**Table 3.** *Source parameters for Newdigate earthquakes with focal mechanisms.*

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

(**Figure 9(b)**). Hicks et al. [1] explained that they based their velocity model on existing regional models, but 'then improved [it] using sonic logs from the Brockham and Horse Hill wells'. However, as **Figure 9(a)** shows, despite this workflow, their velocity model is inconsistent with the records from these wells; they evidently somehow made a mistake over this aspect. Being too slow, their velocity model causes hypocentres to be mislocated too shallow. **Figure 2** includes a depth scale using the preferred velocity model in **Table 1**, based on the HH1 log. At the position of the earthquakes, this depth scale is 400 m deeper than that provided by Hicks et al. [1]. Furthermore, as is discussed in the supplement, Hicks et al. [1] appear to have depth-converted this seismic section using the same velocity model (**Table 2**) that they used for earthquake location. As is also discussed in the supplement, location of the earthquakes using the velocity model in **Table 1** would adjust their hypocentres downward by an estimated 400 m. The hypocentres are thus positioned more-or-less correctly relative to the detail in the seismic section, but they and this detail are now placed 400 m deeper than Hicks et al. [1] envisaged. This adjustment moves the earthquake population from within the Jurassic sedimentary section to within the Palaeozoic 'basement'. These earthquakes presumably occurred on one or more of the steeply north-dipping subsidiary strands of the Newdigate fault zone, given the steeply north-dipping nodal planes, identified as the fault planes, of the focal mechanisms (**Figure 1** and **Table 3**), rather than on the main Newdigate Fault that dips south.

et al. [56] and Fellgett et al. [57] provided syntheses of in situ stress data across much of Britain. However, these authors wrote little about the Weald Basin, Fellgett et al. [57] noting that many hydrocarbon wells in this area have yielded stress data but these data had not yet been placed in the public domain. The stress dataset available for the Weald Basin thus remains that presented by Evans and Brereton [58]. As is detailed in the supplement, this limited dataset indicates a NW-SE maximum principal stress and a NE–SW minimum principal stress. The Newdigate focal mechanisms (**Figure 1**) are consistent with this stress field orientation, given the standard requirement for the maximum principal stress to lie within dilatational

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

As detailed by Hicks et al. [1], the Newdigate seismicity between April 2018 and June 2019 involved four 'bursts' of activity (**Figure 4**). The first began at 11:10 on 1 April (ML 2.66), followed by two events later on the same day, another on 9 April, and a final event on 28 April. The smallest of these events (on 9 April)

> **Longitude (°W)**

*Cataloguing here is complete to 27 August 2020. Data are from https://earthquakes.bgs.ac.uk; these earthquakes have been located using standard BGS procedures, as reported by the International Seismological Centre (http://www.isc.ac .uk). Co-ordinate transformations to the British National Grid, as part of this study, use https://www.bgs.ac.uk/data/*

9 June 2019 02:43:18.2 51.159 0.237 TQ 23382 41449 2.7 -0.5 9 June 2019 23:00:15.0 51.133 0.295 TQ 19393 38462 3.3 -0.1 6 July 2019 01:03:20.4 51.161 0.242 TQ 23027 41663 2.5 -0.7 6 July 2019 01:03:23.7 51.161 0.242 TQ 23027 41663 2.5 -0.7 6 July 2019 01:03:30.1 51.159 0.241 TQ 23102 41442 2.1 -0.8 6 July 2019 01:03:40.2 51.159 0.241 TQ 23102 41442 2.2 -0.7 6 July 2019 03:57:15.3 51.160 0.239 TQ 23239 41557 2.5 0.1 20 July 2019 22:02:26.0 51.158 0.251 TQ 22405 41315 2.1 -0.6 29 July 2019 03:35:25.5 51.160 0.242 TQ 23029 41552 2.2 -0.1 6 Aug 2019 02:32:00.9 51.157 0.239 TQ 23247 41223 2.2 -0.5 12 Aug 2019 00:46:46.6 51.160 0.241 TQ 23099 41554 2.1 -0.7 12 Aug 2019 00:46:49.2 51.160 0.241 TQ 23099 41554 2.1 -1.4 2 Sep 2019 05:13:04.9 51.160 0.237 TQ 23379 41560 2.0 1.1 1 3 Sep 2019 20:19:13.2 51.161 0.237 TQ 23376 41672 2.0 0.2 6 Sep 2019 07:09:45.5 51.161 0.237 TQ 23376 41672 2.0 1.0 21 Sep 2019 14:43:45.2 51.160 0.237 TQ 23379 41560 2.2 0.6 31 Oct 2019 19:25:16.4 51.160 0.238 TQ 23309 41558 2.0 0.8 29 Apr 2020 00:11:25.6 51.172 0.256 TQ 22019 42863 3.1 0.3 24 May 2020 15:16:56.9 51.157 0.250 TQ 22478 41205 2.4 0.6

**BNG Depth**

**(km)**

**ML Note**

quadrants [59].

**3.1 Temporal clustering**

*webservices/convertForm.cfm*

**Table 4.**

**77**

*Note: 1. Felt in Newdigate; maximum EMS intensity 2.*

*Newdigate seismicity since the start of June 2019.*

**Date Time (UTC) Latitude**

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

**(°N)**
