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SAN FRANCISCO -- If an earthquake, similar in size and origin to the magnitude 7.8 1906 San Francisco Earthquake, hit the same area today, the instant wreckage will include as many as 90,000 homes and 26,000 other residential properties destroyed or structurally damaged to the point of displacing as many as a quarter million people in 19 counties.

That's the grim bottom line from "When The Big One Strikes Again" a study released this week at the kick off of the three-day 100th Anniversary Earthquake Conference to recall the devastation of the 1906 quake and discuss measures necessary before the next Big One.

A Big One is coming.

The United States Geological Survey's "Putting Down Roots In Earthquake Country" says there's a 62 percent probability that a quake, magnitude 6.7 or greater, will hit the San Francisco Bay Area by 2032.

The number of casualties could rise to more than 62,000, according to the study released at the conference.

Vulnerable residential structures most likely to cause death or injury in a quake are:

  • Homes built before 1974, but especially those built before 1950, without the benefit of the latest seismic building codes and without seismic "retrofitting" -- improvements to upgrade a home to better withstand a quake. In San Francisco half the homes were built before 1950. In all the Bay Area counties, the vast majority of homes were built before 1974.
  • Homes and residential buildings with weak or "soft story" construction -- open, large, ground floor spaces that are largely unsupported, but have living spaces above. That includes condos and apartments with car ports or garages below living spaces; homes or any structure that has ground floors with large open spaces lacking interior walls or support; tall homes with two or more stories and living areas above a garage; unbraced large openings in the walls of a lower story, such as a two- or three-car garage door; and large decks.
  • Unreinforced masonry and concrete framed structures, few of which are homes, but also residential chimneys, which have not been retrofitted.
  • Manufactured homes without an "engineered tie-down system" or "earthquake-resistant bracing system."
  • Hillside homes not properly designed and anchored.
  • Homes in close proximity to active quake faults, especially those built on or near landfill or silty, sandy and loamy soils that can liquefy in a quake and amplify seismic energy.
  • Home with existing structural damage, caused by a host of factors including extensive termite infestation, mold, dry rot, flooding, soil movement, erosion or other conditions that can weaken the foundation and structural elements.

    Experts say the numbers of homes destroyed or rendered uninhabitable could be substantially reduced if property owners take steps now to help their homes shrug off seismic damage.

    Detailed steps to take are available from the Association of Bay Area Governments' "Home Quake Safety Toolkit" and the United States Geological Survey's "Putting Down Roots In Earthquake Country".

    A structural engineer's inspection initially may be necessary to determine what needs to be done for a given home, but steps generally include:

  • Bolting your home to its foundation at the sill -- the lowest point in the wood frame -- helps it stand up to quakes' lateral energy. "Hold downs" (heavy metal brackets) can be bolted to the ends of structural wall framing to protect against uplifting seismic energy.
  • Shoring up your cripple walls to further protect against lateral movement. Also called a "pony" wall, the cripple wall sits between the first floor and the foundation and generally surrounds a large crawl space, basement or garage. It should be reinforced with shear walls -- plywood properly installed on the inside -- to prevent the wall from buckling and folding over on its foundation during a quake.
  • Fastening together or strengthening the connections of multiple stories and bracing "soft story" construction (decks, garages and other open areas beneath closed in areas), pier construction and other weak points allows the property to move with seismic energy instead of breaking apart at the connections.

    Don't forget, the inherent rigidity of chimneys, masonry walls and other masonry structures, make them more susceptible to seismic activity and require the expertise of a mason or other knowledgeable contractor to inspect and retrofit, often with re-bar and other metal-laden materials.

    Tomorrow: "Reducing Quake Hazards Inside Your Home"

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