Short-Term Probabilistic Hazard Assessment in Regions of Induced Seismicity

This 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.