Beneath the Dragon’s Gate (Longmenshan) Mountains of Sichuan Province, China an apparently ‘stuck’ segment of a major fault complex failed on 12 May 2008 (Stone, R. 2008. An unpredictably violent fault. Science, v. 320, p. 1578-1580). Unprecedented access to the world’s media resulted in our exposure to the full horror of the results of major seismic events in mountainous terrain and on habitations, especially schools, whose building standards were unable to withstand ground shaking. &0 thousand souls died, thousands more are still unaccounted for and more than 1.5 million people have become refugees in a country that is rapidly emerging from Third World status. Now that aftershocks have subsided massive threats remain from the many landslide-blocked rivers and fractured dams. Yet we also witnessed enormous mobilisation of the People’s Army within hours of the earthquake and truly heroic attempts to rescue as many trapped people as possible. Without that swift response the casualties would undoubtedly have been worse.

China boasts one of the most sophisticated seismic warning systems outside of California and Japan, deploying robotic seismometers and GPS recorders in the most risky regions, and with a 10-thousand strong Earthquake Administration. Sadly, Chinese seismologists regarded the faults shown to be accumulating displacement most quickly as those most likely to fail. It is generally ‘stuck’ segments that fail catastrophically. China has a long-respected reputation for gathering data generally regarded as ‘non-scientific’, such as well water levels, and animal behaviour, that might give empirical clues to impending earthquakes. The Tangshan earthquake of 28 July 1976, which killed a quarter of a million people 160 km from Beijing, was preceded by reports of shifts in the water table, odd ‘earthlights’ and unusual animal behaviour. Paying serious attention to reports by ordinary people of such oddities is reported to have avoided untold numbers of deaths in the period since Tangshan, but not in the case of Sichuan. Strangely, a Taiwanese weather satellite detected decreased electrical activity in the ionosphere above Sichuan hours before the recent earthquake (see Clouds and large earthquakes in May 2008 issue of EPN). Geophysicists have noted increased emissions of radon in the period immediately preceding some major earthquakes which might conceivably have an effect on the ionosphere. Whatever, prediction of catastrophic earthquakes has had very few successes in terms of lives saved, and the signal lesson from Sichuan, as from that which destroyed the Japanese city of Kobe in 1995, is that building standards in zones of active faulting must take account of the risk of ground movement.

See also: Stone, R. 2008. Landslide, flooding pose threats as experts survey quake’s impact. Science, v. 320, p. 996-997.

Extraterrestrial impactors

July 2008

June 30, 2008 was the centenary of the mysterious Tunguska event that devastated more than 2000 km2 of forest 1000 km north of Lake Baikal in Siberia at 7 am a hundred years before. Much of the mystery stems from there being no sign of a crater and therefore of the process involved. Speculation about the cause of a massive explosion between 5-10 km above the surface still goes on (Steel, D, 2008. Tunguska at 100. Nature, v. 453, p. 1157-1159). Ideas have ranged over a gamut of high-energy physical processes involved in the explosion: a deuterium-rich, fluffy comet that was ignited as a thermonuclear explosion by hypersonic atmospheric entry; a lump of antimatter; a miniature black hole; explosive release and ignition of natural gas; a ‘Verneshot’, and even an alien space craft involved in an accident. The chances are that the explosion was more mundane, and akin to what occurs inside a diesel engine. Compressive heating of the air in front of a small asteroid or comet travelling at more than 15 km s-1 would generate temperatures around 50 thousand degrees. Flash vaporisation of a small comet or asteroid would add to a massive shock wave at the epicentre, rather than by an intact projectile. It is thought that many small craters, such as Meteor Crater in Arizona, result from impacts by strong metallic asteroids, whereas stony ones or comets easily disintegrate. Whatever, research still goes on at the site, now completely reforested.

The centenary spurred Nature to devote pages 1157-1175 in its 26 June 2008 issue to impact-induced features from Earth and other planets, together with three Letters and two reviews. Topics covered include the search for near-Earth objects and the Spaceguard survey, which is beginning to suggest that humanity can concentrate on global warming for the next century or so, and truly monster impact structures from the Moon and Mars, including evidence for one that may have ‘scalped’ northern Mars. In one of the reviews it is said that a sci-fi novel (Niven, R. & Pournelle, J. 1977. Lucifer’s Hammer. Harper Collins) inspired the Alvarez father-and-son team that first postulated an impact origin for the K-T mass extinction event. The second review is of a highly realistic sculptural depiction of a pope (John Paul II) knocked over by a meteorite: perhaps planetary science’s first involvement, literally, in what some might consider lèse majesté. So, in many ways, quite an event…

See also: Cohen, D. 2008. The day the sky exploded. New Scientist, v. 198, 28 June 2008 issue, p. 38-41.