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, 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.
While it is important to understand the underlying geology and to attempt to quantify the specifc risks we face, the reality is this: quakes will happen here, and we need to prepare.
Q: Is there any good news for the islands when it comes to quakes or tsunamis?
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.
- When it comes to tsunamis, we have two pieces of good news. One is that we’ll feel the quake that causes our tsunami. Communities on the outer coast can be impacted by tsunamis that come from far away, so they won’t know it is coming. Another advantage of the islands is that high ground is never far away. Getting to an elevation that is at least 30 feet above the high tide would get us out of the reach of most if not all tsunamis, and that’s never hard to do in the islands.
- The final piece of good news is that we’re a resourceful, resilient, and common-sensical community where people tend to be good at taking care of themselves and each other. This is critical following an emergency, especially when residents have taken steps ahead of time to prepare (as is the case with many islanders).
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 don’t know for sure, but the best answer is probably. 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: What is the worst case scenario for earthquake in the islands?
A: There are two possibilities:
- One is the 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 a real possibility. 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.
- 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. A localized tsunami could be generated, and it might be far more damaging that the quake itself.
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: Is San Juan County at risk for a tsunami?
A: The short answer is yes. Tsunamis are rare, and there is little historical information as to their extent or severity. Recently scientists have begun using high-powered computers to model various seismic events and the resulting tsunamis. The current thinking is that in the event of a large earthquake (magnitude 9.0) on the subduction zone located off of the outer Washington coast, San Juan County could expect a tsunami anywhere from 90 to 180 minutes later. Tsunamis are drastically affected by bathymetry, topography, and geography- but a best guess says that the waters will reach a height of 10 feet above mean high tide in most places, and potentially a height of 20 feet at the head of long inlets. In most cases a tsunami is not so much a breaking wave, but rather a sudden and fast moving tidal surge (hence the name tidal wave). Picture a vastly amplified tide coming in over the course of minutes not hours.
Canadian Fisheries and Oceans has an animated model of what a subduction zone induced tsunami could look like. Note that this model is somewhat dated, but still gives a good sense of how a tsunami might behave.
There are two other possible tsunami scenarios as well: It is also possible that a quake centered within the Puget Sound region could generate a tsunami that would arrive much sooner and be much bigger. Main thing of course is to head to high ground at the first sign of a major earthquake.
It is also possible that a major quake in the Aleutian Islands of Alaska could generate a quake that would affect the inland waters of the San Juans. Most likely the damage would be limited to docks and marinas. This is the only scenario in which scientists are predicting damage here from a distant quake.
Q: What do I need to know about tsunamis?
A: There are a few key things everyone should be aware of:
· Your tsunami warning is a major earthquake. If there’s a big quake, go to high ground and get away from low bank waterfront, docks, beaches, etc. Head uphill and try to get at least 30’ above the water, but going higher certainly isn’t a bad idea. There are no sirens in San Juan County, and don't count on other forms of technology. Use common sense and caution and head to high ground if you feel a major quake. Stay high until 4 or more hours have passed or you have other confirmation that there's no tsunami (this is where a hand held radio can come in handy).
· If you see the ocean behaving in a strange way, head to high ground. The tide going out, unusual currents, or other odd ocean behavior could be the first sign of a tsunami.
· Remember, the first surge is often not the biggest. Don’t go back to low ground anytime soon. Tsunami impacts can continue for 12 hours or more after the first effects.
· Getting your boat out of the harbor to prevent damage is a bad idea. While that has worked on the outer coast, where deep open water is close at hand, the local waters of the islands will make for highly unpredictable impacts. Be safe, and head to high ground.
Q: How do I know if I’m living in a high-risk tsunami zone?
A: There are a handful of high-risk tsunami areas within San Juan County. The closer you are to the high tide line, the higher the risk. Areas at the head of narrow, deep inlets are more at risk than areas along the outer shore. Places where water is funneled will see an increase in both the height and force of the wave. For more help with understanding the risk in your neighborhood, feel free to contact DEM directly.
Remember that unlike those living on parts of the outer Washington Coast, we in the islands almost always have high ground nearby. And most important of all: we'll feel the earthquake before the tsunami arrives. That's our built in warning system.
Q: How dangerous is a tsunami?
A: In the most likely scenario, San Juan County will have plenty of warning before a tsunami in the form of an extremely large earthquake. This gives us an hour or so to pack up families, pets, and valuables and head away from the shore. The danger from tsunamis comes from rapidly rising water, as well as fast moving debris entrapped in the flow (cars, boats, docks, driftwood logs, and other items become potentially devastating battering rams). It is important that once you leave, you stay high until you are sure that the threat has passed (after a large earthquake, repeated tsunamis are possible over the span of up to 24 hours). Tune in to your local media or your EAS radio for updates.
Q: What can I do to protect against tsunami damage?
A: The highest priority is to protect yourself and your family. After a large earthquake, act as though a tsunami is coming and move to higher ground. Equip yourself with an EAS radio in order to hear tsunami warnings. To mitigate damage to your home, the first thing to do is to think twice before building right on the beach. Ensure that the pilings of any floating dock in front of your house extend 12 feet above mean high tide, enabling the piers to float up with the level of the tsunami and avoiding the release of boats and docks as debris. A tsunami is a relatively rare but massive event; the priority is always to protect lives over property.
Q: Is this info on tsunamis available in a flyer or handout?
A: Yes, click here to link to a short informational flyer on tsunamis in San Juan County.
Q: Where can I learn more?
A: Visit our list of links at: www.sanjuandem.net/quakelinks