What You Should Know About Fault Lines Around Seattle

Image Source: USGS

The Seattle area is located on top of several fault lines, making it a geologically active region. The most significant of these fault lines is the Seattle Fault, which runs along the northern border of Seattle and extends up to the Olympic Peninsula. This fault line is believed to be the source of two major earthquakes that occurred in the Seattle area in the past: the 6.8 magnitude Nisqually earthquake of 2001, and the 6.5 magnitude Puget Sound earthquake of 1965.

According to Harold Tobin, the head of the Pacific Northwest Seismic Network there is an 85% chance of another Nisqually style quake happening in the next 50 years.

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For the past two decades, all has been pretty quiet. Many residents remember that day vividly, but for hundreds of thousands of Washingtonians it’s just a story, either because they have moved here since, or are too young to remember. Most of my students at the University of Washington were just being born around then. The passing of decades quickly lulls us into a false sense of complacency, but another earthquake just like Nisqually — or worse — could happen at any time. Knowing that the 1949 and 1965 quakes were very similar to the one in 2001, seismologists believe that chances are better than even that another Nisqually will happen in the coming few decades.

Recently, there has been a lot of focus on the potential for the “Big One” — a giant offshore magnitude 9 earthquake and tsunami. When that occurs, it will be a calamitous day, and we must certainly anticipate and prepare for it. However, the last time the Cascadia subduction zone fault moved was Jan 26, 1700; on average this happens about every 300-750 years. So, while a giant coastal quake could happen at any time in the next couple of centuries, another like Nisqually will likely happen in mere decades. The Seattle Fault and others like it present hazards, too, but large earthquakes on each of them seem to occur more like every few thousand years.

The Seattle Fault is capable of producing an earthquake of magnitude 7.0 or higher. While the odds of such a large earthquake happening are low, it is still important to be aware of the potential risk. The Seattle Fault has been identified as an area of high seismic hazard, and the US Geological Survey recently released a hazard map that outlines the potential risk and effects of a large earthquake.

In addition to the Seattle Fault, there are several other fault lines located near Seattle. The Tacoma Fault runs along the south side of Seattle and up to the Tacoma area, and is capable of producing a magnitude 6.0 or higher earthquake. The Puget Sound region also has several minor faults, such as the South Whidbey Island Fault, which can produce earthquakes of magnitude 5.0 or higher.

The Cascadia Subduction Zone gets a lot of attention for its large potential. This is the major fault line known as a subduction zone meaning one plate sliding underneath another continental plate. When the plates slip fully its called a full margin rupture and thats how we can get a 9.0 magnitude earthquake. From the data, we know this happens every 300-700 years. We are not overdue but it could happen at any time.

It is recommended that residents of the Puget Sound region have an emergency plan in place, and that they keep emergency supplies on-hand in case of a disaster. Additionally, it is important to take steps to secure your home and property against damage from an earthquake. Taking these precautions can help keep you and your family safe in the event of a large earthquake.

Curious if your property is safe and secure? Book a free seismic retrofit assessment.