**6. Conclusions**

The seismicity at Newdigate, Surrey, during 2018–2019, has been reassessed, amending aspects of the Hicks et al. [1] analysis. First-order correction of their seismic velocity model, which was too slow for the local stratigraphy, adjusts the hypocentres 400 m deeper than previously thought, to depths of 2400 m, placing them within the Palaeozoic 'basement' beneath the Weald Basin rather than within its Jurassic sedimentary sequence. These earthquakes involved mainly rightlateral slip on a steeply north dipping fault, part of the Newdigate fault zone (**Figure 2**).

Oil was produced during 2018–2019 in this vicinity from the Upper Portland Sandstone by the Brockham-X2Y and Horse Hill-1 wells. The correlation between phases of production from this reservoir and 'bursts' of earthquake activity (**Figure 4**) warrants consideration of potential geomechanical mechanisms. A conceptual model that can account for this causal connection is indicated schematically in **Figure 5**. It is thus suggested that the seismicity occurred within a thin (estimated 70 m thick) layer of permeable Dinantian limestone, hydraulically connected to the Portland reservoir via permeable strands of the Newdigate fault zone and by the highly permeable calcite 'beef' fabric within the Portland sandstone. It is hypothesised that past oil production at Brockham depressurized the Portland reservoir around this well and drew groundwater from the Dinantian limestone, causing it to compact and 'unclamp' the seismogenic fault but not sufficiently to reach the Mohr-Coulomb failure criterion to initiate coseismic slip. The resumption of production at Brockham in March 2018 caused a negative pressure pulse to propagate through the hydraulic connection to the Dinantian limestone, which, it is suggested, reached the failure threshold, initiating the first 'burst' of Newdigate seismicity in April 2018. Likewise, negative pressure pulse following resumption of production from the Portland reservoir at Horse Hill in February 2019 initiated a subsequent 'burst' of seismicity. This mechanism requires hydraulic diffusivity <sup>10</sup>–20 m<sup>2</sup> <sup>s</sup> <sup>1</sup> in the calcite 'beef' and 1 m<sup>2</sup> <sup>s</sup> <sup>1</sup> in the Dinantian limestone; it predicts unclamping of fault patches by many megapascals as a result of the Horse Hill production in February 2019 and by up to 0.1 MPa as a result of the Brockham production in March 2018. At other times, the complexity of production patterns (e.g., from both BRX2Y and HH1 in summer 2018) and the absence of pressure data prevent any detailed conclusions being drawn, although the general correlation of seismicity with production from the Portland reservoir (**Figure 4**) is compelling. The proposed 'unclamping' effect requires consideration of the roughness of the seismogenic fault, determined by the height of its asperities and their response to

compaction of the adjoining limestone. Such behaviour is particularly significant in this instance because of the high permeability of the Dinantian limestone; in impermeable rocks a reduction in pore pressure would cause fault clamping rather than unclamping. In principle this model is fully testable, but required data, notably the history of pressure variations in the oil wells, is not currently in the public domain. The recognition that this instance of seismicity is arguably caused by human activity, and the role of highly permeable hydraulic connections extending for many kilometres, has significant implications for regulation to mitigate the potential nuisance from future seismicity caused by oil production in the Weald Basin, and may also inform the understanding of anthropogenic seismicity in other settings.

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