Tsunami from Asteroid Impacts

12 Mar 08 Mader Consulting - Book available 'Numerical modeling of water waves' - second edition. It documents in the problems and limitations of the various modeling methods. 2008 UPDATED DVD AVAILABLE Free to Book Owners + Parts of Chapter 6
12 Sep 07 : A possible Plio-Pleistocene tsunami deposit, Hornitos, northern Chile (Eltanin impact? thanks Nicolás González)
19 Jul 07 New Scientist: Dam-busting 'megaflood' made Britain an island - All the bedrock landforms we see are characteristic of a megaflood," NOT A TSUNAMI but an example of catastrophic erosion caused by water. See also Glacier Garden Lucerrne, Switzerland - Potholes - These impressive potholes were formed at the bottom of the glacier by the sheer force of the water. As is still the case in alpine glaciers today, the melt water initially flowed on the surface of the ice before seeping into the glacier through fissures. At the bottom of the glacier the water was under tremendous pressure. As the flow of water gathered speed, vortices with speeds of up to 200 km/h began to form. + THE FORMATION OF A GLACIAL POTHOLE. (however, note that water alone can cause rock erosion in a very short time - sand and gravel need not be present)
30 Jan 07: Excellent presentation by Steve Ward on the tsunami hazard (19Mb Quicktime movie)
16 Nov 06 AGU 2006 Abstract: Holocene Indian Ocean Cosmic Impacts: The Megatsunami Chevron Evidence From Madagascar
15 Nov 06 ABC Radio: Researchers claim link between tsunamis and outer space + NY Times: Ancient Crash, Epic Wave (thanks David Morrison) + SMH: Big splash theory says meteors hit regularly.
24 Aug 06 Steve Ward: New simulation of Eltanin impact 2 million years ago.
13 May 06 New Scientist: Tsunami risk of asteroid strikes revealed. Natural Hazards: A Quantitative Assessment of the Human and Economic Hazard from Impact-generated Tsunami. Preprint here.
13 May 06 Selection of recent papers from Science of Tsunami Hazards (PDFs):

Confirmation and Calibration of Computer Modeling of Tsunamis Produced by Augustine Volcano, Alaska
Experimental Modeling of Tsunami Generated by Underwater Landslides
Sage Calculations of the Tsunami Threat from La Palma - We find that while high-amplitude waves are produced that would be highly dangerous to nearby communities (in the Canary Islands, and the shores of Morocco, Spain, and Portugal), the wavelengths and periods of these waves are relatively short, and they will not propagate efficiently over long distances.

19 Mar 06 LPSC2006: RUNUP FROM IMPACT TSUNAMI.
5 Mar 06 Steve Ward: New movies of encounter with asteroid Apophis in 2036: Apophis strikes the Pacific ocean just north of Panama + Apophis zooms by Earth missing by just 2500 km.
6 Dec 05 Steve Ward: New impact tsunami movies include blast-wave effects (~3Mb MOV): 10 km diameter Chicxulub + 1 km diameter off Southern California
22 Oct 05 NewSci (subs): The future of Atlantic tsunamis + page 62 of the paper edition (22 Oct 05) has an ad for "Project Scientists - Australian Tsunami Warning System".
20 Oct 05 Essay by Steve Ward & Simon day: Tsunami Thoughts
5 Aug 05 Nature: Hurricanes whip up huge waves - 40-metre monsters [but their period is too short to cause castastrophic erosion]
6 Jul 05 The Australian Museum Society: Tsunami and Sydney: a long-term relationship - Dr Edward Bryant, University of Wollongong - Thursday 7 July, 2005, 6.30pm for 7pm at the Australian Museum, Sydney.
26 May 05 Glencairn Insurance: The Asian Earthquake/Tsunami Disaster An Insurance Perspective (2Mb PDF) - "Santa Cruz, for example, has experienced a number of tsunamis from distant earthquakes in the last 60 years and is also at risk from local tsunamis generated by submarine landslides..." (thanks Steve Ward)
11 May 05 Quaternary Science Reviews (abs): Evidence for three North Sea tsunamis at the Shetland Islands between 8000 and 1500 years ago.
11 May 05 San Gabriel Valley Tribune: Tsunami danger real, but remote, study says Southern California faces a real tsunami threat from offshore earthquakes and submarine landslides...(CC)
6 May 05 Livescience: New Method Predicts Monster Waves (not tsunami)
5 May 05 Tsunami modeller Steve Ward has prepared two movies of potential impacts by asteroid 2004MN4 off the coast of the USA. A-MOV and B-MOV
4 Apr 04 BBC: Flood theory - A tsunami could have hit Bristol and south Wales in 1607 (CCNet)
2 Apr 05 LiveScience: Potential Southern California Tsunami Could Cost up to $42 Billion.
21 Mar 05 Indy Star: Study sees tsunami risk in the Caribbean - Ten destructive tsunamis have been generated in the past 500 years. Nearby undersea landslides are the likely cause.
11 Mar 05 LPSC05: OFFSHORE BREAKING OF IMPACT TSUNAMI: VAN DORN WAS RIGHT.
25 Feb 05 Science (regn): Ancient Alexandria Emerges, By Land and By Sea - subsidence was brought on by a lethal combination of earthquakes, tsunamis, and the slow but relentless sinking of heavy foundations into unstable soil.
15 Jan 05 ABJournal: Asteroid Could Cause Tsunami, LANL Scientist Says - "If we had a monitoring system in the Indian Ocean, maybe 75,000 lives could have been saved in this recent disaster," Gisler said. "It's just ridiculous when you look back that the money wasn't spent." (CC)
10 Jan 05 LA PALMA TSUNAMI: DEBUNKING THE MEGA-HYPED TIDAL WAVE STORY (author not named) + University of Southampton (Aug 04): Canary Islands landslides and mega-tsunamis: should we really be frightened? (CCNet)
7 Jan 05: This page earns Griffith Observatory Star Awards.
3 Jan 05 Financial Express - New Delhi,India: TSUNAMI alarm: desi model or global club?
1 Jan 05 SMH: Anger rises in India over lack of warning - Seismologist Arun Bapat says he has been warning of the risk of a tsunami for decades, yet no one was listening. "There have been four tsunamis in India in the last 100 years, and it is well-known that an earthquake of such a large magnitude generates a tsunami. There was no system in place."
1 Jan 05 World News: Tsunami predicted, not heeded - Scientists at the Pacific Tsunami Warning Centre in Honolulu had forecast Sunday's massive tsunami within 15 minutes of the Indonesian earthquake, but did not know who to pass the information to. A tsunami warning that could have saved thousands of lives was issued, but not acted upon, more than an hour before giant waves hit Sri Lanka and southern India... In Sri Lanka, which suffered the biggest loss of life in the tsunami, crowds had come to the beaches to watch the sea after word spread that it was producing larger-than-normal waves... None of the countries most severely affected had a tsunami warning mechanism.
31 Dec 04 Livescience: Tsunami Kills Few Animals in Sri Lanka (speculative) + Deadly Tsunamis Rival Waves of the Past.
29 Dec 04 Japan Times editorial: Catastrophe without warning.
29 Dec 04 Copy of "The Great Earthquake and Tsunami of 26 December 2004 in Southeast Asia"
by Dr. George Pararas-Carayannis temporarily added due to his website being overloaded.
29 Dec 04 Space.com: Arthur C. Clarke, in Sri Lanka, Calls Tsunami 'Disaster of Unprecedented Magnitude'
27 Dec 04 Links to TerraDaily articles on the SE Asian Tsunami.
27 Dec 04 Channel News Asia: No warning system in place for tsunami-hit countries: USGS (map of affected areas).
27 Dec 04 SMH: Tsunami smashes Asia, more than 11,300 dead - One of the most powerful earthquakes in history hit Asia yesterday,unleashing a tsunami which devastated coastal areas of Sri Lanka,India, Indonesia and tourist isles in Thailand, killing more than 11,300 people.
27 Dec 04 STH Abstract (1999): Tsunamis along the coastlines of India.
26 Dec 04 Yahoo (posted just a few hours after the item below): Indonesia Quake Sparks Fatal Tidal Waves One of the world's most powerful earthquakes in years (magnitude 8.5) rocked northern Indonesia on Sunday and launched tidal waves that slammed shorelines across Asia, killing some 300 in Sri Lanka and almost 100 in Indonesia, officials said... Waves crashed into coastal villages over a wide area of Sri Lanka — some 1,000 miles west of the quake's epicenter
26 Dec 04 Yahoo: One of World's Largest Quakes Hits Near Australia - The earthquake measuring 8.1 on the Richter scale hit near Macquarie Island in the Southern Ocean, more than 500 miles southeast of Tasmania... The earthquake could have caused a tsunami, but no noticeable changes in water levels had been reported in Tasmania or New Zealand.
8 Dec 04 LiveScience: Mystery of Deadly 1946 Tsunami Deepens
5 Nov 04 Deep Sea Research: Special issue on ocean impacts (Vol 49 No 6, Published 2002)
3 Nov 04 AGU Annual Meeting - search for "tsunami": Tsunami Hazards and Probabilistic Analyses + Quantifying Tsunami Impact on Structures + Inundation Modeling for Probabilistic Tsunami Hazard Assessment + Probabilistic Risk Analysis of Run-up and Inundation in Hawaii due to Distant Tsunamis
30 Oct 04 BBC: Tidal wave threat 'over-hyped'
18 Oct 04 Tsunami Society: The Third Tsunami Symposium has been postponed one year to May 2006!
15 Oct 04 Financial Times (subs reqd): Shock Waves - Fears that a volcanic eruption in the Canary Islands could send a 25-metre-high tsunami crashing into the US coast have been gaining momentum. But rival scientists dismiss the prediction as a hugely unlikely worst-case scenario. Who do we believe? ... And even if the La Palma tsunami were not to be quite so immense, it would still be classified as a flood, explains Bob Hartwig, chief economist of the Insurance Information Institute in New York. And floods are, generally, not covered under most property insurance. As far as catastrophic events go, insurance companies are more worried by what lurks in the heavens. "Meteors," says Hartwig, "they're covered by insurance, but not factored into premiums. If a meteor similar to the one that hit Siberia in 1908 were to hit western Europe or the north-east United States, we could go bankrupt."
15 Sep 04 Astrobiology Magazine: Surfing the Wave A tsunami, a series of large waves caused by the disruption of seawater, is one
of the many hazards of living on Earth. Bill McGuire, Director of the Benfield Hazard Research Centre, says that a mega-tsunami could cause death and destruction to both the eastern and western Atlantic coasts. [Prof McGuire claims that the Tsunami Society does not have volcano experts but see the link to Dr. George Pararas-Carayannis's paper below (17 Aug).]
7 Sep 04 Japan Times: Quakes injure 42; tsunami flip 14 boats
3 Sep 04 Science (subs): Tibet's ancient flood - this one was "one of the most erosive events in recent Earth history". See this thesis page by David Finlayson.
1 Sep 04 Geology (abstract): Megatsunami deposits on Kohala volcano, Hawaii, from flank collapse of Mauna Loa (thanks Steve Ward). See also Modeling the 105ka Lanai Tsunami (4.5Mb PDF) and this animation by Steve Ward (large quicktime file - see snapshot at right). Steve also has an animation of a Mt St Helens landslide.
19 Aug 04 Uni Wollongong: Interactive map of Australian Tsunami features.
19 Aug 04 Guardian: Britain being battered by waves hurling giant rocks - Storms are causing waves that are ripping giant boulders from the top of cliffs in exposed areas and hurling them inland.
17 Aug 04 George Mason Uni: Run To The Hills! La Palma "mega-tsunami" + Dr. George Pararas-Carayannis: EVALUATION OF THE THREAT OF MEGA TSUNAMI GENERATION FROM POSTULATED MASSIVE SLOPE FAILURES OF ISLAND STRATOVOLCANOES ON LA PALMA, CANARY ISLANDS, AND ON THE ISLAND OF HAWAII + Royal [Bemuda] Gazette: Expert downplays tsunami threat.
11 Aug 04 Mader Consulting: new book available 'Numerical modeling of water waves' - second edition. It documents in the problems and limitations of the various modeling methods.
10 Aug 04 ABC: Scientist warns of tsunami in the making BUT Tsunami Society: MEGA TSUNAMI HAZARDS - We would like to halt the scaremongering from these unfounded reports [that collapse of volcano flanks might cause ocean-wide "mega-tsunami"]. Also reported in the Guardian (thanks Duncan Steel).
28 Jul 04 SciAm: Old Rivers More Active in Youth ... rapid erosion over a relatively short 20,000-year span. Abstract from Science 23 Jul 04.
27 Jul 04 Yahoo: Ship-Sinking Monster Waves Are Widespread -- ESA (not tsunami)
1 Jun 04 Geology: River incision into bedrock: Mechanics and relative efficacy of plucking, abrasion, and cavitation. Geological Society of America Bulletin: Vol. 112, No. 3, pp. 490–503 (2000!).
28 May 04 AGU: Ocean-Scale Tsunami Propagation: Boussinesq Approximations.
9 Mar 04 ScienceNews: Killer Waves. Thanks Steve Ward.
10 Feb 04 NZ Herald: Expedition hunts giant meteor [sic]. Thanks Steve Ward.
16 Dec 03 Science Now (subs): The Case for Monstrous Hawaiian Waves - Abstract of McMurtry's talk, More details on Hawaii tsunami studies
1 Dec 03 USNews.com: A blast from heaven? (debate over a recent mega-tsunami in the South Pacific - thanks Steve Ward) + GSA: DID A BOLIDE IMPACT CAUSE CATASTROPHIC TSUNAMIS IN AUSTRALIA AND NEW ZEALAND? (abstract) + “Mystic Fires of Tamaatea”: Attempts to creatively rewrite New Zealand’s cultural and tectonic past, by James Goff, Journal of the Royal Society of New Zealand - 17 Jan 04 Earth2Class: DID A BOLIDE IMPACT CAUSE CATASTROPHIC TSUNAMIS IN AUSTRALIA AND NEW ZEALAND?
18 Nov 03 Santa Cruz Sentinel: UCSC seismologist prepares for tsunami.(CC)
12 Nov 03 NewsDay: EXPERTS SEEK TRAIL TO MARK ICE AGE FLOODS (in NW USA) (CC)
29 Aug 03 NASA DPS 2003: Assessing the Human Hazard from Impact-generated Tsunami - by Chesley and Ward.
24 Aug 03: International Seminar/Workshop on Tsunami - “In Memoriam 120 Years Of Krakatau Eruption – Tsunami And Lesson Learned From Large Tsunami“ August 26th – 29th 2003, Jakarta and Anyer.
23 Aug 03 Economist: The next big wave - tsunami detection (CC)
7 Aug 03 Nature (abstracts): Unusually large earthquakes inferred from tsunami deposits along the Kuril trench. Details here.
19 Jul 03: Steve Ward has created a great new animation of the Eltanin ocean impact.(5Mb Quicktime movie)
18 Jul 03 Space.com: Small Stony Asteroids Will Explode and Not Hit Earth, Study Shows. Based on a Nature article (abstract). See also these discussions on CCNet and this press release. In esssence, the overall risk to mankind is not greatly affected by this new work - there will be fewer oceans impacts but more airbursts above the land.
4 Jun 03 SciAm: 2880 Asteroid Impact Simulation Suggests Tsunamis Could Hammer Atlantic Coast. (1.1km asteroid hitting 600km off the US coast).
28 May 03 UCSC: Massive tsunami sweeps Atlantic Coast in asteroid impact scenario.(NNN)
18 Mar 03 Uni Arizona: Worried About Asteroid-Ocean Impacts? Don't Sweat The Small Stuff [note there are many other good reasons for finding asteroids down 200m, even if they don't cause ocean-wide tsunami] (NNN)
25 Feb 03 LPSC2003 Abstracts (selection):

24 Feb 03 Nature (13/2): Long-lost wave report sinks asteroid impact theory (see also CCNet)
24 Jan 03 Tsunami Society: MEGA TSUNAMI HAZARDS - We would like to halt the scaremongering from these unfounded reports [that collapse of volcano flanks might cause ocean-wide "mega-tsunami"]
12 Dec 02: SCIENCE OF TSUNAMI HAZARDS - Vol 21 No2 released (advanced computer modelling of asteroid impacts).
24 Oct 02 ABC (Australian TV): Asteroid Tsunami 8pm tonight - Australian scientist Ted Bryant says it’s not the infrequent big space rocks we really should be concerned about - it’s the smaller ones. Bryant takes Catalyst reporter Graham Philips to the cliff tops of the coastline around Wollongong and finds geological features that he claims were made by Tsunami’s that in turn were caused by asteroid impact.
16 Aug 02 Nando Tines: RESEARCHER STUDIES FOLKLORE FOR CLUES ABOUT TSUNAMI (CCNet)
5 Jun 02 SpaceDaily: New Wave Supercomputers Catch Big Waves (tsunami from asteroid impacts).AAS abstract. LANL Press Release (NNN)
26 Apr 02: Science of Tsunami Hazards Vol 20 No 1 (2002) 'Tsunami book gives a better understanding of ancient floods on Mars' (800K PDF).

Uni of Wollongong.

library of the University of Georgia

26 Apr 02 NYT: EXPERTS FIND CLUES TO CAUSE OF DEADLY PACIFIC TSUNAMI.(CCNet) See also this STH paper by Watt (PDF)
12 Apr 02 The Guardian: A last wave goodbye - An asteroid landing in the ocean would produce the tsunami to end them all, says Duncan Steel. But see below.
3 Apr 02 UniSci: Superfloods Controversial, But Aid Flood Prediction - The massive flood cut deep canyons, stripped away soils, and carried boulders all the way from Idaho to what is now Portland, Ore. In 48 hours, it transformed the landscape of the Pacific Northwest.
30 Mar 02: Science of Tsunami Hazards now has its own website with abstracts for SECOND TSUNAMI SYMPOSIUM.
29 Mar 02 Space.com: Mars, Like Earth, Sculpted by Super Eruptions and Epic Floods
31 Oct 01: Ted Bryant's book Tsunami: the underrated hazard is now available in Australia. It provides a comprehensive reference to the subject of tsunami: mechanisms for generation, evidence of past tsunami in coastal landforms and the effects on humans over millenia. See review on 26 Mar 02.
20 Sep 01 SFC: Threat of killer waves - Recent scientific studies show the power of tsunamis (thanks Steve Ward)
9 Sep 01 BBC: Giant wave hit ancient Scotland.
29 Aug 01 BBC: Giant wave devastation feared (Landslide generated - but there are considerable differences in the tsunami estimates) + MSNBC (more cautious!) + Discovery.
5 Mar 01 ABCNews: Computers Predict Tsunami - NOAA Scientists See Big Wave in West Coast's Future.
11 Oct 2000 BBC: Action urged over giant wave threat (see comments above). Mega-tsunami - Horizon BBC TV.
18 Jul 01 ABCNews: Threat of Giant Waves? - Scientists suspect that an earthquake-triggered tsunami that killed more than 2,000 people in Papua New Guinea two years ago on Monday was bolstered by an undersea landslide.

Giant wave could threaten US

A collapsing volcano in the Atlantic could unleash a giant wave of water that would swamp the Caribbean and much of the eastern seaboard of the United States.

New Scientist

But

a La Palama landslide would not be a threat to the U.S

30 May 2000: Seafloor Off Mid-Atlantic Coast Highly Charged with Gas - ...possible cracks along the outer continental shelf off the mid-Atlantic coast might lead to a tsunami-causing landslide ... discovered that the entire area is charged with gas.

Publication notes

June 2001: New book on tsunami by Ted Bryant.
May 2001: 'The Asteroid Tsunami Project at Los Alamos' by Jack Hills and Patrick Goda, Science of Tsunami Hazards, Vol 19 No.1 (2001) 55-65. Describes the effects ocean impacts by asteroids 5km and 10km in diameter. The authors indicate there is uncertainty about their earlier work involving smaller asteroids and that they are developing improved models. They point out that localised tsunami effects can be delayed and peaks can occur well after the first wave arrives.
Sep 2000: 'The mitigation, management and survivability of asteroid/comet impact with Earth' by V. Garshnek, D. Morrison and F. Burkle, Space Policy (Vol 16 (2000) 213-222) (not online) - uses estimates of risk and fatalities from Table 1 below and refers to this page.
Jan 2000: For alternative estimates of hazards to a populated Earth see my Space.com article Simulating Armageddon on your PC and this extra information.
Dec 1999: This analysis is also described in my paper "Asteroid impacts: the extra hazard due to tsunami" (200K PDF) in the Science of Tsunami Hazards, Volume 17 Number 3 1999 (PDF 9.4Mb) available from LANL.
Nov 1999: For a less technical description of asteroid tsunami please see my Space.com article Asteroids and tsunami: GOOD NEWS AND BAD and references.
Aug 1999. Water crater explained. June 99: Risk to inhabited regions revised - area of devastation for 500m & 1km asteroids reduced. Consequences of 2km impact included.
These estimates assume that impacts are randomly distributed in time. It is possible the Earth may be subjected to a barrage of asteroids (or comet fragments) from time to time. This may have happened over the past few thousand years and could be a source of some of the tsunami that appear to have struck Australia during this period.
May 1999: This page listed in a UK Parliament report on the NEO hazard as "a particularly useful web site" (PDF format).

May 1999: Scientists know of no asteroid or comet that is on a collision course with Earth, at least for the next thousand years or so. The point of the current publicity is that, with limited funding, the search efforts to date have only found about 10% of the large (1 kilometre or bigger) objects orbiting the Sun near the Earth. Amongst the estimated 1,800 or so undiscovered large objects there could be one on a collision course with Earth in the next few decades. This is very unlikely but it is simply crazy for mankind not to look for them with a program like Spaceguard, just in case something needs to be done to avoid a collision. Please don't worry about the hazard or lose sleep but, by all means, if you agree with these comments please write to your local Federal Member of Parliament seeking support for "Spaceguard" from the Australian Government.
April 1999: Risk analysis has been extensively revised, based on recent papers by Crawford & Mader and Ward & Asphaug. Note that risk estimates have been substantially reduced from previous values - the risk of a typical coast dweller dying from an asteroid generated tsunami is probably similar to that of dying from the indirect effects of a large asteroid impact (not necessarily impacting the ocean). For some coastal locations which are particularly vulnerable to tsunami the risk is two or three times greater.

The Tsunami Society held a Symposium May 25-27 1999 in Honolulu. Topics covered (temporary link to abstracts) include: "MEGA-TSUNAMIS FROM ASTEROIDS AND SLIDES", "Eltanin Asteroid Impact Tsunami" and papers on the tsunami risk to numerous coastlines, including Australia. Dr George Pararas-Carayannis's Tsunami Page has more details about the Symposium.

For flexibility in circulating the document non-ASCII characters have been avoided and Fortran style formulae have been used:* means multiply, ^ means"to the power of", Ey means "10 to the power y".

Risk calculations should be regarded as "ballpark" estimates, pending further research on the frequency of impacts and the vulnerability of coastlines to tsunami.

All asteroid sizes refer to diameter. All tsunami wave sizes refer to maximum height above sea level (see diagram).

Nature of Tsunami

Tsunami generally travel very fast across the ocean (typically 500km/h or more). In deep water the tsunami height might not be great but the height can increase dramatically when they reach the shoreline because the wave slows in shallow water and the energy becomes more concentrated. In addition to the inherent increase in the height of the wave from this shoaling effect, the momentum of the wave might cause it to reach a considerable height as it travels up sloping land. It is typical for multiple waves to result from one tsunami-generating event and these could be several hours apart when they reach a distant shore.

*Tsunami is Japanese for "harbour wave" which is misleading to the Japanese because tsunami don't just occur in harbours. Scientists dislike the term "tidal wave" as used by the Press but there is a little logic in the term - some tsunamis do not break when they reach land but surge like a massive, fast moving high tide to flood low-lying areas. Much of the damage comes as they recede back into the ocean.

Illustration of Tsunami Terms (Magnified Vertical Scale)

Amplitude is approximately the maximum height of the wave above sea level when in deep water - see diagram. Note that this is not the same as the "double amplitude" which is the vertical distance between the crest and the trough and is often used to describe the height of a wave).
Run-up height is the vertical height above sea level of the tsunami at its furthest point inland.
Run-up factor is the run-up height divided by the deepwater wave amplitude

The run-up factor can vary considerably, depending on local topography and the direction of travel of the wave. Hills and Goda (1998) note that earthquake-generated tsunami in Japan have an average run-up factor of 10 but sometimes reach 25. In Hawaii run-up factors of 40 have been observed for earthquake-generated tsunami. There is a particular danger to seaports from tsunami because the approach channel to the port can support a much more energetic tsunami (there is less energy dissipated or reflected as it travels over the continental shelf). On the other hand, based on recent assessments of tsunami risks for various locations, Crawford and Mader (1998) estimate the typical run-up factor is only 2 to 3.

Contrary to popular notions, the Australian coastline is vulnerable to tsunami (Nott & Bryant 1999 and Rynn & Davidson 1999). There is also evidence of substantial variations in run-up factor for tsunami along the Australian coast . Along a 40km stretch of coastline the run-up height from one (ancient) tsunami event varied by more than 40 (based on Young et al 1996). The effects are complicated by features such as estuaries, harbours, cliffs and reefs. The topography and features of the continental shelf, the shoreline, an estuary/harbour and the land are all very important is considering the damaging effects of tsunami. Some coastal areas could be vulnerable to relatively small tsunami. Until recently there appears to have very little assessment of this risk except in areas prone to earthquake-generated tsunami such as Japan and Hawaii.

The urgency for increased research on tsunami is reinforced by the devastating tsunami which struck northern New Guinea in July 1998. Scientists are still trying to understand mechanisms of that earthquake-related tsunami.

Asteroid impacts with the Earth

Estimates of risk based on asteroid/comet impact frequency may vary by a factor of ten - "Events like Tunguska occur with uncertain frequency, possibly once every 50 years, if the interpretation of the Spacewatch data is correct, or at most once every 300 to 500 years" (Steel 1995). Subject to this uncertainty, the probability of an impact at a given location can be estimated from
P = P(D) * A_D / A_E (1)
where
P(D) is the probability of an impact by an asteroid of diameter D somewhere on the Earth
A_D is the area of destruction due to the impact
A_E is the total area of the Earth's surface (including ocean).

Applying this to the Tunguska event, and assuming an average interval between Earth impacts of one century, the annual probability of a given location being within the devastation area is P(annual) = 0.01 * 2000 / 5.1E8 = 4E-8 or about 1 in 25 million. .

Steel (1995) provides the following formula for estimating the area of destruction, based on nuclear weapons tests:
A = 400 (Energy)^0.67 (2)
Using this formula the following table sets out the typical values for stony asteroid up to 200m diameter (assuming velocity=20km/s, density=3 g/cc). Again the values are subject to considerable uncertainty and may vary by a factor of ten or more. The area of devastation for 500m and 1km asteroids is derived from the range of values presented by Morrison & Chapman (1995).

Table 1 - Risk of direct impact for a given location

Diameter Kinetic Energy Area Devastated Average. interval (years)

(m) Mt TNT sq km Earth "City" Inhabited Region &
Expected Death toll

50

10

1900

100 yr

30 million yr

900 yr

1 million

100

75

7200

1000 yr

70 million yr

8000 yr

3 million

200

600

29 000

5000 yr

90 million yr

30 000 yr

14 million

500

10 000

70 000

40 000 yr

290 million yr

180 000 yr

30 million

1 km

75 000

200 000

100 000 yr

260 million yr

290 000 yr

60 million

2 km

1 million MT

-

1 million yr

-

1 million yr

1.5 billion

All*

90 yr

14 million yr

800 yr

*All = 1/ ( 1/T₅₀+ 1/T₁₀₀ + 1/T₂₀₀ + 1/T₅₀₀ + 1/T₁₀₀₀) on the basis that probabilities are independent and span the range of asteroid sizes.

# See Appendix for calculation of risk to inhabited regions and predicted death toll.

Morrison and Chapman 1995

There is uncertainty about the diameter of many asteroids when they are initially discovered. These objects are generally so small and so far away that their diameter has to be inferred from their absolute magnitude. Brightness, in turn, depends on the object's albedo (amount of light reflected from the surface). An object of magnitude 23 might have a diameter between 65m and 108m. Furthermore, with limited observations, the absolute magnitude may vary by +/-0.5 therefore the estimated diameter can range from 50m to 190m.

Iron asteroids are more likely to reach the ground intact. They comprise perhaps 5% of the smaller asteroids and are disregarded in this analysis.

Tsunami generated by impacts

bursting a balloon in a bathtub

At this stage there are considerable differences in asteroid/tsunami predictions between the researchers. For a review of the methods see Ward & Asphaug (1999). After presenting their predictions of risk to coastal areas these authors note that "Being about ten times less than Hills et al. (1994) and perhaps ten times greater than Crawford and Mader (1998), our predictions split the field".

The main items of contention appears to be:

the initial size of the wave - based on analysis of the size and shape of the "crater" and the manner in which it collapses, and
the rate at which a tsunami from an asteroid impact dissipates as it travels.

500m diameter asteroid impacting 5km deep ocean.

From Crawford & Mader 1998.

Crawford & Mader 1998. Ocean impact by 500m asteroid

Table 2 - Estimated deepwater wave height (above sea level) at a point 1,000km from an asteroid impact (selected research results)

Stony Asteroid Diameter	Hills & Goda (1998) (their Figure 1)	Ward & Asphaug (1999) (their Figure 6)	Crawford & Mader (1998) (their Table 1)
200m	1m (5m from equation)	5m	negligible

Diameter	Kinetic Energy	Area Devastated	Average. interval (years)
(m)	Mt TNT	sq km	Earth	"City"	Inhabited Region & Expected Death toll
50	10	1900	100 yr	30 million yr	900 yr 1 million
100	75	7200	1000 yr	70 million yr	8000 yr 3 million
200	600	29 000	5000 yr	90 million yr	30 000 yr 14 million
500	10 000	70 000	40 000 yr	290 million yr	180 000 yr 30 million
1 km	75 000	200 000	100 000 yr	260 million yr	290 000 yr 60 million
2 km	1 million MT	-	1 million yr	-	1 million yr 1.5 billion
All*			90 yr	14 million yr	800 yr