Tsunami generation by horizontal displacement of ocean bottom (original) (raw)
Related papers
Tanioka and Satake : Tsunami Generation by Horizontal Movement 863
2008
Tsunami generation by an earthquake is generally modeled by water surface displacement identical to the vertical deformation of ocean bottom due to faulting. The effect of horizontal deformation is usually neglected. However, when the tsunami source is on a steep slope and the horizontal displacement is large relative to the vertical displacement, the effect becomes significant. We show this for two recent earthquakes which generated much larger tsunamis than expected from seismic waves. In the case of the 1994 June 2 Java, Indonesia, earthquake, the focal mechanism was a very shallow dipping thrust and the source was near a very steep trench slope. In the case of the 1994 Nov. 14 Mindoro, Philippines, earthquake, strike-slip faulting extended from ocean to land perpendicular to the coast line. In both cases, we found that the horizontal motion of slope had an important contribution to the tsunami generation.
On the contribution of the horizontal sea-bed displacements into the tsunami generation process
Ocean Modelling, 2012
The main reason for the generation of tsunamis is the deformation of the bottom of the ocean caused by an underwater earthquake. Usually, only the vertical bottom motion is taken into accound while the horizontal co-seismic displacements are neglected in the absence of landslides. In the present study we propose a novel methodology for reconstructing all components of the bottom coseismic displacements field. Then, the sea-bed motion is transmitted onto the free surface using a three-dimensional Weakly Nonlinear (WN) approach. We pay a special attention to the evolution of kinetic and potential energies of the resulting wave while the contribution of horizontal displacements into wave energy balance is also quantified. Approaches proposed in this study are illustrated on the July 17, 2006 Java tsunami and some more recent events.
The contribution of horizontal sea-bed displacements into tsunami generation processes
2010
The main reason for the generation of tsunamis is the deformation of the bottom of the ocean caused by an underwater earthquake. Usually, only the vertical bottom motion is taken into accound while the horizontal co-seismic displacements are neglected in the absence of landslides. In the present study we propose a novel methodology for reconstructing all components of the bottom
Generation of tsunamis by a slowly spreading uplift of the sea floor
Soil Dynamics and Earthquake Engineering, 2001
Generation of tsunamis by slow submarine processes (faulting, slumps or slides) is investigated, in search for possible ampli®cation mechanisms resulting from lateral spreading of the sea¯oor uplift. A linearized solution for constant water depth is derived by transform methods (Laplace in time and Fourier in space), for sea¯oor uplift represented by a sliding Heaviside step function (i.e. a simpli®ed Haskell source model, with zero rise time). The model is used to study the tsunami amplitude ampli®cation (wave amplitude normalized by the ®nal sea¯oor uplift) as a function of the model parameters. The results show that, above the source, the ampli®cation is larger for larger uplifted area and for smaller water depth, and is the largest in the direction of uplift spreading, for velocity of spreading comparable to the long period tsunami velocity. Near the source, this ampli®cation could be one order of magnitude. This ampli®cation mechanism seen in the near-®eld is a form of wave focusing, and is manifested by a high frequency pulse, with amplitude attenuating with distance due to dispersion and geometric spreading. In the far-®eld, the linear theory predicts maximum ampli®cation equal to one, as predicted by point source models. An analogy between this form of focusing and resonance of a single-degree-of-freedom oscillator, and near-®eld radiation patterns are discussed. The magnitudes, seismic movements and source durations of selected earthquakes which generated tsunamis are cited in search of conditions which could lead to slow rupture and unusually large near-®eld ampli®cation. q
In tsunami runup modelling there are still many open questions. Beside bathymetry the influence of the tsunami source description is an important issue. Widely used in tsunami modelling is double-couple model. Usually, it is applied to the sea surface assuming that the sea bottom movement results in an abrupt deformation of the water surface, which is used as an initial condition for tsunami modelling. There may be more exact geophysical models, but as a first guess Okada's method is advantageous because it is fast and has easy access to input parameters. That's why it has been chosen to be first implemented in the tool, called QuakeGen. It calculates variable bathymetry with control of the temporary development of the earthquake. The time variable bathymetry was used to create a tsunami with the landslide module in MIKE 21. The results have been compared to the observed runup heights and arrival times from the 17 July 2006 Java Earthquake tsunami, chosen as a reference case. The generated waves are used as a boundary condition on one bathymetry just beside the generation zone. The runup heights are compared with field survey data reported in and . Furthermore, the influences of time step length during the simulation is investigated. Additionally to the M W = 7.7 earthquake, the first M W = 7.2 earthquake is included into the hydrodynamic simulation. A comparison of the results shows that the tsunami generated using QuakeGen and calculated with MIKE 21 gives the modeller the advantage of further adjustments by controlling the time in source modelling. The combination of QuakeGen and the MIKE 21 landslide module has been proven to yield more reliable results in simulation regarding runup and arrival time due to the possibility of considering all earthquakes which occured within the simulation period.
Horizontal displacements contribution to tsunami wave energy balance
2010
The main reason for the generation of tsunamis is the deformation of the bottom of the ocean caused by an underwater earthquake. Usually, only the vertical bottom motion is taken into account while the horizontal co-seismic displacements are neglected in the absence of landslides. In the present study we propose a methodology based on the well-known Okada solution to reconstruct in more details all components of the bottom coseismic displacements. Then, the sea-bed motion is coupled with a threedimensional weakly nonlinear water wave solver which allows us to simulate a tsunami wave generation. We pay special attention to the evolution of kinetic and potential energies of the resulting wave while the contribution of the horizontal displacements into wave energy balance is also quantified. Such contribution of horizontal displacements to the tsunami generation has not been discussed before, and it is different from the existing approaches. The methods proposed in this study are illustrated on the July 17, 2006 Java tsunami and some more recent events.
2008
In tsunami runup modelling there are still many open questions. Beside bathymetry the influence of the tsunami source description is an important issue. Widely used in tsunami modelling is Okada's (1985) double-couple model. Usually, it is applied to the sea surface assuming that the sea bottom movement results in an abrupt deformation of the water surface, which is used as an initial condition for tsunami modelling. There may be more exact geophysical models, but as a first guess Okada's method is advantageous because it is fast and has easy access to input parameters. That's why it has been chosen to be first implemented in the tool, called QuakeGen. It calculates variable bathymetry with control of the temporary development of the earthquake. The time variable bathymetry was used to create a tsunami with the landslide module in MIKE 21. The results have been compared to the observed runup heights and arrival times from the 17 July 2006 Java Earthquake tsunami, chosen as a reference case. The generated waves are used as a boundary condition on one bathymetry just beside the generation zone. The runup heights are compared with field survey data reported in Fritz et al. (2007) and Lavigne et al. (2007). Furthermore, the influences of time step length during the simulation is investigated. Additionally to the M W = 7.7 earthquake, the first M W = 7.2 earthquake is included into the hydrodynamic simulation. A comparison of the results shows that the tsunami generated using QuakeGen and calculated with MIKE 21 gives the modeller the advantage of further adjustments by controlling the time in source modelling. The combination of QuakeGen and the MIKE 21 landslide module has been proven to yield more reliable results in simulation regarding runup and arrival time due to the possibility of considering all earthquakes which occured within the simulation period.
Wave Motion, 2021
In this study, the propagation of surface water waves initially displaced by a tectonic seafloor deformation of arbitrary geometry was obtained considering the rupture kinematics. The developed solution was applied to a set of problems for wave generation by bottom motion with arbitrary spatiotemporal variations. First, a single bottom motion with different uplift speeds was considered; results showed that relatively fast uplift speed produces increased free surface elevation at the center of the movable bottom. For dual bottom motion with spatial and temporal intervals, the free surface elevation at the end of entire uplift motion has different maxima at different positions depending on their intervals. Then, the bottom motion subdivided into 10 sub-regions with rupture velocity and uplift speed was considered. The result implies that when the rupture process is introduced in the solution, the wave energy in the direction opposite to rupture decreases, while it is enhanced in the rupture direction with higher-frequency components. The solution was applied to the dual-Gaussian-shaped bottom motion with various rupture velocities and directions to demonstrate its prospective use in the numerical models for real tsunami events. Depending on the rupture direction, surface wave propagation exhibits distinct patterns.
Tsunami generation by dynamic displacement of sea bed due to dip-slip faulting
Mathematics and Computers in Simulation, 2009
In classical tsunami-generation techniques, one neglects the dynamic sea bed displacement resulting from fracturing of a seismic fault. The present study takes into account these dynamic effects. Earth's crust is assumed to be a Kelvin-Voigt material. The seismic source is assumed to be a dislocation in a viscoelastic medium. The fluid motion is described by the classical nonlinear shallow water equations (NSWE) with time-dependent bathymetry. The viscoelastodynamic equations are solved by a finite-element method and the NSWE by a finite-volume scheme. A comparison between static and dynamic tsunami-generation approaches is performed. The results of the numerical computations show differences between the two approaches and the dynamic effects could explain the complicated shapes of tsunami wave trains.
Sediment effect on tsunami generation of the 1896 Sanriku Tsunami Earthquake
Geophysical Research Letters, 2001
The 1896 Sanriku earthquake was one of the most devastating tsunami earthquakes, which generated an anomalously larger tsunami than expected from its seismic waves. Previous studies indicate that the earthquake occurred beneath the accretionary wedge near the trench axis. It was pointed out recently that sediments near a toe of an inner trench slope with a large horizontal movement due to the earthquake might have caused an additional uplift. In this paper, the effect of the additional uplift to tsunami generation of the 1896 Sanfiku tsunami earthquake is quantified. We estimate the slip of the earthquake by numerically computing tsunamis and comparing their waveforms with those recorded at three fide gauges. The estimated slip for the model without the additional uplift is 10.4 m, and those with the additional uplit• are 5.9-6.7 m. This indicates that the additional uplift of the sediments near the trench has a large effect on the tsunami generation.