**5. Discussion**

The proposed mechanism for the Newdigate seismicity depends on a pressure drop within the Dinantian limestone alongside the seismogenic strand of the Newdigate fault zone, as a result of depressurization of the water within this fault, caused by oil production from neighbouring wells (**Figure 5**). For the production from BRX2Y to have caused seismicity by this mechanism, the seismogenic fault must already have been extremely close (maybe within 60 kPa; see above) to the Mohr-Coulomb failure condition. It can be inferred that the same mechanism, operating during the previous production from this well, contributed to creating this state of stress by progressively depressurizing the Dinantian limestone. To test this possibility, one may use Eq. (36), noting the 2.7 MPa depressurization of the reservoir over 20 years and estimating from the previous analyses (e.g., **Figure 15 (d)**) that the resulting value of ΔP (and, thus, δP) within this limestone would be 10 kPa. With BE = 27 GPa, <sup>α</sup> = 0.46, H = 70 m, and <sup>Δ</sup>V = 36,900 m<sup>3</sup> to balance the production, depressurization of a 6000 km<sup>2</sup> area would be indicated; if roughly equidimensional, this would have a radius of 40 km. However, in reality, it is to be expected that such depressurization would be largely cancelled by recharge of water into the Dinantian limestone from other directions, which is not incorporated into the model. For example, if after this cumulative production, xM were 6 km and ΔPO were 1 kPa, then substituting Eq. (21) into Eq. (41), and taking b 0.1 m (obtained for L = 1.5 km, the length of the seismogenic part of the Newdigate Fault, using Eq. (49)), ΔΦ would be 6 MPa. Notwithstanding the approximations made in the model, it is thus indeed plausible that the cumulative production at Brockham brought this fault to the condition for shear failure, assuming that it was already critically stressed before this production began.

In principle, testing of the proposed mechanism is possible, given the predicted vertical compaction in the Dinantian limestone (Eq. (35)). Such compaction will cause subsidence of the Earth's surface, and so is in principle observable using multiple techniques, including interferometric synthetic-aperture radar (InSAR) and repeated gravity and GPS measurements. A combined dataset of this type has been analysed for a region of southeast England, including the northern Weald Basin, by Aldiss et al. [129]. At the October 2018 workshop attention was also drawn to an InSAR-derived surface deformation map of Britain by GVL [130], spanning October 2015 to October 2017. However, the predicted subsidence, resulting from the two decades of production at Brockham, will be only a small fraction of 1 mm (36,900 m<sup>3</sup> / 200 km<sup>2</sup> <sup>≈</sup> 0.2 mm), even if none of the fluid withdrawal from this limestone were recharged. The Aldiss et al. [129] analysis revealed vertical crustal motions at 1 mm a<sup>1</sup> , caused by processes such as extraction from or recharge of shallow groundwater reservoirs. Such rates make it impossible to resolve the much smaller effect expected from compaction of the Dinantian limestone at Newdigate.

Much has been made by participants in the OGA [5] workshop regarding the extent to which the Newdigate earthquake 'swarm' might fit the standard criteria identified by Davis and Frohlich [131] for establishing whether instances of seismicity are anthropogenic (e.g., [54]). UKOG [8] have argued that this set of criteria is inapplicable as they relate to seismicity caused by fluid injection, which is not the causal mechanism in this case. However, familiarity with the literature in this field (e.g., [132]) indicates that these criteria are widely used irrespective of the geomechanical cause of any particular anthropogenic earthquake. Verdon et al. [6]

proposed a different approach to assessing anthropogenic seismicity. This approach appears problematic, since it replaces the objective (yes / no) criteria recommended by Davis and Frohlich [131] with subjective numerical scores. The development of a conceptual geomechanical model for the Newdigate earthquakes supersedes the other Davis and Frohlich [131] criteria; nonetheless, an appraisal of this seismicity in terms of these criteria is included in the online supplement.

any arbitrary timescale parameter and in keeping with modern ideas (noted above) that coseismic slip on faults is governed by interactions of asperities. Nonetheless, as is evident, the present analysis makes many simplifying assumptions, not least regarding the geometry of the fluid flow and its variations, for example: recharge of the Dinantian limestone is neglected; and smooth variations in the pressure of the fluid being drawn from the Newdigate Fault (as in **Figure 13(a)**) are approximated as step changes (as in **Figure 13(b)**). Furthermore, as Segall [87] noted, the choice of boundary conditions for analysis of the combined pressure and strain response in the Dinantian limestone is one end member of a range of possibilities. The present analysis is essentially a 'proof-of-concept', to demonstrate that it is plausible that oil production caused the Newdigate seismicity and to shed light on the combination of hydraulic properties that makes this possible; it does not, of course, prove that the

*Seismicity at Newdigate, Surrey, during 2018–2019: A Candidate Mechanism…*

The remainder of this discussion will concentrate on geomechanical issues. Following each of the Newdigate earthquakes, the spatially averaged shear stress on the patch of fault that slipped will reduce by a value equal to the coseismic stress drop Δσ, moving the state of stress away from the Coulomb failure condition. The decline in seismicity in late 2019 suggests that it had reached some form of limiting state; the conditions governing this decline will now be assessed. To determine the area of the patch of fault that slipped in each earthquake, seismic moment MO was first calculated from MW using Eq. (50). For most of the Newdigate earthquakes, MW values are unavailable from Hicks et al. [1], only ML has been determined. Nonetheless, Deichmann [156] has shown that ML can serve as an equivalent proxy of earthquake 'size' as MW, provided it is appropriately calibrated. Hicks et al. [1] used the Luckett et al. [157] ML calibration, which in the UK has superseded the more familiar Ottemöller and Sergeant [158] version. No calibration against MW of

the Luckett et al. [157] ML formula has been reported; nonetheless, for the Newdigate events for which both MW and ML have been determined, both these measures are in close agreement, so ML is used as here a proxy for MW. Next, the radius a of the equivalent circular seismic source is determined from Mo, using Eq. (51). The source area A = π a2 and the mean slip u = Mo / (μ A) are then calculated. For an ideal circular earthquake source, u = 16 (1 – ν) a δσ / (3 (1 – ν) π μ) and the maximum slip u' is 8 (1 – ν) a δσ / ((1 – ν) π μ) (e.g., [128]), so u' = 3 u / 2. Coseismic stress drop δσ is set as 10 MPa (a plausible upper bound), Poisson's ratio ν is set as 0.25, and shear modulus μ BR for ν = 0.25 (Eq. (47)); as before BR = 50 GPa, from Bell [119]. This task was carried out for the complete Hicks et al. [1] earthquake dataset, plus the additional events listed in **Table 4**. The cumulative seismic moment thus obtained was 1.5 1014 N m, equivalent to a single event of MW 3.4, with cumulative area of fault rupture 2.1 <sup>10</sup><sup>5</sup> <sup>m</sup><sup>2</sup> and maximum

Taking 1.5 km as the length of the seismogenic fault, from **Figures 1** and **7**, and 70 m as the thickness of the Dinantian limestone, the area of fault in this lithology is

area of the fault, and would be even greater if lower δσ were adopted. Thus, either patches of fault slipped more than once, or that the seismicity propagated into the overlying and/or underlying lithologies, although the compact hypocentre 'cloud' (**Figure 1**) indicates no clear propagation in any direction. Nonetheless, the calculations indicate that the eight largest earthquakes (with MW ≥ 1.96) have source diameters larger than the estimated 70 m thickness of the limestone; evidently, these events either ruptured outside this layer or ruptured patches of fault that are elongated horizontally. Assuming the latter explanation, and that the overall earthquake population was distributed to produce constant overall coseismic slip across

the seismogenic fault, this amount is determined as was 1.5 1014 Nm/

. Calculated on this basis, the total area of coseismic ruptures exceeds the

coseismic slip in the largest earthquake 3.4 cm.

<sup>10</sup><sup>5</sup> <sup>m</sup><sup>2</sup>

**101**

production caused this seismicity.

*DOI: http://dx.doi.org/10.5772/intechopen.94923*

Although fluid injection is nowadays recognised as a widespread cause of induced seismicity, it is worth noting that reductions in fluid pressure caused by fluid extraction has been linked to seismicity for far longer. The first instance recognised is by Pratt and Johnson [133], for earthquakes accompanying oil production near Houston, Texas. Other case studies subsequently recognised include those by Calloi et al. [134], Kovach [135], Rothé and Lui [136], Simpson and Leith [137], Pennington et al. [138], Wetmiller [139], Grasso and Wittlinger [140], Doser et al. [141], Ottemöller et al. [142], Dahm et al. [143], and Hornbach et al. [144], whereas works discussing physical mechanisms for such seismicity, including compaction of reservoir rocks, include those by Yerkes and Castle [145], Simpson et al. [146], Segall [147], Segall and Fitzgerald [88], Ottemöller et al. [142], and Soltanzadeh and Hawkes [90]. Some of the above works (e.g., [138]) have recognised the significance of processes required in the present conceptual model (e.g., compaction of limestone and failure of asperities), and others (e.g., [148– 150]) have recognised that highly permeable connections can cause seismicity at significant distance from the source of the causative change in fluid pressure. However, no previous case study known to the present author has proposed a geometry between the source of depressurization and the seismogenic fault that resembles the present conceptual model (**Figure 5**).

Hicks et al. [1] considered and rejected the possibility that the Newdigate seismicity was caused by compaction, as a result of depressurization caused by oil production, from one of the oil reservoirs in the area. They reached this conclusion on the basis of the strike-slip focal mechanisms (**Figure 1**), because in their view compaction would be expected to cause dip-slip earthquakes; they thus argued that these strike-slip earthquakes must be natural. They cited in support of this conclusion work on the Groningen seismicity in the Netherlands, where gas field depletion has caused many earthquakes, almost all with normal-faulting focal mechanisms (e.g., [151]). Compaction was initially thought to be the cause of the Groningen seismicity [152]. However, these events were later reinterpreted as caused by the combined effects of compaction and poro-elastic changes to the state of stress on faults [153], the normal faulting focal mechanisms thus reflecting the extensional stress state in the Netherlands. There is therefore no contradiction between this work and the occurrence at Newdigate of strike-slip earthquakes, the focal mechanism orientation again consistent with the local state of stress. Differences between the Newdigate and Groningen case studies concern, for the latter, the much larger scale (reservoir area 900 km<sup>2</sup> ; surface subsidence 30 cm; cumulative production <sup>10</sup><sup>10</sup> <sup>m</sup><sup>3</sup> at reservoir pressure; [154]), and the lower hydraulic diffusivity and elastic moduli (D 0.38 m2 s <sup>1</sup> and E <sup>8</sup>–25 GPa; [153, 155]). Regarding the geomechanics, the principal differences are that the analysis for Groningen has neglected consideration of fault asperities and has assumed that the elastic moduli of the fault wall rocks vary over time following the start of compaction, relaxing from short timescale to long timescale values (this effect being assumed to occur over a characteristic timescale, specified as 7.3 years for no clear reason other than to make the model work). In the present study, the time-dependence of the response develops as a result of the time required for poroelastic compaction in the Dinantian limestone to create fault opening Δx (Eq. (24)) comparable to the typical fault asperity height b. This seems a more physically realistic approach, avoiding

*Seismicity at Newdigate, Surrey, during 2018–2019: A Candidate Mechanism… DOI: http://dx.doi.org/10.5772/intechopen.94923*

any arbitrary timescale parameter and in keeping with modern ideas (noted above) that coseismic slip on faults is governed by interactions of asperities. Nonetheless, as is evident, the present analysis makes many simplifying assumptions, not least regarding the geometry of the fluid flow and its variations, for example: recharge of the Dinantian limestone is neglected; and smooth variations in the pressure of the fluid being drawn from the Newdigate Fault (as in **Figure 13(a)**) are approximated as step changes (as in **Figure 13(b)**). Furthermore, as Segall [87] noted, the choice of boundary conditions for analysis of the combined pressure and strain response in the Dinantian limestone is one end member of a range of possibilities. The present analysis is essentially a 'proof-of-concept', to demonstrate that it is plausible that oil production caused the Newdigate seismicity and to shed light on the combination of hydraulic properties that makes this possible; it does not, of course, prove that the production caused this seismicity.

The remainder of this discussion will concentrate on geomechanical issues. Following each of the Newdigate earthquakes, the spatially averaged shear stress on the patch of fault that slipped will reduce by a value equal to the coseismic stress drop Δσ, moving the state of stress away from the Coulomb failure condition. The decline in seismicity in late 2019 suggests that it had reached some form of limiting state; the conditions governing this decline will now be assessed. To determine the area of the patch of fault that slipped in each earthquake, seismic moment MO was first calculated from MW using Eq. (50). For most of the Newdigate earthquakes, MW values are unavailable from Hicks et al. [1], only ML has been determined. Nonetheless, Deichmann [156] has shown that ML can serve as an equivalent proxy of earthquake 'size' as MW, provided it is appropriately calibrated. Hicks et al. [1] used the Luckett et al. [157] ML calibration, which in the UK has superseded the more familiar Ottemöller and Sergeant [158] version. No calibration against MW of the Luckett et al. [157] ML formula has been reported; nonetheless, for the Newdigate events for which both MW and ML have been determined, both these measures are in close agreement, so ML is used as here a proxy for MW. Next, the radius a of the equivalent circular seismic source is determined from Mo, using Eq. (51). The source area A = π a2 and the mean slip u = Mo / (μ A) are then calculated. For an ideal circular earthquake source, u = 16 (1 – ν) a δσ / (3 (1 – ν) π μ) and the maximum slip u' is 8 (1 – ν) a δσ / ((1 – ν) π μ) (e.g., [128]), so u' = 3 u / 2. Coseismic stress drop δσ is set as 10 MPa (a plausible upper bound), Poisson's ratio ν is set as 0.25, and shear modulus μ BR for ν = 0.25 (Eq. (47)); as before BR = 50 GPa, from Bell [119]. This task was carried out for the complete Hicks et al. [1] earthquake dataset, plus the additional events listed in **Table 4**. The cumulative seismic moment thus obtained was 1.5 1014 N m, equivalent to a single event of MW 3.4, with cumulative area of fault rupture 2.1 <sup>10</sup><sup>5</sup> <sup>m</sup><sup>2</sup> and maximum coseismic slip in the largest earthquake 3.4 cm.

Taking 1.5 km as the length of the seismogenic fault, from **Figures 1** and **7**, and 70 m as the thickness of the Dinantian limestone, the area of fault in this lithology is <sup>10</sup><sup>5</sup> <sup>m</sup><sup>2</sup> . Calculated on this basis, the total area of coseismic ruptures exceeds the area of the fault, and would be even greater if lower δσ were adopted. Thus, either patches of fault slipped more than once, or that the seismicity propagated into the overlying and/or underlying lithologies, although the compact hypocentre 'cloud' (**Figure 1**) indicates no clear propagation in any direction. Nonetheless, the calculations indicate that the eight largest earthquakes (with MW ≥ 1.96) have source diameters larger than the estimated 70 m thickness of the limestone; evidently, these events either ruptured outside this layer or ruptured patches of fault that are elongated horizontally. Assuming the latter explanation, and that the overall earthquake population was distributed to produce constant overall coseismic slip across the seismogenic fault, this amount is determined as was 1.5 1014 Nm/

(50 GPa 1500 m 70 m) or 2.9 cm, roughly equal to the maximum slip in the largest individual earthquake (3.4 cm for MW 3.25) (the value would be 3.3 cm, in better agreement, if the thickness of this lithology were taken as 60 m). It is thus possible that the earthquake swarm was indeed 'self-limiting', and that once the full extent of the seismogenic fault had slipped by this distance, the fault was effectively 'de-stressed' (δσ = 10 MPa having reduced ΔΦ by 6 MPa) and the activity died out, consistent with its observed decline and near cessation in late 2019 (**Table 4**). This seismogenic patch of fault is bounded to the east by the eastern end of the Newdigate Fault, but has no obvious boundary to the west, although it is noted that at its limit the downthrow of the top Portland sandstone is ≤60 m (**Figure 7**), comparable to the thickness of the Dinantian limestone; the state of stress must be different, for some reason yet to be resolved, farther west along this fault. Further analysis of this aspect is evidently warranted, given the possibility that significant seismicity might resume (two very small earthquakes having occurred in the spring of 2020; **Table 4**), as a result of pressure changes and fluid withdrawal arising from the planned increase in production at the Horse Hill site. A simple approach to mitigation, suggested by the present study, would be to ensure that oil production is balanced by reinjection to minimise the spatial extent of subsurface pressure changes. Suitably targeted reinjection could ensure that pressure decreases are confined to the Portland reservoir and do not propagate to the 'beef' and thence to the Newdigate Fault or other faults.

activity began in 2013 in a locality that had experienced both injection (of industrial wastewater) and production (of brine, oil and natural gas). The injection was initially suspected as the cause, on account of its very large volume, but Hornbach et al. [144] deduced that the pressure reduction caused by oil and gas production was the most important individual factor. In other instances, for example that discussed by Justinic et al. [162], authors have emphasised the proximity of hypocentres to injection points to highlight the anthropogenic cause, when many hypocentres are in fact rather deeper and indicate earthquakes within the underlying impermeable basement. Hincks et al. [163] have noted that fluid injection into faults or fractures in basement or near the contact with basement at the base of permeable sediments is statistically much more likely to result in seismicity than injection well above basement. Consideration of poroelasticity provides a natural

*Seismicity at Newdigate, Surrey, during 2018–2019: A Candidate Mechanism…*

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

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

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

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

<sup>1</sup> in the Dinantian limestone; it

highly permeable calcite 'beef' fabric within the Portland sandstone. It is

<sup>1</sup> in the calcite 'beef' and 1 m<sup>2</sup> <sup>s</sup>

explanation for this empirical observation.

*DOI: http://dx.doi.org/10.5772/intechopen.94923*

**6. Conclusions**

(**Figure 2**).

<sup>10</sup>–20 m<sup>2</sup> <sup>s</sup>

**103**

As already noted, the proposed physical mechanism, whereby a decrease in the fluid pressure within a fault can destabilise the fault, is the opposite of what might be termed the 'standard' effect, familiar from many studies of 'fracking': the unclamping that will occur when the fluid pressure within a fault increases and reduces the effective normal stress. The possibility of these two opposite responses to a fluid pressure change in a fault can be seen from inspection of Eq. (41). In the limit of D = 0 this adjusts to the conventional expression ΔΦ = c ΔPO, whereby a pressure decrease by ΔPO would cause fault 'clamping' and a pressure increase would be necessary for 'unclamping', but if D is large enough, a pressure decrease by ΔPO can outweigh this effect, causing fault 'unclamping' and coseismic slip. The 'standard' effect of an increase in fluid pressure causing fault unclamping is to be expected if the fault is in impermeable rocks, where the fluid pressure only acts within the fault and not within the adjoining rock volume (e.g., [11, 159]). In general, for faults within permeable rocks, one can expect these two effects to counteract each other; whether the poroelastic effect will predominate or not will depend on the conditions in each case.

In this context, it is noteworthy that much of the seismicity associated with fluid injection in the USA occurs as a result of pressure increases in faults in impermeable basement rocks, rather than in the permeable rocks into which the injection takes place. A significant case study illustrating this issue is provided by the attempt to control seismicity in the Rangely, Colorado, oilfield by varying the reservoir pressure [160]. In this instance earthquakes occurred on patches of a fault where it transects both the reservoir and deeper crustal basement, their frequency of occurrence in both locations increasing with fluid pressure. The fault unclamping effect of increasing fluid pressure was the same in the reservoir rocks as in the impermeable basement rocks. In this case, evidently, the direct effect of fluid pressure on the Mohr-Coulomb failure condition for the patch of fault in the reservoir rocks outweighed the poroelastic effect (cf. Eq. (40)), as might be expected given the relatively low permeability (1 mD; [160]) of the reservoir rocks, orders-of-magnitude smaller than is expected for Dinantian limestone (see above). A second case study highlighting the complexity of this issue was documented by Hornbach et al. [144] (see also, e.g., [126, 161]), at Azle near Fort Worth, Texas, where earthquake *Seismicity at Newdigate, Surrey, during 2018–2019: A Candidate Mechanism… DOI: http://dx.doi.org/10.5772/intechopen.94923*

activity began in 2013 in a locality that had experienced both injection (of industrial wastewater) and production (of brine, oil and natural gas). The injection was initially suspected as the cause, on account of its very large volume, but Hornbach et al. [144] deduced that the pressure reduction caused by oil and gas production was the most important individual factor. In other instances, for example that discussed by Justinic et al. [162], authors have emphasised the proximity of hypocentres to injection points to highlight the anthropogenic cause, when many hypocentres are in fact rather deeper and indicate earthquakes within the underlying impermeable basement. Hincks et al. [163] have noted that fluid injection into faults or fractures in basement or near the contact with basement at the base of permeable sediments is statistically much more likely to result in seismicity than injection well above basement. Consideration of poroelasticity provides a natural explanation for this empirical observation.
