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Situated at the boundary to the 
The fault was studied again in the late 1990s in the Loma Alta Park near Millard Canyon where a fault scarp larger than was accessible along a (late
Situated at the boundary to the San Gabriel Valley
The San Gabriel Valley ( es, Valle de San Gabriel) is one of the principal valleys of Southern California, lying immediately to the east of the eastern city limits of the city of Los Angeles, and occupying the vast majority of the eastern part ...
and San Fernando Valley, the Sierra Madre Fault Zone (also known as the Sierra Madre-Cucamonga Fault) runs along the southern edge of the San Gabriel Mountains for a total of , where the northwesternmost comprises the San Fernando Fault (the section responsible for the 1971 San Fernando earthquake
The 1971 San Fernando earthquake (also known as the 1971 Sylmar earthquake) occurred in the early morning of February 9 in the foothills of the San Gabriel Mountains in southern California. The unanticipated thrust earthquake had a magnitude o ...
). A 1980s paleoseismic study that included a trench investigation and mapping revealed that a major earthquake had most likely not occurred to the east of the San Fernando rupture area for at least the last several thousand, and possibly the last 11,000 years.
The 1971 event was the first in a series ( 1987 Whittier Narrows, 1991 Sierra Madre, 1994 Northridge) of damaging earthquakes which have occurred on reverse faults in the Los Angeles area. The events triggered discussions concerning the largest magnitude earthquake that could be generated by one of the faults, especially in the Transverse Ranges
The Transverse Ranges are a group of mountain ranges of southern California, in the Pacific Coast Ranges physiographic region in North America. The Transverse Ranges begin at the southern end of the California Coast Ranges and lie within Santa B ...
, but the focal point of earthquake hazard assessments in California are often the San Andreas Fault
The San Andreas Fault is a continental transform fault that extends roughly through California. It forms the tectonics, tectonic boundary between the Pacific Plate and the North American Plate, and its motion is Fault (geology)#Strike-slip fau ...
and other associated dextral
Sinistral and dextral, in some scientific fields, are the two types of chirality ("handedness") or relative direction. The terms are derived from the Latin words for "left" (''sinister'') and "right" (''dexter''). Other disciplines use different ...
faults. Although there is a lack of paleoseismic data on reverse faults in the Los Angeles area, a trench excavation at a site on the Sierra Madre-Cucamonga Fault revealed that two large historic earthquakes occurred in the last 15,000 years.
The fault was studied again in the late 1990s in the Loma Alta Park near Millard Canyon where a fault scarp larger than was accessible along a (late quaternary
The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million years ...
) elevated stream terrace. The clearly defined fault was exposed in the trench and emerged as a band of coarse gravels lining the hanging wall
In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...
. By studying the truncated rocks and a wedge-shaped accumulation of gravel and soil, it was possible to visually reconstruct the original geometry of the rock prior to the thrust and eventual and partial collapse of the hanging wall back onto the footwall. An estimate for the maximum slip of the event was given as .
The evidence found at the Loma Alta trench investigation site brought new information into the deliberation regarding the maximum size of earthquakes near Los Angeles. The large amount of slip observed there did not correspond with a short rupture length of the Sierra Madre Fault Zone, and instead suggested that the historical thrust earthquakes were much larger in magnitude than what was seen with the 1971 event, given its smaller of maximum observed displacement. Two methods were employed to infer the scope of the events at the site (one regression-based and the other based on the seismic moment Seismic moment is a quantity used by seismologists to measure the size of an earthquake. The scalar seismic moment M_0 is defined by the equation
M_0=\mu AD, where
*\mu is the shear modulus of the rocks involved in the earthquake (in pascals (Pa) ...
) and produced a maximum magnitude of 7.5 or 7.6 for the most recent movement of the fault. The results supported an earlier hypothesis that seismic energy release on the Sierra Madre Fault Zone is characterized by infrequent but large earthquakes. A duplicate event in modern times would rupture to the south towards populated areas and would produce strong ground motion
In seismology, strong ground motion is the strong earthquake shaking that occurs close to (less than about 50 km from) a causative fault. The strength of the shaking involved in strong ground motion usually overwhelms a seismometer, forci ...
capable of damaging modern buildings and other critical infrastructure.
References
{{Faults Seismic faults of California San Gabriel Mountains Geology of Los Angeles County, California Geography of the San Fernando Valley Natural history of Los Angeles County, California