CHARACTERISTICS AND RUNOUT MECHANISM OF SUPER-LARGE XINMO ROCK AVALANCHE-DEBRIS FLOW IN DIEXI, SICHUAN PROVINCE

On June 24, 2017 a super-large catastrophic rock avalanche-debris flow of 800×10⁴ m³ took place at Xinmo village, Maoxian county of Sichuan Province. Based on the field investigation, remote sensing interpretation and data analysis, the paper studies the geo-environmental conditions of the rock avalanche, the characteristics of failure, movement and deposition, and the triggering actions of rainfall and earthquakes. The main effects on mobility of rock debris and the hazardous mechanism are discussed. The new technology and method of monitoring and warning for such geohazards are evaluated. It is believed as follows. (1)Xinmo village is located in the deposit of an old large landslide before the 1933 Diexi M_S 7.5 earthquake. The multiple strong historic-prehistoric earthquakes and the annual rainfall not only made the surface unloaded belt of Triassic slate slope spelled, but also damaged and weakened the rockmass integrity and strength. There have developed large tensional fissures in the ridge slope before year 2003 and existed remarkable emplacement portent before failure. Therefore, the event is essentially a post-earthquake, high speed rock avalanche-debris flow. (2)The rock avalanche is about 150×10⁴ m³ but scratched and entrained a total of 600×10⁴ m³ old colluviums. The huge impact on the old colluviums made the inner soil arching effect suddenly lost. The effect of entrainment of the old colluviums greatly increased their mobility and diffusion but the large entrained colluviums conversely made the whole mass not flowing further. Such a failure mechanism of impact-loading and entrainment is similar to the former landslides in Xintan, Yigong and Nayong. (3)The seismic signals generated by rock avalanche clearly display the process of collapsing, impacting, moving and depositing. The emplacement monitoring of multi-temporal InSAR data clearly displayed the displacement precursor on the slope. The combination of two technologies is a promising new method of early identification, evaluation and warning of similar landslide of ultra-sight and in difficult geo-environment. Finally, with regard to the old and new fissures and their separated blocks, it is suggested to immediately make detailed investigation, risk assessment and monitoring-warning so as to avoid similar disasters.