**2.2 Geolocation**

The study area has been illustrated using the map (**Figure 1**), and seismic crosssection (**Figure 2**) from Hicks et al. [1]. However, the original versions of both these figures have required significant amendment regarding accuracy issues. This map was originally geolocated using geographical co-ordinates; to make it easier to use British National Grid (BNG) co-ordinates have been added. This map also shows seismic lines and faults. The information source for seismic lines, including line TWLD90–15 that is illustrated in **Figures 2** and **3**, was not reported by Hicks et al. [1]; it is evident that they are from the UKOGL location map. Hicks et al. [1] also explained that (in lieu of using the existing literature) they identified faults in the study area through their own interpretation of seismic lines. As already

#### **Figure 7.**

*Structure contour map of the top Portland sandstone in the vicinity of the Horse Hill oilfield. Redrawn after part of Fig. 6.6 of Xodus [26]. The contours within the range of reservoir depths are at 530.4, 536.4, 542.5, 548.6, 554.7, 560.8, 566.9 m, 573.0, and 579.1 m TVDSS. Faults are labelled thus: CFF, Collendean farm fault; CFF2, northern strand of Collendean farm fault; HKF, Hookwood fault; NGF, Newdigate fault; and NWF, Nalderswood fault. The key earthquakes depicted are those that initiated the first and third 'bursts' of seismicity, for which analysis of subsurface pressure changes is undertaken: For the first at 11:10 on 1 April 2018 (B; TQ 21992 41976; depth 3.08 km; ML 2.66; MW 2.76); and for the third at 07:43 on 14 February 2019 (H; TQ 21959 41543; depth 2.05 km; ML 2.47; MW 2.52). The event that initiated the second 'burst' of seismicity occurred at 12:28 on 27 June 2018 (TQ 23230 42421; depth 2.39 km; ML 2.52). Earthquake details are after Hicks et al. [1]. Locations of seismic lines are from UKOGL.*

**Figure 8.**

*1.337 s.*

**Table 2.**

**73**

*Velocity Model from Hicks et al. [1]*

*Holmwood fault.*

**H (km) VP (km s<sup>1</sup>**

*Structure contour map of the top Portland sandstone in the vicinity of the Brockham oilfield. Redrawn after part of Fig. 4 of Angus [27]. Faults are labelled thus: BF, Brockham fault; BHF, Box Hill fault; and HWF,*

*This velocity model was used by Hicks et al. [1] for earthquake relocation and moment tensor inversion. H denotes the depth to the top of each layer; VP and VS denote the P-wave and S-wave velocities. Note that this velocity model is significantly slower that that in Table 1; it results in a two-way time to depth 2326.8 m, corresponding to the top of the Carboniferous Limestone at Horse Hill, of 1.466 s rather than the actual*

0.0 2.2 1.2 0.2 2.4 1.4 0.4 2.6 1.5 0.7 2.7 1.5 1.2 3.1 1.8 1.5 3.6 2.0 1.8 4.7 2.7 2.1 5.0 2.8 2.4 5.5 3.1 7.6 6.4 3.7 18.9 7.0 4.1 34.2 8.0 4.6

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

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

**) VS (km s<sup>1</sup>**

**)**

noted, regarding key aspects of the structure the existing interpretations are favoured over these revisions by Hicks et al. [1].

The seismic section in **Figure 2** clearly has higher resolution than older ones, including those which informed earlier fault maps such as that by Butler and Pullan [12] (**Figure 6**). Some of the faults depicted in **Figure 2** are, thus, recognised for the first time. However, additional faults are also evident in the uninterpreted version provided by UKOGL (**Figure 3**). It is thus evident that in the lower part of the Jurassic sediment and upper part of the underlying Palaeozoic basement, the Newdigate Fault consists of multiple fault strands distributed across a zone with

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

#### **Figure 8.**

*Structure contour map of the top Portland sandstone in the vicinity of the Brockham oilfield. Redrawn after part of Fig. 4 of Angus [27]. Faults are labelled thus: BF, Brockham fault; BHF, Box Hill fault; and HWF, Holmwood fault.*


*This velocity model was used by Hicks et al. [1] for earthquake relocation and moment tensor inversion. H denotes the depth to the top of each layer; VP and VS denote the P-wave and S-wave velocities. Note that this velocity model is significantly slower that that in Table 1; it results in a two-way time to depth 2326.8 m, corresponding to the top of the Carboniferous Limestone at Horse Hill, of 1.466 s rather than the actual 1.337 s.*

#### **Table 2.** *Velocity Model from Hicks et al. [1]*

N-S width approaching 2 km. Careful inspection of supplementary Figure S13 of Hicks et al. [1] and **Figure 3** indicates broken and offset seismic reflectors which delineate these subsidiary strands of the Newdigate fault zone, some evidently near the limit of seismic resolution (cf. [51]), which merge upwards by 0.5 s two way time (TWT).

A significant issue to have emerged from checking the Hicks et al. [1] analysis concerns their velocity model used for earthquake location (**Table 2**). As **Figure 9 (a)** shows, this is significantly slower than is expected from the sonic logs for wells BR1, CF1 and HH1, and from the recent depth-conversion analysis by Pullan and Butler [13]. The Hicks et al. [1] velocity model is also significantly slower that that obtained from moveout analysis during the processing of seismic line TWLD90–21

#### **Figure 9.**

*Comparisons of one-way time versus depth for different seismic velocity models. (a) Comparison of the Hicks et al. [1] velocity model with the sonic logs from wells CF1, HH1, and BR1 (from [24], and UKOGL), the velocity model (based on well HH1) adopted for this study, and a representative point from the analysis by Pullan and Butler [13] where a 600 ms one-way time corresponds to a depth of 2100 m. (b) Comparison of the Hicks et al. [1] velocity model with velocity models derived from interval velocities obtained from the moveout analysys of seismic line TWLD90–21 (Fig. 7), for a suite of representative CDPs. These interval velocities were calculated in this study using Dix's formula (e.g., [52, 53]) from root mean square velocities provided by UKOGL. UKOGL did not have such data for the other seismic lines analysed for this study.*

**No.**

**75**

1

2

3

4

5

6 04 May 2019 00:19:19

*Data from*  *ordinates and focal depths. Double difference focal depths (zDD) were not determined (ND). For the other events, the epicentral co-ordinates*

*procedure and the 'Depth' by conventional*

*ray path azimuths to seismograph*

*magnitude. M*

*fault plane, being subparallel to the Newdigate Fault (Figure 2). Mean corner frequency fC, source radius rO, and stress drop Δσ are determined from seismogram*

**Table 3.** *Source parameters*

 *for Newdigate*

 *earthquakes*

 *with focal mechanisms.*

*W 1 and the centroid depth zC are determined from the moment tensor; M*

 *location. ΔN, ΔE and Δz are the uncertainties*

 *stations that recorded each event, NP and NS being the numbers of P- and S-wave records. Δto is the rms residual in origin time. ML and M*

 *in northing, easting, and depth, based for events 3–6 on the double difference solutions. Δα is the maximum gap between*

*W 2 is from P-wave spectra. Strike, Dip and Rake are for the nodal plane of the focal mechanism that is regarded as the*

*supplementary*

 *Table S2 of Hicks et al. [1]. The events are numbered to match Figure 1. For events 1 and 2 only conventional*

 27 Feb 2019 03:42:21

 19 Feb 2019 17:03:57

 14 Feb 2019 07:43:33

 18 Jul 2018

 13:33:18

 TQ 21920 41474

 TQ 22959 41543

 TQ 22872 41538

 TQ 22622 41517

 TQ 22796 41516

 2190

 143 165 294 94 13 10 0.11 2440 2.35 2.31 2.17 2.40

 2110

 286 352 316 98 14 11 0.13 2300 3.18 3.25 2.87 3.60

 2050

 220 429 393 106 5 4 0.04 2040 1.98 1.95 1.77 2.20

 2220

 297 330 379 98 9 7 0.09 2050 2.47 2.52 2.27 2.80

 1860 1463 737 1014 145 15 15 0.02 ND 2.54 2.56 2.45 2.20

 18 Jul 2018

 03:59:56

 TQ 22005 41393

 1990 1397 803 1011 151 15 13 0.04 ND 2.01 2.20 2.03 2.00

 **Date**

 **Time**

**Epicentre**

**Depth**

**Δ**

**N**

**ΔE**

**Δz**

**Δα**

**NP NS Δto**

**zDD**

**ML M**

**W**

**M**

**W**

**zC**

**Strike**

**Dip**

**Rake**

**fC**

**ro**

**Δσ**

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

**1**

**2**

**(m)**

**(°)**

 282

 276

 255

 256

 260

 255

> *hypocentral*

 *and zDD are determined from the double difference location*

 *locations were determined,*

 *which yielded the epicentral co-*

*W are local magnitude and moment*

> *spectra.*

 85 167 16.9 64 8.06

 78 178

 5.8 169 2.62

 61 -163 11.2 86 0.56

 86 173

 7.7 142 1.05

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

 75 169

 4.3 25 0.27

 74 178

 6.4 48 0.25

**(°)**

**(°)**

**(Hz)**

**(m)**

**(MPa)**

**(m)**

**(m)**

**(m)**

**(m)**

**(°)**

**(s)**

**(m)**

**(UTC)**

**(BNG reference)**

