The Role of b-Value Variations in Induced Seismic Hazard Estimation Using Coda-Envelope Derived Relative Magnitudes

In the last decade, the central and eastern United States has experienced an unprecedented rise in seismicity due to subsurface fluid injection. The Permian Basin region of western Texas and southeast New Mexico has now experienced more than 2000 M>2.0 earthquakes in the last two years including 13 M>4.0 earthquakes in 2021. This dramatic rise in seismic activity calls for rapid estimates of seismic hazard for environments where long-term hazard maps are unreliable due to frequent changes in injection activity.

The magnitude-frequency distribution (MFD) describes the relationship between the size of earthquakes and how frequently they occur in a given dataset, and is a necessary step to understanding seismic hazard. The slope of the MFD, i.e., the b-value, quantifies the relative prevalence of large earthquakes to small earthquakes in the region. The MFD and the b-value together contain important implications for earthquake recurrence, magnitude exceedance, and seismic hazard. However, small events suffer from imprecision and large biases due to differences in magnitude estimation methods across different networks and ranges of magnitudes, and these uncertainties propagate directly to the MFD. Therefore, a good estimate of MFD and seismic hazard requires unified and accurate estimates of magnitudes of small induced earthquakes.

Our project focuses on improving hazard estimates for induced seismicity by re-estimating earthquake magnitudes using a relative magnitude method that depends only on waveform data to recalculate magnitudes and benchmarking the results with a newly developed coda-envelope moment magnitude calibration for the Permian Basin. We then use the positive differences of these new magnitudes (Van der Elst 2021) to evaluate spatially and temporally dependent b-values and determine whether small scale variations in the MFD indicate changes in earthquake hazard. These results will (1) provide a unified method of magnitude estimation for induced earthquakes recorded by different networks, (2) contribute to rapid characterization efforts of induced seismicity, and (3) address the role of small-scale spatial and temporal b-value variations in indicating elevated seismic hazard.