Landslide hazard cascades can trigger earthquakes.

While earthquakes are well-known to trigger surface hazards and initiate hazard cascades, whether surface hazards can instead trigger earthquakes remains underexplored. In 2018, two landslides on the Tibetan plateau created landslide-dammed lakes which subsequently breached and caused catastrophic outburst floods. Here we build an earthquake catalog using machine-learning and cross-correlation-based methods which shows there was a statistically significant increase in earthquake activity (local magnitude ≤ 2.6) as the landslide-dammed lake approached peak water level which returned to the background level after dam breach. We further find that ~90% of the seismicity occurred where Coulomb stress increased due to the combined effect of direct loading and pore pressure diffusion. The close spatial and temporal correlation between the calculated Coulomb stress increase and earthquake activity suggests that the earthquakes were triggered by these landslide hazard cascades. Finally, our Coulomb stress modeling considering the properties of landslide-dammed lakes and reservoir-induced earthquakes globally suggests that earthquake triggering by landslide-dammed lakes and similar structures may be a ubiquitous phenomenon. Therefore, we propose that earthquake-surface hazard interaction can include bidirectional triggering which should be properly accounted for during geological hazard assessment and management in mountainous regions.

Growing seismicity in the Sichuan Basin and its association with industrial activities.

In the Sichuan Basin, seismic activity has been low historically, but in the past few decades, a series of moderate to strong earthquakes have occurred. Especially since 2015, earthquake activity has seen an unprecedented continuous growth trend, and the magnitude of events is increasing. Following the M5.7 Xingwen earthquake on 18 Dec. 2018, which was suggested to be induced by shale gas hydraulic fracturing, a swarm of earthquakes with a maximum magnitude up to M6.0 struck Changning and the surrounding counties. Questions arose about the possible involvement of industrial actions in these destructive events. In fact, underground fluid injection in salt mine fields has been occurring in the Sichuan Basin for more than 70 years. Disposal of wastewater in natural gas fields has also continued for about 40 years. Since 2008, injection for shale gas development in the southern Sichuan Basin has increased rapidly. The possible link between the increasing seismicity and increasing injection activity is an important issue. Although surrounded by seismically active zones to the southwest and northwest, the Sichuan Basin is a rather stable region with a wide range of geological settings. First, we present a brief review of earthquakes of magnitude 5 or higher since 1600 to obtain the long-term event rate and explore the possible link between the rapidly increasing trend of seismic activity and industrial injection activities in recent decades. Second, based on a review of previous research results, combined with the latest data, we describe a comprehensive analysis of the characteristics and occurrence conditions of natural and injection-induced major seismic clusters in the Sichuan Basin since 1700. Finally, we list some conclusions and insights, which provide a better understanding of why damaging events occur so that they can either be avoided or mitigated, point out scientific questions that need urgent research, and propose a general framework based on geomechanics for assessment and management of earthquake-related risks. ELECTRONIC SUPPLEMENTARY MATERIAL: Supplementary material is available for this article at 10.1007/s11430-020-9646-x and is accessible for authorized users.

Fault reactivation and earthquakes with magnitudes of up to Mw4.7 induced by shale-gas hydraulic fracturing in Sichuan Basin, China.

This paper presents a timely and detailed study of significant injection-induced seismicity recently observed in the Sichuan Basin, China, where shale-gas hydraulic fracturing has been initiated and the aggressive production of shale gas is planned for the coming years. Multiple lines of evidence, including an epidemic-type aftershock sequence model, relocated hypocenters, the mechanisms of 13 large events (M W > 3.5), and numerically calculated Coulomb failure stress results, convincingly suggest that a series of earthquakes with moment magnitudes up to M W 4.7 has been induced by "short-term" (several months at a single well pad) injections for hydraulic fracturing at depths of 2.3 to 3 km. This, in turn, supports the hypothesis that they represent examples of injection-induced fault reactivation. The geologic reasons why earthquake magnitudes associated with hydraulic fracturing operations are so high in this area are discussed. Because hydraulic fracturing operations are on the rise in the Sichuan Basin, it would be beneficial for the geoscience, gas operator, regulator, and academic communities to work collectively to elucidate the local factors governing the high level of injection-induced seismicity, with the ultimate goal of ensuring that shale gas fracking can be carried out effectively and safely.

Ductile gap between the Wenchuan and Lushan earthquakes revealed from the two-dimensional Pg seismic tomography.

A high-resolution two-dimensional Pg-wave velocity model is obtained for the upper crust around the epicenters of the April 20, 2013 Ms7.0 Lushan earthquake and the May 12, 2008 Ms8.0 Wenchuan earthquake, China. The tomographic inversion uses 47235 Pg arrival times from 6812 aftershocks recorded by 61 stations around the Lushan and Wenchuan earthquakes. Across the front Longmenshan fault near the Lushan earthquake, there exists a strong velocity contrast with higher velocities to the west and lower velocities to the east. Along the Longmenshan fault system, there exist two high velocity patches showing an "X" shape with an obtuse angle along the near northwest-southeast (NW-SE) direction. They correspond to the Precambrian Pengguan and Baoxing complexes on the surface but with a ~20 km shift, respectively. The aftershock gap of the 2008 Wenchuan and the 2013 Lushan earthquakes is associated with lower velocities. Based on the theory of maximum effective moment criterion, this suggests that the aftershock gap is weak and the ductile deformation is more likely to occur in the upper crust within the gap under the near NW-SE compression. Therefore our results suggest that the large earthquake may be hard to happen within the gap.

[Experimental study and clinical application of early pedicle division of skin flap by ligation].

OBJECTIVE: To establish a new method of early cut-off skin flap by ligating to stegnosis pedicle and assess its feasibility and clinical application. METHODS: Twenty New Zealand rabbits were used to make skin flaps with the size of 8.0 cm x 4.0 cm on both sides of the back respectively. And one side was experimental group with the pedicles of skin flaps horizontally oversewn by several pairs of silk thread, the other side was control group. Two pairs of silk thread in the two sides of the pedicle of skin flap of experimental group were ligated on the 3rd day. On the 5th day the pedicle was wholly ligated. All pedicles were divided on the 6th day and the survival area of skin flaps were measured after 3 days. The tissue samples from the skin flaps were collected for histology test on the 4th and 6th day respectively. The pedicles of 78 random flaps from 48 patients were cut off after narrowing them by ligating. RESULTS: The mean flap survival rate of the experimental group was statistically higher than the control group. Histological examination results showed the density and diameter of blood vessel were increased in the skin flaps of the experimental group. The mean time for removal pedicles was shortened to 10 days, and no necrosis was found after resecting. CONCLUSIONS: This method is secure and convenient and the time of pedicle division can be shortened.

[Investigation of a retrograded supra-genicular flap for treatment of scarring contraction of popiteal fossa].

OBJECTIVE: To investigate the blood vascular distribution of supra-genicular flap and to apply the retrograded supra-genicular flap for reconstructive surgery. METHODS: Eight fresh adult cadavers with 16 lower extremities were used for the study, by means of injecting the media of red latex into the main artery of the limb. The skin vascular architecture on the lower two thirds of each thigh was observed to decide the blood supply of this region. Fifty-two patients were also used to treat the scarring contraction of the popiteal fossa with 56 of the retrograded supra-genicular flaps, and long-term follow-ups were carried out. RESULTS: From this study, it was found that there are rich blood vascular networks of the skin above the knee anteriorally, laterally and medially. The abundant vascular anastomoses were also found around the knee. The clinical application of the retrograded supra-genicular flap was successfully carried out for the treatment of the scarring contraction of the popiteal fossa. All of the flaps were survived well. Long-term results were satisfactory. CONCLUSION: The above mentioned flap is a safe and effective flap for repair the defect of the popiteal fossa.