from California Geology, April/May 1971, Vol. 24, No. 4-5.

Special San Fernando Earthquake Edition

THE 1971 SAN FERNANDO EARTHQUAKE

An hour before sunrise, at 6:01 AM, PST, on February 9, 1971, the San Fernando region was struck by one of the most devastating earthquakes in California history. Although the Richter magnitude of the tremblor was 6.6, ranking it as moderate to large, but not great, it shook a wide, heavily populated area, leaving death and destruction in its wake.

As we go to press one month later, 65 lives have been charged to it, and damage has been estimated at more than half a billion dollars. It was California's third worst earthquake in terms of lives lost (exceeded by San Francisco, 1906 and Long Beach, 1933) and second in terms of property damage (exceeded by San Francisco, 1906).

The greatest damage was in the San Fernando area, near the front of the San Gabriel Mountains, where three hospitals were badly damaged (one of them accounting for the greatest loss of lives). Freeway interchanges collapsed (killing two men in a pickup truck under one fallen overpass), reservoirs were in danger of imminent failure, forcing people living below them to evacuate, and houses and commerical buildings collapsed or caught fire.

The earthquake was not, or should not have been, unexpected. In the southern California area--from Point Arguello to Nevada and from the Mexican border to Owens Valley--seismic records suggest that 37 shocks of this magnitude or greater might be expected in a century; indeed, one might expect two shocks with a magnitude of 8.0 or more.

By the time the sun had risen, ten geologists from the Los Angeles District Office of the California Division of Mines and Geology were in the field to appraise the geologic effects and to search for further potential hazards created by the earthquake. They paid particular attention to hillsides that might give way in aftershocks, causing more havoc than already existed. They were supported by aircraft: a fixed wing chartered plane and a helicopter provided and manned by the U. S. Marine Corps. Peter Fischer, of Whittier College, had the foresight and was instrumental in making arrangements for Marine Corps support.

When it was apparent that the landslide potential of the hillsides, though great, did not threaten any urban areas, the geologists turned their attention to the study of geologic effects--so often ephemeral--and to other geologic hazards that may have been developed or increased by the earthquake.

A preliminary report of their investigations is presented in this magazine. Besides those credited as authors, virtually all staff members in the Los Angeles office participated in the study: Wilma Ashby, Pat Caldwell, George Cleveland, Bill Edgington, Jim Evans, Don Fife, Cathy Govaller, Cliff Gray, Ed Kiessling, and Paul Morton. Charlie Bishop came from San Francisco, as well as several staff members from Sacramento-Quint Aune, Chief Wes Bruer, Bob Matthews, and Bennie Troxel. Messrs. Gray and Caldwell went immediately to the Los Angeles District Office where they maintained constant contact with field, administrative, and disaster staffs.

Three Division geologists had an especial interest in the area, as well as especial expertise. Gordon Oakeshott, Deputy Chief had mapped part of the San Fernando 15-minute quadrangle in doing research for his doctoral dissertation in 1936; he revised and extended the map for publication in 1958 as California Division of Mines Bulletin 172, Geology and mineral deposits of the San Fernando quadrangle, Los Angeles County, California. By that time, much of the area had been built over; new base maps were available, and many new roads provided road cuts to give new looks at the geology. The 1971 earthquake has enabled him to add another facet to the study of the area he now knows so well.

In 1962, the Division inaugurated a study of the frontal fault system along the San Gabriel Mountains. As one portion of the study, Dick Saul mapped in detail the southeast quarter of the Oat Mountain 71/2-minute quadrangle, through which the Santa Susana thrust fault cuts obliquely. Upon revisiting the scene of his earlier mapping, Mr. Saul was able to recognize that movement had taken place on the Santa Susana thrust, in the Bee Canyon area.

In 1969, the California Division of Mines and Geology published Map Sheet 15, entitled Preliminary reconnaissance map of major landslides, San Gabriel Mountains, California, by D. M. Morton and R. Streitz. The landslides they mapped included those in the eastern half of the San Fernando quadrangle. Dr. Morton's paper herein describes an additional 1,000 landslides generated by the earthquake--most of them west of the area previously mapped, but many in Big Tujunga Canyon, the site of already mapped slides.

On the day following the earthquake, State Geologist Wesley Bruer convened a meeting of more than 40 geologists and seismologists engaged in studying the earthquake, so that all investigative activities might be coordinated. Among those attending, in addition to Division personnel, were representatives from the California Institute of Technology, the U S. Geological Survey, the National Ocean Survey (a part of the National Oceanic and Atmospheric Administration), the University of California at Santa Barbara, the University of Southern California, California State College at Los Angeles, and the University of Washington; from several state agencies-the Division of Oil and Gas, the Division of Highways, the Department of Water Resources; from Los Angeles County and from Los Angeles City; as well as from several private consulting firms.

The California Division of Mines and Geology is continuing to act as an information exchange center for those who have been and are engaged in geologic, se is mo logic, and geodetic investigations of the earthquake. This early and continuing exchange has added significantly to the effectiveness of the investigations.

It was an expensive and heartbreaking lesson, but California has learned something, and can take heart. Olive View Hospital, a county-owned structure, is such a lesson. Although the county engineer has rated the medical care and treatment center and the psychiatric unit as total losses, for an estimated $31 million dollars, 615 patients and 300 staff members were safely evacuated. Only three lives were lost: two were patients who died when forcibly cut off from their positive pressure breathing equipment, and one staff member was killed by falling debris outside the building. The structure had been built according to earthquake-resistant code provisions; mapped faults, as shown by Dr. Oakeshott on his San Fernando quadrangle, were considered in the engineering design. There was no movement on the faults near the hospital; damage to the building has been attributed to the intense shaking to which it was subjected.

But what we have learned is far surpassed by our astonishing luck. Had shaking of the endangered reservoirs continued for 2 seconds more, it has been estimated that there would have been no time to evacuate those below. Had the earthquake hit at rush hours, when the streets and freeways were full of traffic; had it hit during school hours, when schools were filled with children--our grief might be much greater.

We must hurry to learn what we can, and to put to use what we know, while our luck holds.... M.R.H.