Will Your House Survive an Earthquake? Only a Few Factors Affect Your Fate

This is the first post in a two-part series.

When I meet someone for the first time and mention I’m a structural engineer, they usually nod blankly and change the subject.  But occasionally I get the polite question: “So – will my house withstand the Big One?”

Only a few factors affect your fate.

Post-quake observations show that outcomes are typically ‘all or nothing’ – at least for the wood-framed houses typical in the US, Canada, and New Zealand.

Damaged chimney at my friend Alex's house after the Christchurch quakes

Damaged chimney at my friend Alex’s house after the Christchurch quakes

In real life, quake damage is “nothing, nothing, oops,” as my friend Tom Larsen likes to say.  In an earthquake, most houses experience ‘nuisance’ damage like fallen chimney bricks, diagonal cracks above doorways, and broken dishes.  Only a small minority experience a ‘total loss.’

Let’s look at some numbers:  In the 2011 Christchurch, New Zealand earthquake (a good comparison for reasons stated below), initial reports estimated that 10,000 houses would be demolished, in a region of about 160,000 total houses.  A later report estimated 8,000 demolitions.  That means 5%, or only 1 in 20 were a total loss.

An ‘Oops’ can happen for any of the following conditions (again this applies only to wood-framed houses).  The good news is that most of these conditions still don’t jeopardize your safety.

If you have any of these conditions, then fix it if you can.  Until then, be sure to keep your camping gear where you can access it after the quake!  And, if you don’t already, consider carrying conventional insurance (not just Jumpstart).

UNBRACED WATER HEATER: The gas water heater is not strapped to the wall, which means it can topple and cause a fire.

Cripple Wall Retrofit image courtesy Simpson Strong-Tie

Cripple Wall Retrofit image courtesy Simpson Strong-Tie

UNBRACED CRIPPLE WALL: The house can slide off its foundation, either because:

  • The house has a brick foundation, or
  • The framing is not properly connected to the foundation. This includes bolts from the wood framing down into the concrete, as well as sheathing (plywood) around the perimeter of crawl space, if there is one.

Fix: A cripple wall retrofit with bolts and sheathing typically costs less than a bathroom remodel.  (Replacing a brick foundation can be more.)  Some cities even offer partial rebates.  Next time you schedule work on the house, add a seismic retrofit to the list.

GEOLOGIC HAZARD ZONE: The house is located in one of the following:

  • Active fault zone (Alquist-Priolo zone)
  • Landslide susceptibility zone or on an active landslide
  • Liquefiable soil – unconsolidated soil that can ‘turn to quicksand’ when shaken. This includes, but is not limited to, most sites on ‘reclaimed land’ or ‘land fill’ (not ‘landfill’).
  • Rockfall zone where boulders could tumble from above.

Fix? A geologic hazard is not so easy to fix.  But it was disclosed when you bought the house, right?

example of a soft story apartment building with tuck-under parking

example of a soft story apartment building with tuck-under parking

SOFT STORY: The house has a ‘soft story’ where the first story has few or no walls. This occurs more often for apartment buildings and is less common for single-family houses, except those ‘on stilts.’

  • Soft story conditions can be retrofitted, but each case is different.

URM: The structural walls of the house are made of unreinforced brick masonry (URM).

  • Again, retrofits are possible but each case is different.  Some cities have URM ordinances.

Back to Christchurch: of the 185 fatalities, three were in houses, based on available information.  And all three of those were caused by rockfall.  But there were multiple injuries due to bricks falling from chimneys.  Please don’t sleep near a brick chimney!  Next time you re-roof, replace it with a lightweight enclosure.

From these descriptions, you can see that many – no, most houses don’t have a major vulnerability that could cause a total loss.  Perhaps that 1 in 20 figure is pretty near the mark, but definitely have a look at your cripple wall.

Over to you: Do you think your house will survive an earthquake?  Are there other conditions you’re curious about?  What have you done to protect yourself?

 

Postscript:

I used Christchurch as a point of reference for two reasons:  First, the construction materials and methods are similar to that in the USA and Canada, in contrast to earthquakes in Turkey, Japan, or Chile.  Second, it’s ‘clean’ data – all of greater Christchurch experienced roughly the same shaking, as opposed to the 1989 and 1994 earthquakes in California, where we would have to argue about what proportion of houses actually experienced strong shaking.  Plus, it’s more recent than the California quakes.

The 2010-2011 series of earthquakes in Christchurch included a Magnitude 6.3, six miles from the city center; a Magnitude 7.1, five months earlier, 25 miles west of Christchurch; and several M5+ aftershocks in the months following the M6.3.  All the fatalities and most of the damage occurred in the M6.3 event.