CONTRIBUTION OF ASTEROID GENERATED TSUNAMI TO THE IMPACT HAZARD   David Morrison Ethiraj Venkatapathy NASA Ames Asteroid Threat Assessment Project This paper is a summary from the NASA-NOAA Workshop, August 2016, Seattle Washington, to address the long-standing uncertainty about the nature of asteroid-impact tsunami and their contribution to the impact hazard. Since the majority of impacts take place in the ocean, it is important to evaluate the hazard they represent. The workshop summary is published in NASA TM-219463 (2017). Supported by the NASA Planetary Defense Coordination Office.

Morrison: Hazard of Asteroid Impact Tsunami Session 1: Mark Boslough (Sandia Labs): Chair Charge: Model the tsunami-producing potential of airbursts and direct water impacts and evaluate the nature of the waves produced. Examples: Energies of 5 MT, 100MT, and 250MT (NEA diameters roughly 50m, 125m, and 180m). For a given energy, airbursts were less effective in generating waves, but in both airburst and impact cases, the waves dissipate rapidly. The potential for severe damage from asteroid generated tsunami over the energy range studied is therefore limited to impacts near the shore, and even in these cases the air-blast, fireball, and possible ejection of sediment in shallow water areas may exceed the shore damage from the wave.

Morrison: Hazard of Asteroid Impact Tsunami Session 2: Marsha Berger (New York University): Chair Charge: Determine the ability of near-field impact-produced waves (from Session 1) to propagate over large distances, in deep and shallow water. Examples studied included impacts in South China Sea, the city of Westport on the coast of Washington, and the Long Beach area in southern California. Results depend strongly on local bathymetry. Different computational codes were used, providing a range of results, but in general the conclusion was that the waves resulting from airbursts do not travel for great distances and do not move large quantities of water, so that they produce less inundation and flooding when they reach the shore. Water impacts can lift substantial quantities of water and create localized waves analogous to those from landslides, but the impact damage is limited to areas close to the strike.

Morrison: Hazard of Asteroid Impact Tsunami Session 3: Donovan Mathias (NASA Ames): Chair Charge: Evaluate the threat to human life and infrastructure posed by asteroid-generated tsunami. The ultimate product was a model for the contribution of asteroid ocean impacts to the ensemble impact hazard.   Detailed modeling was done for impacts near Westport WA and Long Beach CA, but again the damage was largely confined to the shore. The global hazard was evaluated by the NASA Ames team using “engineering models” for the waves and actual distributions of coastal population and infrastucture that might be affected by tsunami waves. The assessment concluded: (1) The impact tsunami hazard is negligible for asteroid diameters below 200m. (2) For asteroids larger than about 300m, the risk peaks at about an order of magnitude lower casualty rate than the land impacts. (3) Larger than about 500m the global risk (based on previous work) dominates over either land or ocean impacts. (4) The average annual casualties from land and ocean impacts (not including global effects) are in the range of 1-10.

Morrison: Hazard of Asteroid Impact Tsunami Summary of Workshop Conclusions: The Workshop achieved its primary goals of re-evaluating the tsunami risk from impacts by small (<250m diameter) asteroids using modern codes and simulations, and providing a better estimate of the ensemble risk from water impacts. Any evaluation of impact hazards requires knowledge of the population of impacting asteroids. Recent work by Harris has reduced the population estimates, contributing to lowering the hazard estimates from all impacts.  In the case of airbursts and surface impacts studied here, most damage to coastal populations is limited to surface strikes close to the shore, in which case the direct blast damage may be more important than the wave generated. The risk from near-shore impacts can be important for considering individual cases, but they do not contribute significantly to the ensemble hazard. Current estimates are that the ensemble hazard from ocean impacts is at least an order of magnitude less than from land impacts.