 The pattern of displacement along the Imperial Fault was very similar to that observed for the northern part of the rupture during the 1940 El Centro earthquake , although on this occasion the rupture did not extend across the border into Mexico . This had been explained as the behavior of individual slip patches along the Imperial Fault with two patches rupturing in 1940 and only the northern one in 1979 . The faulting that gave rise to the earthquake has been modeled by comparing synthetic seismograms with near @-@ source strong motion recordings . This analysis showed that the rupture speed had at times exceeded the shear wave velocity , making this the first earthquake for which <unk> rupture was inferred . 
