Location
The Southern California Earthquake Data Center reports that the epicenter was on the other side of the San Gabriel Mountains from Sylmar, as indicated on this Google Map. This Instrumental Intensity Map identifies the same location using a star shaped symbol "centered in Iron Canyon, in the Sand Canyon area of Canyon Country". These maps help explain why the seemingly distant cities of Sylmar and San Fernando received the brunt of the damage: the quake energy was focused within the sparsely populated San Gabriel Mountains, extending slightly into the northeastern end of the densely populated San Fernando Valley.
Read more about this topic: 1971 San Fernando Earthquake