RECENT TRIGGERED (HYDRO)SEISMICITY IN OKLAHOMA: A CAUTIONARY TALE?

Disposal by subsurface injection of wastewater derived from energy technologies poses a risk of induced seismicity but, relative to the large number of operating disposal wells, few events are documented over the past decades. However, where large net volumes of injected fluids are involved, the potential exists for inducing larger seismic events. In Oklahoma, an exponential increase in seismicity coincides with a comparable increase in economic benefit from enhanced oil recovery (EOR) operations, from which an intrinsic by-product is up to an order-of-magnitude greater volume of saline wastewater. An initial (2008-2011) cycle of accelerating seismic-moment release (ASR) culminated in the M_w5.6 Prague earthquake. Since 2012 Oklahoma experiences a second ASR cycle, during which M_w4+ earthquakes - rare in 2013 - have become an almost weekly occurrence in the latter part of 2014. On a 0.02-yr analytical basis, extrapolation of the exponential ASR trend into 2015 and beyond indicates that an M_w5+ threshold may be breached by 2016. The spatial distribution of epicentres illuminates hidden faults, some favourably oriented for re-activation in the roughly WSW-ENE direction of maximum horizontal crustal stress. An epicentral alignment, extending SW-NE through the areas around Guthrie, Langston and Stillwater (GLS), is also recognisable in the pattern of aeromagnetic anomalies, and may be related to splay and transfer structures from the larger Nemaha-Wilzetta fault system. If a major part or the whole of the GLS structure is accidentally re-activated in a low-stress-drop regime by wastewater injection, it is capable of generating a significant earthquake in the range M_w5.5-6.5 or greater, for fault length in the range 20-70 km and fault slip 5-10 cm. The current seismicity gives rise to concern and restlessness in the general population, not easily allayed by calming rhetoric based on standard probabilistic seismic hazard and risk assessment (PSHA). Quantitative loss-modelling of reasonable worst-case (M_w6.5?) scenarios, and motivation of the population to prepare for them, may be psychologically and practically better. In this regard, Oklahoma hydroseismicity provides lessons for other regions (e.g., Karoo Basin, South Africa) where unconventional hydrocarbon development is contemplated.