Matthew Fox, Phil Meredith

The ability to attribute earthquakes to specific causes is challenging. The 2018-2019 earthquake swarm in Newdigate, Surrey, Southern England, coincides with local oil extraction at Horse Hill. Nevertheless, it remains debated whether these earthquakes were triggered by oil extraction or whether they were coincidental. Due to the onset of seismic activity before major oil extraction and the lack of a clear correlation between seismic activity and extraction volume, it has been suggested that the earthquakes may be coincidental. However, we show that time delays between fluid pressure changes and concomitant seismic activity are common in nature. Further, we develop a simple time series model to test whether different units respond differently to oil extraction. We find that extraction from the Portland sandstones at Horse Hill produces earthquakes with a delay of a few days. In contrast, extraction from the Kimmeridge shales produces fewer earthquakes, but with a delay of tens of days. We also show that the occurrence of earthquakes before extraction might be related to surface works. This simple model reproduces the overall trend in seismicity. We are unable to rule out coincidental seismic activity, but our analysis suggests that these earthquakes are triggered by Horse Hill activity.