Tarzana Jolt : Shaking Under Nursery Is Among Strongest Ever

For seven or eight seconds after the Northridge earthquake, the ground beneath the Cedar Hill Nursery in Tarzana shook as violently as almost any location anywhere in recorded seismic history.

It shook hard enough to send buildings airborne and hard enough to launch seismologists from around the world on a quest to explain what they consider to be one of the most puzzling aspects of the devastating temblor.

Earthquakes are generally measured on the familiar scale of magnitude, but the tale of how the ground actually shook--in what direction, how fast and how powerfully--is told by hundreds of devices called strong-motion accelerographs deployed in buildings and in the open throughout the region.

One accelerograph, on a hilltop ranch once owned by author and Tarzan creator Edgar Rice Burroughs, recorded an up-down jolt of 1.82g or nearly twice the strength of gravity. Adding to the data’s significance is the fact that the instrument was bolted into shallow rock, meaning that the reading was not amplified as it would have been if it had been located inside a building.

The instrument also measured several spikes as high as 1.5g in all directions during a period of nearly eight seconds during the 4:31 a.m. Jan. 17 earthquake.

Not only were they the highest ground-motion readings from the earthquake, they were also among a handful of the highest ever recorded. Moreover, seismologists say that sustained shaking of that magnitude is unprecedented.

Even so, houses in the area sustained only minor damage--cracks in walls and driveways and some broken windows.

By studying why the readings were so high, seismologists hope to better understand earthquakes as well as the forces they exert on structures of all types.

“It’s peculiar, and now they want to find out why it is peculiar,” said Ralph Herman, whose family has lived on the ranch and nursery since 1943.

The readings have attracted experts to Herman’s nursery from Japan, New Zealand, England and Africa. Seismologists, engineers and technicians from the U.S. Geological Survey and the state’s Strong Motion Instrumentation Program have been there daily since they saw the abnormally high initial readings and have installed dozens of other accelerographs. They have also tested the instrument that took the first reading.

They used the additional instruments to measure ground motion from aftershocks as well as from artificially induced shear waves--the waves of earthquake energy that cause the most damage. That work ended last weekend.

So far, they have ruled out an instrument malfunction and are confident that the accelerograph was properly anchored so that it measured actual ground movement rather than an effect caused by loose soil.

Rufus Catchings, a Geological Survey seismologist who is overseeing some of the testing, said a “very, very preliminary” analysis of the new data indicates that the high ground-motion readings were quite localized.

“It doesn’t appear to extend very far from that site, so most of Tarzana itself seems fine,” he said.

Further explanation will await the creation of a three-dimensional computer model of the local and regional geology and possibly years of analysis, said Tony Shakal, a seismologist who heads the state’s strong-motion program.

“There’s plenty of room for speculation and I’m sure there is a great deal of it going on,” Shakal said.

The key question is why the ground under the ranch moved as much as it did and whether something there attracts or enhances an earthquake’s energy.

Shakal’s hunch is that the answer will be found in some quirk of local geology. He said the ranch is in an area in which the alluvial soils of the San Fernando Valley give way to the foothills of the Santa Monica Mountains. A layer of siltstone is 19 to 33 feet beneath the ranch.

So far, he said, “we don’t know how much the unusual record . . . was due to the site and how much was due to the (strength or fault movement of the) earthquake.”

This is not the first time the Tarzana site has been the focus of scientific curiosity.

In the magnitude 5.9 Whittier Narrows quake in 1987, the accelerograph at the nursery recorded ground motion of 0.65g, which was stronger than what was picked up by instruments near the quake’s epicenter.

Scientists also studied the site after that quake and determined that it was stable and that the readings were accurate.

Nothing unusual was seen there in other earthquakes, such as the magnitude 7.6 Landers and magnitude 6.7 Big Bear quakes in 1992 or the magnitude 5.8 Sierra Madre quake in 1991.

Seismologists began measuring actual ground movement in 1930. During the Long Beach earthquake three years later, motion equal to one-third the force of gravity was recorded.

The maximum ground-motion readings increased gradually with subsequent earthquakes, topping 1g for the first time at the Pacoima Dam during the 1971 Sylmar quake.

Each time a record was set, Shakal said, seismologists were surprised and skeptical. He said the Tarzana reading “was a higher value than we had expected but recording the forces of ground movement . . . during actual earthquake shaking . . . is a new idea” in scientific terms.

The greatest value of ground-movement readings is for earthquake engineering.

If engineers can predict ground movement, they can design buildings to resist it.

Civil engineers say the high readings from several locations in the Northridge earthquake, including those in Tarzana, may cause them to reconsider how buildings, dams, freeway overpasses and other structures are designed.

Ground Force Earthquakes are most commonly thought of in terms of magnitude, a general measure of a temblor’s size. How the ground actually shook-in what direction, how powerfully and for how long-is measured by sensitive but relatively simple devices called strong-motion accelerographs. The readings show the relationship to the force of gavity. 1 G = a force equal to that of gravity. Newhall Vertical movement: .62V Horizontal movement: .63H Sylmar Vertical movement: .60V Horizontal movement: .91H Tarzana Vertical movement: 1.8V Horizontal movement: 1.2H Arleta Vertical movement: .59V Horizontal movement: .35H Santa Monica Vertical movement: .25V Horizontal movement: .93H Century City Vertical movement: .15V Horizontal movement: .27H Los Angeles Vertical movement: .07V Horizontal movement: .19H Source: California Department of Conservation, Researched by RICHARD COLVIN / Los Angeles Times