TSUNAMI HAZARD ANALYSIS, TSUNAMII HAZARD PLANNING, PROTECTION MEASURES, TSUNAMI EXERCISES AND PUBLIC EDUCATION
There is a need to evaluate the tsunami hazard in order to reduce or mitigate factors endangering public safety and property in coastal communities which have been historically impacted. The risk potential of tsunamis is of extensive interest to governmental, non-governmental agencies and to industries and the public in general. Because the tsunami hazard frequency in the Pacific is the greatest, most efforts in risk assessment and tsunami bazard planning and management have concentrated in this area of the world. Other communities in other parts οf the world are not immune to tbe tsunami bazard. Νο matter how remote the likelihood οf a tsunami may be, the risk should be considered in developing coastal zone management and land use. While some degree οf risk is acceptable. government agencies should promote new development and population growth in areas οf greater safety and less potential risk. These agencies have the responsibility οf evaluating the tsunami bazard and establisbing adequate warning procedures to protect the communities under tbeir jurisdiction. Furthermore, tbese agencies sbould establish proper training for public safety personnel, conduct frequent tsunami exercises and formulate land-use regulations for given coastal areas – particularly if these areas are known to have sustained tsunami damage in the past. Finally, in designing important engineering structures in the coastal zone, the risk resulting from the tsunami bazard should be evaluated and construction should incorporate adequate safety features. Appropriate guidelines need Ιο be developed for the evaluation οf the tsunami risk in terms of frequency of occurrence, severity οf impact, design adequacy οf important coastal structures and, finally, in terms οf preparedness and planning for hazard mitigation. Α program of public education is the most important factor in tsunami bazard preparedness and mitigation.
It has been clearly documented that tsunamis have had a very important and long-term socioeconσmic impact οn the communities οf the Pacific and οn society in general (Pararas-Carayannls. 1983). However, the historical record does not prepare us for the sossible damage that can now be caused by tsunamis in the coastal areas ο? many developing or developed coastal countries where development has taken place in the last 20 years. Ιt is expected that future tsunamis will have a much more severe social and econonιic impact than that of past events. Thus, in assessing the tsunarmi hazard and preparing for such future events is very important.
Tsunami hazard analysis precludes a good understanding οΙ what tsunami events have occurred in tbe past and how often and to what extent these events have affected the region under study. Ιn the absence of adequate historical data. physical and cσmputer tsunami models can be utilized Ιο define quantitatively the tsunami hazard for proper planning and adequate preparedness. Preparedness involves proper hazard plannins, protection measures for the safety of the public and comprehensive programs which include exercises and public education.
TSUNAMI HAZARD ANALYSIS
Tbe risk potential of tsunamis is οf extensive interest Ιο governmental and non-governmental agencies and to industries and the public in general. Management of the tsunami hazard precludes analysis of the risk in planning for mitigation and good perception of the risk by those responsible for the protection of public safety and property. Ιt ix important tbat the hazard is evaluated properly and that the potential threat is correctly estimated. Underestimating the tsunami threat can be both expensive and counterproductive. Reduction of the tsunami risk is the responsibility of government agencies and any misconceptions related Ιο its proper assessment can produce the potential for unnecessary deaths and destruction.
Ιη most comrnunities tsunami hazard mitigation strategies are applied οn an ad hoc basis and in uncoordinated manner – usually following the occurrence of an actual event. However, strategies can be far more effective if they are implemented before the occurrence οf a catastrophic tsunami. The success or failure of any safety program rest οn a valid appreciation of the distribution of the tsunami rlsk. For this reason a specific regional risk assessment must be undertaken Ιο determine how the tsunami hazard differs in its spatial distribution and potential severity οf impact. Rlsk is not an absolute and can be reduced by proper planning, proper public education and proper land use.
Studies of historical tsunamis of local and distant origin ls tbe first priority in tbe analysis of the tsunami bazard. The seismicity of the region should be studied in order Ιο establish the possible threat from local earΙhquakes. Analysls of the seismiclty of the region should go back in time as far as possible, and a historical tsunami data base should be developed consisting of all collected information. Similarly, tsunamls from distant earthquakes should be evaluated by careful review of historlcal information, films, photographs, newspaper articles and diagrams available from government and university archives, newspaper files, television studios, church groups and private collections. Such information may contain data which may shed light Ιη the spatial difference in the distribution of the tsunami impact οf past events which, in turn, may allow proper zonationa of the hazard.
Ιi is possible that major damaging events may not have occurred locally during tbe period οf record taking. However, ΙΙ may be possible to obtaln eye-witness accounts οf past events as remembered by older residents passed down by word of mouth. Such information may be of great value, particularly those ?n which the person lntervlewed was a vlctim or nearly so of the disaster and rememhers witb clarlty the event. Similarly legends of a tsunami catastrophe may have survived. However, such accounts may be distorted and sbould be evaluated carefully in identifying the potential tsunami risk and its recurrence frequency.
Tsunami Hazard Frequency
The next most important information needed ?n tsunami hazard analysis ls the recurrence frequency of tsunaml.s. Assuming that the historic record is long and there have been many years οf direct observations it is possible Ιο establisb the frequency of tsunami events. However, if the bistoric record is limited, planners cannot rely οm such short record alone Ιο evaluate the tsunarni bazard. Large catastrophic events may take place so infrequently in any οne location tbat tbere may be nο locally available data οn which Ιο predict rlsk and produce a zonation οf the hazard. This sbould not be misinterpreted to mean that tbere is nο danger. Α statistical approach may be the only way for the prediction οf the spatial distribution οf the tsunami disaster. The statistical distribution ?n the occurrence οf extreme eνents has been treated by Gumbel (1958). Ιn this, he suggests that the recurrence lntervals οf exceptionally large phenomena bear consistent relationships Ιο their magnitude expressed ?n either arithrnetic or logarithmic terms. Thus, 50 years οf data can be used Ιο extrapolate and determine the once-in-a-thousand-year eνent. Of course this approach ls rather vague as the confidence limits are usually so large so that the resultlng estimates of recurrence are largely meaningless. On this basis ?ι ls very difficult Ιο accept the statlstics of extreme events as the basis for plannlng. Ιn such cases one may have to resort to geological investigations of coastal sediments or tsunami modelling studies as briefly described ?n the following section.
Ιn the absence of historical information, the tsunami disaster may be simulated by making scale models of coastlines and introducing scale models of buildings and other aspects of land use into the physical models. Then, the tsunami may be produced with appropriate wave generators and its impact can be photographed, measured and recorded. Such models have been made. For example for Hilo Bay in Hawaii by the U.S. Corps of Engineers and in close cooperation with the University of Hawaii. Many other hydraulic models have been built Ιο assist in predicting the potential of the tsunami disaster. Such models are expensive to construct and Ιο scale down ?η size both geometrically and kinematically. However, physical models bave been very useful for important coastlines where important engineering structures bave been built.
Computer models permit relatively accurate predictions οf tbe potential tsunaml inundation and can be lnvaluable in the management οf tbe tsunaml bazard. The construction οf such models involves four common elements. The first of tbese elements ls the initial analysis οf the physical characteristics οf tbe tsunaml hazard. This permits the subsequent development οf tbe mathematical model wbich is capable of forecasting tbe severity of the tsunami impact for different events approaching the coastline under study from different directions. Such an approacb leads Ιο the development of tbe spatial pattern οf impact intensity which in turn can be used for tbe microzonation οf the t-sunami hazard. Most nunerlca1 models deal primarily with··the extent and height of tsunami inundation leavlng all otber engineering interpretations to planners and englneers. From such models the extent οf damage can be estimated and evacuation limits can be established to minimize deaths and injuries. This information ls normally presented in a map form with tabulations so that both the spatlal distribution of tbe tsunami rlsk and its gross impact can be established.
Zonation of the Tsunami Hazard
The final product οf the historical studies of the recurrence frequency,and of the hydraulic and numerical modelling, is a representation of tbe spatial varlatlons οf the tsunami hazard along a given coastlne where expected tsunaml height can be quantified and evacuation limits designated. These are prerequisites for proper planning. ·Because of the extreme selectlve nature οf tsunami destruction along given coastlines, a microzonation map οf the tsunaml hazard may be required which will be of great usefulness in planning and management οf the hazard. Similarly, tbe total risk at any point can be established by such studies, as well as the probability of occurrence for insurance purposes. The production of large scale maps depicting variations in the degrees οf tsunami risk are lnvaluable tools in the disaster planning process. Ιn this way, high risk areas can be avoided or used for low intensity development only.
Ιn producing maps οf the tsunami hazard, attention should be paid Ιο scale requirements so that the significance of the hazard can easily be identified and correlated Ιο prominent landmarks. Such maps sbould be sufficient for precise planning ο! land use and should include vertical and horizontal parameters of scaling that are sufficientIy Iarge. For example the tsunami inundation maps that had been produced for Hawaii have been made at scales ο! 1:63,360, and 1:24,000 respectiveIy. ΑΙΙ that can really be suggested is that the seIected scaIe be large enough Ιο make full use ο! the available data and thus permit individual sites and structures Ιο be identified, if possible. ΙΙ should not be so Iarge, however, that it gives an invalid impression of precision in areas where the information does not warrantee such a position. Figure 1 (below) is an example showing potential tsunami inundation maps for Hawaii.
TSUNAMI HAZARD PLANNING AND PREPAREDNESS
There is very little that can be done Ιο prevent the occurrence ο! a tsunami. Ιn the past, before tsunami warning systems were estab1ished, there was a passive approacb Ιο this type of hazard. ΒυΙ while these natural disasters cannot be prevented, their results, such as loss of life and property, can be reduced by proper planning and preparedness.
The tsunami hazard is not frequent and when it does occur ists destructiveness varies from place Ιο place. With proper planning it is a hazard that can be dealt with effectively and its adverse effects can be considerably mitigated. Νο matter how remote, the Iikelihood of a tsunami should be considered in developing plans for public safety and land use management. While some degree ο? risk iss acceptabIe, government agencies should promote new deveIopment and population growth in areas of greater safety and less potential risk (Pararas-Carayannis, 1983). Such government agencies should formulate Iand-use regulations for a given coastal area with the -sunami risk potential in mind, particularly if such an area is known to have sustained tsunami damage in the past.
Tsunami hazard planning and preparedness require proper hazard analysis as outlined previously. This is fairly simple process for certain regions of the Pacific and very difficult for others. Once the historic record of tsunami activity has been examined and proper studies have been completed, only then, fundamental questions οf preparedness can be addressed, such as: What safety measures can be taken by authorities in protecting the coastal population and vital coastal resources, industries and structures? Ηοw can the risk ο! the tsunami hazard be minimized? Are public safety personnel properly trained to deal with the disaster? Are relief facilities adequate to respond in an emergency situation? What level of risk is acceptabIe?
Public safety should be the primary consideration. Government agencies have the responsibility of evaluating the tsunami hazard Ιη accordance with the methodology described and establishing adequate warning procedures to protect the communities under their jurisdiction. It is difficult Ιο establish acceptability of the tsunaml hazard in terms of risk to life. Ιt is the responsibility of the planners Ιο establish standards of an acceptable personal risk and ratio οf injury Ιο fatality. From a moralistic point of view any loss of life is unacceptable – whether directly or indirectly associated with the potential hazard.
The level at which property risk becomes unacceptab1e will generally depend upon the socioeconomic cost of the disaster and οf the size of the benefits accruing from the property Ιη question. For example, loss οf agricultural property and land use may be more acceptable than loss of a nuclear power plant. For such reasons, high rlsk standards may be required for certain lifeline facilities, since this often imposes a great social and economic disruption cost, should they fail. Examples οf this type of land use may include communication centers, chemical factories, nuclear power plants and other important engineering structures. Any unnecessary rlsk Ιο such unique and vital or dangerous properties may result Ιη enormous secondary damages in case of failure (Pararas-Carayannis 1976). For this reason risk should be decreased to the greatest tecbnologically feasible extent by proper design and land utilization. Other lifeline facilities may bear substantial social costs, such as loss οf important facilities, such as hospitals, fire stations, or police services. Such services are vital in disasters and their facilities should be appropriately protected Ιο guarantee their ability to function durlng periods οf emergency.
The key element of a tsunami safety program is a tsunami warning system. For areas where such a system is not functioning it should be established to allow for the monitoring οf potential tsunami disasters and for the issuance οf warnings. Civil Defense Agencies should establish plans for evacuation or other preventative measures to be taken when a tsunami danger arises. Present tsunami protective measures involve promarily existing tsunami warning systems which employ advanced tecbnological instrumentation for data collection and for warnlng communications. Many developed nations of the Pacific have developed sophisticated warning systems and have accepted the responsibility to share tsunami warning information with other countries of the Pacific.
Tsunami Exercises and Public Education
Α program of public education is the minimum requirement to minimize loss οf life. Warning procedures should be established and once established, they shoud be reviewed frequently to define and determine better respective responsibilities between the different government agencies at all levels. These agencies should publish proper training for public safety personnel and for the citizens in general. Tsunami exercises should be held frequently. Warning procedures are inevitably more successful if community awareness has been heightened and a disaster plan drawn up Ιο ensure that all necessary tasks are accomplished with a minimum of delay or confusion during a tsunami disaster. Το accomplish this objective a comprehensive program of public education is necessary.
Gumbel. E.J., 1958. Statistics of Extremes. Columbia University Press. New York. 1958.
Pararas-Carayannls, G., 1976. “Tsunaml Hazard and Design of Coastal Structures. Proceedings, International Conference οη Coastal Engineering. Honolulu, Hawaii. 11-17 July 1976.
Pararas-Carayannis. G., 1983. “The Tsunami Impact οη Society.” Tsunamis -Their Science and Engineering. Tokyo, Terra Scientific Publishing Company, 1983.