Ground-motion difference between two moderate-size intraplate earthquakes in the United Kingdom

Two moderate-size earthquakes occurred in the United Kingdom, the first near Folkestone in 2007 with Mw 4.0 and the second near Market Rasen in 2008 with Mw 4.5. Both were strongly felt and caused some nonstructural damage. The earthquakes occurred at significantly different depths, the Folkestone earthquake at 5 km and the Market Rasen earthquake at 20 km. We determined the seismic moment and the stress drop of the two mainshocks, and two smaller earthquakes in the same locations, by modeling the source displacement spectra. We found stress drops of 30±34 bar and 344±136 bar for the Folkestone and Market Rasen mainshocks, respectively. This is a significant difference considering the earthquakes are only 275 km apart and both are of intraplate origin. We applied the stochastic ground-motion modeling technique and used the stress drop and seismic moment to compute vertical component peak ground acceleration. The modeled ground motions are consistent with the observations. We also computed vertical peak ground acceleration for a hypothetical Mw 6.0 high stress-drop (200 bar) earthquake and found that it would be 4.6 m/sec2 at 20 km hypocentral distance.