 While the focal mechanism of the mainshock was right @-@ lateral fault slip on the northwest trending Imperial fault , a marked change in the distribution of aftershocks occurred with the onset of the Brawley aftershock , which exhibited left @-@ lateral slip . A distinct zone of aftershocks formed a belt from west of Brawley to near Wiest Lake , where sinistral motion on a northeast trending conjugate fault responded to an increase in tension at the northwest end of the Imperial fault . Another line of aftershocks along the projection of the southern San Andreas fault extended south into the valley up to 50 km ( 31 mi ) . Activity in that area of the valley had been aseismic through 1978 , and a few events occurred just prior to the event , and a significant increase in the amount of activity followed the mainshock . 
