San Andreas Earthquake Scenario

When I wrote my book I expected a San Andreas earthquake scenario to be easy to find with all the Hollywood disaster movies. However, official scenario documentation, particularly more current, has been challenging to find.

This particular ShakeOut Earthquake Scenario page on the USGS website is dated January 17, 2018, but much of the information is based on a 2008 ShakeOut Scenario. Since most material I read referenced that 2008 ShakeOut, that is what this scenario post is based on.

The ShakeOut

 The State of California Department of Conservation has a 2008 earthquake scenario that was used for its ShakeOut drill in the event of an earthquake on the San Andreas Fault (2008 ShakeOut Scenario). The scenario projected about 1,800-2,000 deaths, 50-53,000 injuries requiring emergency care, and $191-200 billion in damages. Like other scenarios, this doesn’t include the lasting disruption those damages will cause. And, those numbers are just estimates from the main shock. Additional damaging aftershocks can increase those numbers.

USGS San Andreas Great Southern California ShakeOut 2008
USGS San Andreas Great Southern California ShakeOut 2008

In this scenario, the earthquake is a magnitude 7.8 that lasts about 55 seconds and occurs on the southern 186 miles (300 km) of the San Andreas Fault. Compared to the Northridge quake, which lasted only 7 seconds, shaking lasting this long will be terrifying to many who have never experienced more than a few seconds of ground movement.

“A magnitude 7.8 is not the largest earthquake that the southern San Andreas Fault can produce, nor is the San Andreas the only fault to threaten the populated areas of southern California with very large earthquakes. However, those other faults have recurrence intervals (an estimate of the average time) between larger earthquakes that are considerably longer, measured in thousands of years. By contrast, the southern San Andreas Fault has generated earthquakes of ShakeOut size on average every 150 years—and on a portion of the fault that ruptures in the ShakeOut Scenario, the last earthquake happened more than 300 years ago”.

Loss and Damage Estimations

Some of the expected consequences of the earthquake include the following:

  • At least 10 million people would be within the heavy shaking areas.
  • 1,800 casualties would be expected, (there'd be more if it weren't for strong building codes) but tens of thousands of injuries would also be expected.
  • All unreinforced masonry buildings within fifteen miles of the San Andreas Fault would be destroyed.
  • Hundreds of older buildings will have collapsed into heaps. Thousands of other buildings will be so damaged they’ll eventually need to be torn down.
  • The fault rupture would offset all infrastructure lines going into Southern California, including:
    • Interstate-15 at Cajon Pass
    • Interstate-10 at San Gorgonio Pass
    • Route 14
    • pipelines
    • power lines
    • roads
    • railways
    • telecommunications
    • aqueducts
  • At least 1,600 large fires will likely start, and many small fires will also start, possibly flaring up into larger ones. The fires could double the death count. The fires would contribute about $87 billion in property and business interruption loss. High-density wood building neighborhoods would be at high risk.
  • Two-thirds of the hospitals in Los Angeles, Orange, Riverside, and San Bernardino counties will be non-functional.
  • 50-53,000 people will be trying to get to emergency rooms.
  • The state highway system will mostly survive, due to seismic upgrades, but many bridges and overpasses under the control of cities and counties won’t do as well. 
  • Many roads will be impassable due to damage, collapsed structures, or abandoned vehicles.
  • The water distribution system will also not do well, and some buildings will be six months, or more, without water. 
  • Many of the buried water and sewer pipes will become cracked or broken with the earthquake. Initial cleanup in the newer, relatively undamaged buildings will likely need to be done without running water. 
  • Electricity will be out. Electric trains and elevators will likely not be functional. Non-working traffic lights will gridlock streets, with many drivers eventually deciding to make the long walk home.
  • Phone systems will be unusable or overwhelmed by the number of attempted calls.
  • At the low end (in a 2009 article, referenced at the end of this post) total economic impact and loss would by $191 billion. The ShakeOut Scenario has the economic impact at $213 billion. Most likely it would end up be much higher.
Just because you live or work in a newer building does not mean it be damaged. There are other hazards besides the ground shaking that cause damage. One of those is liquefaction.

The following image is from page 60 of the USGS report. The damage was caused by the magnitude 6.9 Loma Prieta earthquake in 1989. The ground motions weren't really bad enough to cause the damage. What happened is liquefaction induced lateral spreading and that caused the damage. 

Building damage from liquefaction-induced lateral spreading in M6.9 Loma Prieta earthquake in 1989


In the months following the San Andreas earthquake, damage repairs will be extensive. 

About a month after the big one, electricity and water should be restored to most of those who had lost those utilities, particularly in areas with less damage. However, residents will likely need to continue to boil water due to possible contamination from cracks in the water system. It may take a year or longer before the entire system is considered safe.

A month after, tens of thousands of people will still be without permanent shelter. Many will also be without jobs due to damaged buildings. Many roads, freeways, and bridges will still be closed for repairs.

Six months after the quake, many small businesses will have closed, and their closure will contribute to the closure of other businesses.

If you're more into a video depiction of the loss and estimations of the earthquake scenario, here's a YouTube video created by Art Center College of Design Alumnus Theo Alexopoulos that was adapted for The Great California ShakeOut. The video was posted in August 2009.

I still shake my head at preparedness recommendations to have enough water for 1 gallon per person per day for 3 days. That's barely enough time for outside help to come in to a major disaster area. My minimum recommendation is water storage for 2 weeks. And on top of that have means to purify water.

Here's my post on being Water Prepared

For those who want more official (and detailed) information--

Here are the links to the full reports on the USGS site

And here's a link to an article published in Earthquake Spectra. Access to the full article requires an account, institutional access (like through a university), or buying it. 

Much of this information, as well as many of the posts in this blog are in my book, Earthquake! What, Where, and How to Prepare.


Popular Posts

What is the Mercalli Intensity Scale?

Wasatch Fault Earthquake Scenario

Earthquake Hazards: Surface Fault Rupture