China’s Chengdu Earthquake

With the death toll still rising, Monday’s earthquake in China’s Sichuan Basin is the most deadly to strike the quake-prone region in more than 75 years.

The shallow magnitude 7.9 earthquake struck 75 km west-north-west of Chengdu in China’s Sichuan province at 2:28 p.m. local time on Monday May 12th 2008. The 19 km-deep quake caused severe ground shaking, and residents of nearby cities experienced violent tremor which lasted some two to three minutes.

Buildings collapsed, roads were blocked by landslides and dams, bridges, nuclear facilities and other infrastructure sustained serious damage. The earthquake was felt in much of central, eastern and southern China, including the capital Beijing, Guangzhou, Nanjing, Shanghai, Tianjin, Wuhan and Hong Kong. The quake was felt in parts of Bangladesh, Taiwan, Thailand and Vietnam.

Geoscience Australia, which reports the earthquake as magnitude 7.8 at a depth of 10 km, estimates it would have been felt up to 2775 km from the epicentre. Damage would have occurred within a radius of 221 km.

Waves from the earthquake, which occurred at 6:28 p.m. Monday New Zealand time, clearly showed on many of New Zealand’s seismographs as 10 or more minutes of sustained gentle tremor, too light to be felt.

Seismic activity in central and eastern Asia is dominated by the northward convergence of the India plate against the Eurasia plate. The two plates collide at the rate of about 50 mm per year, and the movement causes the Asian highlands to rise up away from the Tibetan Plateau. Monday’s earthquake probably occurred as a result of movement on the Longmenshan fault (or one of its related faults) which runs northeast across the Sichuan Basin. Maps show that the 55 aftershocks of magnitude 4.2 or greater which have occurred up until 11 o’clock Friday morning are spread northeast through the area.

Within 15 minutes, an even shallower magnitude 6.0 aftershock struck near Chengdu, and this was soon followed by 5th magnitude quakes. The sheer size of the quake and the damage to roads has hampered official rescue efforts in this area which, by New Zealand standards, is heavily populated. It is estimated, 3½ days after the initial earthquake, that the death toll has now risen to 20,000 with some tens of thousands still unaccounted for.

The previous most deadly earthquake in the Sichuan Basin occurred in 1933 when the magnitude 7.5 quake of August 25th killed more than 9,300 people.

Monday’s quake is China’s largest since August 1931 when a magnitude 8.0 earthquake in Xinjiang killed 10,000 people. In 1920 the magnitude 7.8 Haiyuan quake killed 200,000 people and seven years later the magnitude 7.6 Tsinghai quake claimed nearly 41,000.

Population density means that some countries suffer tragic consequences when large earthquakes strike, regardless of building standards. Monday’s quake was China’s most deadly in just over 30 years. The Tangshan quake of July 1976 caused 255,000 fatalities. In January 1556, the magnitude 8.0 Shensi quake caused an estimated 830,000 deaths.

By comparison, the magnitude 9.1 Sumatra quake of December 2004 was the world’s third largest since 1900. The earthquake and massive tsunami which followed claimed nearly 228,000 lives. It was the world’s fourth most deadly seismic event during recorded history, ranking behind China’s Shensi and Tangshan quakes and a quake in Syria in the year 1138 which is thought to have claimed 230,000.

Analysis shows that, on average, there is one quake of magnitude 8 or greater in the world each year, and seventeen of 7th magnitude. These are averages, and by their nature, earthqukes can occur in clusters. Worldwide, there have been six quakes of 7th magnitude to date this year. Last year there were four of magnitude 8 or greater and thirteen of magnitude 7. 2006 was a little quieter with two magnitude 8 or greater and eight of magnitude 7. In 2005 there was one of magnitude 8 or greater and ten magnitude 7 quakes were recorded. 2004 was also an active year with two quakes of magnitude 8 or greater and eleven magnitude 7 events.

To put Monday’s quake into perspective for New Zealanders, it released more than 30 times more energy than last December’s earthquake off the coast of Gisborne which claimed one life, and was much shallower and hence caused greater disruption to people and infrastructure.

[Compiled from data supplied by the U.S. Geological Survey and its contributing agencies; and Geoscience Australia.]

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