overdue for significant seismic activity.5

DRR for geophysical disasters primarily involves preparations for these events and monitoring of geological activity. In terms of preparations, this means ensuring the structural stability of buildings and infrastructure (e.g. roadways) and educational efforts on what to do when faced with an earthquake, eruption, landslide, etc. It also means being aware of the locational risks that may increase your chances of experiencing a geophysical disaster. Most efforts to monitor geophysical or geological activity are already done by scientists throughout the world-the information is made available to the public through multiple outlets, including the U.S. Geological Society and the National Geophysical Data Center.

Actions to help decrease risk can also include implementing land-use regulations and restrictions against harmful practices like fracking. Fracking, or hydraulic fracturing, is a process used on shale rock to collect natural gas and oil.6 While fracking allows access natural resources that may otherwise be difficult to get to, it can cause serious harm by pollution of local ground and water sources and destabilization of the earth and soil near fracking sites. In some cases, fracking has caused small earthquakes and has raised concerns about its effects on areas of high seismic activity.7


Earthquakes are probably the most well-known and commonly occurring geophysical disasters in the world. Seismic activity usually generates earthquakes of varying intensities, many of which do not even register on monitoring equipment or have any effect on the surface. This is usually because they are small or have a low magnitude-which is measured to determine the size of the fault and its movement that caused the earthquake to happen.8 A higher magnitude earthquake could mean that it was a large fault or a lot of movement, or both.

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Measurement of earthquake magnitude is done with instruments such as a seismograph and intensity scales. The most common intensity scale is called the Richter scale, which offers a range of 0.0 to 10.0 to denote magnitude. Many of the regularly occurring earthquakes that are not felt at the surface fall on the lower end of the scale, around 2.0 or lower. Minor quakes ranging from 2.0 to 3.9 can be felt by those close to the source of the earthquake-the epicenter-but they rarely do any damage. Earthquakes ranging from 4.0 to 5.9 can cause light to moderate damage or shaking, and will easily be felt for some distance from the epicenter. The ones that are truly devastating and are often deemed disastrous are those that are measured at 6.0 or higher.9 They are significantly more intense and the damage they can cause can be severe. Several 6.0 or higher magnitude earthquakes happen every year throughout the world, but those about 8.0 or higher are rare and are often referred to as "Great" earthquakes. Historically, this has included the 1960 Great Chilean Earthquake (9.5 magnitude) and the 2011 Tohoku Earthquake in Japan (9.1).10

Some effects of earthquakes that should be taken into consideration with DRR include:

Aftershocks-Aftershocks are additional tremors or earthquakes that accompany an earthquake.11 They are often at a much smaller magnitude than the initial earthquake that proceeded them, but they can still strong enough to cause additional damage. Aftershocks can occur anywhere from a few hours after an earthquake to a few days; this was the case with the magnitude 7.0 Haiti earthquake in 2010, which had a 5.9 magnitude aftershock strike eight days later.12

Foreshocks-While not as common or well known as aftershocks, foreshocks are small earthquakes and tremors that proceed a larger magnitude earthquake. They can occur anywhere from a few days to a few weeks ahead of a major earthquake, usually in the same area.13

Soil Liquefaction-Seismic activity can cause solid soil to transform into a pseudo liquid, a phenomenon called soil liquefaction.14 This can happen with soil that is primarily silt, sand, or gravel-like and is often already saturated with liquid (water or otherwise). The shaking from the earthquake causes vibrations that destabilize the ground. Items may easily sink into the ground when soil liquefaction occurs, which may pose issues of safety when those items include buildings and infrastructure.


Volcanic Eruptions

Volcanic eruptions are geophysical disasters that are isolated to very specific areas of the world. There are roughly 1,500 land-based volcanoes in varying stages of activity, ranging from dormancy to currently erupting.15 A volcanic eruption occurs when gas and magma underneath the Earth's crust is released through vents and fissures along tectonic plates. This can involve earthquake-like shaking, intense heat, and large quantities of ash being released from the opening of a volcano or volcanic vent.16

Not all volcanic eruptions come in the form of explosions that rain down death and destruction on neighboring towns; many eruptions are much quieter with a slower lava flow.17 This does not mean that there is no cause for concern, just that these smaller eruptions are often much more contained. Volcanic eruptions should still be taken seriously, even when they are small, as their effects can start before the main eruption and spread as far as 25 miles from the volcano. Typically, there are warning signs that precede an eruption that can occur up to several months in advance and can be just as dangerous as the eruption itself. This can include activity near the volcano such as the release of ash or smoke, raised temperature, tremors, and chemical reactions with the surrounding environment. Scientists actually use this early activity to predict when a volcano will fully erupt.18

Like with earthquakes, the size and intensity of a volcanic eruption can be measured. Instead of the Richter scale, it's the Volcanic Explosively Index or VEI. It was developed in the 1980s and ranges from 0 (non-explosive) to 8 (very large, devastating).19 There fortunately hasn't been many volcanic eruptions above a VEI of 5 or 6 in recent years. Even Mount St. Helens-one of the more active volcanoes on the mainland U.S.-only barely reached a VEI of 5 at most in its series of eruptions in 1979-1980.20 For comparison's sake, Mount St. Helen's was at least 30 times smaller than the largest volcanic eruption of the 20th Century, the Novarupta-Katmai eruption in 1912 that unleashed a destructive force in an Alaskan valley, but much more memorable due to its proximity to urban populations.21

As with earthquakes, there are certain by-products of an eruption that should be taken into account with DRR. They include:

Ash Fall-The ash that is released during an eruption is actually very fine volcanic rock that has been violently ejected from the volcano.22 It is unleashed upwards into the atmosphere, but will come back down in a powdery rain. Ash falls will collect on anything it lands on or in, including ventilation openings. Breathing in ash is like trying to swallow a cup of flour-it coats your mouth and throat, makes it hard to breath, and can damage your airway it's not cleared.

Lava and Pyroclastic Flow-Lava is actually the magma from inside the Earth that has been pushed to the surface, but the two names are often used interchangeably. It is very hot and can burn anything that it comes in contact with, but will harden as it cools or when exposed to cold temperatures (e.g. water). A lava flow is often accompanied by a pyroclastic flow, which is a violently hot mix of volcanic rock, ash, and gases that will quickly flow from an erupting volcano.23

Fire-As mentioned, volcanoes unleash a lot of heat when they erupt and things such as lava can burn material it comes in contact with. Volcanic temperatures often reach anywhere from 1000-2000 degrees Fahrenheit.24 Those high temperatures can cause things to incinerate upon contact, and fires caused by eruption material like lava are not uncommon. This is always a possibility and is often responsible for the destruction that accompanies an eruption.

Mudflows and Landslides

There are some geophysical disasters that do not require seismic or tectonic activity for instances to occur. Landslides and mudflows result when large quantities of debris travels down a slope or incline. A landslide specifically includes material like rocks, earth, or random debris (e.g. vegetation or trees), while a mudflow adds water or some other liquid that turns it into a literal river of mud and/or debris.25 These can be caused by the breakdown of soil and rocks along the sides of hills, cliffs, or other slopes of land. Destabilization of the land by other geophysical disasters-like volcanoes and earthquakes-or human actions like deforestation, excavation, and mining can also cause a landslide or mudflow to happen. Changes in the seasons that produce things like melting snow or high rain may also destabilize the soil, and are more likely to produce mudflows than landslides.

Mudflows and landslides pose a particular threat because they can travel quickly and move anything in its path, most of which is only added to the mix of debris. Trees and rocks are often easily knocked down and swept up by a landslide or mudflow. It is not uncommon for there to be damage to infrastructure, utilities-water, sewage, electrical, and gas-and man-made structures like houses and other buildings from a mudflow or landslide.26 Injuries and fatalities are also possible, as a person may be crushed by the debris, drown or choke on it, or be exposed to bacteria mixed in with the debris. They are heavily disruptive and it is best to avoid areas (e.g. hillsides) that may be prone to their occurrence.