Shaken Awake – Do I Even Need a Retrofit?

If you would like to see exactly where your house is in relation to the Hayward Fault go to this webpage, click “Hayward Fault Map and Tour“, download the kmz. file, and on the upper left type in your address.  The click on SEARCH.  

In 1996 the National Science Foundation funded a project so as to forecast how many people will be displaced if large earthquakes occurred on any of the 11 earthquake faults in the Bay Area and published its findings in a report called Shaken Awake.  The full text of THIS report can be seen here.  The authors of this project included The Association of Bay Area Governments, Bay Area Regional Planners, Washington State University, The United States Geological Survey, Structural Engineers Of California , The California Office of Emergency Services, and relevant agencies.  This effort identified the number of buildings that would be red tagged as uninhabitable after the various Bay Area earthquake faults ruptured so as to calculate the number of people that would require temporary government housing.

The two most important factors that determined whether or not a building would be red-tagged was the type of construction and the projected shaking intensity at its location based on the Modified Mercalli Intensity Scale.  The study included both apartment buildings and 1-3 story houses.

VIDEO ON THE MODIFIED MERCALLI SCALE

The buildings were separated into two groups: houses built before 1940 and those houses built after 1940.  This information is important because we can identify which houses are most at risk and in need of a seismic retrofit.  Once we factor the shaking Modified Mercalli Scale shaking intensity zone together with the year of construction we can determine the risk of damage and therefore the need for a seismic retrofit.  .   The images below were taken directly from Shaken Awake. 

Here are the various faults Shaken Awake looked at and the page number where you can see what they predict.

The damage statistics found in Shaken Awake’s tables assume the worst case scenario.  For example, historically the Northern Hayward Fault has never generated an earthquake larger than a 7.1 on the Richter Scale.  Shaken Awake looked at the  projected impacts created by by this 7.1 worst case scenario earthquake and no greater.  This is what they forecast.

Forecasting damage is a difficult task even under the best of circumstances.  Fortunately, the authors of Shaken Awake had extensive data available to them from both the 1989 Loma Prieta and the 1994 Northridge earthquakes.  This data was used in forecasting the impact of earthquakes along the 11 faults found in the Bay Area and in some cases was remarkably accurate.  Using this data to forecast damage is referred to as modelling.

As previously noted the projected damage to all the structures Shaken Awake is based on two factors: the projected shaking intensity provided by the USGS and the construction type as shown in the first column in the table below.

Dirk, I Don’t Think I Understood This Part of The Report Correctly. 

Please refer to TABLE 2, PAGE 6. The “Loma Prieta Modeled” on the third row  tells us what would happen if a model using empirical data from the Loma Prieta Earthqake were compared to actual damage. .   The second row compares is  what actually happened.  This model is obviously far from perfect and the projection for single family homes with 1-3 storied built before 1940 was remarkably closed to the actual damage.  On the other hand, this same model applied to houses built after 1939 was off by a factor of 2.  Notice how only 301 wood framed 1-3 story houses were red-tagged in the actual damage row.  This is far less than popular opinion.

When evaluating damage for FEMA after the Loma Prieta Earthquake I discover that most of the damage was caused by by deferred maintenance.  In addition, ALL of the houses with collapsed cripple walls had wood siding rather than stucco.  The poor performance of wood siding compared to stucco is born out by this research.  This research is far from conclusive. 

                                                                                                                     PAGE 3

 

The TABLE 2, PAGE 68 shown below tells us that 8.4% of the houses built before 1940 will be deemed uninhabitable if located in Modified Mercalli Scale zone IX and less than 1.8% will suffer the same fate if located in MMI VIII.  In the unlikely event the shaking intensity reacted level X the number of red tagged house would be 12% and 1.8% respectively.  This runs contrary to the popular misconception that most houses will suffer catastrophic damage.   It must be remembered that these are modelled statistics and previously discussed.  Both of these percentages could be reduced to near zero if vulnerable housing stock in MMI zone IX were properly retrofitted.

 

TABLE 3, PAGE 69 looks at yellow tagged homes.  Yellow-tagging is similar to red-tagging except with yellow-tagging the house is still inhabitable.  Yellow tagging means the houses are damaged (usually toppled over chimneys and/or fallen plaster on the ceiling), but they can still be lived in.  Yellow tagged houses were not given much consideration in Shaken Awake because yellow tagged homes will not have an effect on the size of the displaced population.  Some people however don’t want to risk being put in the yellow-tagged category and are willing to pay for that.  I must admit, living in a damaged house for any length of time can be quite annoying and after an earthquake you can buy a contractor for blood or money.

 

 

The table below estimates the total number of uninhabitable houses in Alameda and Contra Costa counties if the entire Hayward Fault ruptures based on type of construction.  Most of these will be in MMI zone IX.

 

In order to find out exactly which zone you are in go to  this page, (Dirk, I do Not Know How to Read This Page And Do Not Know How To Determine Peak AC) determine your Peak Acceleration (Peak Acc), look at the top of the column and you will see where your house is according to the Modified Mercalli Scale.  The information in Shaken Awake along this determination will help you determine your risk.  

Which Zone Is My House In?

The Association of Bay Area Governments has an online tool that will tell you this precisely.  Go to this webpage  and on the far left you will see a drop down menu that looks like this.  Check the two boxes shown by the blue arrows.

Scroll down and check the fault line you are concerned with.  In this case it is “Earthquake Shaking Scenario: Hayward Fault (Southern Segment).  Scroll up and down  the layer list and you will see you can select any of the Bay Area faults including the San Andreas.

Go to the top of the page and directly above where it says “Layer List” type in an address.

In this example we are using Bay Area Retrofit’s address in Albany.  Now click the search tab to the right of the address.  And you will see this.

 

Now, click on the small box 

 

Click on this play button

 

This tells me I am in zone 8 of the Modified Mercalli Scale.  

Zoom out and I will get more detail.  Click elsewhere on the map and look for the surprise. 

According to the TABLE below, I have a 1.8% chance of catastrophic damage or about a 2 in 100 if I don’t retrofit my office.  If I moved to an office that was built after 1940 my chance would be 0.18%.  

Given my office has wood siding it is probably much higher than that.  I suggest a 100% margin of error be applied such that we can predict the probability of catastrophic damage as being 4% or 4 in 100.  Then again, no one really knows what is going to happen for sure.  

What Are My Odds?

The table below provides a good snap shop of how much damage you can expected.  Look up your zone and figure out how much damage you can expect.

 

The San Andreas Fault

The bar graph below tells us the probability of a large earthquake on the 3 segments of the San Andrea Fault that are a threat to homes in San Francisco.

The San Francisco Peninsula Segment is the most serious threat because it has the highest probability of rupturing and is the closest to San Francisco. In other words, its rupture will create the highest shaking intensity based on the Modified Mercalli Intensity Scale (MMI).  The probability of a rupture is only 2% when forecast over the next 30 years. This means there is about is 1 chance in 50 that the fault will rupture in your lifetime.

 

Let’s assume a house is in zone 7.8. on the MMI) Scale according to USGS statistics.  The MMI table shown on the next page uses roman numerals. On that table this house is in Zone VIII.

Finally, houses in this zone houses will be red-tagged as unlivable on average  1.8% of the time or about 1 chance in 55.  To look at it more broadly, this house has a 1 in 50 chance of experiencing a large earthquake in the next 30 years and at the same time there is a 1 in 55 chance of the house being red-tagged when it happens.   I don’t know how statistics work, but overall I think this translates into a 1 chance in 2500 or so of our sample house being red-tagged in the next 30 years.  Red tagged meaning enough damage to be considered uninhabitable.  In short,  I would venture to say the odds of this sample house ever facing the kind of damage most people fear is almost miniscule.

It must also be remembered that the highest percentage of damage will involve hillside homes which are at far more risk than the typical San Francisco House.  This reduces the odds even further.