Comets and Asteroids - Tsunami - page 171

Fig. 9.6 Modeled results of the
initial development of a tsunami
created by the impact of a 200 m
diameter stony asteroid with a
density of 3 g cm -3 traveling at
20 km s -1 . Within 300 s, a
tsunami propagates outwards
more than 50 km. The leading
wave in the bottom panel is about
325 ms high. Based on Ward and
Asphaug ( 2000 ). Dr. Steven
Ward, Institute of Tectonics,
University of California at Santa
Cruz, kindly provided a digital
copy of this simulation

Table 9.2 Crater and tsunami characteristics modeled using ana-
logues to nuclear explosions and the Shoemaker-Levy comet impact
into Jupiter

craters are more likely to approximate the shape of those
formed on land. The size of the resulting tsunami also
varies. Those generated on the continental shelf or in
shallow gulfs and bays may only be 1-2 m in height even
where craters 10 km in diameter have formed. The resulting
features on land cannot be separated easily from those
formed by tsunami generated by large earthquakes or sub-
marine slides. Tsunami propagating landward from impacts
on the edge of the continental shelf are similar. However the
tsunami propagating seaward from these locations may be
much larger and more destructive on distant shores. Impacts
in deeper water generate tsunami originating from a point
source. Here even small impacts generated by low-density
comets 1 km in size may produce tsunami larger than those
generated by the greatest earthquakes of the past century.
Evidence of such a tsunami should be present basin-wide
with features orientated back to a common point source.
Comet and meteor impacts were common in the early
history of the Earth, but within the last 225 million years, the
Earth has only been struck randomly by debris flung into the
inner solar system or by asteroids orbiting between the Earth
and Jupiter (Verschuur 1996 ; Steel 1995 ). One hundred and
two craters have been identified over this period, of which 14
have a marine source. By far the largest of these is Chicxulub
crater, which is 180 km in diameter and is buried beneath the
Yucatan Peninsula in Mexico (Alvarez 1997 ). Chicxulub is
one of the few submarine impacts now preserved under a
continent. This event has been linked to the Cretaceous-

Asteroid diameter (m)
250 500 1000

Analogue to nuclear explosions
Crater size (km) 7.7 14.4 26.9

Tsunami height (m) 11.9 36.7 112.1

Sandia computer simulations
Crater size (km) 5.0 10.0 20.0

Wave velocity (m s -1 ) 166.0 166.0 900.0

Wave period (s) 150.0 180.0 300.0

Tsunami height (m) 1.3 5.0 12.0

Note Latter modeling is the same as that used in Fig. 9.4
Wave heights are taken 500 km from the impact site
Source Based on Crawford and Mader ( 1998 )

Tertiary boundary extinction of the dinosaurs 65 million
years ago. Chicxulub will be described in more detail sub-
sequently. If the distribution of comet/asteroid impacts was
spread evenly over the Earth's surface, and if it can be
assumed that the oceans have continually occupied at least
70 % of the world's surface area, then a minimum of 293
impacts should have occurred in the oceans over the last
225 million years. This represents approximately one ocean
impact event every 0.77 million years.
It is possible to use Eq. ( 9.2 ) and the size of known
impact craters to calculate the energy released by each