A high-resolution wide-angle seismic study of the crust beneath the Northumberland trough
In June 1987, during the BIRPS MOBIL normal-incidence seismic profiling programme, off the East coast of England, the University of Durham recorded simultaneously at several land based seismic stations in Northern England. The resulting wide-angle data, particularly from Line 1, have excellent resolution in both space and time due to the airgun source and 50 m shot spacing. The interpretation of the Line 1 wide-angle data at Durham used BEAM87, Cerveny's Gaussian beam modelling package. The main arrivals interpreted include the upper crustal refraction (Pg), the Moho wide-angle reflection (PmP), the upper mantle refraction (Pn), and a very high amplitude arrival (D) which merges into PmP. Modelling gave a crust about 30 km thick with a change in velocity gradient and a slight velocity contrast at about 20 km depth. There are several wide-angle reflections from interfaces at mid-crustal depths, between 10 and 20 km depth, and the bottom 2 km of the crust has a high velocity of about 7 kms(^-1). Two interesting results are that a lateral velocity change about 40 km offshore is required to fit the Pg travel times; also that arrival D is modelled best as the remnant of a step on the Moho at the same location. These appear to be borne out by the normal incidence data for line 1, which show a lateral decrease in the mid-crustal reflectivity above a set of strong, westerly-dipping reflections at Moho depths. These results suggest the presence of a major crustal fault about 40 km offshore. It is suggested that this fault may be the northward continuation of the Dowsing Fault Zone.