Lecture Series to Focus on Naural Hazards : Geologic History of S.D. Might Surprise Most

Someday there is going to be a major earthquake in San Diego that will cause substantial damage to homes and businesses throughout the county.

And someday sooner, there will be a serious flood in Mission Valley that will cause hundreds of millions of dollars in losses.

And someday, there will be a large landslide in a hillside region, causing many structures to tumble into wreckage.

The geological forecast is plain, says Prof. Patrick Abbott of San Diego State University, even though it would probably surprise most San Diegans.

San Diegans pave over problem soil, build in riverways and cut housing pads from hillsides without sufficient consideration for the probability that some day a cataclysmic event will take place there, Abbott said in an interview. Mother Nature acts according to a clock very different from the one that governs modern civilization. A devastating earthquake or flood happens only once in every 100 or perhaps 1,000 years, but it is the result of millions of years of geological processes. Modern San Diego, however, has sprouted up only in the past 100 years or so. Having experienced no geological disaster here, many San Diegans find it difficult to think of Mission Valley as a natural watercourse that has flooded often (there was a large flood in 1916), to see many hillsides as soft materials inimical to firm foundations, and to understand that the many finger canyons represent subfaults indicative of past earthquake activity.

Abbott, chairman of the geological sciences department at San Diego State, will be the keynote speaker for a monthlong lecture series on geologic hazards in San Diego that begins tonight at the Natural History Museum in Balboa Park.

The risks are not such that people should leave the area, Abbott said. “Rather, what they mean is that I and others should understand more about the processes that formed the area and hope that (such an understanding)will lead to better developments, (which)are inevitable in any case.”

Abbott, who is a native San Diegan, believes that many people have lost touch with the way their physical surroundings behave as a result of natural forces such as rain. This has been especially true since World War II, with the rush to big-city development.

Now, rather than seeing Mission Valley as the bottom area where the water from the San Diego River ultimately collects, residents see it as a place where condominiums can be built next to shopping centers.

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Abbott calls Mission Valley one of the best examples worldwide of how not to treat a river valley. Abbott said that the storage upstream at San Vicente and El Capitan dams would be insufficient to contain a massive rainfall.

One reason for this, he said, is that “as we have paved over the soil, the rain does not seep into the ground and move gradually toward rivers, but flows immediately, resulting in perhaps floods of shorter duration but of higher crests.”

Already, Abbott said, smaller amounts of rain cause closure of north-south local roads in Mission Valley because the water collects and rises more quickly.

Abbott said the spacing as well as amount of rainfall is important in predicting a flood, recalling the warnings to evacuate Mission Valley in February, 1980, when a major storm was predicted to follow several severe storms earlier in the season that had filled the reservoirs to overflowing. The storm instead hit near Ensenada, Mexico, sparing the valley.

San Diego has learned some lessons from the mistakes it has made in Mission Valley, Abbott said, citing the city’s decision to leave the Tijuana river valley undeveloped

Although Abbott says his views on landslides are controversial, he said he personally would never buy a house built on a fill dirt foundation or on a slope.

“Look,” he said, “soil is simply rock that has been broken down by weathering, and what you are doing with fill is trying to compact dirt over a matter of hours versus natural processes that occur over millions of years’ time,” and are therefore stronger.

Soil engineers will disagree with him on the danger of building on soft soils, Abbott said, because they believe that soil can be compacted adequately, and that its suitability for construction can be determined by soil strength tests and improved should those tests show it is necessary. “I’m saying that too often developers pick geologically bad sites and that no matter the engineering done, it is not always sufficient,” he said.

Several kinds of soils can be risky to build on, but Abbott considers clay one of the riskier because it expands easily when it becomes saturated with water. Some of the areas in which clay predominates are Fletcher Hills, Poway, Santee and Rancho Bernardo. The granite or composite soils, found in Mt. Helix and in Del Cerro, are considered safer to build on.

Until recently, Abbott noted, developers found that it was easier to build on the city’s mesas, where the land is firm and soils hard, rather than to attempt to excavate hillsides, which would involve moving millions of cubic yards of soil to cut pads and put in dirt fill.

“So much of what was easiest in the past turned out to be the safest,” Abbott said.

San Diego sits on of the world’s most active earthquake zones, although the area has experienced fewer earthquakes than the Los Angeles or San Francisco areas. Fault lines run throughout San Diego;they are in, for example, the Florida Canyon, the canyon walls of Texas Street running southward out of Mission Valley, and Rose Canyon. Vegetation and construction can obscure fault locations, Abbott said.

Abbott noted that information recently compiled on two major earthquakes in the county--one was near Oceanside in November, 1800, and the other in the San Diego Bay in May, 1862--shows that large temblors have occurred in the past and will occur again, he said.