OR/17/062 Discussion

Baptie, B, Ford, G, and Galloway, D. 2017. The Moidart earthquakes of 4 August 2017. British Geological Survey Internal Report, OR/17/062.

Instrumental data have been used to determine a hypocentre and source mechanism for the Moidart earthquake. The distribution of stations means that the earthquake epicentre is well constrained in a north-south direction, but less so in an east-west direction. Focal depth is also less well constrained, though the distribution of RMS errors with depth shows a clear minimum at 12 km, suggesting that the earthquake most likely nucleated in the mid-Crust. Figure 14(a) shows histograms of earthquake depths for earthquakes with magnitudes of 2.5 ML for both Scotland and all the British Isles. Earthquakes in both Scotland and the wider region have focal depths that are distributed though the upper part of the Crust. There is weak evidence for the hypothesis that larger earthquakes nucleate at greater depths (Figure 14b).

Figure 14    (a) Histogram showing the depth distribution for earthquakes with magnitudes of 2.5 ML or above in the BGS earthquake catalogue. Grey bars show earthquakes in Scotland only, white bars show all earthquakes in the British Isles and immediate offshore area. (b) Magnitude (ML) as a function of hypocentral depth. Grey crosses show all earthquakes. Red crosses show those in Scotland only.

In general, it is difficult to associate earthquakes in the British Isles with specific faults because: (a) no British earthquake recorded either historically or instrumentally has produced a surface rupture; (b) uncertainties in the earthquake location, especially depth, which are typically several kilometres; (c) uncertainties in fault distributions and orientation at depth; and, (d) the limited size of the earthquakes means the rupture dimensions are small. This is also the case for 2017 Moidart earthquake which has a calculated rupture area of approximately 1 km².

Figure 15 shows recorded earthquake activity in northwest Scotland along with major mapped faults from the British Geological Survey DigMapGB series. The faults are interpreted from field mapping, borehole information and seismic data and as such may not capture the full structural complexity of an area. Although the distribution of seismicity is rather diffuse, we suggest that most clusters of earthquake activity are associated with steeply dipping faults that strike approximately southwest-northeast or northwest-southeast and are thought to extend throughout the Crust. For example, the Great Glen fault, the Strathconon fault and Kinloch Hourn fault. Assumpçao (1981) suggests that the proximity of the hypocentre calculated for the 1974 Kintail earthquake to the Strathconon Fault, along with the agreement between the NE-SW strike of the fault and one of the calculated fault planes provides evidence that the earthquake took place on this fault. Similarly, events such as the Oban earthquake of 1986 and the Inverness earthquakes in 1816, 1890 and 1901 could be associated with reactivation of the Great Glen fault. By contrast, low angle thrusts such as the Moine Thrust, the Kishorn Thrust and the Sgurr Beag Slide have significantly less earthquake activity associated with them. However, earthquake activity in the far northwest of Scotland appears to be bounded to the southeast by the Sgurr Beag Slide. No major faults have been mapped in the vicinity of the Moidart earthquake. However, it seems likely that this earthquake also occurred on a steeply dipping fault that strikes southwest-northeast or northwest-southeast.

Figure 15    Instrumentally recorded earthquakes (red circles), from 1970 to present, and historical earthquakes (yellow circles), from 1700 to 1969, in northwest Scotland. Circles are scaled by magnitude. The epicentre of the Moidart earthquake of 4 August 2017 is indicated by the yellow star. Lines show mapped faults from the British Geological Survey DigMapGB series, © NERC 2016. The faults are coloured by geological age.

Earthquake focal mechanisms provide both fault geometries and principal stress directions that can be used to constrain our understanding of the driving forces of current deformation. In areas of low seismicity and sparse station distribution, determining reliable focal mechanisms can be problematic. This means that available mechanisms for earthquakes in Scotland are limited to larger events of ML ≥3.5/4.0, as it is generally not possible to calculate mechanisms for smaller events (Baptie, 2010). Figure 16 shows focal mechanisms available for earthquakes in Scotland. Focal mechanisms for the Kintail, Carlisle and Aberfoyle earthquakes were published by Assumpçao (1981), King (1980) and Ottemöller and Thomas (2007). The mechanisms for the Dunoon, Arran, Dumfries and Mallaig earthquakes were published by Baptie (2010). The mechanism shown for the Moidart earthquake is the one determined using the program HASH.

Figure 16    Focal mechanisms available for earthquakes in Scotland (Baptie, 2010). The blue and white areas show the compressional and dilatational quadrants and the lines between the quadrants show the strike and dip of the two possible fault planes. The axes of maximum and minimum compression are indicated by the blue and white squares respectively. The blue squares on the map show the location of the earthquakes. The blue lines show the orientation of the maximum horizontal compressive stress, sH, taken from smoothed stress orientations published in the World Stress Map (Heidbach et al., 2010).

All the focal mechanisms consistently show strike-slip faulting with N-S compression and E-W tension. This results in either left-lateral strike-slip faulting along near vertical NE-SW fault planes, or right-lateral strike-slip faulting along near vertical NW-SE fault planes. The fault planes are generally steeply dipping, though there is some variation in both the angle and direction of dip (e.g. Dunoon, 1985; Aberfoyle, 2002). Although the mechanism determined for the Moidart earthquake is probably poorly constrained, it does show good agreement with the previously published mechanisms.

The World Stress Map (Heidbach et al., 2010) contains sH (maximum horizontal compressive stress) orientations for the British Isles determined from a variety of stress indicators including borehole breakouts (e.g. Williams et al., 2015), drilling induced fracturing and hydro-fracturing as well as previously published focal mechanisms. Figure 15 also shows smoothed stress orientations on a 0.5° grid from the World Stress Map. The orientation of sH is roughly N-S in the north of Scotland, rotating to a more NW-SE direction in the north of England. This agrees reasonably with the focal mechanisms in Scotland. However, the smoothed sH values are based on sparse data, with most borehole breakout data from offshore areas (e.g. Williams et al., 2015).

Left-lateral strike-slip along a NE-SW trend is consistent with the prevailing N- to NNW-oriented horizontal tectonic stresses of western Europe (Grunthal and Stromeyer, 1992), and the earlier Palaeocene and Oligocene Alpine-related compression. This trend matches the recent geological history of the large-scale fault structures in British Isles where Alpine-related compression has driven faulting. This means that faults such as the Great Glen fault or Strathconon fault, which strike NE-SW are likely to show left-lateral strike-slip. Similarly, steeply dipping faults that strike NW-SE will have right lateral strike-slip.

This suggests that earthquake activity in northwest Scotland is a result of re-activation of a number of favourably oriented, steeply dipping fault systems, by deformation associated with first order plate motions rather than deformation associated with glacio-isostatic recovery (e.g. Muir-Wood, 2000).