Significant Earthquakes and Faults
Chronological Earthquake Index
Sierra Madre Earthquake
Time: June 28, 1991 / 7:43 am PDT
Location: 34° 15.54' N, 118° 00.06' W 19 km (12 miles) northeast of Pasadena
Magnitude: ML 5.8
Type of Faulting: reverse
Depth: 11.97 km
This earthquake occurred on the Clamshell - Sawpit Canyon fault, an offshoot of the Sierra Madre fault zone in the San Gabriel Mountains. Because of its depth and moderate size, it caused no surface rupture, though it triggered rockslides that blocked some mountain roads. Roughly $40 million in property damage occurred in the San Gabriel Valley. Unreinforced masonry buildings, like those seen in the photographs below, were the hardest hit. Two deaths resulted from this earthquake -- one person was killed in Arcadia, and one person in Glendale died from a heart attack. In all, at least 100 others were injured, though the injuries were mostly minor.
The aftershock sequence of this earthquake was very small, with none of the aftershocks large enough to cause additional damage. This scarcity of aftershocks can be seen on a map of all earthquakes in 1991. Normally the symbol for a large earthquake is completely buried by the symbols of its aftershocks. That's not the case with the Sierra Madre earthquake; its symbol (the bright yellow circle) shows up quite clearly on the map.