Oklahoma earthquake rates in 2019-2026 fell faster than injection-pressure forecasts made in 2018; lower injection explains little of the error, and rates stay 4-18 times above the 1980-2008 level

Question: did the decline of injection-induced earthquakes in Oklahoma after the 2015 peak follow injection-driven forecasts, and is the rate back at the pre-2009 level? Approach: we compared the published Langenbruch, Weingarten and Zoback (2018) forecast with ComCat counts, and we ran honest out-of-sample tests of 24 simple pressure-model variants (2-D diffusion from reported Arbuckle injection of 814 wells; memoryless, maximum-pressure (Kaiser), delayed, non-linear and depleting responses) against exponential and power-law decay baselines, for four calibration end dates and three magnitude thresholds (OGS catalog, 0.2-degree cells, monthly). Results: observed M>=3 counts were 181, 57 and 34 in 2018-2020 against the published 190, 130 and 100. Reported injection was 10% and 30% below the assumed rate in 2019 and 2020, but in our models this lowers the forecast by only 7-14%, while observed M>=2.5 counts were 20-28% of the forecast. For forecasts from 2017, 2018 and mid-2019, decay of the recent rate beat the selected pressure model in every forecast year. For a forecast from January 2021, a strongly non-linear, depleting pressure model beat both decay baselines in all 6 years at M>=2.5 and M>=3, but its forecast hardly depends on post-2018 injection (2.6%). In 2021-2025 the M>=2.5 rate (31-125 per year) was still 4-18 times the 1980-2008 rate (7.1 per year). The residual events occur in cells that were active in 2014-2016; their link to current local injection is uncertain (slope -0.01 to 0.40, all 95% upper bounds below 1). Limitations: the pressure model is 2-D and Oklahoma-only; the LWZ study polygon and forecast uncertainty are not available; OGS magnitudes mix ML and copied Mw for larger events; plug-backs cannot be tested with OCC depth fields.

induced seismicity · Oklahoma · wastewater injection · Arbuckle Group · earthquake forecasting · forecast verification · pore pressure diffusion · Kaiser effect · seismicity decay · seismogenic index