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Common Earthquake Questions Print E-mail


Q: What is the risk of earthquake in San Juan County?

A: The Pacific NW is a geologically active area and earthquakes are a part of life here. They are more infrequent than in some other parts of the world (California for instance), but when they do happen here, they are every bit as damaging and in some cases more so. During any given year, the risk of a quake is low, but the chance of a major quake happening in the future is 100%. It is only a question of when. It's a hard thing to quantify, but it's been said that someone born today has a 50% chance of experiencing a major Pacific NW quake in their lifetime.


While it is important to understand the underlying geology and to attempt to quantify the specific risks we face, the reality is this: quakes will happen here, and we need to prepare. You can learn more about the basic geology and types of quakes the Pacific NW faces by clicking HERE.


Q: What is the worst case scenario for earthquake in the islands?

A: There are two possibilities:


One is a subduction zone quake. These occur every 300-600 years (last one in 1700) and are capable of generating a 9.0+ quake and a major tsunami. The 2011 Japan quake and 2004 Indonesia quake were subduction zone quakes. This quake would cause major damage throughout the Northwest. Large sections of the outer coast would be devastated by the tsunami, and a smaller but still damaging tsunami within the islands is likely. There would likely not be widespread destruction of homes and businesses in the islands, but there would be lots of damage, and certainly we would be without ferries, power, phone, internet for an extended period of time. Highways and briges on the mainland would be severely damaged. Click HERE for a report that details impacts of a subduction zone quake in the Pacific NW.


The second possibility is a localized fault generating a large quake. The damage would be more limited and would not affect all of the Northwest, but those near the epicenter could expect significant damage. There are several faults near the islands that could cause a 7.0 or large quake. Click HERE to view a summary report of what a 7.4 quake on the Devil's Mountain Fault might look like.


Q: Is there any good news for the islands when it comes to earthquakes?

A: Absolutely. There are a number of ways in which we’re relatively better off than some of our neighbors. For example:


Bedrock is good for reducing damage from quakes. It doesn’t liquefy and tends to dampen the motion. Loose unconsolidated soils like those found on river deltas do the opposite. Fortunately for us in the islands, we have lots of rock, and relatively few areas with lots of loose soils.


Single family wood frame construction tends to do well in earthquakes. Larger concrete or masonry construction- particularly if the building is older tend to suffer. Again, here in the islands we have much of the former, and not much of the latter.


Q: OK, so what is the bad news?

A: The main piece of bad news is that eventually we’ll experience a major quake here and our geographic isolation will mean we’re going to be on our own. That quake will impact us locally, but much of the real devastation will happen on the mainland, effectively cutting us off from the rest of the world, and also ensuring that we’re going to be on our own for some time before help arrives. Much of the I-5 corridor could be knocked out, the city of Seattle could be impacted, and the marine, rail, air, and highway links our economy is so dependent on may be severely impacted. Ferries could be out of operation, and resources to help out affected areas may be focused on more populated areas first. All of this points to the need to be prepared to take care of ourselves for at least a week, and ideally longer.


Lack of fuel, food, power, and communications technology will cause obvious inconveniences. The challenges that come from not having ferries will only make things more difficult. While it is unlikely that there will be widespread destruction of houses, there will be some fires, and some older buildings may fail. If there is a tsunami, there will be extensive damage to waterfront and low bank infrastructure and residences.


Q: Are there active faults that go through or near the San Juans?

A: We're aware of the Devil's Mountain Fault that runs through the north end of Whidbey island and out just south of Lopez and San Juan Islands. We don’t know for sure of others, but the best answer is that more are likely to exist. New crustal faults are being mapped all of the time in the Northwest. Much of this work has been focused on faults near cities, but in the years to come, most seismologists expect that a wide ranging network of faults will be found, including some that directly impact the San Juans.


The Devil’s Mountain fault complex is a series of associated faults that stretch from the Cascades through Whidbey Island and NW towards Victoria. While this fault does not cross the islands, it is close enough that it’s impact would certainly be felt here. This fault is capable of generating a major quake, up to 7.5, and there is some indication that it has the potential to generate a local tsunami.


Again, the important thing is for us to know that quakes are possible, and that our knowledge of the specific risks is still very much incomplete.


Q: How do I prepare for an earthquake?

A: Most importantly, check our individual preparedness page, then go to our earthquake preparedness page for info. Call us for advice or with questions (370-7612)


Q: Where can I learn more?

A: Visit our list of links at: www.sanjuandem.net/quakelinks