Date of Award

2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Ocean Engineering

Department

Ocean Engineering

First Advisor

Stéphan T. Grilli

Abstract

This body of work consists of three manuscripts regarding numerical modeling of landslide-induced tsunamis.

Landslide Tsunami Hazard Along the Upper US East Coast: Effects of Slide Deformation, Bottom Friction, and Frequency Dispersion

Numerical simulations of Submarine Mass Failures (SMFs) are performed along the upper US East Coast to assess the effect of slide deformation on predicted tsunami hazard. Tsunami generation is simulated using the three-dimensional nonhydrostatic model NHWAVE. For rigid slumps, the geometry and law of motion are specified as bottom boundary conditions. Deforming slide motion is modeled using a depth-integrated bottom layer of dense Newtonian fluid, fully coupled to the overlying fluid motion. Once the SMFs are no-longer tsunamigenic, tsunami propagation simulations are performed using the Boussinesq wave model FUNWAVE-TVD, using nested grids of increasingly fine resolution towards shore and employing a one-way coupling methodology. Probable maximum tsunamis are simulated for Currituck SMF proxies sited in four areas of the shelf break slope that have enough sediment accumulation to cause large failures. Deforming slides have a slightly larger initial acceleration, but still generate a smaller tsunami than rigid slumps due to their spreading and thinning out during motion, which gradually makes them less tsunamigenic. Comparing the maximum envelope of surface elevations along a 5 m isobath, consistent with earlier work, the bathymetry of the wide shelf is found to strongly control the spatial distribution of tsunami inundation. Overall, tsunamis caused by rigid slumps are worst case scenarios, providing up to 50% more inundation than for deforming slides having a moderate level of viscosity set in the upper range of debris flows. Tsunamis from both types of SMFs are able to cause water withdrawal to the 5 m isobath or deeper. Bottom friction effects are assessed by performing some of the simulations using two different Manning coefficients, one 50% larger than the other. With increased bottom friction, the largest tsunami inundations at the coast are reduced by up to 15%. Selected simulations are rerun by turning off dispersion in the model, which leads to moderate changes in maximum surface elevations nearshore (- 10 to + 5% changes), but to more significant effects in the far field (- 40 to 80% changes). Onshore, dispersion causes the appearance of short period undular bores that eventually break nearshore without significantly affecting inundation at the coast. However, these bores increase wave-induced maximum flow velocity and impulse forces, the latter by up to 40%, which may affect the design of coastal structures.

New simulations and understanding of the 1908 Messina tsunami for a dual seismic and deep submarine mass failure source

Over 100 years after the event, the mechanism of the 1908 Messina tsunami remains unresolved. The up to 12 m runups observed along the coasts of Sicily and Calabria cannot be explained by the coseismic tsunami, so recent studies have proposed a dual earthquake/submarine mass failure (SMF) mechanism. Here we propose a new dual source and use it to simulate tsunami generation with a three-dimensional non-hydrostatic model, coupled to a two-dimensional fully nonlinear and dispersive model, to simulate tsunami propagation to shore. We first reanalyze observations of tsunami arrival times from eyewitnesses acquired shortly after the 1908 event, and a tsunami record at a tide gauge in Malta. Similar to earlier work, this data is used to locate the likeliest tsunami source area by inverse wave ray tracing, but accounting for frequency dispersion effects on wave celerity, uncertainty in reported arrival times, and a time delay between the EQ and SMF triggering. Analyzing the sea floor morphology in this area, we identify a new SMF at the foot of the Fiumefreddo Valley, northeast of Mount Etna. The general location is consistent with earlier studies, however our SMF is much smaller (~2 km3) than, e.g., that of Billi et al. (2008) and is a fairly rigid-block-slump, rather than a translational SMF. We model the block motion and simulate tsunami generation from a dual EQ/SMF source, and its propagation to shore, in higher resolution grids and based on more accurate bathymetry and topography than in earlier work. Runups and travel times agree well with observations, except for runups on either side of the Messina Straits north of the SMF, which are still underpredicted. In the far field, simulations reproduce well the arrival time and initial wave amplitudes at the Malta tide gauge. Our newly parameterized SMF and modeling improve tsunami runups simulated near the SMF location and south of it. However, as with all previous modeling of this event, additional sources are required to explain runups in the northern Messina Straits, which we suggest might be smaller and shallower SMFs located in this area. These will be considered in future work.

Dual earthquake/landslide source modeling of the 2018 Palu tsunami generation and hazard

The Mw 7.5 earthquake that struck Central Sulawesi, Indonesia, on September 28, 2018, was rapidly followed by coastal landslides and destructive tsunami waves within Palu Bay. This earthquake was supershear and predominantly strikeslip, with most published mechanisms predicting limited seabed uplift/subsidence, which make it an unlikely source of the up to 10.5 m runups recorded in the southern portion of the bay. Scientific debate has continued over the tsunami mechanism; earthquake, coastal landslides, or a combination of both. Published research has been inconclusive, with some studies simulating an earthquake generated tsunami as explaining most observations, with others focusing solely on landslide sources. For the latter, most simulations are based on hypothetical landslides not identified in post-tsunami onland field and bathymetric surveys. In this work, we simulate the tsunamis generated by the earthquake models of Jamelot et al. (2019), Socquet et al. (2019), and Ulrich et al. (2019), alone and in combination with seven coastal landslides that were confirmed by the field and bathymetric surveys (Liu et al., 2020; Takagi et al., 2019) which, from video evidence, produced significant waves. To generate and propagate the tsunamis, we use a combination of two numerical models, the 3D non-hydrostatic wave model NHWAVE and the 2D Boussinesq wave model FUNWAVE-TVD. Both models address the physics of wave frequency dispersion, which is important for modeling landslide tsunamis identified in the event. The coastal landslides are modeled in NHWAVE as granular material. Our combined earthquake and coastal landslide cases recreate the observed tsunami runups except for those in the southeast of the bay where they were most elevated (10.5 m). With regard to the timing of tsunami impact on the coast, results for the dual landslide/earthquake sources are in reasonable agreement with reconstructed time series at several locations around the bay, particularly using the model of Ulrich et al. (2019). In agreement with other studies, our work suggests an additional tsunami mechanism is necessary in the southeast of Palu Bay to explain observations there. Using partial information from bathymetric surveys, we site an additional landslide in the SE of Palu Bay and show that, when simulated together with the other slides and Ulrich et al. (2019)'s earthquake, results can better explain observations in the Southeast. This supports the need for future marine geology work in this area.

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