Hazards, Disasters, and Risks

Abstract

In this chapter, we will elaborate on three basic terms in the field of disaster risk science: hazards, disasters, and risks. We will also discuss the classification, indexes, temporal and spatial patterns, and some other fundamental scientific problems that are related to these three terms.

In this chapter, we will elaborate on three basic terms in the field of disaster risk science: hazards, disasters and risks. We will also discuss the classification, indexes, temporal and spatial patterns, and some other fundamental scientific problems that are related to these three terms.

1.1 Hazards

According to the United Nations International Strategy for Disaster Reduction (UNISDR), a hazard is a natural process or phenomenon that may pose negative impacts on the economy, society, and ecology, including both natural factors and human factors that are associated with the natural ones. Hazards are the origins of disasters. Hazards are detrimental to the development of human beings and hinder the sustainability of the world.

During the development of human beings, people have experienced and gradually understood all kinds of hazards. From different perspectives, disaster risk scientists studied on the classification, temporal and spatial patterns, and causes of hazards.

In this section, we will focus on the different classifications of hazards. Refer to research in natural disaster science and disaster geography for the temporal and spatial patterns of hazards. If you are interested in the causes of hazards, you may look up related research findings in geoscience, life science, and environmental science.

1.1.1 Classification of Hazards by Causes

There are all kinds of hazards in human society. However, from the perspective of causes, hazards can be divided into two types, that is, hazards caused by natural factors and hazards caused by human factors that are associated with natural environments. In fact, the percentage of the former type of hazards is dwindling, while that of the latter type of hazards is increasing.

• (1) Classification of hazards by ICSU-IRDR research program

The Integrated Research on Disaster Risk (IRDR) program of the International Council for Science (ICSU) classified hazards into 6 families, 20 main events, and 47 perils (UN-ICSU 2012).

There are six broad hazard categories within the family group:

Geophysical hazard: a hazard originating from solid earth. This term can be used interchangeably with the term geological hazard.

Hydrological hazard: a hazard caused by the occurrence, movement, and distribution of the surface and subsurface freshwater and saltwater.

Meteorological hazard: a hazard caused by short-lived, micro- to mesoscale extreme weather and atmospheric conditions that last from minutes to days.

Climatological hazard: a hazard caused by long-lived, meso- to macro-scale atmospheric processes ranging from intra-seasonal to multi-decadal climate variability.

Biological hazard: a hazard caused by the exposure to living organisms and/or the toxic substances or vector-borne diseases that they may carry.

Extraterrestrial hazard: a hazard caused by asteroids, meteoroids, and comets as they pass near earth, enter the earth’s atmosphere, and/or strike the earth, or change in interplanetary conditions that affect the earth’s magnetosphere, ionosphere, and thermosphere.

There are 20 main events. They are earthquake, mass movement, volcanic activity, flood, landslide, wave action, convective storm, extratropical storm, extreme temperature, fog, tropical cyclone, drought, glacial lake outburst, wildfire, animal incident, disease, insect infestation, extra impact, airburst, and space weather.

• (2) Classification of hazards by Joel C. Hill

Gill and Malamud (2014) divided natural hazards into six groups. In the paper, he also estimated the temporal and spatial scales of different hazard groups and types.

The 6 hazard groups and 21 hazard types are:

Geophysical hazard: earthquake, tsunami, volcanic eruption, landslide, and snow avalanche.

Hydrological hazard: flood and drought.

Shallow earth processes hazard: regional subsidence and uplift, local subsidence and heave, and ground collapse.

Atmospheric hazard: tropical cyclone, tornado, hail, snow, lightning and thunderstorm, long-term climatic change, and short-term climatic change.

Biophysical hazard: wildfire.

Space hazard: geomagnetic storm and extra impact events.

The hazard groups proposed by Joel C. Gill et al. are almost equivalent to the hazard families of ICSU-IRDR classification except for two differences. One difference is that the meteorological and climatological families of ICSU-IRDR were combined into a single atmospheric group in Gill’s classification. The other difference is that the hazard group of shallow earth processes was added in Gill’s classification in order to emphasize the hazardous impacts of shallow earth changes (Table 1.1).

Table 1.1 Definitions of 21 hazards in the classification by Gill and Malamud (2014)

• (3) Classification system of hazards by Kenneth Hewitt

In the book Regions of Risk by Hewitt (1997), hazards were divided into the following categories:

Natural hazards include four types (meteorological, hydrological, geological and geomorphological, biological and disease hazards)

Technological hazards include hazardous materials, destructive processes, and hazardous designs.

Social violence hazards include weapons, crime, and organized violence.

Compound hazards include fog, dam failure, and gas explosion.

Complex disasters include famine, refugees, poisonous flood, nuclear wastes and explosion of nuclear power plants (Table 1.2).

Table 1.2 Classification system of hazards by regions of risk (Hewitt 1997)

1.1.2 Classification of Hazards by Occurrences

Another way to categorize hazards is based on the environment where hazards occur (also called disaster-formative environment). The classification based on causes emphasizes the origin of hazards, that is, whether the hazards are caused by natural factors, human factors, or the interaction between natural and human factors. In contrast, the classification based on disaster-formative environment lays stress on the environmental basis of hazards, especially the distinctions among different spheres of the earth, and relatively ignores the causes. Actually, different kinds of hazards nowadays contain effects from both the natural and human factors to different degrees. And this is one of the important reasons why UN changed the goal of the global disaster reduction activities from natural disaster reduction to disaster risk reduction.

• (1) Classification of hazards by Peijun Shi

In Shi’s paper (1991) published on the Journal of Nanjing University (Natural Sciences, Special Issue on Natural Hazards), hazards were divided into four levels: systems, groups, types, and kinds. This classification highlights not only the occurrence environment but also the causes of hazards (Shi 1991).

The first level of this classification is focused on the causes, the second level the environments, the third level the types, and the fourth level the detailed hazards.

The hazard system is composed of three systems: nature, human, and environment.

The natural hazard system is then divided into four groups: atmosphere, lithosphere, hydrosphere, and biosphere. The hazards are mainly caused by natural environmental factors.

The human hazard system includes three groups: technology, conflicts, and wars. The hazards are mainly caused by human environmental factors.

The environmental hazard system is made up of five groups: global change, environmental pollution, desertification, vegetation degradation, and environmental diseases. The hazards are due to integrated natural and human factors.

• (2) Classification of hazards in Zhang Lansheng and Liu Enzhen

The Atlas of Natural Hazards in China edited by Zhang and Liu, as a result of the cooperation between Beijing Normal University and the People’s Insurance Company of China, was published by China Science Press (Beijing) in 1992. Based on the atlas, the paper A Research on Regional Distribution of Major Natural Hazards in China by Wang et al. (1994) was published, and the classification system of major natural hazards in China consisting of types and subtypes (Table 1.3) was built. The major natural hazards in China can be divided into 5 environments, 31 types, and 108 subtypes based on the differences in disaster-formative environments.

Table 1.3 Major natural hazards in China (Wang et al. 1994)

Atmosphere including nine natural hazards—drought, typhoon, rainstorm, hailstorm, extreme low temperatures, frost, ice and snow, sandstorm, and dry-hot wind.

Hydrosphere including five natural hazards—flood, waterlogging, storm surge, sea wave, and tsunami.

Lithosphere including five natural hazards—earthquake, landslide, debris flow, subsidence, and wind-drift sand.

Biosphere including six natural hazards—crop diseases, crop pests, forest diseases and pests, rodents, poisonous weeds, and red tide.

Geosphere including six natural hazards—soil erosion, desertification, soil salinization, frozen soil, endemic disease, and environmental pollution.

1.1.3 Intensity Classification of Hazards

• (1) Intensity classification of single hazard

The intensity classification of single hazard is based on the measurement specifications and standards of hazards. Hazards of different origins and in different environments are measured by different indicators. For example, earthquakes are measured in magnitude, rainstorms in rainfall intensity, typhoons in maximum sustained wind, and floods in flood stage. Those hazard measurement specifications and classification standards can be found on the Web sites of international or national departments of measurement standards.

Generally speaking, meteorological departments set up the measurement specifications and classification standards for atmospheric hazards; hydrological or water resources, and oceanic administrations for hydrosphere hazards; geological and earthquake administrations for lithospheric hazards; agricultural, forestry, and health administrations for biosphere hazards; and environmental and land resources administrations for geosphere hazards.

A large number of observations show that there is a negative correlation between hazard intensity and frequency. In other words, the higher the intensity is, the lower the occurring frequency is and the longer the repeating period is. There is a power function relationship between the hazard intensity and the occurring frequency (Chen and Shi 2013).

Refer to textbooks or monographs on geoscience, life science and resources and environmental science for the intensity classification of single hazard.

• (2) Intensity classification of multi-hazards

The regional and integrated disaster risk research requires scientists to understand the diversity of hazards of different spatial and temporal scales and classify the intensities of multi-hazards. Because the measurement indicators vary among different hazards and there is no universal indicator, the intensity classification method for single hazard mentioned in the previous section will not be able to meet the needs of the regional and comprehensive studies of the diversity of hazards.

Based on current data, it is very difficult to synthesize various hazard intensities measured in different indicators. One way to get around this problem is to divide each kind of hazard intensity into relative levels and then calculate the average of levels weighted by the area that respective type of hazard covers during a certain period of time. This method can approximately reflect the regional overall hazard intensities in a certain space and a certain period of time. But there is one problem with this method; that is, different hazards with the same level of relative intensity might have different impacts on hazard-affected bodies. Therefore, in order to eliminate this effect, another term is added—the weighted average of the loss rate of each hazard in a certain space and time period.

Referring to the quadrat method in the vegetation investigation, we proposed to use multiple degree to describe the abundance of hazards in a region. Another way to do this is similar to the multiple cropping index calculation in land-use research. Based on Wang et al.’s paper (1994), in this book, we propose to use multiple degree and covering index of hazards to express the clustering degree and influence of multiple hazards in a region.

Multiple degree (H_D): the clustering degree of hazards in a certain region. As a relative value changing with the compared region, it can be expressed as

$$ H_{\text{D}} = n/N $$

(1.1)

where H_D is the multiple degree of hazards in a region (%), n is the number of hazards in the region, and N is the number of hazards in a higher level of region (e.g., World, Asia, China). The value of N is set to be 108 (Table 1.3) for the calculation of county-level multiple degree of natural hazards in China.

Relative intensity (H_i): the relative destructive or damaging ability of hazards. Relative intensity is a relative value and only a quantity of the hazard per se. It is not an obvious positive correlation with the disaster loss or damage but is the basic reason (condition) for the regional loss. It can be calculated as follows:

$$ H_{\text{i}} = \sum\limits_{i = 1}^{n} {P_{i} \cdot S_{i} } \quad i = 1,2, \ldots ,n $$

(1.2)

where H_i is the relative intensity (level) of hazards in a region, P_i is the relative intensity of hazard i, and S_i is the area ratio of hazard i, ranging from 0.01 to 1.0, i.e., 1–100% and i is the number of hazard types.

Covering index of hazards (H_c): the percentage of covering area of hazards in a region. It can be expressed as

$$ H_{\text{C}} = \sum\limits_{i = 1}^{n} {S_{i} } \quad i = 1, \, 2, \, \ldots , \, n $$

(1.3)

where S_i is the percentage of covering area of a type of hazard in a region and i is the number of hazard types.

Composite index (H): the sum of the three indexes mentioned above divided by the respective maximum values. The formula is

$$ H = H_{\text{D}} /\hbox{max} (H_{\text{D}} ) + H_{\text{i}} /\hbox{max} (H_{i} ) + H_{\text{C}} /\hbox{max} (H_{\text{C}} ) $$

(1.4)

where H_D is the hazard multiple degree, H_i is the relative intensity, H_C is the covering index of a hazard in a region, and max () is the maximum value of the respective index.

1.1.4 Regional Difference in Multiple Natural Hazards of China

We will use the calculated results in Wang et al.’s paper (1994) to demonstrate the practical application of the four indexes–multiple degree, relative intensity, covering index, and composite index of hazards.

Multiple degree of natural hazards. In Fig. 1.1, the maximum value of the natural hazard multiple degree is about eight times as large as the minimum value in China. The value ranges from below 0.04 to above 0.30. This large variation shows that there is an obvious spatial clustering feature of natural hazards in China. Generally speaking, the high values are centered in North China and decrease toward northeast, northwest, and southeast. Ninety percent of the districts and counties with H_D value greater than 20% are located in the middle latitude belt (25°–45°N). In Southwest China where the H_D values are relatively low, the H_D value increases in some topography-transition areas. Thus, it can be seen that natural hazards relatively cluster in natural environment transition zones, such as the middle latitude belt, sea–land transition zones, topography-transition areas, and semiarid farming–pastoral ecotone. In the transition regions of several natural environments, there exist continuous areas with high H_D values. North China is right in such location and thus becomes the most concentrated area of natural hazards in China, also an important part of the Pacific Rim and mid-latitude multiple hazard belt. Therefore, regional natural hazards’ factors are of important value to the degree of regional natural environmental change.

Fig. 1.1

Multiple degree of China’s natural hazards (Shi 2011)

Covering index of natural hazards. Figure 1.2 shows that there is a large variation in covering index of natural hazards in China, ranging from less than 0.02 to more than 11.0 and indicating obvious regional differences. On the whole, the trapezoid region with Qiqihar, Harbin, Tianshui, and Hangzhou as four vertexes has the highest H_C values (>8.0) in the country. In this high-value region, the Northeast China Plain and the North China Plain have values usually greater than 9.0. The regions with H_C values greater than 10 display a Lambda-shaped layout; that is, one line is Qiqihar–Tongliao–Beijing–Taiyuan–Baoji–Tianshui, and the other line stretches from southern Hebei Province to Hangzhou along the Grand Canal. The low-value regions are centered in the Northern Tibetan Plateau, from which H_C value increases outwards. In regions south of the Yangtze River, there are two high-value belts: southeast coastal belt and southwestern provinces including Yunnan, Guizhou, and Sichuan. There is a positive correlation between the H_C value and the H_D value. It can be seen from Figs. 1.1 and 1.2 that H_C values and H_D values share the same regional distribution, especially in North China, and the distributions of H_C values and H_D values are related to those of different kinds of natural hazards. Usually areas influenced by natural hazards of atmosphere, hydrosphere, and biosphere tend to have relatively high H_C values. Examples are the previously mentioned high-value regions including North China Plain, Northeast China Plain, and Loess Plateau, where meteorological, floods and biological hazards are concentrated and influence extensively.

Fig. 1.2

Covering index of China’s natural hazards (Shi 2011)

Relative intensity of natural hazards. Figure 1.3 shows that H_i values are within the range of 0.8–24.0. Regions with a H_i value greater than 19.0 are sparsely distributed. One high-intensity area (H_i > 16.0) stretches from the northeast to the southwest, and another one is in the southeastern side of the first one—Hunan and Jiangxi Provinces. The relative intensities in the vast north-central Tibetan Plateau and Northwest China are relatively low. The regional differentiation of relative intensity is tightly associated with the regional distribution of several major hazards. First of all, the seismically active belts of China, i.e., the Pacific Rim belt and Himalayan seismic belt, have the correspondingly high intensities. Seismic regions, once having an earthquake with a magnitude greater than 8, usually become small high-intensity centers, such as West China, Tangshan. Secondly, the high-intensity regions are overlapped with the regions concentrated with cloudbursts. For example, the coastal typhoon belt, the northern Hebei Mountains–Taihang Mountains–Dabie Mountains cloudburst belt, and the cloudburst belts in western Sichuan and western Hunan. Thirdly, the frequently flooded areas also correspond to high relative intensity areas. These areas include Liaohe Plain, North China Plain, Northern Jiangsu Plain, and Hubei and Hunan Plains. Finally, areas with frequent debris flows and landslides, mainly in the “Second Step” east to Tibetan Plateau, have high values of relative intensity. Therefore, the overall relative intensity of natural hazards is controlled by several major natural hazards. However, these major natural hazards may also interact in the same region, which makes the regional differentiation of relative intensity of China’s natural hazards more complicated and also expands the high-intensity regions. In every high relative intensity area, there is at least one dominant natural hazard.

Fig. 1.3

Relative intensity of China’s natural hazards (Shi 2011)

Relationships among multiple degree, relative intensity, and covering index. The interaction of the three indexes varies among regions. Figure 1.4 shows the regional distribution of the composite index of natural hazards in China. North China has the highest values of all three indexes and thus is affected by frequent and catastrophic hazards. Coastal areas have the second highest values of three indexes and are subject to frequent and severe hazards. The third highest value regions include the farming–pastoral ecotone in northern and western Sichuan, Yunnan, western Guizhou, and southeastern Tibetan Plateau in the southwest. Whereas, the northern Tibet is a low-value region. The above outlines the basic regional differentiation of natural hazards in mainland China.

Fig. 1.4

Regional pattern of the composite index of China’s natural hazards (Shi 2011)

There are differences in natural hazards between eastern and western China and between southern and northern China. As for east–west differentiation, the values of multiple degree, relative intensity, and covering index are higher in the east and lower in the west. The high values in the east are centered in North China while the low values in the west are centered in northern Tibet. As for north–south differentiation, the vast area within 25° to 45°N in the east has values of all three indexes higher than areas to its south and north. Among this vast area, the highest values exist in range of 30° to 40°N. However, the north–south differentiation in the west is not obvious, since there are incomplete data records, especially in the border area among Tibet, Qinghai, and Xinjiang, or the Hoh Xil region. Due to inadequate data, this region has the lowest values of all three indexes nationwide.

The regional differentiation of natural hazards is closely associated with the environments where hazards develop. The environmental evolution-sensitive zones usually have high multiple degree, high relative intensity, and high covering index, suffering frequent or severe hazards. However, a small number of ecologically vulnerable areas have low values of multiple degree and intensity. One outstanding example is eastern Guizhou. In areas with harsh environment, such as the vast West China, the multiple degree and relative intensity are not necessarily high. Therefore, there is no direct relationship between environmental conditions and the impacts of natural hazards.

1.2 Disasters

Disasters are direct or indirect results of hazards. Disaster impacts include human losses, property losses, resources and environmental destruction, ecological damages, disruption of social order, and threats to the normal functioning of lifelines and production lines.

The classification of disasters is closely associated with hazards and disaster-affected bodies. In Chinese literatures, “Zaihai” is used to refer to both hazards and disasters. However, in Western literatures, hazard and disaster are two terms used separately. Most researches in the West are focused on the classification of hazards, rarely on the classification of disasters. Whereas, in Chinese literatures, the classification of disasters takes the place of the classifications of both hazards and disasters. This confusion of hazards with disasters, or the confusion of hazard science with disaster science (e.g., seismology substitutes for earthquake catastrophology, and rainstorm meteorology for rainstorm catastrophology), negatively affects the development of disaster risk science.

With the development of human society, the types of disaster-affected bodies (exposure) have increased and the distribution of disaster-affected bodies has expanded. At the same time, human’s ability of disaster prevention has also been improved. Therefore, even the same hazard could induce varying degrees of disasters. When analyzing the disasters, people stress on the disaster-affected bodies; namely, focus on human’s disaster prevention level, which is referred as the vulnerability, resilience, and adaptation of human beings to hazards in the Western literatures.

1.2.1 Classification of Disasters

As mentioned above, in Western research there is more of an emphasis on the classification of hazards than that of disasters. In Chinese official documents or research literatures, the majority is the classification of disasters based on the causes and scales of disasters. The genetic classification of disasters according to the causes in Chinese literatures is basically the same as that of hazards in Western literatures.

• (1) Classification of disasters by Zongjin Ma

In the book Introduction to Catastrophology by Ma (1998), according to the causes, disasters can be divided into natural disasters and man-made disasters. Natural disasters can be further categorized into natural disasters and man-made natural disasters, while man-made disasters are composed of man-made disasters and natural man-made disasters. When the management of disasters is taken into account at the same time, disasters can be divided into 5 classes and further 30 types. In the book, the author also clearly pointed out the administration departments in charge of each type of disaster (Table 1.4). This classification is different from others in the classification of the disaster-formative environments, with an inclusion of ocean sphere instead of hydrosphere. Another difference is that sources of flood and drought are attributed to the atmosphere in Ma’s classification (Ma 1994).

Table 1.4 China’s disaster classification and professional management (Ma 1998)

Besides, this classification is basically in accordance with the classifications in PRC Disaster Reduction Report (1993) and Major Natural Disasters and Disaster Reduction in China (1994) (Table 1.5).

Table 1.5 Classification of major natural hazards in China

Similar to the classification of Introduction to Catastrophology, in the book Natural Disasters by Chen (2013), based on the differences between the internal, external, and gravitational energy of the earth, natural disasters were divided into seven major categories: earthquakes, tsunamis, volcanos, meteorological disasters, floods, landslide and debris flow, and spatial disasters. This classification does not only reflect the holistic view of disasters but also emphasizes the timescales of disasters and environmental processes of the earth system.

• (2) Classification of disasters in Chinese state standards

For the dual purposes of comprehensive prevention, reduction and relief of disasters and counting the losses and damages caused by natural disasters, experts organized by State Disaster Reduction Center of Ministry of Civil Affairs drew up the classification standards of natural disasters in China, in which the definition and code of each disaster are also given. In this classification, natural disasters in China are divided into 5 groups and 40 specific types, including 13 specific meteorological and hydrological disasters, 9 seismic and geological disasters, 6 ocean disasters, 7 biological disasters, and 5 eco-environmental disasters (Table 1.6).

Table 1.6 Classification, definitions, and codes of natural disasters in China (The State Standard of the People’s Republic of China 2012)

Table 1.7 Disability types in UNISDR disaster indicators (UNISDR 2015)

From the comparison between the classification of the Twelfth Five-Year Special Plan and that of the State Standards, it can be seen that they share the same five big groups of natural disasters, but the latter one has 15 more specific types than the former one.

Besides, an emergency incident is defined as “a natural disaster, accidental disaster, public health incident or social safety incident, which takes place by accident, has caused or might cause serious social damage and needs the adoption of emergency response measures” in Emergency Response Law of the People’s Republic of China (2007).

1.2.2 Classification of Disaster Scale

There is no universal standard for the classification of disaster scale. Although there are different standards in different fields, the major factor considered is the scale of the hazardous event-induced disasters. Generally, the classification indicators include the number of casualties, the amount of property loss, disaster-affected area, and hazard intensity.

• (1) Indicator system of UNISDR

In the Sendai Framework for Disaster Risk Reduction 2015–2030, there are seven disaster reduction indicators, four of which are related to the measuring of disasters, namely disaster mortality, disaster-affected people, direct disaster economic loss in relation to global gross domestic product (GDP), damage to critical infrastructure and disruption of basic services (especially health and educational facilities) (UNISDR 2015). The disaster events include the natural or man-made disasters in specific spatial and temporal conditions (Table 1.7).

Mortality. Number of people killed or missing from a hazardous event. The death toll refers to the number of death population during or after the event, while the missing toll only refers to the total number of missing people during the event.

Besides counting the total number of dead and missing people, it is also important to calculate the percentage of killed and missing people per 100,000 people. Thus, the effect of population base can be eliminated in temporal and spatial comparison of mortality.

Affected people. It refers to the total population that are affected directly or indirectly by disasters. Directly affected people are those whose health was affected, such as injured and sick people, and those evacuated, displaced or relocated, and those who suffered from the disaster-induced direct damages to livelihoods, infrastructure, social culture, environment, and properties. At the same time, disaster statistics also need to include people whose houses were destroyed or collapsed and people who receive food aid.

Indirectly affected population are those suffered from the additive effects of disasters, namely people affected by disaster-induced disruption or modification of economy, critical facilities, basic services, business, work, society, and health.

In practice, due to the difficulty in counting indirectly affected population, only directly affected population are included in the disaster statistics. Likewise, it is also worth calculating the percentage of affected people per 100,000 people.

In addition to counting the killed and missing people and affected people, it is also common to specify their ages, genders, residence addresses, and disabilities.

Direct economic loss. Direct economic loss refers to disaster-induced loss of materials or properties, such as houses, factories, and infrastructures. Usually after the occurrence of a disaster, it is advised to assess the property loss as soon as possible to facilitate the cost estimation for disaster recovery and insurance claims processing.

It is also recommended to calculate the percentage of direct economic loss accounting for the global or national gross domestic product (GDP).

Direct economic loss can be further divided into agriculture loss, loss of industrial and commercial facilities, houses, critical infrastructure damaged or destroyed by disasters.

Direct agriculture loss: It refers to crop and livestock losses and also includes the losses of poultry, fishery, and forestry.

Industrial facilities damaged or destroyed: It refers to the loss of manufacturing and industrial facilities damaged or destroyed by hazardous events.

Commercial facilities damaged or destroyed: It refers to the loss of commercial facilities (including storage, warehouse, cargo terminal, etc.) that are damaged or destroyed by hazardous events.

Houses damaged: It refers to the loss of houses slightly affected by hazardous events and subject to no structural or architectural damages. After repair or cleanup, these damaged houses can still be habitable.

Houses destroyed: It refers to the loss of houses that collapsed or were burnt, washed away, and severely damaged and are no longer suitable for long-term habitation.

Critical infrastructure damaged or destroyed: It refers to the loss of educational and health facilities, and roads damaged or destroyed by hazardous events.

Educational facilities damaged or destroyed: It refers to the number of educational facilities damaged or destroyed by hazardous events. Educational facilities include children’s playroom, kindergarten, elementary school, high school (junior and senior), vocational school, college, university, training center, adult education school, military school, and prison school.

Health facilities damaged or destroyed: It refers to the number of health facilities damaged or destroyed by hazardous events. Health facilities include health centers, clinics, local or regional hospitals, outpatient centers, and facilities that provide basic health services.

Roads damaged or destroyed: It refers to the length of road networks in kilometers that are damaged or destroyed by hazardous events.

Infrastructure damaged or destroyed: It refers to the loss of infrastructures other than the critical infrastructures, such as railways, ports, airports.

Railways damaged or destroyed: It refers to the length of railway networks in kilometers that are damaged or destroyed by hazardous events.

Ports damaged or destroyed: It refers to the number of ports that are damaged or destroyed by hazardous events.

Airports damaged or destroyed: It refers to the number of airports that are damaged or destroyed by hazardous events.

Basic services. Basic services refers to the disruption of public services or time loss due to low-quality services, which are caused by hazardous events. Basic services include health facilities, educational facilities, transportation system (including train and bus terminals), ICT system, water supply, solid waste management, power supply system, emergency responses, etc.

The health facilities, educational facilities, transportation system are mentioned above in the critical infrastructure loss and infrastructure loss sections.

ICT system refers to communications and the associated equipment network, including radio and TV stations, post offices, public information offices, Internet, landline and mobile telephones.

Water supply includes drinking water supply and sewerage systems.

Drinking water supply system includes drainage system, water processing facilities, water transporting channels (channels and aqueducts) and canals, water tank, or tower.

Sewerage system includes public sanitary facilities, sewerage treatment system, collection and treatment of solid wastes from public sanitation.

Solid waste management refers to collection and treatment of solid wastes that are not from public sanitation.

Power/energy system includes power facilities, electrical substations, power control centers, and other power services.

Emergency response includes disaster management offices, fire departments, police stations, military, and emergency control centers.

• (2) Indicator system of statistical system of damages and losses of large-scale natural disasters in China

The Ministry of Civil Affairs and National Bureau of Statistics of China jointly introduced the regulation Statistical System of Damages and Losses of Large-scale Natural Disasters in 2013, which brought the comprehensive assessment of natural disaster loss into the regulation system (Shi and Yuan 2014). This Statistical System explains the purpose and meaning of statistics of large-scale disasters and defines the statistical scope and major indicators. Other contents described in this regulation include the submission procedure, forms of organization and data collection, 26 loss statistical report forms (1 of which is the loss summary table), 1 basic report, and 738 indicators. Some examples of these indicators are affected people, houses damaged and destroyed, household property loss, agriculture loss, industry loss, service loss, infrastructure loss, loss of public service system, resources and environmental loss, and so on (Table 1.8).

Table 1.8 Report system in China’s statistical system of damages and losses of large-scale natural disasters (Shi and Yuan 2014)

Figure 1.5 shows the changes in the percentage of direct economic loss accounting for GDP and human mortality caused by disasters in China (1990–2012, among which Wenchuan earthquake data are not included). The overall decreasing trends of the two items demonstrate a good result of comprehensive disaster reduction.

Fig. 1.5

Percentage of direct economic loss accounting for GDP and human mortality caused by disasters in China (1990–2012, Wenchuan earthquake data are not included) (Shi 2011, 2012)

Compared to the disaster indicators in Sendai Framework for Disaster Risk Reduction 2015–2030 that incorporates both the human-made and natural disasters, the Statistical System can only be applied to natural disasters. In contrast to the emphasis of the latter one on the comprehensiveness, the former one only highlights the key points. Another difference between these two is that the latter one includes the resources and environmental damages caused by natural disasters, while the former one stresses on the effectiveness and quality losses of infrastructure and services caused by disasters. Therefore, there are similarities in the disaster indicators of these two regulations, and there are also differences due to social and cultural differences. Even though some indicators share the same name in two systems, the actual meanings might be different. In practice, people need to be cautious in choosing the right indicator(s).

At present, the classification of disaster grade mainly adopts the standardized division method of disaster risk factors of each disaster, while there is no standard division for multi-hazard classification. The qualitative approach is usually used to classify disaster intensity levels, that is, the use of continuous quantitative or semiquantitative indicators, such as Applied Multi-Risk Mapping of Natural Hazards for Impact Assessment (ARMONIA) that categorizes a disaster into high, medium, or low level according to its intensity. Another example is hazard score proposed by Odeh Engineers Inc. (2001) that takes into account the level, frequency, and percentage of the affected area in the total research area. A higher score means the hazard has a higher intensity. The world natural disaster hotspots identified by the World Bank are based on 2.5° × 2.5° grid cells for risk assessment. In each grid cell, the hazard indexes of all types of hazards occurred are summed to give a score for the determination of hotspots. The hazard index of each type of hazard is established according to the corresponding data.

• (3) Very large-scale disaster indicator system

The term very large-scale disaster emerged in the beginning of the twenty-first century.

At the end of the twentieth century, a series of disasters happened worldwide and caused great impacts on the human society and economy. For example, Hurricane Andrew occurred in USA in 1992 claimed 65 lives and caused 26-billion-dollar losses (1992 price). The Kobe earthquake with magnitude 7.3 that struck Japan in 1995 caused the death of 6434 people and losses of 130 billion dollars (1995 price). And 1562 people were killed in the 50- to 100-year floods in South China in 1998 that also caused the death of 1562 people and losses of 107 billion Chinese yuan (1998 price, equivalent to 16 billion US dollars of 1998). The subsequent 7.4- and 7.2-magnitude Marmara earthquakes happened to the middle west of Turkey in 1999 caused the death of 18,373 people and damages of 13–19 billion US dollars of 1999 (OECD 2004).

Since the beginning of the twenty-first century, no decreasing trend has been seen in the impacts of disasters on the human society and economy. The terrorist attacks upon USA on September 11, 2001, caused the death of 2996 people and economic losses of 120 billion US dollars of 2001 (OECD 2004). The outbreak of Severe Acute Respiratory Syndrome between the end of 2002 and the spring of 2003 in China resulted in 8437 cases (among which 5327 cases occurred in mainland China) and 813 deaths (of which 348 deaths occurred in mainland China). SARS is named by World Health Organization (WHO) to describe a viral respiratory disease caused by the SARS coronavirus (SARS-CoV) (Baidu Baike 2016). In the same year, the widespread heat waves in Europe killed 70,000 lives (Robine et al. 2008).

Because these large-scale or very large-scale disasters have great and wide impacts, people are becoming more and more interested in the studies of them. In 2004, OECD published a book named Large-Scale Disasters—Lessons Learned. In 2008, Cambridge University Press in UK published a book called Large-scale Disaster—Prediction, Control, Reduction compiled by Mohamed Gad-el-Hak. The international research project Integrated Risk Governance was launched in 2010 to cope with very large-scale disasters. Afterward, academia paid more attention to the study of very large-scale disasters.

Definition of Very Large-scale Disaster. The Chinese word “juzai” appeared in 1986 in China for the first time and was used to mistranslate the word “Catastrophic disaster” in the Western literatures. The appearance of “juzai” in Chinese media and academia is closely related to the founding and explanation of the catastrophic disaster insurance funds. According to the statistical data from cnki.com.cn, as of the end of December 2011, there were up to 1359 literatures that include “juzai” in the titles. And the number of papers increased annually with the peak of 504 publications in the year 2008. More than half of these 504 papers are related to the catastrophic disaster insurance. Due to the frequent occurrences of very large-scale disasters in recent years, the new words such as “juzai prevention,” “juzai relief,” and “juzai assessment” are becoming more and more widely used in scientific publications.

In the Chinese academic literatures, it is the author of this book that first introduced the word “dazai” for “large-scale disaster” and “juzai” for “very large-scale disaster” in the Western literatures after attending the High Level Advisory Board Seminar of Financial Management of Large-scale Catastrophes held by OECD in Paris in the July 2006 (Shi et al. 2006, 2007).

Although a lot of work has been done in the definition and classification of very large-scale disasters, there are no well-recognized definition and classification standards of very large-scale disasters in the fields of academia or finance. Different scholars have their own angles.

In Western literatures, the following definitions have great influences.

In the book Large-Scale Disasters–Lessons Learned published by OECD in 2004, the terms large-scale disasters (or Megadisasters) and very large-scale disasters were used, but the specific quantitative criterion was not provided. In OECD’s opinion, very large-scale disasters can cause a great number of casualties, property losses, and widespread infrastructure damage. The impacts are so great that governments of the affected area and neighboring regions become unable to cope with; even public panic occurs. OECD also emphasizes the importance of cooperation and assistance among the member countries in response to the very large-scale disasters (OECD 2004).

In the book Large-scale Disaster—Prediction, Control, Reduction by Mohamed Gad-el-Hak (2008), disasters are divided into large-scale and very large-scale disasters based upon the disaster scope and death toll (Fig. 1.6). A very large-scale disaster is defined as a disaster with the death toll more than 10,000 or the affected area over 1000 km².

Fig. 1.6

Criteria of very large-scale disasters in the book Large-scale Disaster—Prediction, Control, Reduction (Gad-el-Hak 2008)

The definition of catastrophic disaster is usually based on the scale of the insured property losses by experts on insurance and financial management and development. The Insurance Services Office (ISO) of USA defines a catastrophic disaster as an event that causes insured property losses of 25 million dollars or more and affects a significant number of property/casualty policyholders and insurers. Swiss Re uses losses more than 38.7 million US dollars as a standard. From the amount of property losses, it can be seen that the scale of a catastrophic disaster cannot reach that of a large-scale disaster or megadisaster, let alone a very large-scale disaster. This also shows that the term “juzai” mentioned in the Chinese literatures in the late 1980s has a scale of the catastrophic disaster and was only paid attention to by experts on insurance and financial management and development.

Therefore, before the use of large-scale disasters or megadisasters and very large-scale disasters in the Western literatures in the early twenty-first century, the term “juzai” in the Chinese literatures only refers to a catastrophic disaster.

From the angle of geoscientists, very large-scale disasters are usually defined according to the hazard intensity, casualties, property losses, and affected scope. A very large-scale disaster in Ma’s opinion must reach two of the following criteria: over 10,000 deaths, direct economic losses of more than 10 billion Chinese yuan of 1990, economic losses of more than the average annual fiscal revenue of the previous three years of a Chinese province, drought disaster rate more than 70%, or flood disaster rate more than 70%, crop losses of more than 36% of the average annual crop production of the previous three years of a Chinese province, more than 300,000 houses collapsed, and livestock death toll of more than 1 million (Ma et al. 1994). Shi et al. defines a very large-scale disaster as a great disaster caused by a 100-year hazard (e.g., a 7.0-magnitude or stronger earthquake) and resulting in a great number of casualties and large and widespread property losses (Shi et al. 2010). Also in Shi’s definition, the impacts of a very large-scale disaster are so great that the affected area is unable to respond by itself and has to resort to outside help (Table 1.9). According to the classification standard in Table 1.9, the very large-scale disasters caused by natural hazards worldwide between 1990 and 2015 are listed in Table 1.10.

Table 1.9 Classification standards of disasters (Shi et al. 2010)

Table 1.10 List of worldwide very large-scale disasters between 1990 and 2015

From Table 1.10, we can see that one of the characteristics of the very large-scale disasters is the big hazard intensity. A very large-scale disaster can be a disaster chain composed of a very large hazard and its induced secondary disasters. It can also be a superposition of multiple types of disasters that are triggered by multiple hazards in a specific region and during a specific period of time. Besides, very large-scale disasters usually cause a great number of deaths and injuries, a huge amount of property losses, severe impacts on economy, society and natural environment, and a large disaster area. The emergency aids and reconstruction when or after the occurrence of the very large-scale disasters usually need help from a larger region or the whole country. In some cases, even international aids are indispensable.

All the very large-scale disasters mentioned so far are caused by sudden hazards. The indicators and classification standards for disasters caused by the accumulation of gradual hazards should be different (Zhang et al. 2013).

However, there are few discussions about the classification standards of gradually generated very large-scale disasters. Drought is one of the major natural disasters in both China and the world. Since 1949, a number of severe droughts causing great number of casualties and huge property losses have happened in China. For example, more than tens of thousands of people were killed due to the three-year great drought from 1959 to 1961. Based on the case of drought, we will discuss the classification standard of gradual very large-scale disasters below.

We cannot use hazard intensity to measure or to classify very large-scale droughts. This is because the forming process of a drought is very complicated. A drought hazard could be meteorological, or hydrological. It can also be soil drought or socioeconomic drought. The indicators and measurement criteria vary among different types of droughts. The data and studying methods are also different. What’s more, there is no linear relationship between the drought intensity and drought losses. And there is no definite relationship between the drought hazards and the formation of drought disasters neither.

The impacts of a very large-scale drought disaster can be represented in crop losses and population in need of aids. Drought could result in a bad harvest or total crop failure and water shortage for both human beings and livestock. Industrial production, urban water supply, and ecological environment could also be affected to varying degrees if a drought lasts for a long time. In the Statistical System of Damages and Losses of Natural Disasters by PRC Ministry of Civil Affairs (2013), the following items are included in the statistics of droughts: affected population, population affected by water shortage, number of livestock affected by water shortage, affected crop area, crop disaster area, total crop failure area, affected grassland area, and population in need of food and water aids. The inclusion of population affected by water shortage and population in need of aids in this Statistical System demonstrates the “people-oriented” disaster relief philosophy. In the State-level Contingency Plan for Natural Disaster Relief by General Office of the State Council of PRC, it is mentioned that when the number of people in need of food and water aids from governments accounts for a certain percentage of the agricultural population or reaches a designated magnitude, the state will initiate emergency response of the corresponding level (Table 1.11).

Table 1.11 List of severe drought losses in China since 1949 (Zhang et al. 2013)

Based on the severe droughts in China in Table 1.11, five criteria are used to define very large-scale drought disaster, crop disaster ratio, crop disaster area, disaster population, population in need of aids ratio, and direct economic loss (Table 1.12).

Table 1.12 Classification standards of very large-scale disasters (Zhang et al. 2013)

1.3 Risks

Risk is the probability of disaster loss in a future period of time in a region, or the future disaster. Essentially, risk is the probability of occurrence of a future hazardous event and its impacts (loss and/or damage). UNISDR (2004) defines risk as the probability of harmful consequences resulting from interactions between natural or human-induced hazards and vulnerable conditions. Two aspects that need special attention are the influence of social factors on risk and the estimation of hazard intensity and distribution.

Disaster risk usually refers to natural disaster or environmental risk that is associated with natural factors. The wide attention which disaster risk receives is related to the disaster (especially catastrophic disaster) insurance and the risk governance of emerging risks and very large-scale disasters.

The International Risk Governance Council, founded in 2003 in Geneva, Switzerland, paid high attention to the governance of emerging risk and slow-developing catastrophic risks and also established the transition from risk management to risk governance.

In 2006, Chinese National Committee for the International Human Dimensions Program on Global Environmental Change (CNC-IHDP) proposed to IHDP to undertake the Integrated Risk Governance (IRG) research under the background of global environmental change. This international scientific program proposal was approved by the scientific committee of IHDP and launched in 2010 (Shi et al. 2012). This program especially emphasizes the risk governance of very large-scale disasters. In 2015, this program was listed as a core program in the ICSU Future Earth Scientific Plan and focuses more on the risk governance of very large-scale disasters and green development.

In 2006, the Davos World Economic Forum issued the Global Risk Report for the first time (WEF 2006). And then it issued one report every year afterward. Until 2016, there have been 11 global risk reports being published. The series of reports deal with both traditional and non-traditional risks.

1.3.1 Risk Classification System of Davos World Economic Forum

In early 2014, the Davos World Economic Forum published the Global Risk Report 2014, where 31 global risks are grouped into five categories (Table 1.13). The top ten global risks of highest concern in 2014 picked out by the World Economic Forum are fiscal crises in key economies, structurally high unemployment/underemployment, water crises, severe income disparity, failure of climate change mitigation and adaptation, greater incidence of extreme weather events (e.g., floods, storms, fires), global governance failure, food crises, failure of a major financial mechanism/institution, and profound political and social instability. It can be seen from the above that, besides the continuing focus on the traditional risks, we need to accelerate the study of response to a series of non-traditional risks.

Table 1.13 Global risk classification system of Davos World Economic Forum (WEF 2014)

The five categories of risks were not changed in the Global Risk Report 2016, but the number of specific risks was decreased from 31 to 29 (Global Risk 2016). In this report, a global risk is an uncertain event or condition, if occurring, which can cause significant negative impact for several countries or industries within the next 10 years. A global trend is a long-term pattern that is currently taking place and that could contribute to amplifying global risks and/or altering the relationship between them (Table 1.14).

Table 1.14 Global risk classification system of Davos (WEF 2016)

The Global Risks Landscape 2016 was proposed in the Global Risk Report 2016. From the landscape, it can be seen that the risks with the highest impact and likelihood are failure of climate change mitigation and adaptation, water crises, large-scale involuntary migration, fiscal crises, interstate conflict, profound social instability, cyber attacks, and unemployment or underemployment (Global Risk 2016). In the Global Risks Interconnections Map 2016, the most strongly connected risks are failure of climate change mitigation and adaptation, profound social instability, large-scale involuntary migration, and unemployment or underemployment (Global Risk 2016).

1.3.2 Risk Classification of International Risk Governance Council

For the purpose of improving the governance of emerging risks and slow-developing catastrophic risks, the International Risk Governance Council (IRGC) provided the risk taxonomy in 2005. This classification system includes six categories: physical factors, chemical factors, biological factors, natural forces, social-communicative hazards, and complex hazards. Based on these general categories, there are 33 specific risks (Table 1.15).

Table 1.15 Risk taxonomy of International Risk Governance Council (IRGC 2005)

The Davos World Economic Forum Reports involve a wide range of global risks covering the fields of economy, politics, culture, society, and ecology and could be corresponding to the economic development, political development, cultural development, social development, and ecological development proposed by the Chinese government, respectively. Thus, it can be seen that the risk classification of the World Economic Forum emphasizes the combination with practice.

The risk taxonomy of IRGC is from the perspective of hazards, similar to the disaster classification in Sect. 1.2. This classification stresses on the causes of risks and thus lacks in combination with practice. However, it pays attention to emerging risks and slow-developing catastrophic risks, including the governance of very large-scale disaster risks. At the same time, it provides the framework for systematic risk assessment and governance.

In China, the classification of risks is tightly associated with the security and disaster classifications. For example, the overall national security concept proposed by the Chinese government is in a one-to-one correspondence with the global risks in the World Economic Forum Report. In detail, the political security, homeland security, and military security correspond to geopolitical risks; economic and resource security to economic risks; cultural and societal security to societal risks; technology, information, and nuclear security to technological risks; ecological security to environmental risks (Xi 2016). Another example is the four public securities proposed by the Chinese government corresponding to five of the six risk categories of IRGC, that is, natural disaster of the former corresponds to the natural forces of the latter, accidental disasters to physical risks, public health accidents to chemical and biological risks, and social security incidents to social-communicative hazards. The complex hazards are usually related to the four public securities proposed by the Chinese government, and also to the integrated disasters.

The classification system of risks is built upon the hazard and disaster classifications in China. For example, if hazards are divided into natural, man-made, and environmental ones, risks can be classified into the corresponding three types. In the same way, risks can also be divided into four categories of natural, accidental, public health, and social security ones based on the four-type classification of hazards.

1.3.3 Classification Criteria for Risk Levels

The natural disaster risk level is usually expressed in exceedance probability or return period, the same way as the intensity level of natural hazards. For example, the meteorological, hydrological, and ocean disaster risks can be divided into 10-year level (small-scale disaster), 20-year level (medium-scale disaster), 50-year level (large-scale disaster), and 100-year level (very large-scale disaster). The earthquake disaster risk level is usually expressed in earthquake magnitude. For example, a magnitude 7.0 or above earthquake poses a very large-scale disaster risk, 6.5–7.0 large-scale, 6.0–6.5 medium-scale, and 6.0 or below small-scale disaster risks. The natural disaster risk level does not only depend on the natural hazard intensity but also count on the vulnerability and exposure of the hazard-bearing bodies. In practice, the classification of natural disaster risk levels is even more complicated and thus usually resorts to the relative levels such as the first-level risk, the second-level risk, the third-level risk, the fourth-level risk, and the fifth-level risk. The larger the number is, the higher the risk level is. In the Atlas of Natural Disaster Risk of China by Peijun Shi (Chinese–English bilingual version, Shi 2011) and the World Atlas of Natural Disaster Risk by Peijun Shi and Roger Kasperson (Shi et al. 2015), the temporal and spatial patterns of natural disaster risks of China and the world are displayed by using indicators including risks, risk grades, and risk levels (Qin et al. 2015; Shi 2011, 2015).

It is more difficult to classify man-made and environmental risk levels by using quantitative criteria. A common way is to use relative level, or using the trends and changes of man-made and environmental risks to describe their levels. The Global Risk Trends 2015 in the Davos World Economic Forum Risk Report is an example of this kind of way of reflecting global risk levels. In detail, increasing global risk levels at 2015 are aging population, changing landscape of international governance, climate change, environmental degradation, growing middle class in emerging economies, increasing national sentiment, increasing polarization of societies, rise of chronic diseases, rise of cyber dependency, rising geographic mobility, rising incoming and wealth disparity, shifts in power, and urbanization (WEF 2015).

The top three most likely global risks in 2016 in each region are reported in the Global Risk Report 2016 of WEF (WEF 2015). In North America, the top three are cyber attacks, extreme weather events, and data fraud or theft. In Latin America and the Caribbean, the top three are failure of national governance, profound social instability, and unemployment/underemployment. In Europe, the three are large-scale involuntary migration, unemployment/underemployment, and fiscal crisis. In the Middle East and North Africa, they are water crises, unemployment/underemployment, failure of national governance, and profound social instability. In sub-Saharan Africa, they are failure of national governance, unemployment/underemployment, and failure of critical infrastructure. In Central Asia (including Russia), they are energy price shock, interstate conflict, and failure of national governance. In East Asia and the Pacific, they are natural catastrophes, extreme weather events, and failure of national governance. In South Asia, the top three are water crises, unemployment/underemployment, and extreme weather events.

The exceedance probability mentioned previously, a concept usually used in the study of natural disaster risks, refers to the likelihood of the intensity or motion parameters of an earthquake, or the flood level, or the maximum wind speed at the center of a typhoon exceeding a designated value or values in a specific location and during a certain period of time. In other words, it is the probability of the required value exceeding the given value and can be mathematically expressed as

$$ P_{\text{exceed}} = P (u > u_{\text{limit}} ) $$

(1.5)

where P_exceed is the likelihood of the required value (u) of a data series exceeding the limit value (u_limit).

For example, a set of data X (x₁, x₂,…x_n) have n raw data points that are arranged from the lowest to the highest. The exceedance probability of data point x_i is

$$ P = \left[ {\frac{n - i + 1}{n}} \right] \times 100\% $$

(1.6)

The following takes the earthquake as an example for the calculation of exceedance probability.

Within t years, the probability of earthquake occurrence for n times P(n) in a region is

$$ P\left( n \right) = F\left( n \right) $$

(1.7)

In the same way, within t years, the likelihood of no earthquake happening in this region is

$$ P\left( 0 \right) = F\left( 0 \right) $$

(1.8)

Then, the likelihood of at least one earthquake within t years, or the exceedance probability, is

$$ F\left( t \right) = 1 - P\left( 0 \right) = 1 - F\left( 0 \right) $$

(1.9)

The probability density is

$$ f\left( t \right) = F^{\prime}\left( t \right) $$

(1.10)

Poisson distribution is widely used in earthquake studies. Within t years, the probability P(n) of n earthquakes occurring in a region can be expressed in the Poisson distribution form as below:

$$ P\left( n \right) = \frac{{e^{ - u} \times vt^{n} }}{n!} $$

(1.11)

Then, within t years, the likelihood of no earthquake happening in this region is

$$ P\left( 0 \right) = \frac{{e^{ - vt} \times vt^{0} }}{0!} = e^{ - vt} $$

(1.12)

So the likelihood of at least one earthquake happening or the exceedance probability within t years is

$$ F\left( t \right) = 1 - P\left( 0 \right) = 1 - e^{ - vt} $$

(1.13)

The corresponding probability density is

$$ f\left( t \right) = F^{\prime}\left( t \right) = v \times e^{ - vt} $$

(1.14)

The variable v mentioned above is the annually averaged occurrence probability of earthquake in a region, which has an inverse relationship with the return period T₀:

$$ T_{0} = \frac{1}{v} $$

(1.15)

From here, we can see that the relationship between the return period T₀ and the exceedance probability F(t) can be expressed as

$$ T_{0} = \frac{1}{v} = \frac{ - t}{{{ \ln }\left( {1 - F\left( t \right)} \right)}} $$

(1.16)

Based on the equation above, we can calculate the return periods of different exceedance probabilities for a period of time.

For example, the exceedance probability of 63% for 50 years is equivalent to a 50-year disaster, 10% means a 474-year disaster, and 2–3% means a 1600–2500-year disaster.

In summary, hazards are negative factors to human beings, and the temporal and spatial patterns of hazards can be studied by comparing with historical observed data. Disasters are the impacts of hazards on human beings and can be measured in terms of losses and damages. Risks are future hazard-induced disasters in a specific location. In short, disaster risk science is a discipline studying the mechanics, processes, and dynamics of the interactions among hazards, disasters, and risks, as well as disaster risk prevention and reduction. The relationships among hazards, disasters, and risks are shown in Fig. 1.7.

Fig. 1.7

Relationships among hazards, disasters, and risks