Resolution vs. image quality in pre-tsunami imagery used for tsunami impact models in Aceh,Indonesia

Land cover roughness coefficients (LCRs) have been used in multivariate spatial models to test the mitigation potential of coastal vegetation to reduce impacts of the 2004 tsunami in Aceh, Indonesia. Previously, a Landsat 2002 satellite imagery was employed to derive land cover maps, which were then combined with vegetation characteristics, i.e., stand height, stem diameter and planting density to obtain LCRs. The present study tested LCRs extracted from 2003 and 2004 Landsat (30 m) images as well as a combination of 2003 and 2004 higher spatial resolution SPOT (10 m) imagery, while keeping the previous vegetation characteristics. Transects along the coast were used to extract land cover, whenever availability and visibility allowed. These new LCRs applied in previously developed tsunami impact models on wave outreach, casualties and damages confirmed previous findings regarding distance to the shoreline as a main factor reducing tsunami impacts. Nevertheless, the models using the new LCRs did not perform better than the original one. Particularly casualties models using 2002 LCRs performed better (δAIC > 2) than the more recent Landsat and SPOT counterparts. Cloud cover at image acquisition for Landsat and low area coverage for SPOT images decreased statistical predictive power (fewer observations). Due to the large spatial heterogeneity of tsunami characteristics as well as topographic and land-use features, it was more important to cover a larger area. Nevertheless, if more land cover classes would be referenced and high resolution imagery with low cloud cover would be available, the full benefits of higher spatial resolution imagery used to extract more precise land use roughness coefficients could be exploited.     

Warning systems for natural threats

This paper is a somewhat revised version of the one previously given as a keynote lecture in the ECI Conference, ‘Geohazards: Technical, Economical and Social Consequences’. Some changes have been made to the original paper and the oral presentation: several of the figures and pictures illustrating the recent events have been eliminated, only one decision analysis example is shown and the review of literature on other warning systems has been expanded. Readers interested in the conference paper and oral presentation are referred to the website: http://services.bepress.com/eci/geohazards/     

Numerical simulation of the landslide-induced tsunami of 1988 on Vulcano Island, Italy

 On 20 April 1988 a landslide of approximately 200,000 m³ occurred on the northeastern flank of the volcano La Fossa on the island of Vulcano. The landslide fell into the sea, producing a small tsunami in the bay between Punte Nere and Punta Luccia that was observed locally in the neighbouring harbour called Porto Levante. The slide occurred during a period of unrest at the volcano that was monitored very accurately. The study of this event is composed of two parts, the simulation of the landslide and the simulation of the ensuing tsunami; the former is studied by means of a Lagrangian-type numerical model in which the landslide is seen as a multibody system, an ensemble of material-deforming blocks interacting together during their motion; the latter is simulated according to the Eulerian view by solving the shallow-water approximation to Navier-Stokes equations of fluid dynamics, with the incorporation of a forcing term depending on the slide motion. Technically, the slide evolution is computed first, and this result is then used to evaluate the excitation term of the hydraulic equations and to calculate the tsunami propagation. Computed wave fronts radiate both toward the open sea, with rapid amplitude decay, and along the shore, in the form of edge waves that lose energy slowly. Comparison between model outputs and observations can be carried out only in a qualitative way owing to the absence of tide-gauge records, and results are satisfactory. Received: 14 September 1998 / Accepted: 18 December 1998     

Practical modified scheme of linear shallow-water equations for distant propagation of tsunamis

A simple but practical numerical model describing a distant propagation of tsunamis is newly proposed by introducing an additional term to the existing modified scheme. The numerical dispersion of the proposed model is manipulated to replace the physical dispersion of the linear Boussinesq equations without any limitation. The new model developed in this study is applied to propagation of a Gaussian hump over a constant water depth and the predicted free surface displacements are compared with available analytical solutions. A very reasonable agreement is observed.     

Ethnic groups’ response to the 26 December 2004 earthquake and tsunami in Aceh, Indonesia

The 26 December 2004 earthquake and tsunami unfairly hit the different ethnic groups of Aceh, Indonesia. About 170,000 Acehnese and Minangkabau people died in the Northern tip of Sumatra while only 44 Simeulue people passed away in the neighbouring Simeulue island located near the earthquake epicentre. Such a difference in the death toll does not lie in the nature of the hazard but in different human behaviours and ethnic contexts. The present study draws on a contextual framework of analysis where people’s behaviour in the face of natural hazards is deeply influenced by the cultural, social, economic and political context. Questionnaire-based surveys among affected communities, key informant interviews and literature reviews show that the people of Simeulue detected the tsunami very early and then escaped to the mountains. On the other hand, Acehnese and Minangkabau people, respectively in the cities of Banda Aceh and Meulaboh, did not anticipate the phenomenon and were thus caught by the waves. The different behaviours of the victims have been commanded by the existence or the absence of a disaster subculture among affected communities as well as by their capacity to protect themselves in facing the tsunami. People’s behaviours and the capacity to protect oneself can be further tracked down to a deep tangle of intricate factors which include the armed conflict that has been affecting the province since the 1970s, the historical and cultural heritage and the national political economy system. This paper finally argues that the uneven impact of the 26 December 2004 earthquake and tsunami in Aceh lies in the different daily life conditions of the ethnic groups struck by the disaster.     

The great 1787 Mexican tsunami

Tsunamis have proven to represent a significant hazard around the globe and there is increased awareness about their occurrence. The Pacific coast in southern México is no exception, because there is firm evidence of the effects of past large tsunamis. Here we present results from computer-aided modeling of the March 28, 1787-“San Sixto” earthquake and tsunami, and focus on the regions of Acapulco, Corralero, Jamiltepec, and Tehuantepec, located along the Guerrero-Oaxaca coast. The theoretical waveforms suggest wave heights in excess of 4 m and 18 m at specific locations in Acapulco and Corralero, respectively, and wave heights of at least 2 m at locations in Jamiltepec and Tehuantepec. From our modeling results and based on historical documents and the topography of the area, we conclude that these wave heights would have been sufficient to cause inundations that in the case of Acapulco were restricted to several meters inland, but in other areas like Corralero reached at least 6 km inland. Our results are consistent with published and unpublished damage reports that attest to the hazards associated with great earthquakes and tsunamis along the subduction zone in Mexico     

Method of calculating tsunami travel times in the Andaman Sea region

A new model to calculate tsunami travel times in the Andaman Sea region has been developed. The model specifically provides more accurate travel time estimates for tsunamis propagating to Patong Beach on the west coast of Phuket, Thailand. More generally, the model provides better understanding of the influence of the accuracy and resolution of bathymetry data on the accuracy of travel time calculations. The dynamic model is based on solitary wave theory, and a lookup function is used to perform bilinear interpolation of bathymetry along the ray trajectory. The model was calibrated and verified using data from an echosounder record, tsunami photographs, satellite altimetry records, and eyewitness accounts of the tsunami on 26 December 2004. Time differences for 12 representative targets in the Andaman Sea and the Indian Ocean regions were calculated. The model demonstrated satisfactory time differences (<2 min/h), despite the use of low resolution bathymetry (ETOPO2v2). To improve accuracy, the dynamics of wave elevation and a velocity correction term must be considered, particularly for calculations in the nearshore region.     

Earth observation data based rapid flood-extent modelling for tsunami-devastated coastal areas

Earth observation (EO)-based mapping and analysis of natural hazards plays a critical role in various aspects of post-disaster aid management. Spatial very high-resolution Earth observation data provide important information for managing post-tsunami activities on devastated land and monitoring re-cultivation and reconstruction. The automatic and fast use of high-resolution EO data for rapid mapping is, however, complicated by high spectral variability in densely populated urban areas and unpredictable textural and spectral land-surface changes. The present paper presents the results of the SENDAI project, which developed an automatic post-tsunami flood-extent modelling concept using RapidEye multispectral satellite data and ASTER Global Digital Elevation Model Version 2 (GDEM V2) data of the eastern coast of Japan (captured after the Tohoku earthquake). In this paper, the authors developed both a bathtub-modelling approach and a cost-distance approach, and integrated the roughness parameters of different land-use types to increase the accuracy of flood-extent modelling. Overall, the accuracy of the developed models reached 87–92%, depending on the analysed test site. The flood-modelling approach was explained and results were compared with published approaches. We came to the conclusion that the cost-factor-based approach reaches accuracy comparable to published results from hydrological modelling. However the proposed cost-factor approach is based on a much simpler dataset, which is available globally.     

Tsunami databases: The problems of acceptance and absence

Tsunami databases are not dissimilar from a wide variety of other data sources used by researchers, and yet users rarely, if ever, use them appropriately. They contain summary information of individual tsunamis, but they will always be incomplete because of the fragmentary data related to numerous past events and the difficulty of updating event information for those already included. Examples from AD1917 and AD1892 throw some light on this dilemma and show that not only is the uncritical use of such databases inappropriate, but that the data they contain may not be fit for the research questions being asked. This is unfortunate because it has probably led to at least one significant misconception developing within the tsunami community - that earthquakes are by far the most significant tsunami-genic source. When used appropriately though, tsunami database have the potential to inform and empower coastal communities.     

Remote sensing-based assessment of tsunami vulnerability and risk in Alexandria, Egypt

Tsunamis can cause catastrophic loss of life, destruction of property, engineered structures and coastal infrastructure, and they can lead to major economic losses. Even though tsunamis are relatively rare in the Mediterranean Sea, their potential danger to cities along the Mediterranean coast cannot be neglected. In order to create awareness among the potentially affected people it is important to know the risk and vulnerability of the population and infrastructure related to a possible tsunami impact. In this work a hazard, vulnerability and risk analysis for buildings in two districts of Alexandria was carried out. Relevant input parameters were derived mainly from remote sensing and field data and were analyzed with a geographical information system (GIS). Based on historical records of past tsunamis, two inundation scenarios of 5 m and 9 m were defined and modeled applying a bath-type model. The resulting tsunami building risk zone maps showed that 12% of the buildings in El Gomrok district are at high or very high risk for the 5 m scenario, while the risk for El Montazah area is low. For the 9 m scenario, on the other hand, the majority of the buildings in both districts, 56% of El Gomrok, and 60% of El Montazah, are in the high or very high risk zone. An analysis of the building use indicated that the majority of these buildings are residential and commercial types, highlighting that the potential consequences of a tsunami could be severe. Due to the scarcity of historical data no frequency could be associated with the two selected scenarios. While both are credible we consider the 5 m scenario as possible but unlikely and the 9 m scenario as unlikely.