Sichuan One Month Later: Aftershocks, Barrier Lakes and an Impetus to Reevaluate the Seismic Code
The strongest, deadliest earthquake to strike China in more than three decades hit Sichuan province on May 12th. The ground has not stopped shaking. Aftershocks continue to jolt the region[1]-a common phenomenon for severe quake events at shallow depths-and some of these aftershocks are causing landslides that have blocked rivers, leading to the creation of barrier lakes. Officials are particularly concerned about one barrier lake in the highlands of Sichuan Province, at Tangjiashan. They fear the lake could burst and flood cities and villages downstream. These threatened areas- from which more than 250,000 people have been evacuated in advance of potential flooding-are the same cities and villages that have been struggling with the devastation caused by the earthquake’s initial shock.
That shock rocked Sichuan at 2:28 p.m. local time on May 12th, about 90km west northwest of Chengdu, the capital of the province. The USGS initially estimated its magnitude at 7.5, but revised that to 7.8 and then to 7.9. Its depth was estimated at 10km. The quake lasted only two minutes, but was felt in several countries, including Vietnam, Thailand and Russia. In China, high-rise buildings in Beijing and Shanghai-some 1,500km from the epicenter-swayed back and forth. The following day, 12,000 people were reported dead, many of them buried under collapsed buildings. The death toll continued to rise, reaching nearly 70,000 after a month. About 17,000 people are still missing today.
Several factors compounded the severity of the earthquake, including the shallowness of the epicenter and the density of the population in the Sichuan region. With dense population comes dense construction. Apart from recently engineered structures in Chengdu, which withstood the earthquake fairly well, many buildings in the province toppled completely. As many as eight schools collapsed. Factories and hospitals were reduced to rubble, and five million people lost their homes.
Some new buildings did survive the earthquake, suggesting that the current building code- revised in 1976 after the deadly Tangshan quake[2] -might be appropriate if followed rigorously. However, the collapse of so many recently engineered buildings in this region, including schools and hospitals, suggests that either construction quality was inferior, or the seismic codes were not thoroughly enforced. Buildings in Sichuan have escaped adherence to codes for several reasons. First, except for key structures such as power plants, building codes were not strictly enforced before 1976. (Still, some old residential buildings built pre-code actually performed better than newer ones in the May 12th quake. The older buildings tended to be smaller and comprised of more walls per area, while newer buildings tended to have larger rooms and fewer walls, increasing building vulnerability. ) Second, seismic design and construction quality are only strictly enforced for engineered structures built by relatively large construction companies. Third, some recently built single family buildings-struggling with cost and limited resources-were not seismically designed and constructed under professional supervision. Third, seismic provision for public buildings such as schools and hospitals is not clearly regulated for higher seismic criteria, leaving local practitioners free to interpret the code based on budget and other constraints. Apparently, Chinese building code regulators have noticed this gray spot and requested clarification in future revisions. Finally, some recently built single family homes-struggling with cost and limited resources-were not seismically designed and constructed under professional supervision.
For people who lost their homes, mortgage payments are an immediate concern. In principle, residents are still liable for their mortgage payments, but-during this period of disaster relief-the China Banking Regulatory Commission has ruled that banks cannot enforce mortgage payment. The banks, observers believe, will eventually have to write off these mortgages as bad debt, prompting speculation about what would happen if a similar-sized earthquake occurred in Beijing, another seismically active region in China, where property values are even higher. (AIR estimates that a large earthquake event in Beijing-an M7.3 earthquake for example- would result in $68 billion in insured losses.)
Despite huge economic losses, insurance companies in the earthquake-ravaged region remain healthy because insurance take-up rates there are below the national average, especially on the residential side. (Earthquake insurance is excluded from most standard policies.) Nonetheless, this event-the first earthquake to hit a metropolitan region since 1976-will likely motivate Chinese insurers to evaluate their risk management practices and to design innovative earthquake-related products that will cater to the increasing demand for earthquake insurance, going forward.
AIR has not been permitted to conduct a damage survey in Sichuan, in part because aftershocks continue to make the region dangerous. Such persistent tectonic activity is not unexpected; the quake occurred on the Longmeng Shan fault in the North-South Seismic belt of Central China, the most seismically active area in the country. The Longmen Shan fault itself is located along the eastern margin of the Tibetan Plateau. Many faults have developed here, but while the majority of these are strike slip faults that are very active (moving at up to 10 mm/yr), the Longmen Shan fault displays primarily dip slip (reverse) movement, and its current slip rate is uncertain; some researchers suggest it moves at less than 1 mm/yr-a relatively low rate compared to other active faults along the plateau margin. The May 12th earthquake is the first direct evidence that the Longmen Shan fault zone-like other fault zones in this belt- is accommodating a significant amount of crust movement.
.MGW.
[1] As of 9 June, 206 aftershocks greater than or equal to M4 have hit Sichuan, as well as 5 aftershocks greater than M6. The greatest of these was an M6.4 tremor (source: CEA official website).
[2] China’s building code evolved throughout the late 20th century and the evolution of this code can be broken down into four periods: the pre-code period, the adoption of a code in 1978 (TJ 11-78), a 1989 amendment (GBJ 11-89), and the 2001 revision (GB 50011-2001). The 1978 code was a response to the 1976 Tangshan tragedy, an earthquake that killed an estimated 250,000 people. The 1989 amendment resulted from the adoption of the concept of probabilistic hazard, and the 2001 revision reflected the advancement in research, construction materials and practice during the current period of rapid economic growth.
Leave a Reply