Spatiotemporal variations in shear-wave splitting during the 2018-9 Surrey, UK earthquake sequence

Between July 2018 and September 2019, a natural swarm of shallow seismicity, with event depths between 1.5 and 3.6 km, was recorded primarily along the Newdigate Fault in Southeastern England (Hicks et al., 2019). After the first nine events, a monitoring network of five stations was installed. This network recorded approximately 280 earthquakes, with a maximum magnitude of 3.2. This wealth of data, in a seismically quiet region of the UK, gives an opportunity to use shear-wave splitting analysis to improve constraints on the state of stress in the Weald Basin — a region with limited data on the 2022 Stress Map of Great Britain and Ireland — and to study the change in local stress during the sequence. We acquire new stress data from across the Weald Basin using borehole breakout analysis of dual calliper logs for six wells across the basin. This analysis gives a mean regional S_Hmax orientation of 142° with a circular standard deviation of 15°. 

We present shear-wave splitting measurements for 108 earthquakes in the sequence, which produce two intriguing features. Firstly, there is a significant (near 90°) rotation in fast polarisation directions for shear-wave splitting measured at stations north of the Newdigate Fault, which are aligned with the regional S_Hmax,and measurements made at stations south of the fault. This stark, but consistent change in fast polarisation directions over a 3–4 km region demonstrates the potential of shear-wave splitting to resolve local variations in stress around the Newdigate Fault. Secondly, we observe temporal variations in the measured anisotropy, with percentage anisotropy increasing and then decay after the larger events in the earthquake sequence. Combining these observations, we unravel the evolution of the state of stress during the Newdigate earthquake sequence and highlight the power of shear-wave splitting to constrain crustal stress.