Person:

Plesch, Andreas

The Longmen Shan is the steepest topographic front at the India-Asia collision zone and the site of the Mw 7.9 Wenchuan earthquake. Here to explain the interseismic GPS velocities across the greater Longmen Shan region, we develop a boundary element model including earthquake cycle effects, topography, the westward dipping Beichuan Fault and a ∼20 km deep, shallowly dipping, detachment, inferred from observed afterslip and from structural considerations. Previous analyses which neglected the detachment and earthquake cycle effects have found shortening rates near zero. In contrast, we find that interseismic GPS data are consistent with a shortening rate of 5.7±1.5mm/yr and maximum surface slip-deficit rate of 9.5±2.5mm/yr. This model unifies the interpretation of geodetic deformation throughout the earthquake cycle and suggests that the Longmen Shan is an active fold-and-thrust belt with of Wenchuan-like recurrence intervals as short as 600 years.

There is growing concern about seismicity triggered by human activities, where small stress increases bring tectonically loaded faults to failure. Examples include mining, impoundment of water, stimulation of geothermal fields, extraction of hydrocarbons and water, and injection of water, CO 2 , and methane into subsurface reservoirs 1 . In the absence of sufficient information to understand and manage the processes triggering earthquakes, authorities have set up empirical or statistical regulatory monitoring-based frameworks, with varying success 2 , 3 . Field experiments quantifying the effects of increased pore fluid pressure on triggering earthquakes carried out in the early 1970’s at the Rangely, Colorado (USA) oil field 4 suggested that, if conditions in the subsurface are known sufficiently well, seismicity can be turned on or off by cycling subsurface fluid pressure above or below a threshold. We report here development, testing, and implementation of a novel multidisciplinary methodology for managing triggered seismicity using unusually comprehensive and detailed information about the subsurface to calibrate geomechanical and earthquake source physics models. We then validate these models by comparing their predictions to subsequent observations made after calibration. We employ our approach in the Val d’Agri oil field in seismically active southern Italy, providing the first documented case of successful management of triggered seismicity based on a process-based method applied to a producing hydrocarbon field. Applying our approach elsewhere could help to manage and mitigate triggered seismicity.