Quantifying the Seismic Hazard From Natural and Induced Earthquakes (Invited)
Abstract
In the past 12 years, seismicity rates in portions of the central and eastern United States (CEUS) have increased. In 2011, the year of peak activity, three M ≥ 5 earthquakes occurred, causing millions of dollars in damage. Much of the increase in seismicity is believed to have been induced by wastewater from oil and gas activity that is injected deep underground. This includes damaging earthquakes in southern Colorado, central Arkansas, and central Oklahoma in 2011. Earthquakes related to oil and gas activities contribute significantly to the total seismic hazard in some areas of the CEUS, but most of the tens of thousands of wastewater disposal wells in the CEUS do not cause damaging earthquakes. The challenge is to better understand this contribution to the hazard in a realistic way for those wells that are inducing earthquakes or wells that may induce earthquakes in the future. We propose a logic-tree approach to estimate the hazard posed by the change in seismicity that deemphasizes the need to evaluate whether the seismicity is natural or man-made. We first compile a list of areas of increased seismicity, including areas of known induced earthquakes. Using areas of increased seismicity (instead of just induced earthquakes) allows us to assess the hazard over a broader region, avoiding the often-difficult task of judging whether an earthquake sequence is induced. With the zones of increased seismicity defined, we then estimate the earthquake hazard for each zone using a four-branch logic tree: (1) The increased seismicity rate is natural, short-term variation within the longer-term background seismicity rate. Thus, these earthquakes would be added to the catalog when computing the background seismicity rate. (2) The increased seismicity rate represents a new and permanent addition to the background seismicity. In this branch, a new background seismicity rate begins at the time of the change in earthquake rate. (3) Induced earthquakes account for the increased rate and they should be expected to continue. Estimating the hazard from these induced events requires the specification of a magnitude-frequency distribution specific to the induced earthquakes. The hazard from these earthquakes would be added to that from the background seismicity. (4) The increased seismicity rate will decay back to the background seismicity rate. This would apply to the decay of a natural aftershock sequence or earthquake swarm, as well as the decay of an induced earthquake sequence where the fluid injection has been terminated. Based on the understanding of an individual earthquake sequence, one could apply these logic tree branches as appropriate. For some earthquake sequences, some branches might have zero weight, e.g., if one is certain that the earthquakes are induced. Varying the weights of the logic-tree branches gives a natural way to integrate uncertainty into the calculations of hazard. Although an imperfect solution, this approach is a first step in accounting for the contribution of both induced earthquakes and variations in natural earthquake rates to seismic hazard.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.S32C..05R
- Keywords:
-
- 4302 NATURAL HAZARDS Geological;
- 4315 NATURAL HAZARDS Monitoring;
- forecasting;
- prediction;
- 7223 SEISMOLOGY Earthquake interaction;
- forecasting;
- and prediction;
- 7200 SEISMOLOGY