Re: the 1979 6.4 quake:
“The Imperial Valley is surrounded by a number of interconnected fault systems and is vulnerable to both moderate and strong earthquakes as well as earthquake swarms.
“Four of the region’s known strike-slip faults and one additional newly discovered normal fault all broke the surface during the earthquake.
“The earthquake was caused by rupture along parts of the Imperial Fault, the Brawley fault zone and the Rico Fault, a previously unknown normal fault near Holtville, though slip was also observed on the Superstition Hills Fault and the San Andreas Fault.”
Thank you.