Comets and Asteroids - Tsunami - page 170

Asphaug 2000 ). The leading edge of the resulting tsunami
travels 25 km from the center of the impact within 5 min
and is over 300 m high. At distances of 50, 500, and
2,000 km the wave is still over 100, 11, and 6 m high,
respectively. These heights are substantially greater than the
calculated heights based upon nuclear explosions (Fig. 9.5 ).
At the other extreme, simulations of asteroid impacts into
an ocean have been performed on a supercomputer at the
Sandia National Laboratories in New Mexico, using algo-
rithms that modeled accurately the impact of the Shoe-
maker-Levy 9 comet into Jupiter's atmosphere in July 1994
(Crawford and Mader 1998 ). These are the same calcula-
tions used in the simulation of splash effects shown in
Fig. 9.4 . Despite an uncertainty factor of two, the computer
modeling yields tsunami heights that are smaller than those
derived using Eq. ( 9.6 ) by a factor of almost ten. These
differences are summarized in Table 9.2 for a small range of
asteroid sizes. The differences have been calculated 500 km
from the center of impact for an iron asteroid striking an
ocean 5,000 m deep at a velocity of 20 km s -1 . The mod-
eled tsunami wave heights span an order of magnitude—a
fact indicating that there is not a broad consensus amongst
researchers on the heights of asteroid-generated tsunami.

100

50

20
10

5
2

1

0 100 200 300 400 500 600 700 800 900 1,000

Diameter of an iron asteroid (m)

Fig. 9.5 The size of tsunami, above sea level, generated by iron
asteroids of various diameters striking the ocean. The heights are at
distances of 500, 1,000, 2,000, and 5,000 km from the center of
impact. The asteroids have a density of 7.8 g cm -3 and impact with a
velocity of 20 km s -1

If the height of a tsunami at shore were approximately ten
times its open ocean height, then according to Eq. ( 2.14 ), the
tsunami wave produced by a 100 m diameter iron asteroid
(Table 9.1 ) would penetrate inland 890 m on any flat coastal
plain within 2,000 km of the impact. An iron asteroid,
500 m in size can generate a tsunami wave that is approxi-
mately 35 m high leaving the impact site. 2000 km away,
this tsunami would still be approximately 9 m high. If this
size asteroid landed in the middle of the Pacific Ocean, its
tsunami would be just less than 5 m in height approaching
any of the surrounding coastlines. Theoretically, this wave
could sweep inland over 12 km across any flat coastal plain
in the Pacific Ocean region. Stony asteroids are less effective
at generating tsunami than iron asteroids, but the impact of
their run-up is just as impressive. A 500 m stony asteroid
would generate a tsunami that is 5.5 m high, 2,000 km from
the impact site. This is bigger than any historical tsunami in
the Pacific. Such a wave could sweep inland more than 6 km
over any flat coast surrounding the Pacific Ocean. Asteroids
larger than 1 km in diameter will produce catastrophic tsu-
nami in any ocean even if tsunami generation is depth lim-
ited. Modeling, using incompressible, shallow-water
long-wave equations indicates that a significant amount of
this wave's energy would be reflected from the front of flat
continental shelves. Hence, coasts protected by wide
shelves, such as those found along the east coast of the
United States and northern Europe, are less affected by
cosmogenic tsunami than are steep coasts such as those
found along the coasts of Australia or Japan. However, even
on the most protected coastline, the wave formed by a large
asteroid would mimic the effect of tsunami generated by
mega-landslides.
Recent mathematical and computer modeling indicates
that there is a wide range in the calculated heights of tsu-
nami emanating from asteroid impacts (Paine 1999 ; Gu-
siakov 2007 ). For example, Fig. 9.6 presents a simulation of
the wave height produced by a stony asteroid 200 m in
diameter traveling at a speed of 20 km s -1

9.4 Geological Events

9.4.1 Hypothesized Frequency

Impact craters are profoundly difficult to identify because of
active erosion of the Earth's surface and recycling of the
Earth's crust through plate tectonics. As of the beginning of
2013, 183 impact craters had been identified on the surface
of the Earth, excluding the polar icecaps (Planetary and
Space Science Centre 2013 ). Identification is presently
proceeding at the rate of 15 new craters per decade. As of
2003, only 7 of these were identified as marine, while 20
originally occurred in the ocean, but are now preserved on
land (Dypvik and Jansa 2003 ). Only marine impacts gen-
erate tsunami. While some of the preserved craters may
have occurred on the margins of oceans, no deep-ocean-
basin impact structure has been recognized. Impacts in the
ocean produce variable crater forms depending upon the
depth of water. Impacts in shallow water produce craters
with low or absent rims because currents and back flow into
the crater smooth out topography. Such currents can also cut
gullies through the crater rim. Infilling of the crater with
sediment is also a dominant process, so crater relief is
shallow. If an impact occurs on the edge of the continental
shelf, then that part of the crater furthest from land may
collapse sending shelf material cascading onto abyssal
plains to build up sediments hundreds of meters thick. In the
deepest ocean, water does not absorb the impact. Here

(Ward and