S51A-4395:
Was the Timpson, Texas, M4.8 event induced by fluid injection?
Friday, 19 December 2014
Zhiqiang Fan1, Peter Eichhubl1, Julia Gale1, Jon E Olson2, Cliff Frohlich3 and Valerie Gono2, (1)University of Texas at Austin, Bureau of Economic Geology, Austin, TX, United States, (2)University of Texas at Austin, Dept. of Petroleum and Geosystems Engineering,, Austin, TX, United States, (3)University of Texas at Austin, Institute for Geophysics, Austin, TX, United States
Abstract:
A M4.8 earthquake with dominant strike-slip near Timpson, east Texas, the largest documented earthquake to date in that region, has received extensive attention due to the possible linkage to waste water injection. The reliably located aftershocks align along a previously mapped fault striking about N42°W. Two active injection wells are located within 3 km of the aftershocks. One injection well became operational in August 2006 with an average injection rate of 42,750 m3/mo at an average pumping pressure of 12.4 MPa at depths between 1853 and 1868 m. Six months later, the second well started injection at 15,600 m3/mo. To investigate the causative relationship between fluid injection and possibly induced seismic fault slip, we integrated geologic and geophysical data into a poroelastic finite element model to simulate the spatial and temporal evolution of pore pressure and stress fields and analyze the stability of fault by applying the Coulomb failure criterion. Parametric studies were performed to analyze the sensitivity of Coulomb failure stress to the variability of input parameters including permeability of injection layer, fault orientation and permeability, and orientation and magnitude of stress state prior to injection. Assuming a Byerlee friction coefficient of 0.6, and using best available estimates of layer permeability, fault orientation, and stress tensor orientation and magnitude, we calculated fault slip occurs 55 months after the start of injection in the model, close to the observed delay of 69 months between injection and the M4.8 event. However, even with principal stress directions and fault orientation being reasonably well constrained, Coulomb failure stress is highly sensitive to input parameters resulting in large uncertainties in correlating injection rate and volume with the onset of induced seismic events. In addition, injection layer and fault zone permeability has a profound effect on the pore pressure evolution. These results demonstrate the benefit of detailed geomechanical site characterization for assessing and managing induced earthquake risk.
