How are natural disasters classified?

Natural disasters are classified in many ways, but the most common classification is based on the origin of the atmosphere where the disaster occurs. In addition, natural disasters are also classified according to intensity and level of damage, and classified according to territories.

Classification of natural disasters by origin

Natural disasters are classified in many ways, but the most common classification is based on the origin of the atmosphere where the disaster occurs. Accordingly, the following main types of natural disasters can be divided:

• Natural disasters occur in the atmosphere (thunderstorms, tornadoes, hurricanes, waterspouts, severe cold, frost, forest fires, lightning, .). In particular, forest fires here have natural causes occurring in especially hot and dry weather conditions, many forest fires in the United States and Australia belong to this category.
• Natural disasters occur in the Hydrosphere (hail, floods/floods, flash floods, droughts, high tides, rising sea levels, etc.). In cold climate countries, other types of natural disasters exist such as ice floods in high mountainous terrain.
• Natural disasters related to the biosphere (epidemics, outbreaks of flora and fauna, etc.). Diseases caused by viruses and parasites that cause disease usually exist in nature, but when the weather and climate change suddenly, they can develop into epidemics. Similarly, outbreaks of harmful flora and fauna can become natural disasters for humans, such as: pest outbreaks, locust outbreaks, etc.
• Natural disasters occur in the Lithosphere (earthquakes, volcanoes, tsunamis, landslides, avalanches, etc.). Natural disasters such as earthquakes, volcanoes and tsunamis are often only related to special structures of the earth's crust, so they only exist in a small range, but the level of destruction is very large.
• Natural disasters originating from the Universe : meteorites, gamma-ray bursts, waves and radiation from the Sun, etc. These natural disasters are rarely noticed by humans because they rarely occur, however, currently natural disasters from the Universe are being concerned by developed countries (Box 3).

Meteorite falls in Chelyabinsk, Russia. The meteorite explosion occurred at 9:20 a.m. (03:02 GMT) on February 15, 2013 in the city of Chelyabinsk, Russia, causing 2,000 houses in the city to have broken windows and 500 people to be injured by flying glass. According to scientists, the explosion was caused by a 17-meter-diameter meteorite that exploded at an altitude of 30 km in the sky over the city of Chelyabinsk. The energy of the explosion was equivalent to the energy of 300 atomic bombs dropped by the United States on Hiroshima in 1945. Russian scientists have recovered the largest piece of meteorite that fell in an icy lake near the city of Chelyabinsk, weighing nearly 500 kg. Research results show that the meteorite is mainly composed of silicate - a stony meteorite according to the classification of geologists.

• The first three types of natural disasters are often related to Earth's climate change.
• The last two types of natural disasters are often not much related to climate change, occur rarely and occur in a small range, but when they do occur, they often cause great damage to people, sometimes catastrophically.

For example, the 2011 Fukushima earthquake and tsunami in Japan, the 2004 Indonesia earthquake and tsunami, the 1906 Tungust event in the Russian Federation, etc.

Classification of natural disasters by intensity and extent of damage

Hazard: A natural phenomenon that can cause loss of life, property, the environment, living conditions and disruption of economic and social activities. The term "Natural Hazard" often refers to extreme events or processes of meteorological, hydrological or geological origin, and is often divided into two main groups based on the speed at which the hazard occurs, including:

• Sudden hazards are natural hazards that occur rapidly (e.g. earthquakes, hurricanes, flash floods) .
• Slow-onset hazards  are hazards that develop over a long period of time, leading to dangerous conditions (e.g., droughts that last for a long time, causing food shortages, malnutrition, and even famine). It is becoming increasingly difficult to distinguish between natural and man-made hazards.

For example, deforestation on mountain slopes can lead to flash floods and landslides during heavy rains. Landfills, blockages in drainage systems, or improper construction can also lead to flooding. Landslides, floods, droughts, and fires are examples of combinations of causes that can be natural phenomena and human activities. One hazard can lead to another, for example, an earthquake can lead to tsunamis, landslides, and fires; while a hurricane can lead to floods and storm surges.

Catastrophe: A disaster is a state of great damage to a country, region, or community when a natural disaster occurs, for example: great loss of life, housing, and construction works, pollution, and destruction of the living environment, etc. when a natural disaster occurs. A disaster is the highest manifestation of natural hazards, causing serious impacts on the community and having a wide range of impacts, leading to great damage and disruption of normal community life. The impacts of a disaster can include loss of life, injury, disease, and adverse impacts on human health, mental health, and social well-being, damage to material assets, destruction of property, disruption of services, disruption of socio-economic activities, and environmental degradation.

According to the Law on Natural Disaster Prevention and Control of Vietnam in 2013, the level of natural disaster risk is divided into 5 levels according to the following criteria: intensity or danger level of natural disaster, scope of impact of natural disaster, ability to cause damage to life, property, infrastructure and environment. At the international level, the level of natural disaster risk is often divided into 5 levels (from 1 to 5) according to the increasing level of natural disaster intensity, in which level 5 is the level of natural disaster in an emergency situation where response must be decided by the Head of State. For example, the Saffir Simpson hurricane rating scale applied in the United States and many Western countries is divided into 5 levels based on wind speed and storm surge at the center of the storm (Table 1).

Table 1: Saffir Simpson Hurricane Rating Scale.

Classification of natural disasters by regions

In the activities of natural disaster risk prevention according to the Law on Natural Disaster Prevention and Control 2013, Point b, Section 1, Article 17 clearly states the task of 'risk assessment, zoning of natural disaster risks, and making natural disaster warning maps'. Zoning activities create conditions for local authorities and communities to come up with optimal solutions for prevention, control and mitigation of risks with high probability of occurrence. This work will be carried out for the entire territory of our country in the coming time. In this document, we propose to classify natural disasters by territory based on two groups of parameters: the frequency of natural disasters and the expected intensity/level of impact of natural disasters.

• The frequency of natural disasters is divided into levels: Very frequent (RT), frequent (T), rarely occurring (H) and unlikely to occur (KT) .
• The intensity/impact level of natural disasters is divided into levels : Very strong (RM), strong (M), moderate (V), weak (Y), and zero.

The qualitative classification table according to the above levels for 6 specific territories of our country is presented in Table 2.

Table 2: Proposed classification of natural disasters according to specific regions in Vietnam.

According to Table 2, for example, for tsunami disasters, the Northern mountainous region, Central Highlands and Southeast regions are unlikely to experience them, while the remaining four regions rarely experience them, but if they do occur, the impact will be huge.