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Tsunami hazard: A practical guide for tsunami hazard reduction
Physics of the Earth and Planetary Interiors, 72 (1992) 299—301 Elsevier Science Publishers B.V., Amsterdam
Tsunami Hazard: A Practical Guide for Tsunami Hazard Reduction, E.N. Bernard (Editor), Kiuwer Academic Publishers, Dordrecht, 1991, 326 pp., hardback. Dfl. 140 (US $94/UK $49). ISBN 0792311744.
The book is formed by a selection of papers presented at the XIV International Tsunami Symposium (ITS) held in Novosibirsk, Russia from 31 July to 3 August 1989 and sponsored by the Tsunami Commission (TC), a body of the International Union of Geodesy and Geophysics (IUGG). The book is edited by E. Bernard, Chairman of the Tsunami Commission. The work is completed by two short reports regarding two companion Tsunami Meetings held in Novosibirsk, following this Symposium; the II International Intergovernmental Oceanographic Commission (IOC) Workshop on Technical Aspects on Tsunami Warning Systems, Tsunami Analysis, Preparedness, Observation and Instrumentation, 4—5 August 1989, and the XII Session of the International Coordinating Group, Tsunami Warning System in the Pacific (ICG/ITSU), 7—10 August 1989. Only 13 of the 62 presentations given at the symposium were selected for inclusion in the book, which begins with E. Bernard’s Opening Address, schematically and incisively highlighting the history of the Tsunami Commission since its creation in 1960 and tsunami research over the following 30 years. The 13 papers may be grouped into three main categories according to their subject, namely, observations (two), physical processes (six) and hazard mitigation (five). This classification is a small implicit criticism of the volume title and subtitle which are slightly misleading, since they improperly suggest that the book is focussed on tsunami hazard problems and on systems/ strategies for tsunami risk reduction, while other topics are also treated satisfactorily and deserve readers’ attention. Good examples of how to examine observations from tsunami recorders are found in the
299
first two papers, respectively, by American and Russian scientists. The USA team composed of Gonzales, Mader, Eble, and Bernard analyzes deep ocean records from a network of bottom pressure recorders established for tsunami monitoring in the northern Pacific. Signals from two tsunamis produced in the Alaskan Gulf in November 1987 and in March 1988 are compared with synthetic waveforms calculated by means of SWAN, a numerical model for tsunami generation/ propagation solving non1inear shallowwater equations. Agreement was good for the first few waves of the second tsunami, while significant differences are found for the 1987 event. Since the computed waves are sensitive to the earthquake source parameters assumed, it follows that (1) a better fit may be found only after an improved knowledge of the seismic source is provided by seismic data analyses or that, alternatively (2) tsunami waveforms may be used to put constraints on the source mechanism. The analysis of signals from bottom cable stations on the southwestern shelf of Kamchatka performed by a Russian group (Kovalev, Rabinovich, and Shevchenko) is used to demonstrate that to establish an inexpensive, long-term tsunami network in difficult environmental conditions is a feasible task. In the absence of tsunamis, the tsunami frequency band in the recorded signals is shown to be weakly correlated with atmospheric fluctuations, but strongly correlated with sea-surface activity: a sort of ‘negative viscosity’ being invoked as the mechanism to transfer energy from storm-generated short gravity waves to the tsunami frequency window, according to the Longuet-Higgins and Hasselmann theories. The observed long-wave spectra exhibit stable features that may be explained in terms of near-shore bathymetry. A linear bottom slope model for standing leaky waves instead of the more usual model for trapped edge waves is used to compute spectra that are in satisfactory agreement with the observations. Physical processes concern generation, propagation, and impact of long waves on the coasts.
300
These aspects are addressed by six papers on numerical models and laboratory experiments. A precious of the state of the art of numerical models of tsunamis together with a discussion of near-future perspectives is given by Shuto. Simulations of Pacific Ocean tsunamis, such as the 1960 Chilean event, the 1964 Alaskan case, and the 1983 Japan Sea tsunami, are used (1) to discuss the influence of the seismic source description on tsunami nucleation, (2) to examine deep-sea propagation and, specifically, the limits of the linear long-wave approximation, numerical amplitude decay, theoretical and numerical wave dispersion, and Coriolis force, (3) to review nearshore and run-up problems with inclusion of instabilities arising near the boundaries or near the wave fronts and of moving boundary approximation. Shuto also contributes to another paper together with Nagano and Imamura where the 1960 Chilean tsunami is simulated. They devote particular attention to dispersion in the deep-ocean tsunami propagation and to an accurate treatment of coastal transformation processes, such as energy dissipation due to sea-bottom scouring in long bays. It is stressed that accurate current velocity estimates may be used to explain damage to coastal aquacultures, namely, to pearl culture rafts. Satake and Kanamori use tsunami waveform inversion techniques to evaluate the spatial distribution of coseismic slip on the fault plane. After calibrating the method by examining the 1968 Tokachi-oki and 1983 Japan Sea tsunamis, for which enough seismic and tidegauge data are available, they study the anomabus 1984 Torishima tsunami that was unusually large for the earthquake size. As a result of their inversion, they are able to prove that the largeamplitude tsunami observed was mainly due to a propagation effect. This technique is quite promising since it also enables one to study historical tsunamigenic earthquakes using only tidegauge records and no seismic data. As an illustration of this principle, an application to the 1944 Tonankai and 1946 Nankaido earthquakes gives the estimated slip distributions on the fault. Tsunami run-up is dealt with in three papers. Transition from tsunami bore to run-up is studied by means of laboratory experiments by Yeh.
Though the motion near the run-up water line is very complicated, nevertheless shallow-water wave theory may be of help in predicting the maximum run-up height. This is also shown in a companion paper by Sinolakis who applying linear and non-linear theory approximations demonstrated clearly the validity of the linear computations to explain Yeh’s results of long waves on steep slopes. Bores of undular and non-undular type owing to tsunamis ascending rivers are studied by a Japanese team (Tsuji, Yanuma, Murata, and Fujiwara), using the numerous bores formed by the 1983 Japan Sea tsunami. Their theoretical approach is based on solving the Korteweg—de Vries—Burger’s equation under different conditions, that give rise to numerical solutions ranging from undular to non-undular water surface profiles, and on comparing their results with laboratory experiments. The problem of tsunami hazard is treated in five papers, two concerning the Mediterranean Basin and three the Pacific Ocean. Tsunamis in, and near, Greece are investigated by Papazachos and Dimitriu, who study the relation between tsunamis and focal mechanisms. By examining historical and instrumental earthquakes, on the basis of seismotectonic considerations and faultplane solutions estimated for earthquakes from 1962 to 1986, they are able to distinguish various alignments of seismic sources. The available tsunami catalogs are used to establish their tsunamigenic potential and to identify the most active and dangerous tsunamigenic zones affecting Greece. The same problem of tsunami potential evaluation is also addressed by Tinti in another paper, concerning seas surrounding Italy. It should be stressed that Greece and Italy are the countries with the highest tsunami activity in the Mediterranean. The method followed in this second case is essentially based on the statistical analysis of the Italian seismic catalog. This is studied to determine the strongest seismic sources near-shore and off-shore and then to evaluate their ability to produce tsunamis. The method may be successfully applied in all regions where tsunami catalogs are low in data, while seismic data are abundant and cover long periods of time. From the analysis, it results that Calabria
301
and the coasts of eastern Sicily as well as the Gargano promontory coast in Puglia, southern Italy, are the most exposed to tsunami attacks. Farreras and Sanchez investigate tsunami hazard for the Mexican west coast. Information on events in the last 200 years reveals the existence of two zones, south and north of the Rivera fracture, that are mainly affected by local and by remotely generated tsunamis, respectively. Tsunami Warning Systems (TWS) of different types are needed for the two zones. The paper illustrates the system presently in operation on the northern coast, mentions a project to establish a real-time system in the south, and gives a useful example of microzonation, vulnerability assessment, and risk analysis for the coastal town of Marina Cruz. THRUST (Tsunami Hazards Reduction Utilizing Systems Technology) is the subject of two papers. It is a real-time TWS whose history, from
the past justification and design phase to the present implementation in Chile and future devebopments is reported by Bernard. The system, originally devised to provide a quick response time of about 1 mm by using satellite communication links, has been enhanced to an even shorter response time of about 20 s, thanks to improvements in satellite operations. The THRUST systern operating in Chile and its integration into the National Chilean TWS is discussed by Lorca, who describes the standard operations plan and its testing during a disaster simulation exercise. This volume is recommended to students, researchers, scientists, and engineers involved in studies in tsunami science, seismology, oceanography, and natural hazards assessment and mitigation. Stefano Tinti (Università di Bologna, Italy)
Tsunami Hazard: A Practical Guide for Tsunami Hazard Reduction, E.N. Bernard (Editor), Kiuwer Academic Publishers, Dordrecht, 1991, 326 pp., hardback. Dfl. 140 (US $94/UK $49). ISBN 0792311744.
The book is formed by a selection of papers presented at the XIV International Tsunami Symposium (ITS) held in Novosibirsk, Russia from 31 July to 3 August 1989 and sponsored by the Tsunami Commission (TC), a body of the International Union of Geodesy and Geophysics (IUGG). The book is edited by E. Bernard, Chairman of the Tsunami Commission. The work is completed by two short reports regarding two companion Tsunami Meetings held in Novosibirsk, following this Symposium; the II International Intergovernmental Oceanographic Commission (IOC) Workshop on Technical Aspects on Tsunami Warning Systems, Tsunami Analysis, Preparedness, Observation and Instrumentation, 4—5 August 1989, and the XII Session of the International Coordinating Group, Tsunami Warning System in the Pacific (ICG/ITSU), 7—10 August 1989. Only 13 of the 62 presentations given at the symposium were selected for inclusion in the book, which begins with E. Bernard’s Opening Address, schematically and incisively highlighting the history of the Tsunami Commission since its creation in 1960 and tsunami research over the following 30 years. The 13 papers may be grouped into three main categories according to their subject, namely, observations (two), physical processes (six) and hazard mitigation (five). This classification is a small implicit criticism of the volume title and subtitle which are slightly misleading, since they improperly suggest that the book is focussed on tsunami hazard problems and on systems/ strategies for tsunami risk reduction, while other topics are also treated satisfactorily and deserve readers’ attention. Good examples of how to examine observations from tsunami recorders are found in the
299
first two papers, respectively, by American and Russian scientists. The USA team composed of Gonzales, Mader, Eble, and Bernard analyzes deep ocean records from a network of bottom pressure recorders established for tsunami monitoring in the northern Pacific. Signals from two tsunamis produced in the Alaskan Gulf in November 1987 and in March 1988 are compared with synthetic waveforms calculated by means of SWAN, a numerical model for tsunami generation/ propagation solving non1inear shallowwater equations. Agreement was good for the first few waves of the second tsunami, while significant differences are found for the 1987 event. Since the computed waves are sensitive to the earthquake source parameters assumed, it follows that (1) a better fit may be found only after an improved knowledge of the seismic source is provided by seismic data analyses or that, alternatively (2) tsunami waveforms may be used to put constraints on the source mechanism. The analysis of signals from bottom cable stations on the southwestern shelf of Kamchatka performed by a Russian group (Kovalev, Rabinovich, and Shevchenko) is used to demonstrate that to establish an inexpensive, long-term tsunami network in difficult environmental conditions is a feasible task. In the absence of tsunamis, the tsunami frequency band in the recorded signals is shown to be weakly correlated with atmospheric fluctuations, but strongly correlated with sea-surface activity: a sort of ‘negative viscosity’ being invoked as the mechanism to transfer energy from storm-generated short gravity waves to the tsunami frequency window, according to the Longuet-Higgins and Hasselmann theories. The observed long-wave spectra exhibit stable features that may be explained in terms of near-shore bathymetry. A linear bottom slope model for standing leaky waves instead of the more usual model for trapped edge waves is used to compute spectra that are in satisfactory agreement with the observations. Physical processes concern generation, propagation, and impact of long waves on the coasts.
300
These aspects are addressed by six papers on numerical models and laboratory experiments. A precious of the state of the art of numerical models of tsunamis together with a discussion of near-future perspectives is given by Shuto. Simulations of Pacific Ocean tsunamis, such as the 1960 Chilean event, the 1964 Alaskan case, and the 1983 Japan Sea tsunami, are used (1) to discuss the influence of the seismic source description on tsunami nucleation, (2) to examine deep-sea propagation and, specifically, the limits of the linear long-wave approximation, numerical amplitude decay, theoretical and numerical wave dispersion, and Coriolis force, (3) to review nearshore and run-up problems with inclusion of instabilities arising near the boundaries or near the wave fronts and of moving boundary approximation. Shuto also contributes to another paper together with Nagano and Imamura where the 1960 Chilean tsunami is simulated. They devote particular attention to dispersion in the deep-ocean tsunami propagation and to an accurate treatment of coastal transformation processes, such as energy dissipation due to sea-bottom scouring in long bays. It is stressed that accurate current velocity estimates may be used to explain damage to coastal aquacultures, namely, to pearl culture rafts. Satake and Kanamori use tsunami waveform inversion techniques to evaluate the spatial distribution of coseismic slip on the fault plane. After calibrating the method by examining the 1968 Tokachi-oki and 1983 Japan Sea tsunamis, for which enough seismic and tidegauge data are available, they study the anomabus 1984 Torishima tsunami that was unusually large for the earthquake size. As a result of their inversion, they are able to prove that the largeamplitude tsunami observed was mainly due to a propagation effect. This technique is quite promising since it also enables one to study historical tsunamigenic earthquakes using only tidegauge records and no seismic data. As an illustration of this principle, an application to the 1944 Tonankai and 1946 Nankaido earthquakes gives the estimated slip distributions on the fault. Tsunami run-up is dealt with in three papers. Transition from tsunami bore to run-up is studied by means of laboratory experiments by Yeh.
Though the motion near the run-up water line is very complicated, nevertheless shallow-water wave theory may be of help in predicting the maximum run-up height. This is also shown in a companion paper by Sinolakis who applying linear and non-linear theory approximations demonstrated clearly the validity of the linear computations to explain Yeh’s results of long waves on steep slopes. Bores of undular and non-undular type owing to tsunamis ascending rivers are studied by a Japanese team (Tsuji, Yanuma, Murata, and Fujiwara), using the numerous bores formed by the 1983 Japan Sea tsunami. Their theoretical approach is based on solving the Korteweg—de Vries—Burger’s equation under different conditions, that give rise to numerical solutions ranging from undular to non-undular water surface profiles, and on comparing their results with laboratory experiments. The problem of tsunami hazard is treated in five papers, two concerning the Mediterranean Basin and three the Pacific Ocean. Tsunamis in, and near, Greece are investigated by Papazachos and Dimitriu, who study the relation between tsunamis and focal mechanisms. By examining historical and instrumental earthquakes, on the basis of seismotectonic considerations and faultplane solutions estimated for earthquakes from 1962 to 1986, they are able to distinguish various alignments of seismic sources. The available tsunami catalogs are used to establish their tsunamigenic potential and to identify the most active and dangerous tsunamigenic zones affecting Greece. The same problem of tsunami potential evaluation is also addressed by Tinti in another paper, concerning seas surrounding Italy. It should be stressed that Greece and Italy are the countries with the highest tsunami activity in the Mediterranean. The method followed in this second case is essentially based on the statistical analysis of the Italian seismic catalog. This is studied to determine the strongest seismic sources near-shore and off-shore and then to evaluate their ability to produce tsunamis. The method may be successfully applied in all regions where tsunami catalogs are low in data, while seismic data are abundant and cover long periods of time. From the analysis, it results that Calabria
301
and the coasts of eastern Sicily as well as the Gargano promontory coast in Puglia, southern Italy, are the most exposed to tsunami attacks. Farreras and Sanchez investigate tsunami hazard for the Mexican west coast. Information on events in the last 200 years reveals the existence of two zones, south and north of the Rivera fracture, that are mainly affected by local and by remotely generated tsunamis, respectively. Tsunami Warning Systems (TWS) of different types are needed for the two zones. The paper illustrates the system presently in operation on the northern coast, mentions a project to establish a real-time system in the south, and gives a useful example of microzonation, vulnerability assessment, and risk analysis for the coastal town of Marina Cruz. THRUST (Tsunami Hazards Reduction Utilizing Systems Technology) is the subject of two papers. It is a real-time TWS whose history, from
the past justification and design phase to the present implementation in Chile and future devebopments is reported by Bernard. The system, originally devised to provide a quick response time of about 1 mm by using satellite communication links, has been enhanced to an even shorter response time of about 20 s, thanks to improvements in satellite operations. The THRUST systern operating in Chile and its integration into the National Chilean TWS is discussed by Lorca, who describes the standard operations plan and its testing during a disaster simulation exercise. This volume is recommended to students, researchers, scientists, and engineers involved in studies in tsunami science, seismology, oceanography, and natural hazards assessment and mitigation. Stefano Tinti (Università di Bologna, Italy)