Probing Into the Crust Through eastern Scotland: seismological contraints on the Highland Boundary Fault

The Highland Boundary Fault (HBF) delineates a fundamental division in the topography and surface geology in Scotland, separating 1000-500Ma metamorphic rocks to the north from predominantly ~440-360Ma sedimentary rocks of the Midland Valley to the south. Despite detailed geological mapping of the HBF and surrounding areas, the role(s) of the HBF in the tectonic history of Scotland is contested. On one hand, the HBF may represent a major plate boundary that was active initially as a strike-slip, then reactivated as a high angle thrust fault. On the other hand, some argue that lateral movement on the HBF was limited, and the topographic break seen at the HBF is primarily due to differences in erosion rates. Seismicity on the HBF has been reported in both the instrumental and historical records, including a M4.8 earthquake in Comrie in 1839 and an earthquake swarm in Aberfoyle in 2003. Notably, no seismicity has been observed in northeast Scotland. It may be that there is no seismicity in this region, or that the distribution of seismic instrumentation has been insufficient to detect very small magnitude earthquakes (<M2).

 

To address these questions, in March-May 2022 we deployed a new network of 10 seismometers in north eastern Scotland as part of the PICTS (Probing Into the Crust Through eastern Scotland) project, which, together with a BGS Seismology permanent station, DRUM, form three transects across the HBF. These instruments form the first dense seismometer deployment in this region and data from them will allow us to place high-resolution constraints on the structure of the crust and uppermost mantle across the HBF, determine crustal thickness in this region, and to investigate if any seismicity is occurring on the eastern portion of the HBF.

 

Here we present preliminary results from the data recorded on seismometers from the PICTS project, including images of crustal structure from receiver function analysis that show differing crustal structure to the north and south of the HBF.