POINT MUGU EARTHQUAKE

21 FEBRUARY 1973

More than 30 geologists, seismologists, and engineers convened in the evening in Oxnard, California, after the Point Mugu earthquake of 21 February. The purpose of the meeting was to share information gathered during the first-day investigation following the earthquake that had shaken a large part of southern California early that Wednesday morning.

The meeting was called by State Geologist Wesley G. Bruer and was co-chaired by Don Moran of the Earthquake Engineering Research Institute,in accordance with procedures developed by the Governor's Earthquake Council. Investigators included representatives of local, federal, and state agencies, universities, and private firms.

Although much smaller in terms of energy released, the Point Mugu earthquake of 21 February, with a Richter magnitude of about 5.75, bears several similarities to the 1971 San Fernando earthquake of magnitude 6.6. Both quakes appear to have taken place on frontal fault systems of mountain blocks in the Transverse Ranges-the San Gabriel Mountains in the San Fernando quake and the Santa Monica Mountains in the Point Mugu quake.

Preliminary analysis of early aftershock data by the U.S. Geological Survey and by Cal-Tech, suggests that the fault movement which caused the Point Mugu earthquake was of the reverse or thrusting type, as was the movement in the San Fernando quake.

Near Point Mugu, about 10 miles southeast of Oxnard, California, the western Santa Monica Mountains appear to have been thrust up and southward relative to the ocean floor near the coastline - possibly along the east-west trending Malibu Coast fault zone. This is similar to the movement that took place in the San Fernando quake in which the western San Gabriel Mountains were thrust up and southward over the San Fernando Valley.

Both quakes were the result of north-south compressional forces being exerted in the Earth's crust in the Transverse Ranges - mountain ranges bounded roughly by Santa Barbara, Los Angeles, and San Bernardino. The Transverse Ranges comprise one of the 11 geomorphic provinces of California. Many other faults, similar to the Malibu Coast fault and the San Fernando fault, exist in this area and are subject to these same forces.

"Although the Point Mugu quake was felt over a wide area of southern California - as far away as San Luis Obispo, Bakersfield, Barstow, Indio, and San Diego - the estimated $1 million in property damage is surprisingly small,'' said State Geologist Bruer. The 1971 San Fernando quake, felt over an area several times larger, inflicted damages amounting to more than $500 million. This difference is partly due to the less urbanized nature of the area near the epicenter of the Point Mugu quake.

Don Moran said the limited structural damage which did occur in the Point Mugu quake, was to old, unreinforced masonry buildings. "This again emphasizes the extreme life hazard presented by this type of construction," Moran said.

Damage to newer, earthquake resistive buildings was mostly confined to suspended ceilings, light fixtures, partitions, and windows. "There is a need for further investigation and code improvement relative to non-structural building components," Mr Moran added.

"The apparent greater depth of origin of the Point Mugu quake - estimated at 10 to 15 miles- relative to the 8-mile focal depth of the San Fernando quake may, in part, account for the fairly large area over which shaking was felt from the moderate Point Mugu quake," Bruer said.

Another contrasting feature is the apparent lack of ground rupture due to fault displacement in the Point Mugu area, although it is possible that as yet undetected ruptures may have occurred on the ocean floor. The San Fernando quake caused numerous fault breaks in the ground surface. (See CDMG Preliminary Report 11.) However, numerous rock falls, cracks and fissures, and sand boils caused by the shaking of unconsolidated ground were observed in the Point Mugu area. A maximum ground acceleration of 13 percent of gravity was recorded at Port Hueneme about 9 miles northwest of the epicenter.

Various scientific and engineering investigations continued after the first day. These included geologic field reconnaissance and the recording and analysis of seismological data from aftershocks. Several dozen seismological instruments of various types have been deployed by Cal-Tech, the California Division of Mines and Geology, the U.S. Geological Survey, the National Oceanic and Atmospheric Administration, and U.C. San Diego.

Analysis of the data gathered by these and other investigations should add to our knowledge about the mechanics of earthquakes and hopefully lead to more effective reduction of future earthquake losses.

In accordance with a recommendation of the Governor's Earthquake Council, the Los Angeles Office of the California Division of Mines and Geology, in conjunction with the Earthquake Engineering Research Institute, is serving as an information clearing house for the various post-earthquake investigations underway.

The accompanying map shows the general location of geophysical and seismological, geological, and structural effects as compiled after one day of field investigation by the earth scientists and engineers who attended the meeting.