OR/17/062 Introduction

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

On 4 August 2017 at 14:43 UTC, an earthquake of magnitude 4.0 ML occurred in the locality of Moidart on the west coast of mainland Scotland (Figure 1). The epicentre was approximately 22 km south of Mallaig, 50 km west of Fort William and 145 km northwest of Glasgow. The earthquake was the largest event in the region since a magnitude 4.1 earthquake near Oban on 29 September 1986 and it was the felt widely across the west of Scotland.

Figure 1    Instrumentally recorded earthquakes (red circles), from 1970 to present, and historical earthquakes (yellow circles), from 1700 to 1969, within a 100 km square centred on the epicentre of the Moidart earthquake of 4 August 2017 (yellow star). Circles are scaled by magnitude.

The earthquake was followed by at least four aftershocks, the largest of which had a magnitude of 3.4 ML and which occurred two minutes after the mainshock. The two largest aftershocks were also felt by people across the region.

Analysis of the BGS earthquake catalogue shows that there have been only five other earthquakes with magnitudes of 4 ML or above in the period of instrumental monitoring from 1970 to present. The largest of these was a magnitude 4.4 ML earthquake near Kintail in 1974. This was one of a sequence of over 20 earthquakes that occurred over several months in 1974/1975. Two other earthquakes in this sequence also had magnitudes of above 4.0 ML. A magnitude 4.1 ML earthquake near Oban in 1986 was 24 km south-southeast of the Moidart earthquake. More recently, a magnitude 4.0 ML earthquake occurred near Arran in 1999. As a result, the Moidart earthquake of 4 August 2017 was the largest earthquake in Scotland in 18 years and, as such is a relatively rare event.

Similarly, historical observations of earthquake activity in Scotland date back to the 16th century (Musson, 1996). These show that despite many accounts of earthquakes felt by people, damaging earthquakes are relatively rare. Scotland's largest recorded earthquake, a magnitude 5.2 ML event in Argyll in 1880, was 75 km to the southeast of the Moidart earthquake. Only two other earthquakes with a magnitude of 5.0 ML or greater have been observed in the last 400 years. As a result, the risk of damaging earthquakes is low.

Musson (1994) lists 19 earthquakes with magnitudes of 4.0 ML or above in the period from 1800 to 1969. Combining these with the six instrumentally recorded earthquakes ≥4.0 ML suggests that there is an earthquake of this magnitude or greater somewhere in Scotland roughly every 8–9 years.

The aim of this report is to provide an overview of the source parameters determined for the Moidart earthquake along with an interpretation of the available macroseismic data.