Y. Tony Song

The 26 December 2004 tsunami source estimated from satellite radar altimetry and seismic waves

[1]xa0The 26 December 2004 Indian Ocean tsunami was the first earthquake tsunami of its magnitude to occur since the advent of both digital seismometry and satellite radar altimetry. Both have independently recorded the event from different physical aspects. The seismic data has then been used to estimate the earthquake fault parameters, and a three-dimensional ocean-general-circulation-model (OGCM) coupled with the fault information has been used to simulate the satellite-observed tsunami waves. Here we show that these two datasets consistently provide the tsunami source using independent methodologies of seismic waveform inversion and ocean modeling. Cross-examining the two independent results confirms that the slip function is the most important condition controlling the tsunami strength, while the geometry and the rupture velocity of the tectonic plane determine the spatial patterns of the tsunami.

Detecting tsunami genesis and scales directly from coastal GPS stations

[1]xa0Different from the conventional approach to tsunami warnings that rely on earthquake magnitude estimates, we have found that coastal GPS stations are able to detect continental slope displacements of faulting due to big earthquakes, and that the detected seafloor displacements are able to determine tsunami source energy and scales instantaneously. This method has successfully replicated three historical tsunamis caused by the 2004 Sumatra earthquake, the 2005 Nias earthquake, and the 1964 Alaska earthquake, respectively, and has been compared favorably with the conventional seismic solutions that usually take hours or days to get through inverting seismographs. Because many coastal GPS stations are already in operation for measuring ground motions in real time as often as once every few seconds, this study suggests a practical way of identifying tsunamigenic earthquakes for early warnings and reducing false alarms.

Merging tsunamis of the 2011 Tohoku‐Oki earthquake detected over the open ocean

[1]xa0Tsunamis often travel long distances without losing power and severely devastate some coastal areas while leaving others with little damage. This unpredictable situation has been a major challenge for accurate and timely tsunami forecasting to facilitate early-warning and possible evacuations of affected coastal communities without disturbing the lives of others. Here we show evidence from satellite altimetry observations of the 2011 Tohoku-Oki earthquake-induced tsunami that sheds light on this issue. Three satellites observed the same tsunami front, and for the first time, one of them recorded a tsunami height about twice as high as that of the other two. Model simulations, based on the GPS-derived earthquake source and constrained by measurements of seafloor motions near the hypocenter, confirm that the amplified tsunami is one of several jets formed through topographic refraction when tsunamis travel along ocean ridges and seamount chains in the Pacific Ocean. This process caused the tsunami front to merge as it propagates, resulting in the doubling of the wave height and destructive potential in certain directions. We conclude that the potential of merging tsunamis should be emphasized in mapping tsunami hazards and assessing risk levels at key coastal facilities.

A theoretical study of topographic effects on coastal upwelling and cross-shore exchange

Abstract The effects of topographic variations on coastal upwelling and cross-shore exchange are examined with a theoretical, continuously stratified, three-dimensional coastal ocean model. The model takes into account topographic variations in both alongshore and cross-shore directions and allows analytical solutions with an Ekman surface layer that faithfully represents the physical nature of the coastal upwelling system. Theoretical solutions with any analytical form of alongshore-varying topography can be solved based on the perturbation method of Killworth [J. Phys. Oceanogr. 8 (1978) 188]. Analyses of the model solutions lead to the following conclusions: (1) The variation of upwelling fronts and currents is shown to be caused by the combined effect of topography and stratification. Topographic variation causes uneven upwelling distribution and leads to density variation, which results in a varying horizontal pressure gradient field that causes the meandering currents. The variation index is dependent upon a bilinear function of their physical parameters––the ratio of the topographic variation depth to the total depth and Burger’s number of stratification. (2) Cross-shore slope is found to play a role in maintaining the meandering structure of the alongshore currents. The anticyclonic circulations can further induce downwelling on the offshore side of the current, while the cyclonic circulations enhance upwelling and form upwelling centers on the inshore side of the current. (3) Alongshore topography does not change the total upwelled water, i.e., the total Ekman pumping is conserved. However, it increases cross-exchange of water masses by transporting inshore (offshore) water near topographic features far offshore (inshore) from the mean position of the front. The applicability and limitations of the theory are also discussed.

Ocean bottom pressure waves predicted in the tropical Pacific

[1]xa0Using a mass-conserving (non-Boussinesq) Pacific Ocean model and TOPEX/POSEIDON sea-surface height measurements, we document previously unreported bottom pressure waves (BPWs), associated with large-amplitude eddies (vortices) both north and south of the Equator. The bottom pressure signals, defined as the vertically integrated water-mass anomalies, are found to propagate westward across the Pacific Ocean with an approximate 4-mbar amplitude (1 mbar = 100 Newton/m2, approximately equivalent to 1 cm of sea surface height), continuously interacting with topographic features. Their wavelength is about 1100 kilometers, phase speed is about 0.5 m/s, and period is about 30-days, all consistent with the known properties of tropical instability waves (TIWs). These results suggest that the constantly evolving tropical instability eddies might have strong bottom pressure signals, therefore, likely alias the U.S.-German Gravity Recovery and Climate Experiment (GRACE) measurements, which have a monthly-sampling period.