Source parameters of the 1926 and 1927 Jersey earthquakes from historical, instrumental, and macroseismic data

English Channel Islands are located off Normandy coast of France within an intraplate area not associated with high seismicity rate and active tectonics. However, in July 1926 a damaging and well-documented earthquake occurred there, followed by a strongly felt event in February 1927. In this paper, we reprocess macroseismic observations, analog seismograms and bulletin data in order to re-appraise the location and magnitude of these two earthquakes. We find that the macroseismic epicentre of the 1926 Jersey earthquake is shifted to the East when compared with the location offshore South of Jersey given in both the French database SisFrance and the recent French catalog FCAT-17. Arrival-times from published data, together with our own onset-time readings, are processed in order to obtain probabilistic hypocentral locations. The maximum-likelihood instrumental epicentre of the 1926 event is located about 15 km East of Jersey Island (49.20°N, 1.82°W). This epicentre is well constrained within a 10 km radius area. The location of the 1927 epicentre is slightly south of Jersey, at a location similar to that of SisFrance, but this latter epicentre is less constrained as fewer observations are available (the Probability Density Function for the 1927 epicentre is relatively “diluted”). Focal depth remains very poorly determined for both the 1926 and 1927 events. Analysis of historical seismograms is also performed in order to determine both surface-wave and moment magnitudes, MS and MW. We find MS = 5.6 ± 0.2 for the 1926 event and MS = 5.0 for the 1927 event. Waveform fitting of some of the highest quality seismograms of the 1926 event gives us MW in the range [5.0, 5.5], depending on the chosen focal depth and focal mechanism. The 1927 Jersey earthquake is expected with an intensity magnitude MI about 0.6 smaller. •In July 1926 a damaging and well-documented earthquake occurred there, followed by a strongly felt event in February 1927•The macroseismic epicentre of the 1926 Jersey earthquake is shifted to the East when compared with the location offshore South of Jersey given in both the French database SisFrance and the recent French catalog FCAT-17.•The maximum-likelihood instrumental epicentre of the 1926 event is located about 15 km East of Jersey Island (49.20°N, 1.82°W) and well constrained within a 10 km radius area.•Waveform fitting of some of the highest quality seismograms of the 1926 event gives us MW in the range [5.0; 5.5]•The 1927 Jersey earthquake