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Background
Tsunamis are among the most dangerous and complex natural phenomena, being responsible for great losses of life and extensive destruction of property in many coastal areas of the World Ocean . They are typical example of the “low probability – high consequence” hazard. On average, 2-3 damaging tsunamis occur in the World Ocean annually, but at each particular coastal location the recurrence interval between destructive impacts can vary from 30-50 to 200-300 years.
Two main practical tasks associated with tsunami problem are the operational (short-term) tsunami warning and the assessment of long-term tsunami risk (coastal tsunami zoning). In terms of early warning systems, the tsunami community is much ahead of other communities dealing for natural hazards. Since the middle of 60 th , the International Tsunami Warning System in the Pacific provides the operational warning within 1 hour after occurrence of potentially dangerous submarine earthquake anywhere in the Pacific basin. The system works under the auspices of the IOC/UNESCO International Coordination Group for the TWS in the Pacific and currently includes 28 nations. In 2005, similar groups were created in other main tsunamigenic regions, like the Indian Ocean , the North-West Atlantic and the Mediterranean .
However, in terms of assessment of long-term tsunami hazard and risk evaluation the situation is not so favorable. There is no generally adopted approach to this problem, no well developed common methodology for assessment of tsunami risk, and no any internationally coordinated program. In the Pacific region, several countries (USA, Canada, Japan, New Zealand, Australia) having long-term experience in implementation of tsunami mitigation programs and advance computational technologies widely use the tsunami inundation maps for coastal cities and other critical facilities obtained on the basis of so-called “scenario approach”. In other countries, there are just solitary attempts to evaluate tsunami hazard for selected coastal locations based on different approaches and methodologies sometimes specially developed for a particular case study.
Since the beginning of 1990 th , the seismological community has the internationally coordinated program GSHAP (Global Seismic Hazard Assessment Program) that a demonstration project of the UN/International Decade of Natural Disaster Reduction (UN/IDNDR) conducted by the International Lithosphere Program (ILP). The program resulted in the development of standard methodology for assessment of seismic hazard, its application on the regional and world-wide basis and the development of the Global Seismic Hazard Map (Giardini, Grunthal, Shedlock, Zhang, 1999).
The initiative of a newly formed ICG/IOTWS for development of common methodology and guidelines for tsunami risk assessment is very timely and its importance can not be overestimated. This initiative is especially useful and valuable for the countries of the Indian Ocean region that have very different level of tsunami hazard, different seismotectonic settings, availability of historical data on earthquake and tsunami occurrence, level of available expertise in research aspects of tsunami problem.
The tragic event of the December 26, 2004 Sumatra tsunami clearly showed that almost all parts of the Indian Ocean coast are exposed to the attacks of tsunamis and increasing exploitation of coastal areas for urban, industrial, commercial and resort facilities is demanding for more reliable and operational tsunami prediction and reliable long-term estimation of tsunami risk.
While the progress in the short-term tsunami prediction (operational warning) is made due to the continuous technical, methodological and organizational improvements, the state-of-the-art in long-term tsunami prognosis (coastal tsunami zoning) is almost the same as 20 years ago.
Existing methods of tsunami hazard assessment fall into three main categories. The first is application of a simple historical determinism method that generally means just mapping of what was happened in the past. The method implies the mapping of tsunami run-up effects historically known for a particular coastal area, to represent (with some corrections) the highest possible run-up to be expected in the future.
The second category consists of straightforward historical stochastic approach which is construction of a statistical model to reproduce available historical observations. This method is entirely based on available run-up statistics and does not consider any seismotectonic of the source area. An application of the historical stochastic method consists of the following basic steps:
(1) compilation of historical catalog tsunami observation (run-ups and tide-gauge measurements) for a particular site;
(2) assuming type of statistics and calculation of t sunami run-up frequency function ( empirical frequency of recurrence );
(3) calculation of “tsunami hazard function” (historical exceedance rate function)
(4) obtaining annual probability of exceedance for different run-up values.
The primary disadvantage of the stochastic method is its unreliability at lower annual probabilities than the inverse period of the catalog. This reliability can be extended somewhat by fitting a distribution to the tail (in which case the method is called “the parametric historic method”), but this does not relieve concerns that issues as seismic gaps or uncertainties in tectonics have been neglected. However, the most severe limitation of this method is that it is inapplicable for areas with limited or no historical tsunami observations (that is very common situation along the most part of the western and northern Indian Ocean coastline and the whole coast of Australia ).
The third method is based on the fully deterministic “scenario” approach and on intensive application of numerical models for calculation of tsunami generation, propagation and flooding. This method uses a single-valued event (a design earthquake) to occur at the most probable (or most dangerous) location. The parameters of this event are determined on the basis of “expert judgment” and their uncertainties are rarely taken into account. Tsunami run-up heights at the site of interest implied by such an event are then calculated. The frequency of event occurrence is usually not taken into account (or evaluated in very crude way), and there is no formal and open way of treating uncertainties. This method is widely used for calculation of tsunami inundation maps for coastal cities and other critical facilities located at close proximity of the coastline.
The modern approach to the similar problem of long term estimation of seismic hazard is based on seismotectonic consideration of seismogenic structures (blocks and faults) and processes (seismic cycles, gaps, migration, etc.) and application of numerical models to estimate ground shaking produced by an earthquake with expected source parameters. Further development of this approach employs non-Poissonian (time-dependent) earthquake recurrence evolving the full spatial-temporal properties of earthquake activity.
The proposed Tsunami Hazard Assessment Project for the Indian Ocean (THAP/IND) is directed to the development of modern common methodology applicable to the long- term tsunami risk assessment problem for areas with different level of tsunami hazard.
2. Objectives
The ultimate goal of the THAP/IND is the development of regionally coordinated, homogeneous methodology to the tsunami hazard evaluation to assess properly the long-term tsunami risk of different part of the Indian Ocean coast and elsewhere for further implementation in risk mitigation strategies. This approach will be based on the state-of-the art of similar methodologies for risk assessment in other disciplines (seismology, first of all), application of modern computational techniques and databases, as well as GIS technology for visualization and mapping.
The principal targets for THAP/IND are the countries locating near the main tsunami-prone areas in the Indian Ocean region, that presently do not have adequate national programs for tsunami hazard evaluation. Countries with moderate or low tsunami hazard will especially benefit from THAP/IND, since in these countries the tsunami risk to the coastline is not normally estimated and taken into account. Meanwhile, some of them have very densely populated coastline ( Sri Lanka , India , Bangladesh ) and critical coastal facilities (nuclear power plants, chemical plants, oil and gas terminals) that can be severe damaged or even destroyed by tsunami impact.
3. Benefits
The benefits of the THAP/IND will be standardized THA methodology implemented into the software packages for regional and national assessment of tsunami risk, that can be brought to the attention of national decision makers for the implementation in risk mitigation strategies.
The THAP/IND will fill a critical gap that many countries faced to in attempting to assess properly the tsunami hazard and risk along their coastlines.
The THAP/IND will promote a regionally coordinated, homogeneous approach to tsunami risk evaluation; the ultimate benefit will be national assessment of tsunami risk in a standardized form that can be brought to the attention of national decision makers for the implementation of risk mitigation strategies.
The THAP/IND will provide a framework for data exchange, will facilitate the creation of unified seismic and tsunami catalogs and databases, will facilitate the coordination in across-boundaries hazard assessment and the implementation of unified assessment procedures.
To achieve all these potential benefits, it is important to ensure that the THAP/IND operates in a cooperative fashion within national projects and programs for assessment and mitigation of tsunami risk as well as it is well coordinated with other IOC/UNESCO programs related to the tsunami hazard mitigation in the Indian Ocean region.
4. Methodology
The tsunami hazard can be defined as the probabilistic measure of coastal flooding associated with tsunami recurrence. Tsunami hazard for a particular coastal location is expressed as probability of exceedance of chosen run-up height and velocity of water current that likely will, or will not, be exceeded during the specific expose time. Tsunami hazard is usually expressed in the forms of tsunami hazard maps that depict the level of tsunami hazard over the selected coastal area.
The assessment of tsunami hazard is the first step in evaluation of the tsunami risk that is defined as convolution of tsunami hazard with vulnerability and the coast of restoration of damaged coastal facilities and infrastructure. The vulnerability is a specific factor that includes type, value and age of buildings and infrastructures, population density, land use, date and time of an event occurrence.
The proposed methodology of tsunami risk assessment for seismically induced tsunamis, that are up to ¾ of all historical tsunamis globally occurred, can be based on the seismotectonic-probabilistic approach that since the middle of 80 th is widely used in seismology for calculation of seismic hazard risk maps. The basic steps involved in the seismotectonic-probabilistic method are as follows:
(1) designation of zones of uniform seismicity within the area under investigation and describing it in terms of geometry and seismic sources distribution;
(2) designation of the set of active faults specified by geometry, sense of slip (type of mechanism), segmentation, and a function describing rupture length as a function of magnitude;
(3) establishing magnitude-frequency relation for each zone and indication the magnitude of largest possible earthquake;
(4) calculation of synthetic catalogs of run-up heights on the basic of numerical models of tsunami generation, propagation and run-up, adding the actual historical observations where available;
(5) application of statistical methods for evaluation the maximum expected run-up height per unit time.
Regarding the stage 4 (calculation of synthetic catalog of run-up heights at the coast, the proposed THA methodology is in more favorable position as compared with seismic hazard assessment methodology that is needed to predict the ground shaking from the source magnitude. This is due to availability of advanced numerical models for calculation of tsunami propagation and run-up allowing the precise prediction of expected run-up height at the coast for a given source mechanism and magnitude. However, the diversity of possible tsunami sources (volcanic explosions, submarine landslides, oceanic impacts, low-pressure atmospheric anomalies) implies the serious additional problems in assessment of overall tsunami risk that are still to be overcome.
On the other hand, even for one type (seismic) sources, the development of unified THA methodology presents a considerable difficulties due to the diversity of seismotectonic settings of the Indian Ocean region that includes areas with World's most active seismicity (Indonesian arc), high seismic risk areas like southern borders of Iran and Pakistan and north coast of Oman and almost non-seismic continental margins along eastern coast of Africa. The evaluation of tsunami hazard for the whole Indian Ocean region will require the characterization of the earthquake cycle over recurrence times from 10 2 years in active subduction areas (Indonesia) to 10 3 -10 4 years in slow crustal deformation areas (east Africa). For these areas, the incorporation of geological input is necessary to characterize the earthquake occurrence in space and time. |
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