State Map Details Valley’s Geologic Makeup

To mark today’s 30th anniversary of the devastating San Fernando-Sylmar earthquake, the state Department of Conservation released a new map Thursday identifying in great detail the geologic materials in the San Fernando Valley and how they respond to seismic activity.

With the map, geologists and engineers have the most complete picture to date of the Valley’s unique geological makeup, giving them another tool to evaluate an area’s susceptibility to shaking during an earthquake, experts say.

“This is the background data that will help engineers and geologists who will do further studies,” said Christopher J. Wills, a senior engineering geologist for the Department of Conservation’s Division of Mines and Geology.

“This map gives geologists, engineers, builders and the general public a jumping-off point to better understand what materials are out there and where,” he said. The information, for instance, will help in determining building locations and retrofitting.

The map uses color gradations to differentiate soil types that have developed in the Valley over the past 1.8 million years, Wills said.

The types of soil most vulnerable to shaking during an earthquake are artificial fill, man-made basins, silt, sand and clay, Wills said. The most stable in a temblor are sedimentary rock and crystalline rock.

For the most part, the Valley soil is a combination of all of the most vulnerable types, covering a wide swath from Calabasas to Burbank and from Sylmar to the Santa Monica Mountains.

“We can see that the San Fernando Valley is a trough, where over thousands and thousands of years the mountains have come up and the Valley floor has dropped and filled with sediment . . . which is prone to liquefaction,” he said. Liquefaction occurs when ground water and such soils as silt or sand mix during an earthquake.

Wills said the map is not to be used as a divining rod to determine whether one area is more dangerous than another, but rather as an informational tool for residents, builders and public safety officials to use in mitigating earthquake damage.

He also said geology enthusiasts may be interested in learning what lies beneath their homes and how it got there.

The information for the map was obtained over five years from records filed by work crews digging trenches for sewer lines, building foundations or investigating underground storage tank leaks.

“We used all the information we could find in describing the [geology] out there,” Wills said.

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The map’s detailed information expands the data used to prepare the Valley liquefaction hazard zone maps released in 1998 and 1999, Wills said. The Department of Conservation regularly updates its Web site as new information is obtained. The department’s maps are available on paper or on its Web site (https://www.consrv.ca.gov).

In addition to the Valley soils map, the Department of Conservation unveiled another map showing the epicenters of potentially damaging earthquakes--magnitude 5.0 or greater--that California has recorded since 1800.

By combing through journals, diaries, official records and newspaper articles, Tousson Toppozada, a leading expert on California seismology, compared shaking activity from early quakes to modern temblors to develop a map of the most devastating quakes in the past two centuries.

“The new map allows us to plot the epicenters, record their magnitudes and see where some areas overlap,” Toppozada said. “Some areas in Los Angeles and the San Francisco Bay have been damaged six times over.”

The map indicates California, on the average, has had a magnitude 6 or greater earthquake each year since 1850, he said. It also shows some areas have been relatively quiet seismically in the past 90 years, but were active in the previous 100 years.

The epicenter map includes the magnitude 6.7 San Fernando-Sylmar quake, which occurred 30 years ago today. The temblor resulted in 65 deaths, more than 2,500 injuries and $550 million in damage.

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The earthquake destroyed Veterans Hospital, where two major buildings collapsed, killing 49. The hospital was never rebuilt, and the land on which it stood is now Veterans Memorial Park. Olive View Hospital was also leveled in the quake, and has since been rebuilt.

While there have been more disastrous earthquakes, perhaps no other temblor has led to more advancements in seismic safety.

“The San Fernando quake triggered earthquake zoning maps so that we would avoid building on top of or astride fault lines,” Toppozada said. “We started retrofitting and strengthening bridges and freeway overpasses, and there was more emphasis on public safety.”

Although the historical map does not advance earthquake prediction efforts, Toppozada said the map is extremely useful in long-term hazard and risk assessment.

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Soil Stability During an Earthquake

With this new map, geologists and engineers for the first time have a complete picture of the Valley’s unique geological makeup, giving them another tool to evaluate an area’s susceptibility to shaking during an earthquake. Scientists reviewed soil samples from 40 to 50 feet below the surface.

For the most part, Valley soil is a combination of silt, sand, clay, man-made basins and artificial fill--the soils most vulnerable to shaking during an earthquake--covering a wide swath from Calabasas to Burbank and from Sylmar to the Santa Monica Mountains.

The most stable soils during a temblor include sedimentary rock and crystalline rock.