Earthquakes occur in the crust where deformation is by brittle fracture. Beyond a 'transition zone' earthquakes are no longer possible and plastic deformation occurs. Onshore UK seismicity generally occurs within the seismogenic zone, to mid-crustal depths. However, the activity on the Lleyn Peninsula following the 1984 mainshock and subsequent aftershock series occurred at depths of around 22 km in the lower crust. Other well constrained deep activity has occurred in the Welsh Borders around Newtown, suggesting a lowering of this brittle-ductile transition zone. In contrast, shallower than average focal depths of around 6 km are obtained for Cornwall where radiogenic granites are responsible for the highest heat flow in the UK. Variation in the cut-off depth for crustal seismicity is thought to be due to a combination of heat flow and chemical/mineralogical differences (decreased quartz levels) in the crustal rocks.