January 3, 2014
Multi-Study Focuses On Link Between Lushan, Wenchuan Earthquakes
April Flowers for redOrbit.com - Your Universe Online
New research, published in Seismological Research Letters, suggests that the 32-mile segment of the fault northeast of the 2013 Lushan rupture is the place to watch for the next major earthquake in the region. Papers published in the special section of the journal suggest that the 2008 Wenchuan earthquake triggered the magnitude M6.6 Lushan quake.Huajian Yao, professor of geophysics at the University of Science and Technology of China, guest edited the eight articles of the special section, which present current data, description and preliminary analysis of the Lushan event and discuss the potential for future earthquakes in the region.
The M7.9 Wenchuan earthquake in China's Sichuan province was the largest quake to hit China since the 1950s. More than 87,000 people were killed or went missing as a result of the quake. The 2013 Lushan quake, 56 miles to the south, caused 203 deaths, injured another 11,492 and affected the lives of more than 1.5 million people.
"After the 2008 magnitude 7.9 Wenchuan earthquake along the Longmenshan fault zone in western Sichuan of China, researchers in China and elsewhere have paid particular attention to this region, seeking to understand how the seismic hazard potential changed in the southern segment of the fault and nearby faults," said Yao. "Yet the occurrence of this magnitude 6.6 Lushan event surprised many. The challenge of understanding where and when the next big quake will occur after a devastating seismic event continues after this Lushan event, although we now have gained much more information about this area."
Only moderately understood, the southern portion of the Longmenshan fault zone is very complex. The 2008 Wenchuan event occurred along the central segment of the zone, which is similar to the southern segment where the Lushan rupture was generated. The southern segment includes the Wenchuan-Maoxian fault, Beichuan-Yingxiu fault, the Pengxian-Guanxian fault and Dayi faults, a series of sub-parallel secondary faults.
The main shock of the Lushan earthquake did not break to the surface; however, the strong shaking still caused significant damage and casualties in the epicentral region. Three of the research papers describe the rupture process of the Lushan quake:
* Libo Han, China Earthquake Administration, worked with colleagues to provide a preliminary analysis of the Lushan main shock and two large aftershocks, which appear to have occurred in the upper crust and terminated at a depth of approximately 5 miles. The team suggests, that although the Lushan event cannot be associated with any identified surface faults, the may have occurred on a blind thrust fault subparallel to the Dayi fault, which lies at and partly defines the edge of the Chengdu basin.
* Chen Lichun and a team of researchers from the China Earthquake Administration suggest the Lushan quake spread in a "piggyback fashion" toward the Sichuan Basin—although with weaker activity and lower seismographic potential than the Wenchuan quake—based on observations from extensive trenching and mapping of fault activity after both the Wenchuan and Lushan earthquakes.
* Junju Xie, from the China Earthquake Administration and Beijing University of Technology, led a team who examined the vertical and horizontal near-source strong motion from the M 6.8 Lushan earthquake. Likely due to the fact that seismic energy dissipated at the depth of 7-15 miles and the rupture did not break through the ground surface, the vertical ground motion is relatively weak for this event.
Scientists have questioned if the Lushan and Wenchuan earthquakes were related and if so, what was the manner of that relationship. The scientific community has been divided, with some considering the Lushan quake to be a strong aftershock of the Wenchuan quake, while others see them as independent events. The papers of this special section attempt to answer this question from different perspectives.
An accurate picture of where the Lushan quake originated was needed in order to discover whether the Lushan earthquake was truly independent from the Wenchuan quake. A team led by Yong Zhang from the GFZ German Research Centre for Geosciences and the China Earthquake Administration began this process by confirming a new hypocenter for Lushan. They analyzed near-fault strong-motion data (movements that took place at a distance of up to a few tens of miles away from the fault) as well as long distance (hundreds of miles) teleseismic data to find the location where the fault first began to rupture.
The newly calculated location of the hypocenter led Zhang and his team to agree with previous studies, which suggested the initial Lushan rupture was a circular rupture event with no predominant direction. They note, however, that their calculations place the major slip area of the Lushan quake approximately 25 to 31 miles away from the southwest end of the Wenchuan quake fault.
The relationship of the two quakes was explored by Ke Jia of Beijing University using a statistical analysis of aftershocks in the region as well as the evolution of shear stress in the lower crust and upper mantle in the broader quake region. The team's analysis suggests the Lushan quake was affected by the Wenchuan quake in an immediate sense by changing the overall background seismicity in the region. The team calculates a 62 percent probability that Lushan is a strong aftershock of Wenchuan using the background seismicity changes.
Yanzhao Wang, of the China Earthquake Administration, led a team that quantified the stress loading of area faults due to the Wenchuan quake, suggesting the change in stress may have caused the Lushan quake to rupture approximately 28.4 to 59.3 years earlier than expected. The Lushan earthquake is at least 85 percent a result of a delayed aftershock of the Wenchuan event, the team concluded, rather than due solely to long-term tectonic loading.
Researchers began immediately calculating stress changes on the major faults surrounding the rupture zone after the Wenchuan quake, partially to identify where dangerous aftershocks might occur. They also wanted to test how well these stress change calculations might work to predict new earthquakes.
Tom Parsons, of the US Geological Survey, and Margarita Segou, of GeoAzur, participated in the analysis by comparing data collected from the Wenchuan and Lushan quakes with data on aftershocks and stress changes in four other major earthquakes, including the M 7.4 Landers and Izmit quakes in California and Turkey, respectively, and the M 7.9 Denali quake in Alaska and the M 7.1 Canterbury quake in New Zealand.
These comparisons demonstrate that strong aftershocks similar to Lushan are likely to occur in three places: where there is the highest overall aftershock activity, where stress change is the greatest and where fault zones are already well-developed. The researchers also note, however, that by these criteria, the Lushan quake would only have been predicted by stress changes, and not the clustering of aftershocks following the 2008 Wenchuan event.
Seismologists and other scientists question where they should begin looking for the next big quake in the region. The Wenchuan event primed the researchers with data to help predict where and when the next rupture might be in the region, suggesting that the earthquake would increase seismic stress in the southern Longmenshan fault that was the site of the 2013 Lushan quake. That information alone, however, could not accurately predict that the southern Longmenshan fault would be the next to rupture after Wenchuan, say Mian Liu of the University of Missouri and colleagues, because the Wenchuan earthquake also increased the stress on numerous others faults in the region.
Liu and colleagues note that additional insights can be gained by seismic moment studies. Moment balancing is a comparison of how much seismic strain energy is accumulated along a fault over a certain period and the amount of strain energy released over the same period. There had been a slow accumulation of strain energy without release by a major seismic event in over a millennium in the case of the Longmenshan fault. Following the Wenchuan event, the southern segment of the Longmenshan fault became the fault with the greatest potential for a quake. Liu's team says that following Lushan, the 37-mile-long segment of the fault northeast of the Lushan rupture is the most likely place to see the next major earthquake.