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Faults that Slip Slowly - an Unrecognised Danger

The Wenchuan Earthquake of 2008 was hosted by a fault zone that had previous to the 7.9 earthquake been assigned a hazard rating of moderate to low, as it had previously been known to only slip slowly, but it was shown by the large 2008 earthquake that it was capable of producing surprises.

Striking the Chinese province of Sichuan in 2008 this 7.9 magnitude earth quake surprised everyone, including the scientists who had been studying the seismic risk of the region (Kirby, Whipple & Harkins, 2008). The Longmen Shan Fault zone that hosted the quake had been assigned the moderate-to-low rating (Global Seismic Assessment program; Zhang et al., 1999) as it deforms very slowly, but in 2008 the quake killed more than 80,000 people and injured 370,000 people. 5 years after the quake it is still uncertain how such a large and devastating earthquake could be hosted on a fault that deforms so slowly, and why the seismic risk of the fault could be so wrongly assessed. The China Earthquake Administration set up new projects following the 2008 quake to improve understanding of seismic rick. The author1 suggests Longmen Shan earthquake cycles are characterised by long intervals of recurrence and minor interseismic deformation that results from the relatively high strength of the rock in the earthquake source area, which is combined with interseismic locking of the fault that is unusually tight and loading of the stress that is very gradual. If this does indeed prove to be the case then other faults that also slip slowly may also pose an unrecognised risk and need to to have their seismic hazard reassessed.

The eastern margin of the Tibetan Plateau is marked by the Longmen Shan Mountains. The impression that the fault zone beneath these high peaks does not represent a significant, imminent seismic hazard is based on 3 observations. The deformation of the crust near the fault zone prior to the earthquake, a measure of the strain buildup in the crust was indicated by GPS observations to be close to negligible (Zhang et al., 2010). In the Longmen Shan fault zone the individual faults had slipped at slow rates, over geological timescales, of less than 1 mm/year (Densmore et al., 2007). And finally it had been at least 2,700 years since a devastating earthquake had occurred in this part of the Longmen Shan fault zone (Wen et al., 2009).

The author1 describes the surface deformation and strain accumulation before the Wenchuan earthquake as striking (Zhang et al., 2010). The deformation rate occurring in eastern Tibet shows a distinct contrast between the deformation rates in eastern Tibet compared with those of the long Shan fault zone. The eastern section of the Tibetan Plateau deformed at about 10 mm/year, the deformation spreading over an area of about 500 km, though there was only minor deformation totalling 1-2 mm/year on the Longmen Shan fault zone that had hosted such a devastating earthquake. It had seemed reasonable to expect large earthquakes to be more likely in parts of eastern Tibet that are expanding rapidly instead of the comparatively narrow fault zone where deformation was occurring at a rate that was so close to being almost undeforming. The location of Wenchuan earthquake stood out in the earthquake record in recent history because it was a slowly deforming region that was so close to a region that was deforming much more rapidly.

The contrast of speed in the deformation rate between eastern Tibet and Longmen Shan has been suggested to be the result of regions of rocks with contrasting rock strength. In eastern Tibet the entire lithosphere, which includes the rocks of the crust and the mantle, are relatively weak (England & Molnar, 1997; Flesch, Haines and Holt, 2001; Clark et al., 2005) which predisposes this region to easy, rapid deformation as a response of the Tibetan Plateau as it moves to the east, as the Indian and Asian tectonic plates continue to converge long after the Indian landmass collided with Asia. It is believed the rocks of the Longmen Shan, composed of crystalline rocks from the Precambrian, are stronger than those of eastern Tibet (Burchfiel, 2008). This suggestion is supported by seismic studies that have found that seismic waves move faster through the crust beneath the Longmen Shan than the rocks of the eastern Tibetan Plateau, indicating that the cold, strong rocks comprise the seismic source region deep beneath the Longmen Shan (Liu et al, 2009). Rapid accumulation of strain and deformation would be prevented by such strong rocks. The rocks of the Sichuan Basin, neighbouring the Longmen Shan on the east, act as a mechanical barrier as the rocks of this basin are also mechanically strong (Clark et al., 2005; Liu et al, 2009; Li & van der Hilst, 2010). The author1 suggests strain analysis alone for seismic hazard can be misleading in regions where the rocks are strong or of contrasting strength, as slow straining in these settings doesn't necessarily mean there is no accumulation of stress.

The author1 suggests the pattern of accumulation of interseismic strain may have been influenced by the geometry that ruptured during the Wenchuan earthquake. Following the earthquake investigations (Zhang et al., 2010) revealed that the Yingxiu-Beichuan fault that hosted the earthquake has a geometry that is curved. Below about 15 km deep in the crust the fault inclines downwards at about 30o from the horizontal, though at shallower depths the same fault curves up steeply at an angle of about 70o to the surface. The fault would lock tightly, deforming only slowly, as a result of the compression that would be applied to such a steep fault, thereby increasing the frictional resistance of the fault. The author1 suggests the tight locking of the seismogenic fault is attested to by the relatively sparse small quakes that occurred above the fault between 1970 and 2008. It is also suggested by the author1 that the unusual fault geometry that contrasts with that of the Main Himalaya Thrust Fault is further evidence supporting the inference that scientific understanding in one setting cannot be automatically applied to another (England & Jackson, 2011).

A small amount of strain caused by the growth of the Tibetan Plateau causing eastern Tibet to deform is transferred to the Longmen Shan Fault Zone (Zhang et al., 2010). As a result of this faults in the Longmen Shan are loaded gradually and slip slowly. It has been shown by palaeoseismic investigations that the recurrence of large earthquakes in the Longmen Shan Fault Zone occur about every 3,000 years (Ran et al., 2013), as would be expected in this fault zone. All the strain that accumulates on these faults in the period between large earthquakes must be released only during large earthquakes, given the lack of such earthquakes in the Longmen Shan Fault Zone larger than magnitude 7 earthquake for at least the past 2,700 years (Wen et al., 2009). The author suggests a quick calculation can test this theory. It is known that deformation occurs at an average rate of about 1.5 mm/year (Densmore et al., 2007; Ran et al.,. 2013) and this has been happening for about 3,000 years, the slip required to release the strain that has accumulated is about 4.5 m, and this is close to the about 5 m displacement that has been observed in relation to the 2008 earthquake.

The false sense of security regarding the Longmen Shan faults in terms of the seismic hazard of this region prior to the Wenchuan earthquake was contributed to by incomplete knowledge of the structure, palaeoseismic history and dynamics of the Longmen Shan faults. It is now known that faults that slip only slowly can be dangerous and it is now necessary to reassess the seismic risk at other slowly slipping faults that are believed be of low seismic hazard.

 

Sources & Further reading

  1. Zhang, Pei-Zhen. "Beware of Slowly Slipping Faults." Nature Geosci 6, no. 5 (05//print 2013): 323-24.

 

Author: M. H. Monroe
Email:  admin@austhrutime.com
Last updated 19/05/2013
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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading