A Magnitude 6.7 Earthquake on the Seattle Fault (Part 1)

On April 29, 1965, I was sitting in Mr. Peterson's class at Mann Junior High, in Lakewood, Washington, when we were hit by a deafening noise.  My first thought was one of the military transport planes from McChord Air Force Base was landing on the school.  In somewhat of a daze I heard Mr. Peterson yelling from under his desk for us to get under ours.  I did and from that vantage point I could see out into the parking lot at what appeared to be waves flowing through the pavement, the cars being bounced up and down.  It lasted about 45 seconds and was terrifying.

Each wing of the school was built on separate concrete slabs and the structure was designed to "float" in an earthquake.  No doubt, part of the motion of the parking lot and cars was our wing floating up and down as a result of the seismic activity.  Some damage to the buildings occurred, but was mostly between the sections which connected one wing to another.

Later we learned we had experienced a magnitude 6.5 earthquake, which had been centered near Olympia; the approximate location of a magnitude 7.1 earthquake occurring in April of 1949.  Although it was a significant event, there were only three people killed and around $12.5 million in damages.

Let's look at three types of earthquakes we can experience in the Northwest.  Deep earthquakes occur more than 300 km or about 186 miles beneath the earth's surface, and most of these we don't even feel, because they never make it to the earth's surface.  The earthquakes in 1949, 1965 and the recent Nisqually earthquake in 2001 were all quakes that occurred deep in the earth and as a result the damage and loss of life was relatively minor.

Next are subduction zone earthquakes, which produce the largest quakes ever recorded, and in the Northwest they result from the Juan de Fuca plate being forced under the North American Plate.  It is called the Cascadia Subduction Zone and it extends from southern British Columbia down to norther California.  In the past, it produces an earthquake roughly every 300 years, and it has been over three hundred since the last earthquake along this subduction zone.  The great Alaska earthquake of 1964 registering moment magnitude 9.2,  was was the result of the Pacific Plate being forced under the southwest crust of Alaska.  The megathrust, moment magnitude 9.2 earthquake that occurred on December 26, 2004, off the coast of Sumatra, Indonesia was a subduction zone earthquake.  It created a tsunamis resulting in 230,000 deaths and an untold loss of property in 14 countries bordering the Indian Ocean.  On February 27, 2010, the Chilean subduction zone earthquake registered a moment magnitude 8.8, and it devistated much of the country and resulted in a tsunami that destroyed several coastal towns in Southern Chili.

The third type is a shallow fault earthquake.  These tremors occur very close to the earth's surface, less than 70 km or roughly 43.5 miles deep, in cracks or faults in the rock and several new shallow faults have been discovered in the Puget Sound area in the past ten years.  These findings tells us we have an even greater risk of this type of quake.  Since these earthquakes are so close to the earth's surface, even small quakes can cause a great amount of shaking and destruction.  The Seattle Fault is one of these, and the last major quake on this fault occurred about 1,100 years ago.

The Seattle Fault runs from the Hood Canal, east, through the Puget Sound, south Seattle and parallels I-90 through Mercer Island, Bellevue, and Issaquah.  In 2005 the Earthquake Engineering Research Institute and the Washington Military Department Emergency Management Division published a book entitled Scenario for a Magnitude 6.7 Earthquake on the Seattle Fault, describing the effect of one of these quakes.  Let me share some of these scientific predictions of a magnitude 6.7 earthquake along this shallow fault.

 The Nisqually earthquake was a magnitude 6.8 and caused $2 to $4 billion in damages.  Many of us have thought, because this earthquake was relatively mild, earthquakes of that magnitude wouldn't cause a large amount of damage or affect life to any significant degree.  By contrast, a magnitude 6.7 quake along the Seattle Fault would result in an estimated $33 billion in damage.  That's quite a difference.

The Nisqually earthquake did not cause extensive damage to most buildings and property, but the scenario for a Seattle Fault earthquake predicts landslides, severe damage to roads and bridges, including collapsing overpasses and a tsunami in the Puget Sound.  It will rupture water and sewer lines resulting in local flooding and contaminated water.  This quake will result in over 1,600 deaths, 24,000 injuries, 9,700 buildings destroyed, another 29,000 buildings not safe to be occupied, and 153,000 structures will have their use restricted.  It's estimated there will be 130 fires, and with many of the water lines broken, the fires will spread, burning nearly a half-billion dollars in property. 1  This would be comparable to the Northridge earthquake which occurred northeast of Los Angeles in 1994, and was a 6.9 moment magnitude quake. 

Part 2 of "A Magnitude 6.7 Earthquake on the Seattle Fault" discusses the effects on those living in the Puget Sound area. 

1 Earthquake Engineering Research Institute and the Washington Military Department Emergency Management Division. Scenario for a Magnitude 6.7 Earthquake on the Seattle Fault. First edition, 2005, p1

Photos in this article are used with permission of USGS

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