|Mechanisms of induced seismicity Both wastewater injection and gas extraction can cause induced earthquakes. Detailed observations from the Midwestern United States and Groningen, Netherlands, show that in both cases, preexisting conditions in Earth's crust are of central importance
Topics: Alternative Energy, Earthquake, Geophysics, Green Energy
Since 2009, the Midwestern United States has seen a dramatic rise in earthquakes induced by human activities. Most of these events were caused by massive reinjection of wastewater produced during oil and gas extraction (1, 2). In February 2016, regulators in Oklahoma called for an injection rate reduction after several major events up to moment magnitude 5.8 (Mw 5.8) occurred. On the other side of the Atlantic, an unprecedented number of earthquakes has followed gas extraction from the Groningen field in the Netherlands (3). The Dutch government imposed production cuts after a Mw 3.6 event in August 2012 caused structural damage to houses. Intensive research of these two instances of induced seismicity points to contrasting mechanisms, but in both cases, the natural conditions prior to subsurface activities play a dominant part.
Fifty years ago, Healy et al. determined that fluid injection at depth causes the pore pressure to rise in a preexisting fault, reducing its strength and potentially leading to its failure (4). In contrast, fluid extraction at depth reduces the pore pressure, leading to compaction of the rock mass; the increased rock stress can drive a preexisting fault to failure. In both settings, the two factors that control induced earthquakes are operational parameters, such as the volume that is injected or produced, and natural conditions, such as the presence of preexisting faults and their ambient stress level. Operational parameters are often assumed to dominate, but that notion may reflect limited knowledge of the locations of preexisting faults and their ambient stress level. For regulatory measures to be effective in mitigating induced seismicity, it is crucial to understand the role of the natural conditions that existed before human activities.
How earthquakes are induced
Thibault Candela, Brecht Wassing, Jan ter Heege, Loes Buijze