Short-Term Probabilistic Hazard Assessment in Regions of Induced Seismicity

Abstract

ABSTRACTThis project introduces short-term hazard assessment frameworks for regions with induced seismicity. The short-term hazard is the hazard induced during the injection for hydraulic-fracturing-induced earthquakes. For wastewater-disposal-induced earthquakes, it is the hazard within a few days after an observed earthquake. In West Texas, hydraulic-fracturing-induced earthquakes cluster around the injection activities, and the earthquake occurrence varies greatly in time and space. We develop a method to estimate the hazard level at the production site during the injection, based on past injection and earthquake records. The results suggest that the injection volume has a negligible effect on short-term earthquake occurrence in this case, because injection volumes per well fall within a relatively narrow range, whereas the regional variations in seismic productivity of wells and b-values are important. The framework could be easily modified for implementation in other regions with hydraulic-fracturing-induced earthquakes. We then compare the framework with wastewater-disposal-induced earthquakes in Oklahoma–Kansas and natural earthquakes in California. We found that drivers of short-term seismic hazard differ for the three cases. In West Texas, clustered earthquakes dominate seismic hazards near production sites. However, for Oklahoma–Kansas and California, the short-term earthquake occurrence after an observed mainshock could be well described by the mainshock–aftershock sequence. For Stillwater in Oklahoma, aftershocks contribute less to the hazard than San Francisco in California, due to the high Poissonian mainshock rate. For the rate of exceeding a modified Mercalli intensity of 3 within 7 days after an M 4 earthquake, the aftershock sequence from natural earthquakes contributed 85% of the hazard level, whereas the aftershock contribution was only 60% for induced earthquakes in Oklahoma. Although different models were implemented for hazard calculations in regions with hydraulic fracturing versus wastewater injection, injection activities could be drivers of short-term hazard in both cases.