Introduction

A tsunami, the Japanese word for “large harbor wave,” is a series of large water waves produced by a sudden vertical displacement of water. Aquatic earthquakes are the most common cause, but volcanic activity, landslides and impacts of meteorites may also generate tsunamis. Earthquake-generated tsunamis develop when tectonic plates, either deep sea, continental shelf, or coastal, move abruptly in a vertical direction, and the overlying water is displaced. Waves created by these disturbances move in an outward direction, away from the source. In deep waters, the surface disturbance of water is relatively unnoticeable and may only be felt as a gentle wave. As the wave approaches shallow waters along the coast, it rises above the surface related to the amplitude of the underwater waves. The speed of the tsunami diminishes and the height of the wave increases as it reaches the shore line. The extent of inundation that occurs is largely dependent on local topography; in low lying areas flooding can be extensive and can reach far inland disrupting even non-coastal communities ^1,2,3 .

While rare, high-impact tsunamis have the potential to cause widespread destruction and affect hundreds of thousands ^4,5 . The 2004 Indian Ocean tsunami resulted in more than 225,000 deaths across twelve nations, and the 2011 Japan tsunami caused an estimated 28,000 deaths ⁶. Displacement and damage to infrastructure are also important contributors to the human, social, and economic effects of tsunamis ⁷. Few reviews of the impacts of tsunamis on human populations exist. Given the recent tsunami disasters in 2004 and 2009, a broader understanding of the characteristic effects of tsunamis on human populations could inform preparedness and response efforts. The objectives of this review were to describe the impact of tsunamis on the human population, in terms of mortality, injury, and displacement and to identify risk factors associated with these outcomes. This is one of five reviews on the human impact of natural disasters, the others being volcanoes, floods, cyclones, and earthquakes.

Methods

Data on the impact of tsunamis were compiled using two methods, a historical review of tsunami events and a systematic literature review for publications relating to the human impacts of tsunamis with a focus on mortality, injury, and displacement.

Historical Event Review

Data for the historical event review were obtained from two sources. The National Oceanic and Atmospheric Administration’s National Geophysical Data Center (NOAA-NGDC) tsunami database ⁸ consists of two sets of related files on tsunami events and a tsunami run-ups. The event file lists the cause (almost exclusively undersea earthquakes) that triggered tsunamis together with the total impact of a single event (i.e. the aggregate impact from multiple wave run-up locations together with the coordinates of the originating event). The run-up file includes wave characteristics and impacts in each affected location (multiple run-up reports per event). Records from the tsunami run-up file that met all of the following criteria were retained: 1) occurred between 1900 and 2009; 2) reported as definitely or probably occurring; 3) had a wave height ≥2.0m; and 4) resulted in ≥1 deaths. This yielded a total of 116 run-up reports from 82 different tsunami events.

The Centre for Research on the Epidemiology of Disasters’ Emergency Events (CRED EM-DAT) was the second data source used in the review. All wave/surge events that were reported between 1900 and 2009 in EM-DAT were included (n=58); data were initially exported in 2008 when CRED reported wave/surge events; this category was subsequently discontinued. For tsunami impacts reported by EM-DAT, zeroes were treated as missing values because they were used as placeholders and their inclusion in the analysis could contribute to the under estimation of tsunami impacts. The NOAA-NGDC run-up database was subsequently searched for events reported by EM-DAT that were previously excluded due to wave height <2.0m or uncertain reporting criteria, and NOAA-NGDC event data were added to the EM-DAT records. This process yielded a total of 151 records, including 58 events reported by EM-DAT and 134 run-ups reported by NOAA-NGDC. The run-up file was used to assess wave characteristics and outcomes. A separate event file comprised of 81 events was created by combining multiple reports of tsunami impact within a country into a single event. To create a summary record for each of event with multiple reports, human impacts at each location were summed, and the maximum wave height and inundation depth were applied. The event file had 94 events, including 58 reported by EM-DAT and 71 by NOAA, and was used to assess frequency and distribution of tsunamis and their impact by country. Findings presented in this review are based on the 151 run up file. Both run-up and source data can be accessed online at https://www.jhsph.edu/refugee/natural_disasters/_Event_Tsunamis.html.

In order to examine country- and event-specific characteristics associated with low and high levels of tsunami mortality, deaths were categorized as follows: low (<10 deaths), medium (11-75 deaths) and high (>75 deaths). Bivariate tests for associations between tsunami mortality and the following characteristics were performed using χ² (categorical measures) and ANOVA (continuous measures): time period (dichotomized, 1900-1955 and 1956-2009), region as defined by the World Health Organization (WHO), income level (World Bank), gross domestic product (GDP), GINI (measure of income inequality), distance from source (quartiles), wave height (dichotomized, <6.65 and >6.65) and earthquake magnitude. For the region variable, only three events were reported in Africa and because all were related to the 2004 Indian Ocean tsunami, they were grouped with Southeast Asian; only one event was recorded in the Eastern Mediterranean region which was grouped Europe .All analyses were performed using Stata Statistical Software, Version 11.0 ⁹

Systematic Literature Review

Key word searches in MEDLINE (Ovid Technologies, humans), EMBASE (Elsevier, B.V., humans), SCOPUS (Elsevier B.V., humans), and Web of Knowledge, Web of Science (Thomson Reuters) were performed to identify articles published in July 2007 or earlier that described natural hazards and their impact on human populations. Key words used to search for natural hazards included natural hazard(s), natural disaster(s), volcano(es), volcanic, volcanic eruption, seismic event, earthquake(s), cyclone(s), typhoon(s), hurricane(s), tropical storm(s), flood(s), flooding, mudslide(s), tsunami(s), and tidal wave(s). Key words included for impact on human populations were affected, damage(d), injury, injuries, injured, displaced, displacement, refugees, homeless, wounded, wound(s), death(s), mortality, casualty, casualties, killed, died, fatality, fatalities and had to be used in either the title, abstract or as a subject heading/key word. The search resulted in 2,747 articles from MEDLINE, 3,763 articles from EMBASE, 5,219 articles from SCOPUS, and 2,285 articles from ISI Web of Knowledge. Results from the four databases were combined and duplicates were excluded to yield a total of 9,958 articles. . One search was done for all the five natural hazards described in this set of papers. This paper describes the results for tsunamis. The systematic review is reported according to the PRISMA guidelines.

Title screening was performed to identify articles that were unrelated to natural disasters or human populations. Each title was screened by two independent reviewers and was retained if either or both reviewers established that inclusion criteria were met. To ensure consistent interpretation of inclusion criteria, percent agreement was assessed across reviewers for a small sample of articles, and title screening began after 80% agreement on inclusion was achieved. A total of 4,873 articles were retained for abstract review. Articles that met one or more of the following criteria were excluded in the abstract screening: language other than English; editorial or opinion letter without research-based findings; related to environmental vulnerability or hazard impact but not human populations; individual case report/study; focus on impact/perceptions of responders; and not related to human or environmental vulnerabilities or impacts of hazards. As with the title screening, overall percent agreement between reviewers was assessed, and abstract screening began after achieving 80% agreement. Each abstract was screened by two independent reviewers and was retained if either or both reviewers established that inclusion criteria were met. During the abstract review, included abstracts were coded for event type, timeframe, region, subject of focus, and vulnerable population focus.

A total of 126 articles were retained for full article review. Articles discussing the impacts of natural disasters on human populations in terms of mortality, injury, and displacement were prioritized for review. A total of 64 articles on tsunamis meeting the aforementioned subject focus criteria were retained for full review. Upon full review, 27 articles were retained including 23 that underwent dual review and standardized data abstraction, two identified as review articles ^10,11, one policy article ¹² and one article on mitigation ¹³ (Figure 1). Following the systematic review, a hand search was conducted using the databases and key words listed above to identify relevant articles published between July 2007 when the initial search was conducted and October 2012; seven additional articles were identified that met criteria for full review. In total, 34 articles relating to risk factors for mortality, injury or displacement were identified; summaries of articles with primary data (n=30) are presented in Table 1.

Results

Historical Event Review

Between 1900 and 2009, 94 tsunamis that affected human populations were recorded. The frequency of events was relatively constant through the 1980s, after which a dramatic increase was reported (Figure 2). This increase is likely the result of improvements to monitoring and reporting systems. Tsunami frequency and mortality were concentrated in the Western Pacific, Southeast Asia, and Americas regions, each of which accounted for almost one third of tsunami events and deaths, but Southeast Asia accounted for 52% of the tsunami-affected population from 1900 – 2009 and 95% of the tsunami affected population from 1980 – 2009 (Figure 4). An estimated 2.5 million people were affected by tsunamis between 1900 and 2009. A sharp increase in tsunami mortality and affected populations was observed from 2000 to 2009 as a result of the 2004 Indian Ocean tsunami (Figure 3). The overall impact of tsunamis on human populations is summarized in Table 2.

Tsunami Characteristics. The physical characteristics of tsunamis were reported by NOAA-NGDC and included distance of the tsunami run-up from source, wave height, and earthquake magnitude. The median distance from source was 119 km (mean 810 km; range 7-10,621 km), and the median wave height was 6.7m (mean 13.0m; range 1.8 -67.1m). The majority of the tsunamis reported were due to earthquakes (95.5%), with small minorities resulting from landslides (3.0%), volcanoes (0.8%), and meteorological events (0.8%). Median magnitude for earthquake generated tsunamis was 8.1 (mean 8.1; range 6.3-9.5).

Mortality. Deaths were reported in 92.6% (n=94) of tsunamis occurring between 1900 and 2009. There were an estimated 255,195 deaths (range 252,619-275,784) resulting from tsunamis recorded in the historic event review, with the 2004 Indian Ocean tsunami accounting for an estimated 227,497 deaths (89%) of all mortality. Deaths were concentrated in Indonesia (62% or 170,689 deaths), Sri Lanka (13% or 35,399 deaths), Thailand (3.2%, 8,876 deaths) and the Philippines (3% or 8,137 deaths). Apart from rare high-impact events such as the 2004 Indian Ocean tsunami, mortality levels in tsunamis tend to be relatively low with a median of 50 deaths per event (mean=2,963,range 0-165,708) when using the highest reported death toll.

Table 3 presents results of the bivariate analyses between tsunami characteristics and mortality. Time period, and GINI coefficient were not statistically associated with tsunami mortality. There were considerable differences in mortality levels by WHO region, with the majority of tsunami events that occurred in the Americas resulting in low (<10) deaths and 50% of events in the South East Asian region resulting in high (>75) deaths (p<.001). Both World Bank-defined income level and per capita GDP were significantly associated with tsunami mortality. Whereas the majority of events in high income countries resulted in ten or fewer deaths, a considerably greater number of events in low and lower-middle income countries resulted in greater than ten deaths (p=.009). Similarly, events that resulted in greater than 75 deaths occurred in countries or territories with per capita GDPs that were more than two times lower than countries that experienced fewer tsunami deaths (p<.001). While the data show an inverse associations between tsunami mortality and distance from source (p=.001) and a positive association between mortality and wave height (p=.016), it is important to note that information pertaining to both measures were missing in a considerable proportion of events, particularly those occurring earlier in the study period.

Injury. Injury reports were only available in 22 (23.4%) events for a total of 48,462 injuries (range 45,466-51,457). A typical tsunami caused 218 injuries (median value) but the distribution was skewed by rare large-scale events (mean injured=2,339, range=0-23,176). To more accurately estimate the total number of injuries due to tsunamis, it was presumed that injuries would occur in events where deaths were reported. There were 87 tsunami events with fatalities; when the median and mean for injuries were applied to the remaining 65 events with fatalities, it was estimated that between 14,170 and 152,035 unreported tsunami-related injuries occurred worldwide between 1900 and 2009.

Systematic Literature Review

Mortality. Of the 33 articles reviewed, 12 reported on mortality, including mortality counts or rates and/or risk factors for death (Table 4). Age and sex were described as risk factors for death in three articles, all from the 2004 Indian Ocean tsunami ^28,32,37. In both Indonesia and Sri Lanka, significantly higher death rates were reported among females who were 1.4 to 2.1 times more likely to perish than males. Significantly elevated risk of death was also observed in children (1.8 to 4.3 times) and older adults (2.1 to 3.1 times) who were more likely to die compared with younger/middle age adults. Other risk factors included education which was inversely associated with mortality risk ²⁸; fisheries-based livelihoods ^28,32indoor location at the time of the tsunami and home destruction ²⁸ and the physical environment ¹⁵. The majority of tsunami deaths were due to drowning ^11,14.

Injury. Tsunami-related injuries were reported in 18 articles though only ten provided detailed information on injury types or risk factors (Table 5). Only one study estimated a population-based injury rate of 8.5% in Indonesia following the 2004 tsunami ⁴². Other articles described the proportion of care seekers with tsunami-related injuries or included only patients with tsunami-related injuries, preventing comparisons in injury patterns across events and locations. Only two studies described risk factors for tsunami-related injury, both with similar findings, in the 2004 Asian tsunami. The first, a study of patients seen by a medical team in Sri Lanka, observed that males and adults (15-64 years) seeking care for injuries were over-represented, suggesting these population sub-groups had higher injury rates ²⁶. The second used household survey data from Indonesia and reported lower injury risk among females (OR=0.81) and the highest injury rates among those 20-49 years of age. The risk factors for injury were opposite those of mortality suggesting that those more likely to survive the tsunami were also more likely to be injured ⁴². The most common types of injuries were wounds due to physical impact with debris, fractures, and near drowning and/or aspiration ^{21,22,23,25,26,30,42} . As compared to other types of natural disasters, tsunamis often result in relatively high mortality but have lower rates of injury ¹¹.

Discussion

Main Findings

From 1900 to 2009, approximately 2.5 million people were affected by tsunamis including over 255,000 deaths and an estimated 50,000 injuries. The mortality and population affected estimates presented in this study are consistent with other reviews of global tsunami events ¹¹ It is likely that the number injured is underestimated, given the low frequency with which this figure was reported, particularly in the first half of the 20^th century. When the mean and median numbers injured were applied to events that resulted in deaths it was estimated that between 10,900 and 116,950 unreported tsunami-related injuries occurred worldwide during this time period. It is important to note, that studies have shown that that ratio of deaths to injuries following a tsunami is typically significantly higher compared with other natural disaster types ¹¹.

This study is the first to examine the influence of place and event characteristics on tsunami mortality. Analyses of tsunami run-up data from 1900 to 2009 reveal that events occurring in the South East Asian region were significantly more likely to result in greater numbers of deaths compared to other regions, and this finding persisted even after excluding the 2004 Indian Ocean tsunami. Examination of the relationship between mortality levels and the two poverty measures (World Bank income level and per capita GDP) demonstrate that risk of mortality event is significantly higher in low-income countries. Lastly, certain event characteristics are more likely to be predictive of mortality than others. Increased wave height and closer proximity to the source were associated with higher mortality levels whereas earthquake magnitude was not associated mortality. A number of other factors that were not examined in this study have been shown to influence tsunami damage and impacts including wave velocity, water depth and submarine topography ¹⁰. Ecological assessments such as this review may mask significant in-country differences in mortality risk (e.g. population density, rural or urban location, geographic variations and individual-level socio-demographic factors).

Findings from the systematic literature review of studies examining tsunami-related mortality and injury contribute to understanding the primary causes of death and types of injury, as well as factors that place certain populations at increased risk. The most common cause of tsunami-related death was drowning, and the most frequently reported injuries included wounds and lacerations, fractures, and near drowning and/or aspiration. When reported the mortality risk was higher among females and the very young and old and injuries were more common among middle-aged men. Additional mortality risk factors included location during the event and fisheries-based livelihood. Few studies assessed or found relationships between socioeconomic status and mortality risk, although that one study found an inverse association between education level and mortality ²⁸ which suggests that low socioeconomic status may place individuals at increase mortality risk. This suggests that preparedness efforts target specific population groups for the prevention of deaths and injuries. Future studies on both tsunami-related injury and mortality risk would benefit from incorporating additional socio-demographic measures to gain a more comprehensive understanding of risk factors.

A number of strategies could be adopted by the international community and vulnerable countries to mitigate the short and long term impacts of future tsunami events. In the 1998 Papua New Guinea tsunami factors that contributed to higher mortality during this event included concentration of populations in vulnerable areas and failure of residents in affected areas to timely evacuate. In contrast, deployment of medical assistance and international support immediately following the tsunami played a large role in preventing further loss of life ¹³.With respect to public health in particular, a greater focus on ongoing disease surveillance, the appropriate targeting of aid to those in most need, and strengthening of health care systems ¹¹ can help to mitigate the medium to long term health impacts of tsunamis. Advanced tsunami warning systems may vastly improve early detection, and education campaigns can play a crucial role in improving awareness about tsunami risk and mitigation ¹⁰. Policies enacted by the Sri Lankan government following the 2004 tsunami highlight some of the challenges to longer term disaster mitigation where enactment of a buffer zone policy that forced the relocation of coastal communities had deleterious social and economic impacts ¹². In adopting disaster mitigation policy, governments should consider the contribution not only of physical vulnerability (i.e. distance of communities from coastal areas) but also social vulnerability, such as livelihoods, to increased disaster risk and the potential short and long term impacts such policies may have on affected communities.

Limitations

Systematic reviews face numerous limitations. The effects of tsunamis are the subject of gross approximations and aggregations which result in a great deal of imprecision. The availability and quality of data has likely increased and improved over time, however, in many events deaths are unknown or unrecorded. For a significant number of events no data are reported for injured, displaced, and affected populations; this likely contributes to a substantial underestimation of the impacts of tsunamis on human populations. Inconsistencies and errors were common in the data files from the two different sources, and in some cases inclusion criteria were not ideal for the purposes of this review which created a challenge in reconciling event lists. Additionally, mainly due to the small number of tsunami run-up events reported over the study period, we were unable to perform more complicated statistical analyses that would have provided estimates of the independent effects of place and event characteristics on tsunami related death. When combined with the relatively small number of tsunami events, uncertainty in the historical record limits the conclusions that can be drawn about the impact of tsunamis on human populations. A principal limitation of the literature review is the fact that only English language publications were included; this likely contributed to incomplete coverage of studies published in other languages originating from low and middle income countries.

Conclusions

From 1900 to 2009, a total of 250,000 tsunami-related deaths and close to 50,000 injuries, respectively, were reported, the majority of which were concentrated in the 2004 Indian Ocean tsunami. An estimated 2.5 million people were affected by tsunamis during this time period. While mortality estimates presented in this study are consistent with those reported in other studies, particularly for the 2004 Indian Ocean Tsunami, the injury figure may be an underestimate of the true value given low reporting levels. The distribution of tsunami related deaths varied greatly by region and economic development level. Findings from the historical event review indicate that the South East Asian region and poorer countries were more likely to experience higher mortality was associated with larger wave height and closer proximity to the source.

The primary cause of tsunami-related mortality was drowning and, although a number of injury types were reported following tsunamis, the ratio of dead to injured is much greater in tsunamis as compared to other natural disaster types. Risk factors for tsunami-related death included female sex and very young and old age. Tsunami losses are likely to increase in future years due to population growth in high risk seismic areas. Increased attention to tsunami prevention and mitigation strategies, with a focus on areas most prone to tsunamis and populations at greater risk is necessary. While strategies that are specific to the development level and country context are important, global initiatives such as early warning systems are essential for further tsunami risk mitigation.

Competing Interests

The authors have declared that no competing interests exist.

Correspondence

Shannon Doocy, Johns Hopkins Bloomberg School of Public Health, 615 N. Wolfe St, Suite E8132, Baltimore, MD 21230. Tel. 410-502-2628. Fax: 410-614-1419. Email: [email protected].