OR/17/062 Conclusions

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

The 2017 Moidart 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.

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.

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.

Although there are no mapped major fault systems in the immediate vicinity of the Moidart earthquake, the similarity of the focal mechanism to others calculated in the region suggests that it also resulted from slip on an unmapped fault that strikes NE-SW or NW-SE.

The mainshock and the aftershocks all occurred within a small source volume, of the order of a few hundred metres in extent and had similar source mechanisms.

The modelled source displacement spectra provide a good fit for the observed displacement spectra and suggests a moment magnitude of 3.6 ± 0.1. This is slightly less than that expected for an earthquake with a local magnitude of 4.0 ML using the well-known empirical relationship determined by Grunthal et al (2009), which gives an expected moment magnitude of 3.7 Mw. This discrepancy has implications for seismic hazard estimation and requires further research.