Sierra Madre Fault Zone

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.

[1] 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 evidence found at the Loma Alta trench investigation site brought new information into the deliberation regarding the maximum size of earthquakes near Los Angeles.

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) 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.