Sebastien Allgeyer

Numerical tsunami simulation including elastic loading and seawater density stratification

Systemic discrepancies between observed and modeled tsunami wave speeds were previously identified for two recent major tsunamis: the 2010 Maule and 2011 Tohoku events. To account for these discrepancies, we developed a numerical tsunami propagation code solving the shallow water equation and including the effects of elastic loading of the seafloor by the tsunami as well as a linear density profile in the seawater column. We show here that both effects are important to explain the commonly observed difference between observations and simulations. We conclude that the density variation in the seawater column affects the wave speed without changing the waveform, whereas the loading effect has an effect on the wave speed and the waveform showing a negative phase before the main arrival due to the depression of the seafloor surrounding the tsunami wave. The combination of both effects is needed to achieve a better match between observations and simulations.

Improved water balance component estimates through joint assimilation of GRACE water storage and SMOS soil moisture retrievals

The accuracy of global water balance estimates is limited by the lack of observations at large scale and the uncertainties of model simulations. Global retrievals of terrestrial water storage (TWS) change and soil moisture (SM) from satellites provide an opportunity to improve model estimates through data assimilation. However, combining these two data sets is challenging due to the disparity in temporal and spatial resolution at both vertical and horizontal scale. For the first time, TWS observations from the Gravity Recovery and Climate Experiment (GRACE) and near-surface SM observations from the Soil Moisture and Ocean Salinity (SMOS) were jointly assimilated into a water balance model using the Ensemble Kalman Smoother from January 2010 to December 2013 for the Australian continent. The performance of joint assimilation was assessed against open-loop model simulations and the assimilation of either GRACE TWS anomalies or SMOS SM alone. The SMOS-only assimilation improved SM estimates but reduced the accuracy of groundwater and TWS estimates. The GRACE-only assimilation improved groundwater estimates but did not always produce accurate estimates of SM. The joint assimilation typically led to more accurate water storage profile estimates with improved surface SM, root-zone SM, and groundwater estimates against in situ observations. The assimilation successfully downscaled GRACE-derived integrated water storage horizontally and vertically into individual water stores at the same spatial scale as the model and SMOS, and partitioned monthly averaged TWS into daily estimates. These results demonstrate that satellite TWS and SM measurements can be jointly assimilated to produce improved water balance component estimates.

The 1755 Lisbon Tsunami in Guadeloupe Archipelago: Source Sensitivity and Investigation of Resonance Effects

On the 1 st of November 1755, a major earthquake of estimated Mw=8.5/9.0 destroyed Lisbon (Portugal) and was felt in whole Western Europe. It generated a huge tsunami which reached coastlines from Morocco to Southwestern England with local run-up heights up to 15 m in some places as Cape St Vincent (Portugal). Important waves were re- ported in Madeira Islands and as far as in the West Indies where heights of 3 m and damages are reported. The present knowledge of the seismic source(s), presented by numerous studies, was not able to reproduce such wave heights on the other side of the Atlantic Ocean whatever the tested source. This could be due to the signal dispersion during the propaga- tion or simply to the lack of simulations with high resolution grids. Here we present simulations using high resolution grids for Guadeloupe Archipelago for two different sources. Our results highlight important wave heights of the range of 1 m to more than 2 m whatever the source mechanism used, and whatever the strike angle in some particular coastal places. A preliminary investigation of the resonance phenomenon in Guadeloupe is also presented. In fact, the studies of long wave impact in harbours as rissaga phenomenon in the Mediterranean Sea leads us to propose the hypothesis that the 1755 waves in the West Indies could have been amplified by resonance phenomenon. Most of the places where amplification takes place are nowadays important touristic destinations.

Time Reversal Imaging of the Tsunami Source

In this paper, we apply time reversal imaging (TRI) to the problem of reconstructing the initial sea surface displacement that generates a tsunami. We discuss theoretical considerations in the application of TRI to the tsunami problem, including time reversibility and reciprocity of the shallow-water equations. Several numerical experiments are conducted to establish the efficacy of TRI in the tsunami context. TRI is applied to observations of the tsunami generated by the Tohoku earthquake on March 11, 2011, for which an unprecedented number of high-quality observations are available. Finally, we compare the findings of the TRI results with other, more conventional methods of source inversion. Results indicate that TRI is effective for imaging a tsunami source when a sufficient number of observations are available. Because it involves fewer assumptions about the nature of the tsunami source, in particular those regarding source location and fault geometry, we believe that TRI has the potential to improve our understanding of tsunami generation—for example, through detection of non-seismic components of the tsunami source.

Slow slip events and the 2016 Te Araroa Mw 7.1 earthquake interaction: Northern Hikurangi subduction, New Zealand

Following a sequence of three Slow Slip Events (SSEs) on the northern Hikurangi Margin, between June 2015 and August 2016, a Mw 7.1 earthquake struck ~30 km offshore of the East Cape region in the North Island of New Zealand on the 2nd September 2016 (NZ local time). The earthquake was also followed by a transient deformation event (SSE or afterslip) northeast of the North Island, closer to the earthquake source area. We use data from New Zealands continuous Global Positioning System (cGPS) networks to invert for the SSE slip distribution and evolution on the Hikurangi subduction interface. Our slip inversion results show an increasing amplitude of the slow slip towards the Te Araroa earthquake foreshock and mainshock area, suggesting a possible triggering of the Mw 7.1 earthquake by the later stage of the slow slip sequence. We also show that the transient deformation following the Te Araroa earthquake ruptured a portion of the Hikurangi Trench northeast of the North Island, further north than any previously observed Hikurangi margin SSEs. Our slip inversion and the coulomb stress calculation suggest that this transient may have been induced as a response to the increase in the static coulomb stress change downdip of the rupture plane on the megathrust. These observations show the importance of considering the interaction between slow slip events, seismic and aseismic events, not only on the same megathrust interface, but also on faults within the surrounding crust.

Tsunami Hazard in La Réunion Island (SW Indian Ocean): Scenario-Based Numerical Modelling on Vulnerable Coastal Sites

Several major tsunamis have affected the southwest Indian Ocean area since the 2004 Sumatra event, and some of them (2005, 2006, 2007 and 2010) have hit La Réunion Island in the southwest Indian Ocean. However, tsunami hazard is not well defined for La Réunion Island where vulnerable coastlines can be exposed. This study offers a first tsunami hazard assesment for La Réunion Island. We first review the historical tsunami observations made on the coastlines, where high tsunami waves (2–3 m) have been reported on the western coast, especially during the 2004 Indian Ocean tsunami. Numerical models of historical scenarios yield results consistent with available observations on the coastal sites (the harbours of La Pointe des Galets and Saint-Paul). The 1833 Pagai earthquake and tsunami can be considered as the worst-case historical scenario for this area. In a second step, we assess the tsunami exposure by covering the major subduction zones with syntethic events of constant magnitude (8.7, 9.0 and 9.3). The aggregation of magnitude 8.7 scenarios all generate strong currents in the harbours (3–7 m s

Three-dimensional numerical modeling of tsunami-related internal gravity waves in the Hawaiian atmosphere

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Could a 1755-Like Tsunami Reach the French Atlantic Coastline? Constraints from Twentieth Century Observations and Numerical Modeling

-1) and about 2 m of tsunami maximum height without significant inundation. The analysis of the magnitude 9.0 events confirms that the main commercial harbour (Port Est) is more vulnerable than Port Ouest and that flooding in Saint-Paul is limited to the beach area and the river mouth. Finally, the magnitude 9.3 scenarios show limited inundations close to the beach and in the riverbed in Saint-Paul. More generally, the results confirm that for La Runion, the Sumatra subduction zone is the most threatening non-local source area for tsunami generation. This study also shows that far-field coastal sites should be prepared for tsunami hazard and that further work is needed to improve operational warning procedures. Forecast methods should be developed to provide tools to enable the authorities to anticipate the local effects of tsunamis and to evacuate the harbours in sufficient time when such an earthquake occurs.