The 4.0 ML Arran Earthquake of 4 March 1999

On 4 March 1999, an earthquake of local magnitude (ML) 4.0 occurred at 00:16 GMT about 3.5 km offshore of the SW corner of the Isle of Arran. The earthquake was located at a depth of 19 km and was felt over an area of about 50,000 km2 (isoseismal 2 EMS) across western and central Scotland and Northern Ireland. The maximum intensity is reported as 5 on the European Macroseismic Scale (EMS) (Grünthal, 1998) at Lamlash, on the eastern side of the Isle of Arran but in Kilmory, which was closer to the epicentre, it was only felt at 4 EMS. Only two aftershocks were recorded, a 1.6 ML event about 15 minutes after the mainshock, and a 1.1 ML earthquake one week later. The focal mechanism computed for the mainshock is not tightly constrained but indicates dominantly strike-slip faulting with varying degrees of dip slip motion, on either a NW or a NE-striking focal plane. The earthquake occurred in a region where there has been recurrent small magnitude earthquake activity during the 1990’s. Earthquake activity peaked in early 1993 (12 events with a maximum magnitude of 1.7 ML) and in the summer of 1996 (14 events with a maximum magnitude of 2.2 ML). This event occurred in a region where no historical earthquakes approaching this size have been located.