| Abstract |
The Moidart earthquake of 4 August 2017 (4.0 ML) was the largest earthquake in Scotland for 18 years. The earthquake was felt widely across the west of Scotland. Only five other earthquakes of this size or greater have been observed in the period of instrumental recording from 1970 to present. Historical observations and instrumental recordings have been used to estimate that an earthquake of 4.0 ML or greater occurs somewhere in Scotland roughly every 8-9 years on average._x000D_
The earthquake hypocentre was calculated using an iterative linearized method. The results suggest that the earthquake occurred in the mid-Crust at a depth of approximately 12 km. This is largely consistent with observed focal depths for other earthquakes in the region, which are distributed throughout the upper 20 km of the Crust._x000D_
The strong similarity between the recorded ground motions from the mainshock and the four recorded aftershocks suggests that they all occurred within a small source volume, of the order of a few hundred metres in extent and had similar source mechanisms._x000D_
The modelled source displacement spectra provide a good fit for the observed displacement spectra and suggest a moment magnitude (Mw) of 3.6 ± 0.1. This is slightly less than that expected for an earthquake with a local magnitude of 4.0 ML using commonly used empirical relationships relating local and moment magnitude, which gives an expected moment magnitude of 3.7._x000D_
The calculated focal mechanism suggests that the earthquake resulted from strike-slip faulting on a fault plane that strikes either SW-NE or NW-SE and dips steeply, although the dip of both fault planes is rather poorly constrained. This is in good agreement with focal mechanisms calculated for other earthquakes across the region, which all show similar solutions._x000D_
Seismicity in northwest Scotland is clustered around a number of large, steeply dipping major faults that strike either NE-SW or NW-SE suggesting that earthquake activity across the region is driven by reactivation of such fault systems by deformation associated with first-order plate motions rather than deformation associated with glacioisostatic recovery._x000D_
Although there are no mapped major fault systems in the immediate vicinity of the Moidart earthquake, it seems likely that the earthquake also occurred on a steeply dipping fault that strikes either NE-SW or NW-SE but remains unmapped. |
