Liquefaction Risks

* The letter from Prof. C.R. Berger (Dec. 11) expresses concern that recycling water by underground water recharge in the San Fernando Valley could significantly increase damage potential during earthquakes due to liquefaction.

Liquefaction induced by an earthquake can be a danger only when the water table is close to the ground surface. But due to many years of pumping ground water in the Valley, the water table in the areas where recharge is practiced (the East Valley) is hundreds of feet below the ground surface. So there is no cause for worry about liquefaction in those areas. Artificial recharge will never be able to bring the water table back to a shallow depth in the East Valley, because there never will be that much water available for recharge.

Insofar as depth to the water table is concerned, there is more danger of liquefaction in the West Valley, where the water table is much closer to the surface. But no artificial recharge is practiced there, because almost all of the soil in the West Valley is clayey and does not accept water rapidly. And clayey soils are not as susceptible to liquefaction as the sands of the East Valley, so few areas of the West Valley are in danger of liquefaction.

IRVING SHERMAN

Woodland Hills