Continuing hazards in the wake of the 2008 Sichuan earthquake

CURRENT AND FUTURE HAZARDS in the region of the event are likely to derive from three different sources: damaging earthquakes in the densely populated Sichuan Basin, continued landsliding of weakened or fractured rock, and widespread deposition of sediment in river valleys.

The Sichuan earthquake of 12 May 2008 had a Moment Magnitude of 7.9 and devastated a large swath of the Longmen Shan mountains in Sichuan Province, southwestern China. The official death toll is at least 70,000, with nearly 375,000 injured, although both numbers are expected to rise. Up to 8m of slip occurred along two geological faults, the Beichuan and Pengguan faults. This rupture, which extended over a distance of nearly 250km, led to widespread structural damage, disruption of transport links, and landsliding.

The effect of the earthquake on infrastructure was surprisingly heterogeneous, in large part because of large variations in fault slip along the length of the rupture. In some areas, catastrophic building failure was pervasive, whereas in others the only structures that failed were those immediately adjacent to the faults. Aftershock activity extended more than 300km northeast of the epicentre into neighbouring Gansu and Shaanxi provinces. The earthquake occurred in a region with a complex history of faulting, driven by the slow, inexorable collision of India with Eurasia. That long term history provides a useful context for understanding the types and patterns of past and future catastrophic risks in the region.

The Beichuan and Pengguan faults run southwest to northeast along the eastern margin of the Tibetan Plateau. These faults originated some 200 million years ago but have become reactivated over the past 40 million years as India has collided with Eurasia, pushing up the Tibetan Plateau and moving it northeastwards relative to the lowlands of central and eastern China.

Quiet beforehand

Before 12 May, the region was notable mostly for its lack of seismic activity; small earthquakes, of the type that occur continuously in active mountain belts worldwide, were conspicuously absent, and the last major earthquake in the region occurred in 1933. Work published by our group last year, however, showed that the Beichuan and Pengguan faults had been active in the past 10,000 to 15,000 years, with evidence for at least one earthquake on each fault during that time interval.

Recurrence times between large earthquakes on any one fault in the area are probably of the order of a few thousand years, although we lack the data to make more precise estimates. These relatively long recurrence times mean that the 12 May earthquake has released a considerable amount of stress on the Beichuan and Pengguan faults; therefore, they are likely to remain quiet for some time. Aftershock activity on both faults has now declined to pre-earthquake background levels, so the likelihood of repeated large earthquakes on these faults in the next decades to centuries is very low.

“Two aspects of these landslides will give rise to longer term hazards that will persist over the next 10 to 50 years.

Worryingly, however, the Beichuan and Pengguan faults form only part of a network of faults in the eastern Plateau region. Some of these faults occur beneath the densely populated western Sichuan Basin, which contains the major cities of Sichuan (Chengdu, Mianyang, Deyang, Dujiangyan, and Leshan) with a combined population of more than 15 million people. Recent work has shown that the 12 May earthquake increased the level of stress on some of these faults, enhancing the probability of future earthquakes. These results must be treated with caution because of our poor current understanding of the faults in the region.

Our own preliminary investigations, however, show that the faults in the Sichuan Basin have been active over the past few million years. It thus seems clear that future earthquake activity is much more likely along faults in the Sichuan Basin than along the Beichuan and Pengguan faults. Although the faults in the Sichuan Basin are fairly short (30-100 km) and are, therefore, expected to sustain only moderate (Mw 6.5 to 7.0) earthquakes, their proximity to major population centres makes them a significant source of seismic hazard.

Landslides

The 12 May earthquake triggered landslides over an area of at least 25,000km2, centred on the surface trace of the Beichuan and Pengguan faults. On the basis of experiences in previous, smaller earthquakes, such as 1994 Northridge, California and 1999 Chi-Chi, Taiwan, the total number of landslides probably exceeded 10-20,000. In places, such as along the Min Jiang valley near the town of Yingxiu, the density of landsliding approached 50% - that is, nearly half of the ground surface was disrupted or affected in some way.

The landslides represented a major secondary hazard to inhabitants and infrastructure, as seen most notably in the destruction of the town of Beichuan and the damage to roads throughout the region. They also caused widespread damming of rivers, leading to fears of catastrophic flooding once the loosely compacted landslide dams began to fail. Most, if not all, of these dams have now been removed, either naturally or aided by excavations and explosives, and the risk of catastrophic flooding due to dam failure is largely gone.

It is important to recognise that the current and future landslide hazard in the region appears to depend strongly on the type of bedrock. In the southwestern part of the rupture area, which is underlain by resistant granite and limestone bedrock, landslides tended to be widespread but relatively shallow, involving just the upper few metres of soil or rock, and delivering relatively small amounts of debris to the foot of the slope.

In contrast, the northeastern part of the rupture area, between the towns of Beichuan and Qingchuan, is underlain by weak sedimentary rocks and was characterised by very large, deep-seated landslides that affected areas up to several square kilometres, and in some cases buried entire villages. Intervening areas between these large landslides were surprisingly unaffected by mass movement, despite severe shaking and close proximity to the active faults.

“Their proximity to major population centres makes them a significant source of seismic hazard.

Longer term hazards

Two aspects of these landslides will give rise to longer term hazards that will persist over the next 10 to 50 years. First, the shaking during the earthquake has left large areas of fractured, weakened rock on the hill slopes that are prone to failure in large rainstorms. Again, the extent and degree of risk will vary considerably over the region. The shallow failures in the southwest have been repeatedly reactivated during storms in summer 2008, and frequent continued landsliding and consequent disruption of roads and other infrastructure can be expected in the short to medium term.

The size, and thus to a large extent the hazard, of any one event, however, is likely to be small. The large landslides in the northeast have also been affected by rainfall, although catastrophic reactivation of an entire slide mass is unlikely. Instead, it can be expected that these large failures will continue to move episodically over the next few decades and supply sediment to the foot of the slope, particularly through smaller debris flows and slides.

Second, the landslides delivered a large volume of sediment to the rivers of the Longmen Shan region. Because this sediment far exceeds the normal capacity of the rivers to transport it away, widespread accumulation of sediment is now occurring in the regional river network. In areas near Qingchuan, as much as 3m of deposition has now occurred along large stretches of river floodplains, burying roads and buildings. Over time this sediment will be washed downstream, eventually entering the Sichuan Basin and the Yangtze River system.

The large quantity of landslide-derived sediment poses two long term problems: it will accumulate in the many man-made reservoirs in the Longmen Shan, drastically decreasing their effectiveness for flood control and hydroelectric power generation; and it will decrease the capacity of the river channels to transport flood waters, making catastrophic flooding more likely during any given storm event. The rate at which sediment will pass downstream and be removed from the region is not yet known, but comparison with similar past earthquakes suggests that the effects are likely to persist for at least the next 10-50 years, possibly as long as several centuries.

In summary, the Sichuan earthquake caused catastrophic damage due to a combination of severe ground shaking, large surface displacements, and widespread landsliding. Repeated earthquakes on the faults involved are unlikely in the short to medium term future because of the relatively low rates of long term fault activity in the region. Instead, future hazards are most likely to include (1) enhanced probability of damaging earthquakes in the densely populated Sichuan Basin, (2) continued landsliding of fractured rock, particularly during rainstorms, and (3) widespread deposition of sediment on river valleys and floodplains, which has led to reduced capacity to deal with major flood events.