High-Resolution Numerical Analysis of the Triggering Mechanism of M (L)5.7 Aswan Reservoir Earthquake Through Fully Coupled Poroelastic Finite Element Modeling

摘要

In 1981, a powerful M (L)5.7 earthquake occurred 50 km away from the Aswan Reservoir dam. After the statistical analysis on the correlationship between long-term continuous seismicity occurrence and the reservoir water level variation attributed to the impoundment and drainage procedures, researchers believe that this event is a typical reservoir-triggered seismicity (Nature 301(6):14, 1983; Earthquake Activity in the Aswan Region, Egypt. Birkhauser, Basel, pp. 69-86, 1995), although its triggering mechanism is poorly understood to date. To quantitatively address the triggering mechanism as well as its relationship with the characteristics of local geological settings around the reservoir region, in this paper, a fully coupled three-dimensional poroelastic finite element model of the Aswan reservoir is put forward by taking the consideration of the realistic observation data, for example, the high-resolution topography, water level fluctuation history, flood zone boundary and water depth variation, fault parameters, etc. Meanwhile, the change of Coulomb Failure Stress (Delta CFS) in correspondence to elastic stress and pore pressure variations induced by fluid diffusion is calculated. And the elastic strain energy accumulation in the reservoir region due to the impoundment load is obtained as well. Our primary results indicate that both the pore pressure and the coulomb stress on the seismogenic fault plane gradually increase with the respect of time while the water level rises. The magnitude of Delta CFS at the hypocenter of this major event is around 0.1 MPa, suggesting that the impoundment of the Aswan Reservoir possibly triggered the M (L)5.7 earthquake. The contribution of the elastic load is less than 3 percent of the total Delta CFS; on the other hand, the dynamic pore pressure change predominantly accounts for the contribution. The accumulative maximum surface deformation beneath the Aswan reservoir is up to 80 cm since its impounding began until the M (L)5.7 earthquake occurred. Although the total elastic strain energy accumulation caused by the impoundment water load is around 1.0 x 10(10)J, this energy density still insignificant compared to that of the vast reservoir inundation area, as it is only less than few percent of the total energy released by the major event, which confirms that the sustained regional geological loading controls the occurrence of this large reservoir-induced event. Furthermore, elastic loading and pore fluid pore pressure diffusion due to the impoundment of the Aswan reservoir might accelerate its occurrence.