Abstract:

Scientific interpretations are required for some abnormal phenomena at the XI intensity region of the Wenchuan earthquake: explosion bangs, large size circular pits and far distance flying rock avalanches. During field surveys we found circular pits and ejecta distributed above the Shuijingyan rock avalanche at Beichuan County in June 20, 2008, about one month after the main shock. According to field investigation and lab test results, the fired ejecta around craters are mainly carbonate manganese mixed of corrosion limestone and manganese underground surface. The brown porous ejecta are featured by intensive thermophilic degradation because they had high content of polycyclic aromatic hydrocarbons. If the regional geological conditions and coalbed gas distribution are considered, together with some updated findings of abnormal high content of methane monitored at the depth of central active fault F₂ during aftershocks, it is inferred that the main shock induced gas explosion, the latter triggered and enhanced some huge scale rock avalanches. The methane is stored within corrosion caves in limestone which interbedded with carbonaceous rocks, and closed fissures near the F₂. At the lower wall the local explosions left circular pits and fired eject around the pits, triggered and enhanced the following rock avalanches to fly far distances. The result presented is significant for monitoring the natural gas explosions in great shock and understanding its functions to large scale rock avalanches.

Key words: Wenchuan earthquake, crater, carbonate manganese, coalbed gas, thermophilic degradation