Tsunami Generation by Combined Fault Rupture and Landsliding
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Tsunami generation and propagation mechanisms need to be clearly understood in order to inform predictive models and improve coastal community preparedness. In the last two decades, unexpectedly large tsunamis have devastated localised coastal communities claiming thousands of lives (i.e. the Japan Tsunami of 2011 and Palu Bay Tsunami 2018). An earthquake primarily caused these devastating tsunamis, although the moderate earthquake magnitude for case of (Palu Bay Tsunami 2018), wave directivity and wave arrival time, and extremely large run-up heights in localised areas did not agree with predictions or later tsunami simulations. However, evidence has been later found to suggest that localised submarine landslides, probably triggered by the earthquake, might be responsible for the unforeseen tsunami effects from those events. Physical experiments, supported by mathematical models, can potentially provide valuable input data for standard predictive models of tsunami generation and propagation. A unique two-dimensional experimental set-up has been developed to reproduce a coupled-source tsunami generation mechanism: an underwater fault rupture followed by a submarine landslide. The test rig was located in a 20 m flume in the COAST laboratory at the University of Plymouth.
The objective of this research are to carry out experiments involving individual and coupled sources, to provide quality data for developing a parametrisation of the initial conditions for tsunami generation processes which are triggered by a dual-source. The fault rupture was replicated by the sudden uplift of a plate, which was controlled by an actuator. The submarine landslide was modelled by a solid block and a granular landslide. Two landslide triggering mechanisms were designed to automatically release the landslide once the uplift reached the final programmed position.
During the test programme, several test configurations, uplift distances, water depths and the landslide density and granularity were varied. The position of the uplift plate was retrieved from the actuator feedback and the landslide model was tracked with a digital camera. Theoretical expressions for the fault rupture motion and the landslide motion were formulated and compared to previous investigations. The free surface elevation of the water body was measured using resistance wave gauges. Hence the individual source motion parameters and the generated wave characteristics were determined. The fault rupture generated wave was crest-led followed by a trough of smaller amplitude, measured closest to generation. The uplift displacement scale controlled the wave amplitude. For the case of landslide generated tsunamis, the generated wave was trough-led which increased in amplitude until it reached the end of the slope, where its amplitude started decreasing. The trough amplitude was controlled by the landslide relative density and granularity. The trough was followed by a crest of smaller amplitude which propagated similarly to the trough.
For a coupled-source scenario, the generated wave was crest led, followed by a trough of smaller amplitude decreasing steadily as it propagated along the flume. The crest amplitude was shown to be influenced by the fault rupture uplift displacement scale, whereas the trough was influenced by the landslide's relative density. At the closest wave gauge to generation, 83\% of the uplift displacement was transferred to the generated crest amplitude. The fault rupture generated wave periods of approximately 3.2 s whereas the landslide generated wave had a period of approximately 1.7s. The wave period remained unchanged between uplift only and coupled source scenarios.
The granular landslide was observed to have a reduced peak velocity when compared to the equivalent solid landslide, probably owing to the fragmentation into smaller masses. This peak velocity was reached at the end of the slope for both landslide models (solid and granular). The effect of the landslide granularity on the generated leading crest amplitude was observed to be 15 mm smaller when compared to the equivalent solid landslide.
During this investigation, the combination of the fault rupture and the landslide was confirmed to contribute to the tsunami amplification process. This finding contributes to the growing body of literature that recognises the importance of coupled source mechanisms to explain unexpectedly large tsunamis.