Maximum Magnitudes for Fluid-Induced Earthquakes Revisited

Abstract:

Assessing the seismic hazard of earthquakes induced by fluid injection requires estimating realistic maximum magnitudes, Mmax. Is Mmax limited by the industrial activity that induces the seismicity or is it limited by the same factors that constrain the magnitudes of natural earthquakes? McGarr (2014) analyzed evidence from many case histories of earthquakes induced by a variety of fluid injection activities into deep formations, to argue that Mmax is limited by the total volume of fluid injected before the largest earthquake. Moreover, these earthquakes appear to result from the migration of pore pressure increase into seismogenic fault zones usually located in the shallow crystalline basement. A loophole in this argument is the possibility that a small-scale injection activity might trigger a vastly larger-scale earthquake. Although this cannot be ruled out, there are reasons why such a disproportionate seismic response to fluid injection is highly improbable, for example, the differences between natural and induced earthquakes. Natural earthquakes are the result of regional-scale strain accumulation that finally results in rupture that nucleates at depths in the range of 10 to 15 km where fault zones are strongest. Fluid-induced earthquakes, in contrast, are caused by localized pore pressure increases that perturb faults in the shallowest part of the seismogenic crust as evidenced by their hypocentral depths, typically in the top 5 km. For natural earthquakes, Mmax for mapped active faults is estimated from fault area (e.g., Hanks and Bakun, 2014) and the frequency of earthquake occurrence from the long-term slip rate. For earthquakes induced by fluid injection, it is straightforward to show that the volume of injected fluid plays the same role as fault area, but this area, and correspondingly Mmax, increases with time as long as injection continues. Similarly, injection rate plays the same role as the long-term slip rate on mapped faults.