’Quake caused by deep slip

Press, Issue 32535, 19 February 1971, Page 9

’Quake caused by deep slip

<N.Z. Press Assn.—Copyright) LOS ANGELES, Feb, 18. Scientists said today that preliminary reports on last week’s disastrous earthquake showed that a piece of the earth’s crust lying 10 miles below the surface suddenly slipped, causing a rupture in the surface and setting off intense shaking. Mr Douglas Morton, a seismologist with the California Division of Mines and Geology, which is co-ordinat-ing studies of the earthquake, told a news conference that the slip of the crust caused the San Gabriel mountains to move upward 3ft and then southward another 3ft over the floor of the San Fernando Valley.

The preliminary report of what happened during the earthquake on February 9 that killed 64 people was based on data compiled by instruments and field studies by staff from universities and staff and Federal agencies. The crust movement was of the relatively rare faultthrust type, the scientists said, and explained why the Sylmar area of the valley was severely jolted, although the

earthquake’s epicentre was several miles away. The intense shaking at Sylmar caused a veterans’ administration hospital built in 1926 to collapse, killing 45 persons, and destroyed a new $23.5m county hospital. Several hundred homes were severely damaged and roads and utilities suffered extensive damage.

Scientists said that in Soledad Canyon, 10 miles north of the Sylmar area, a piece of the earth’s crust suddenly slipped at a point about 10 miles below the surface. The hunk of crust was lying at an angle running southward and upward. When the block of material moved, it caused a break in the surface just south of Sylmar in little Tujunga Canyon, setting up the intense shaking. Such thrust-fault earthquakes account for only about one in each 10 earthquakes in California. In most earthquakes a vertical crack, or fault, slips, causing one side of the crack to move horizontally relative to another. In these earthquakes, the epicentre and the area of maximum damage generally coincide. The movement of the San Gabriel mountains is a normal part of the mountain building process on the earth’s surface that has been going on for millions of years.