Asteroid Impact Tsunami: A Probabilistic Hazard Assessment

We investigate the generation, propagation, and probabilistic hazard of tsunami spawned by oceanic asteroid impacts. The process first links the depth and diameter of parabolic impact cavities to asteroid density, radius, and impact velocity by means of elementary energy arguments and crater scaling rules. Then, linear tsunami theory illustrates how these transient cavities evolve into vertical sea surface waveforms at distant positions and times. By measuring maximum wave amplitude at many distances for a variety of impactor sizes, we derive simplified attenuation relations that account both for geometrical spreading and frequency dispersion of tsunami on uniform depth oceans. In general, the tsunami wavelengths contributing to the peak amplitude coincide closely with the diameter of the transient impact cavity. For the moderate size impactors of interest here (those smaller than a few hundred meters radius), cavity widths are less than or comparable to mid-ocean depths. As a consequence, dispersion increases the 1/ √ r long-wave decay rate to nearly 1/r for tsunami from these sources. In the final step, linear shoaling theory applied at the frequency associated with peak tsunami amplitude corrects for amplifications as the waves near land. By coupling this tsunami amplitude/distance information with the statistics of asteroid falls, the probabilistic hazard of impact tsunami is assessed in much the same way as probabilistic seismic hazard, by integrating contributions over all admissible impactor sizes and impact locations. In particular, tsunami hazard, expressed as the Poissonian probability of being inundated by waves from 2 to 50 m in height in a 1000-year interval, is computed at both generic (generalized geography) and specific (real geography) sites. For example, a typical generic site with 180◦ of ocean exposure and a reach of 6000 km, admits a 1-in-14 chance of an impact tsunami exceeding 2-m in height in 1000 years. The likelihood drops to 1-in35 for a 5-m wave, and to 1-in-345 for a 25-m wave. Specific sites of Tokyo and New York have 1-in-24 and 1-in-47 chances, respectively, of suffering an impact tsunami greater than 5 m in the next millennium. c © 2000 Academic Press