Numerical simulations are performed with a two-dimensional ͑2D͒ fully nonlinear potential flow ͑FNPF͒ model for tsunami generation by two idealized types of submarine mass failure ͑SMF͒: underwater slides and slumps. These simulations feature rigid or deforming SMFs with a Gaussian cross section, translating down a plane slope. In each case, the SMF center of mass motion is expressed as a function of geometric, hydrodynamic, and material parameters, following a simple wavemaker formalism, and prescribed as a boundary condition in the FNPF model. Tsunami amplitudes and runup are obtained from computed free surface elevations. Model results are experimentally validated for a rigid 2D slide. Sensitivity studies are performed to estimate the effects of SMF-shape, type, and initial submergence depth-on the generated tsunamis. A strong SMF deformation during motion is shown to significantly enhance tsunami generation, particularly in the far-field. Typical slumps are shown to generate smaller tsunamis than corresponding slides. Both tsunami amplitude and runup are shown to depend strongly on initial SMF submergence depth. For the selected SMF idealized geometry, this dependence is simply expressed by power laws. Other sensitivity analyses are presented in a companion paper, and results from numerical simulations are converted into empirical curve fits predicting characteristic tsunami amplitudes as functions of nondimensional governing parameters. It should be stressed that these empirical formulas are only valid in the vicinity of the tsunami sources and, because of the complexity of the problem, many simplifications were necessary. It is further shown in the companion paper how 2D results can be modified to account for three-dimensional tsunami generation and used for quickly estimating tsunami hazard or for performing simple case studies.
The source of the local tsunami of 17 th July 1998 that struck the north shore of Papua New Guinea remains controversial, and has been postulated as due either to seabed dislocation (fault) or sediment slump. Alternative source mechanisms of the tsunami were addressed by offshore investigation using. multibeam bathymetry, sub-bottom profiling, sediment sampling and observation from the JAMSTEC Dolphin 3K Remotely Operated Vehicle and Shinkai 2000 Manned Submersible. The area offshore of Sissano is a complex active convergent margin with subduction taking place along the New Guinea Trench. Dominant transpressional convergence results in diachronous collision of the highstanding North Bismarck Sea Plate in a westerly direction. The result is a morphological variation along the Inner Trench Slope, with the boundary between eastern and western segments located offshore Sissano in an area of on-and offshore subsidence. This subsidence, together with nearshore bathymetric focusing, is considered to increase the tsunamigenic potential of the Sissano area. The offshore data allow discrimination between tsunami generating mechanisms with the most probable source mechanism of the local tsunami as a sediment slump located offshore of Sissano Lagoon. The approximately 5-10 km 3 slump is located in an arcuate, amphitheatreshaped structure in cohesive sediments that failed through rotational faulting. In the area of the amphitheatre there is evidence of recent seabed movement in the form of fissures, brecciated angular sediment blocks, vertical slopes, talus deposits and active fluid expulsion that maintains a chemosynthetic vent fauna. Dating of the slump event may be approximated 2 by the age of the chemosynthetic faunas as well as by a seismic signal from the failing sediment mass. Faults in the area offshore Sissano are mainly dip-slip to the north with recent movement only along planes of limited lateral extent. A possible thrust fault is of limited extent and with minimal (cm) reverse movement. Further numerical modeling of the tsunami also supports the slump as source over fault displacements.
[1] Tsunamis generated in lakes and reservoirs by subaerial mass flows pose distinctive problems for hazards assessment because the domain of interest is commonly the ''near field,'' beyond the zone of complex splashing but close enough to the source that wave propagation effects are not predominant. Scaling analysis of the equations governing water wave propagation shows that near-field wave amplitude and wavelength should depend on certain measures of mass flow dynamics and volume. The scaling analysis motivates a successful collapse (in dimensionless space) of data from two distinct sets of experiments with solid block ''wave makers.'' To first order, wave amplitude/water depth is a simple function of the ratio of dimensionless wave maker travel time to dimensionless wave maker volume per unit width. Wave amplitude data from previous laboratory investigations with both rigid and deformable wave makers follow the same trend in dimensionless parameter space as our own data. The characteristic wavelength/water depth for all our experiments is simply proportional to dimensionless wave maker travel time, which is itself given approximately by a simple function of wave maker length/water depth. Wave maker shape and rigidity do not otherwise influence wave features. Application of the amplitude scaling relation to several historical events yields ''predicted'' near-field wave amplitudes in reasonable agreement with measurements and observations. Together, the scaling relations for near-field amplitude, wavelength, and submerged travel time provide key inputs necessary for computational wave propagation and hazards assessment.
Abstract. Case studies of landslide tsunamis require integration of marine geology data and interpretations into numerical simulations of tsunami attack. Many landslide tsunami generation and propagation models have been proposed in recent time, further motivated by the 1998 Papua New Guinea event. However, few of these models have proven capable of integrating the best available marine geology data and interpretations into successful case studies that reproduce all available tsunami observations and records. We show that nonlinear and dispersive tsunami propagation models may be necessary for many landslide tsunami case studies. GEOWAVE is a comprehensive tsunami simulation model formed in part by combining the Tsunami Open and Progressive Initial Conditions System (TOPICS) with the fully nonlinear Boussinesq water wave model FUNWAVE. TOPICS uses curve fits of numerical results from a fully nonlinear potential flow model to provide approximate landslide tsunami sources for tsunami propagation models, based on marine geology data and interpretations. In this work, we validate GE-OWAVE with successful case studies of the 1946 Unimak, Alaska, the 1994 Skagway, Alaska, and the 1998 Papua New Guinea events. GEOWAVE simulates accurate runup and inundation at the same time, with no additional user interference or effort, using a slot technique. Wave breaking, if it occurs during shoaling or runup, is also accounted for with a dissipative breaking model acting on the wave front. The success of our case studies depends on the combination of accurate tsunami sources and an advanced tsunami propagation and inundation model.
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