At 8:02 a.m. on April 20, 2013, a magnitude 7.0 earthquake occurred in Lushan, Ya'an, Sichuan, located on the Longmenshan fault zone on the eastern edge of the Qinghai-Tibet Plateau, with a focal depth of 13 kilometers and a focal area of IX. degrees (9 degrees).
Aftermaths of the Lushan earthquake continued, with more than 8,000 aftershocks recorded in less than a month after the earthquake. Due to the shallow source, high magnitude and high intensity of the Lushan earthquake, a total of 96 deaths were caused, inducing more than 2,500 collapses, landslides, mudslide disasters and more than 400 unstable slopes in 8 counties (districts) including Lushan County and Baoxing County.
The Lushan earthquake caused serious casualties and damage to buildings, but the real culprit of the earthquake hid underground and did not show its true face. Fortunately, the earthquake did not cause more casualties due to the surface rupture of the fault.
Destructive inverse faults
Before introducing the "invisible" faults of the Lushan earthquake, it is necessary to introduce the "visible" faults.
The epicenter of the Lushan 7 earthquake is about 85 kilometers away from the epicenter of the Wenchuan 8 earthquake that occurred 5 years ago, And it belongs to the Longmenshan Fault Zone, which is also an earthquake caused by reverse fault activity, but the difference is that the reverse fault of the Lushan earthquake is not exposed on the surface, which reduces the loss caused by the earthquake to a certain extent.
Distribution of Wenchuan earthquake geological disasters
The Wenchuan earthquake disaster loss is large, in addition to the high magnitude of the earthquake, the earthquake release energy is more, and it is related to the movement of the seismic fault. The seismic fault of the Wenchuan earthquake is a reverse fault, which is a type of geological structural fault layer, and its movement form is the upper disk rising, the lower disk is relatively falling, and the surface damage is more serious than the positive fault and the slip fault.
The Wenchuan earthquake triggered tens of thousands of landslides and landslide geological disasters, with a distribution range of about 100,000 square kilometers, and the occurrence of these geological disasters was certainly affected by topography, stratigraphic lithology and human activities, but the most important thing was affected by fault activities. The earth's enormous force tore at the ground, creating multiple rupture zones on the ground, with a total length of about 275 kilometers and a width of about 15 kilometers. The closer you are to the seismic fault, the greater the number of geological disasters.
In the Wenchuan earthquake, the induced geological disasters also had a significant upper/lower disk effect, that is, the upper disk of the earthquake fault had a higher distribution density, wide range and large scale than the lower plate geological disaster.
Comparison of aerial images of the upper and lower disks of the Duwen Highway fault after the earthquake - the geological disasters in the lower plate of the fault on the left are few and small in scale, and the geological disasters in the upper plate of the fault on the right are many and large
"Invisible" faults
Both the Lushan earthquake and the Wenchuan earthquake occurred on the Longmenshan fault zone of the "Y" shaped geotectonic structure at the eastern edge of the Qinghai-Tibet Plateau, which is located at the junction of the Songpan-Ganzi orogenic Belt and the Yangtze land mass, west of the eastern Qinghai-Tibet Plateau, east of the Sichuan Basin, about 500 kilometers long and 70 kilometers wide, composed of three large faults, from west to east, long and long, the Houshan fault, the central fault and the Qianshan fault, which are the strong seismic high-incidence areas and one of the most important seismic activity zones on the mainland.
Large earthquakes often occur along the surface activity of faults, and often form surface fracture zones up to tens to hundreds of kilometers. However, unlike the Wenchuan earthquake, the Lushan earthquake did not produce a significant surface fracture zone.
Within 10 days after the earthquake, the China Earthquake Administration organized an emergency scientific investigation of the Lushan earthquake site, and in the Lushan earthquake intensity map issued by the China Earthquake Administration, no obvious seismic surface fracture zones were found along the VIII. and IX. zones and along a number of fault lines in the southern section of the Longmenshan fault zone. Ground cracks have appeared in some places in the polar earthquake zone, and after investigation, these ground cracks have been confirmed to be secondary surface ruptures caused by the liquefaction of sandy soil, which have little to do with fault movement.
The expedition did not find obvious seismic surface rupture zones, but instead saw some folds and uplifts on the surface (as shown above), such as cement pavements in some areas that were broken due to extrusion. Combined with multiple analysis and verification such as aftershock positioning and judgment of the seismic structure model, it was finally determined that the fault of the Lushan earthquake was hidden underground and did not expose the surface, which we called the blind reverse fault.
Equilibrium profile of Lushan earthquake seismic tectonic model - small white circle is the aftershock of Lushan earthquake after repositioning; The yellow five-pointed star is the location of the source of the Lushan earthquake
When the blind reverse fault movement, the fault displacement decays rapidly upwards, gradually tends to zero near the surface, which does not cause or will only form a small surface fault displacement, and the surface deformation during the earthquake is dominated by fold uplift.
Earthquakes caused by blind faults are not uncommon, such as the 1985 Ucha 7.4 earthquake, the 1906 North Tianshan Manas earthquake, the 2017 Jiuzhaigou Ms7.0 earthquake and the 2017 Milin Ms6.9 earthquake. A common feature is that there is no surface rupture zone formed by the active fault seen along the surface, but an anticline-like uplifts can be observed near the fault points on the blind inverse faults, so that anticline ridges can be seen near the epicenter, and aftershocks on vertical profiles across blind inverse faults are often distributed in a faceted manner.
If an earthquake of magnitude greater than 6.5 is caused by an active fault at the exposed surface, not only a surface rupture zone is formed along the active fault, but also an aftershock on the profile is arranged in a linear dominant manner along the fault concentration.
It can be generally said that compared with faults on the exposed surface, the geological disasters caused by blind fault activity are relatively mild in the case of elements such as the same seismic magnitude. That doesn't mean that blind ruptures aren't worthy of our attention.
For a long time, when people studied earthquakes, they often thought that the faults of earthquakes in the depths of the Earth must show up on the surface, or at least show some signs of fault activity. More researchers believe that the fault where the earthquake occurs is a kind of surface fault, which means that it is buried deep, and the fault that is hidden underground without a fault fault on the surface is not an active fault and a seismic fault. However, a growing number of examples of earthquakes suggest that blind faults are also active and can also trigger large earthquakes.
Correct understanding of blind faults is a breakthrough in our understanding of fault activities and the principle of earthquake occurrence, which is of great scientific significance for revealing the incubation and causes of earthquakes, as well as the determination of regional earthquake risk.
bibliography:
Sichuan Lushan 7.0 magnitude earthquake: a typical blind reverse fault type earthquake Xu Xiwei, et al
Analysis of the fault effect of geological disasters triggered by the Wenchuan earthquake Huang Runqiu and Li Weile
Detection of blind fault activity and its significance Gu Menglin, Zhou Xuesong
The basic characteristics of geological disasters induced by the "4.20" Lushan earthquake and the comparative analysis with the "5.12" Wenchuan earthquake Yin Zhiqiang, et al