Geoscience Reference
In-Depth Information
Table 9.1
Tsunami heights (m) generated at distances of 500, 1000, and 2,000 km from the impact site of iron or stony asteroids with the ocean
Iron asteroid characteristics
Iron asteroid
Stony asteroid
Asteroid
diameter (m)
Kinetic energy
(Gigatons TNT)
Crater
diameter
(km)
500 km
distance
1000 km
distance
2000 km
distance
500 km
distance
1000 km
distance
2000 km
distance
100
0.2
3.4
2.7
1.4
0.7
1.6
0.8
0.4
200
1.6
6.3
8.3
4.2
2.1
5.0
2.5
1.2
300
5.3
9.1
16.1
8.0
4.0
9.6
4.8
2.4
400
12.7
11.8
25.6
12.8
6.4
15.2
7.6
3.8
500
24.7
14.4
36.7
18.4
9.2
21.9
10.9
5.5
600
42.7
17.0
49.4
24.7
12.3
29.4
14.7
7.3
700
67.8
19.5
63.4
31.7
15.8
33.4
18.9
9.4
800
101.2
22.0
78.7
39.3
19.7
46.8
23.4
11.7
900
144.1
24.5
95.2
47.6
23.8
56.6
28.3
14.2
1000
197.7
26.9
112.9
56.5
28.2
67.2
33.6
16.8
Note Velocity of impactor = 20 km s
-1
, Density of iron asteroid = 7.9 g cm
-3
, Density of stony asteroid = 3gcm
-3
, Ocean depth at
impact = 5000 m
Source Based on Shoemaker (
1983
) and Hills and Mader (
1997
)
1.2 km wide Barringer crater in Arizona—is equivalent to
12.5 megatons of TNT. Equations (
9.2
) and (
9.3
) indicate
that
that are 60 % smaller than those produced by denser
asteroids. The following equations more realistically model
these three conditions:
this
object
would
have
produced
a
pseudo-crater
1.5 km in diameter if it had hit the ocean.
When Eq. (
9.2
) is substituted into Eq. (
9.1
), the size of
the tsunami is over-estimated for small asteroids. Equa-
tion (
9.1
) also does not account adequately for tsunami
generated by stony asteroids or asteroids in shallow seas.
Stony asteroids smaller than 100 m in diameter tend to
fragment in the ocean (Hills and Mader
1997
). Larger ones
tend to dissipate some of their energy in the atmosphere.
For example, a 200 m diameter stony asteroid traveling at
25 km s
-1
would impact with a force equivalent to
940 megatons of TNT. If this asteroid exploded as an air-
burst, then only 20 % of its energy would reach the surface
of the ocean. This latter component is more than 30 times
greater than the energy of the Chilean Tsunami of 1960 and
approximately equal to the largest Krakatau eruption of
1883. The diameter of the smallest object that can reach the
Earth's surface virtually intact is 40 m for an iron meteorite,
130 m for a stony asteroid, and 380 m for a short-period
comet. The recurrence interval for a stony asteroid, 130 m
in diameter, is about every 1000 years. The tsunami created
by large asteroids impacting in the ocean are also depth
limited. As a rule of thumb, depth becomes a limiting factor
when it is less than 12 times the diameter of the asteroid.
For example, asteroids greater than 167 m in diameter will
be depth limited if the asteroid falls into water less than
2,000 m deep. In this case, the resulting tsunami is 60 %
smaller than if the asteroid had fallen into deeper water.
Asteroids larger than 500 m in diameter would be depth
limited in most oceans. Stony asteroids also produce waves
0
:
54
R
1
t
10
9
W
Iron asteroid H
m
¼
6500
ð
9
:
6
Þ
h
i
0
:
54
R
1
t
ð
9
:
7
Þ
Þ
3
Þ
2
Stony asteroid H
m
¼
7800
ð
0
:
049r
a
ð
0
:
05v
a
ð
0
:
33q
a
Þ
0
:
25
dR
1
t
10
9
W
Shallow water H
m
¼
1450
ð
9
:
8
Þ
These relationships are tabulated for asteroids of
between 0.1 and 1.0 km in diameter in Table
9.1
. The
heights of tsunami, above mean sea level, produced by an
iron asteroid are also plotted in Fig.
9.5
. This size range
covers those objects that could realistically strike the
Earth's ocean in the near future. Tsunami wave height
quickly attenuates away from the site of impact. It also
increases sizably as the diameter of the asteroid increases.
For example, a 100 m diameter iron asteroid would produce
tsunami of 2.7, 1.4, and 0.7 m in height, within 500, 1000,
and 2,000 km respectively of the center of impact. This size
asteroid is the minimum limit for the detection of NEAs and
one equivalent in energy to the Krakatau eruption of 1883.
The wave heights are equivalent to tsunami generated by
the Krakatau eruption over similar distances. The 0.7-m
height is much higher than the 0.2-0.4 m open ocean height
postulated for the Chilean Tsunami of 1960, 2,000 km away
from its source. As mentioned in
Chap. 6
, this latter tsu-
nami's run-up measured 3 m high on the Hawaiian Islands
and 1-2 m in Japan, and killed 61 and 190 people respec-
tively in each area.
