San Diego Seismic Study Shows ‘High Potential’ for Liquefaction in Midway District

By Virginia Adams Wilson

On the official sandiego.gov website you may find the Seismic Safety Study, a map of geologic hazards.

This information is valuable when choosing a location suitable for development. Instinct or common sense would have us avoid areas marked as hazardous, such as the Midway District of San Diego, labeled “High Potential” for liquefaction.

Liquefaction occurs where a loose sandy soil surface is close to the level of the water table below, and earthquake vibrations stir up the sand and water. The resulting slurry acts as a temporary quicksand. Buildings are thrown off balance and sink in at odd angles. Gas and electrical lines are torn loose, and fires erupt. Roads and bridges are broken, making evacuation difficult. Water mains fail, cutting off the supply of fresh water for days or weeks.

San Francisco’s Marina District suffered extensive damage during the 1989 Loma Prieta earthquake, due to liquefaction, 90 miles from the epicenter. Note that the buildings in the Marina District were mostly three story (30 feet), with a small percentage being four story.

Enlargement of section of map showing it is labeled #31.

Now try to visualize a new neighborhood, just the first 4,250 homes, nestled in the former wetlands between our San Diego River and the San Diego Bay, with the Point Loma fault on the West side, the Rose Canyon fault zone on the East, the Spanish Bight fault to the South. Remember the scattered network of smaller faults throughout Point Loma to the Southwest.

Take a deep breath and relax, aware that the San Andreas fault lies just 100 miles away, and that the 4,250 homes are centrally located within thousands of acres of land known to be a Geologic Hazard with “High Potential” for liquefaction if an earthquake were to happen. We have seen the Marina District, the destruction of its housing built 30 to 40 feet high; but this is San Diego’s future we are imagining.

Partial legend for map shows area #31 has “High Potential” for liquefaction.

Allow the first 4,250 housing units to form buildings of any height they wish to be. Observe as the buildings cluster together, assuming various heights: 3 story, 3 story times three, 3 story times five. (Well, surely, 150 feet is high enough.)

Now for the hard part: contemplate, if you dare, the consequences of the inevitable earthquake occurrence. As above: “quicksand, damage, injury, gas leaks, fires, no power or water, and no way out”, except with the wreckage of much taller buildings, fortunately occupied by just the first 10,000 or so residents of our imaginary neighborhood.

We cannot predict when such an earthquake will strike. It may be years or even decades away. But if this seismic hazard area is open to development allowing population density similar to the current proposal, the scale of a potential disaster will expand as time passes. The expected population growth is more than an abstract number. It should be viewed as the additional number of lives endangered by reckless development.

Virginia Wilson is a member of the OB Planning Board and a former member of the San Diego Community Forest Advisory Board.