Mechanism of the strong earthquake triggered by high pressure fluid in reservoir: A case study of the 5.12 Wenchuan earthquake

Abstract

At present, the mechanism of earthquake is still based on elastic rebound theory—Earthquake is caused by the release of elastic energy of rock strata due to fault dislocation. But more and more scholars began to question whether the elastic energy after fault faulting can really reach the huge energy released by the actual earthquake. Therefore, it is necessary to study the nature of destructive strong earthquake and its real energy source after the initial movement. According to the characteristics of the reservoir and its pressure in the sedimentary strata, it is concluded that there are a lot of high-pressure fluid in the reservoir, and its pressure can be released under certain conditions, resulting in fluid physical explosion, which may be an important part of strong earthquake energy. The calculation results show that when the fault ruptures and penetrates the reservoir with large area, the elastic energy produced by the pressure release can reach the energy released by the earthquake with magnitude above 8.0; Artificial engineering activities can also lead to the release of small-scale fluid pressure, such as blowout during drilling, earthquake induced by hydraulic fracturing, etc. At the same time, according to the analysis of the waveforms and propagation ray paths of the seismic stations close to the epicenter, it is considered that the strong earthquake wave may not be S-wave, but P-wave. Therefore, it cannot be ruled out that the strong earthquake might be caused by explosion. A large number of direct or indirect evidence, such as the time domain and frequency domain characteristics of seismic waves recorded by several stations during the Wenchuan earthquake, the explosion phenomena observed on the ground, and the cores obtained by scientific drilling after the earthquake, indicate the possibility of the release of this kind of fluid explosion energy. Finally, this paper proposes that the seismicity can be divided into three stages: The stage Ⅰ of micro rupture, in which there is fluid activity and electrokinetic effect, but the initial earthquake motion is not triggered; The stage Ⅱ of fault rupture after the initial earthquake motion; The strong earthquake stage Ⅲ, which is caused by the release of fluid pressure.

Keywords

• wenchuan earthquake
• time frequency analysis
• initial fracture
• initial motion
• reservoir
• fluid overpressure