Geoscience Reference
In-Depth Information
Fig. 9.2
Probability of comets
or asteroids of given diameter
striking the Earth. The sizes
generating significant tsunami are
shaded. Based upon Verschuur
(
1996
) and formulae presented in
the text. Estimates derived from
Michael Paine's web page at
Kinetic energy in megatons
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
10
5
10
6
10
7
10
8
10
8
K/T boundary impact
Point where tsun
ami
generated by
an impact
become significant
10
7
10
6
Every million years
10
5
10
4
Barringer
10
3
Tunguska
Every millennium
Point where tsunami
generated by airbursts
also become significant
10
2
Every century
Annual
event
10
1
Every decade
10
0
Annually
10
100
1,000
10,000
Asteroid diameter (m)
Known events
Range in estimates
9.3
How Do Extraterrestrial Objects
Generate Tsunami?
respectively, begin to distort or fragment traveling through
the atmosphere. Any object entering the atmosphere at a
shallow angle is more likely to reach the ocean without
breaking up; however, this does not necessarily lead to
bigger tsunami. Distortion without fragmentation leads to a
pancake-shaped body of greater diameter impacting into the
ocean. Even if an asteroid fragments, the fragments can hit
the ocean as a hollow shell, creating a cavity that can be ten
times greater than the radius of the original asteroid or
comet. Theoretically, an iron meteorite with a diameter of
less than 30 m could generate a tsunami by this mechanism.
The initial waves formed in this case are technically not
tsunami, as they are formed by the air blast. The real tsunami
comes about 5 s later when the cavity in the water collapses.
These fragmentation and distortion aspects have not yet been
modeled in the generation of tsunami, but may be important.
Asteroid-generated tsunami can be modeled using
incompressible, shallow-water long-wave equations descri-
bed in
Chap. 2
(Hills and Mader
1997
; Crawford and Mader
1998
; Ward and Asphaug
2000
). In fact, meteoritic tsunami
are similar to those generated by rockfalls such as the Lit-
uya Bay Tsunami of July 9, 1958 discussed in
Chap. 7
. For
example, a small meteoroid of only 500 m diameter falling
in the ocean at 20 km s
-1
, at a low angle of entry, could
carve a path at least 12 km long across the ocean. The
horizontal and vertical accelerations of the resulting seismic
waves would be much greater than any known earthquake.
An important similarity between low-angled asteroid
impacts and the Lituya rockfall-induced Tsunami is the role
9.3.1
Mechanisms for Generating Tsunami
There are four types of extraterrestrial objects based upon
density (Verschuur
1996
): comets (*1.0 g cm
-3
), carbo-
naceous bodies (2.2 g cm
-3
), stony asteroids (3.5 g cm
-3
),
and iron asteroids (7.9 g cm
-3
). Comets are generally
considered dirty snowballs; however, larger comets, in
breaking up in successive orbits within the inner solar
system, may produce many of the asteroids of various
densities that strike the Earth. These four classes of objects
also have different yield strengths that can vary over several
orders of magnitude. The yield strength determines how
easily the objects will fragment when they hit the atmo-
sphere. Finally, these objects travel through space at dif-
ferent speeds. Comets travel at 25-50 km s
-1
, while the
near Earth objects move at slower speeds of 15 km s
-1
.If
objects fragment and explode in the atmosphere as bolides
before striking the Earth's surface, they can still generate
tsunami. In this case, the size of the wave depends very
much upon the height of the explosion in the atmosphere.
Any object greater than 1 km in diameter tends to inter-
sect the Earth's atmosphere without fragmenting or
exploding. In effect, these large objects travel fast enough
that they do not have time to see the atmosphere before
striking the surface. Comets less than 580 m in diameter, and
stony and iron asteroids less than 320 and 100 m in diameter
