San Andreas Fault VS Cascadia Subduction Zone

With the recent release of the hit blockbuster movie, San Andreas, there has been some discussion in the media about the reality of the movie and if any of it could actually happen.  Since its release on May 29th, news outlets around the country have been interviewing geologists and seismic experts to get to the bottom of the movie’s mega quake portrayal.

In the movie, chunks of California a decimated, the earth splits open, sky scrapers fall like dominoes, a massive tsunami strikes San Francisco, sympathy earthquakes are triggered, and seismographs on the east coast go bonkers.  All with the latest and greatest in CG and Surround Sound.   Meanwhile, Dwayne ‘The Rock’ Johnson jumps through every imaginable hoop to save the planet.  Don’t get me wrong, I like The Rock – they guy has got some serious swagger and is at the top of his game.  But I digress, we’re not here to talk about The Rock.

Just about everyone knows what the San Andreas Fault is and where it is located.  It has been the centerpiece of several movies over the years, probably most notably, the 1974 blockbuster hit, ‘Earthquake’.  Again, the gist being a mega earthquake strikes, destroying much of southern California, while the greatest Hollywood stars of the time work to save the planet.   The last major earthquake attributed to the San Andreas Fault would be the 2004 magnitude 6.0 Parkfield, CA quake, preceded in the more destructive 1989 magnitude 6.9 Loma Prieta quake that created severe damage throughout much of the Bay Area.

Comparatively speaking, few people know what the Cascadia Subduction Zone is and where it is located.   Outside of correlating Cascadia with the Pacific Northwest, my first impression of the term was the Cascade Mountains, which has little to do with what is possible one of the most potentially lethal fault zones on the planet.  The last earthquake attributed to the Cascadia Subduction Zone occurred on January 26th, 1700.  The quake created a tsunami that struck the shores of Japan within hours of the event, which was documented in written accounts of the time.  Geologic evidence in the region of SW Washington and Northern Oregon supports the time frame of the event, as does stories passed down from generation to generation among the local Native American Tribes.

So, what is each fault really capable and not capable of creating?


We’ll start with the SAN ANDREAS FAULT:

*The movie, San Andreas depicts a mammoth magnitude 9.1 quake.  According to USGS Seismologist Lucy Jones, the San Andreas Fault is not capable of producing a magnitude 9.1 quake.  The largest quakes on record for the San Andreas Fault were the 1857 Fort Tejon quake at magnitude 7.9, and the infamous 1906 San Francisco earthquake at magnitude 7.8.  While still terrifyingly destructive, a magnitude 7.9 quake pales compared to the devastating potential of a 9.0+ quake.

*The movie depicts a massive tsunami striking San Francisco.  There are a couple of problems with this scenario:

1) For a tsunami to be created by an earthquake, the fault line affecting the quake would need to be under water – the San Andreas Fault is no where near the ocean.  Even then, if it were capable of creating a tsunami from its location, it would most likely travel west across the Pacific Ocean toward Japan, not back toward the origin of the quake.

2) Tsunami’s are generally created either by plate lift action – like the one created by the massive magnitude 9.0 quake that struck the Indian Ocean in 2004 or a land slide type of action, like the 1958 Lituya Bay, Alaska mega tsunami (

* Most of the sky scrapers within the affected areas collapsing.  According to Lucy Jones, “Most coming down, fiction.  A few, potentially fact.”  They age, height, construction composition, and retrofits, if any, will determine if a building will be more likely to fall in a seismic event.  It does not provide all the answers, but a bit of insight can be found in this article from Exploratorium:

* Earthquake felt on the US east coast.  Again, Lucy Jones explaines: “Sorry, ain’t going to happen. The Great 1906 Earthquake that devastated San Francisco was felt into Nevada, and that is as far East as it got.”  Seismographs may sense large, distant seismic events, but for the person walking on the street to feel it thousands of miles away is total fiction.

*San Andreas shows Hoover Dam collapsing.  Again, per Lucy Jones, “The complete collapse of Hoover Dam extremely unlikely. People have worked very hard to prevent that.”  While I don’t completely write off the possibility of a catastrophic failure of the Hoover Dam, I find the possibility pretty far fetched, just from the immensity, depth, and design of the structure.

We’ve contested some of what the movie portrays; so what could the San Andreas Fault do?  Easily, cause extensive damage or complete destruction to unreinforced masonry buildings; damage and or destroy much of the infrastructure, including rail, surface roads, highways, overpasses, and bridges; cause severe damage to many residential structures, both single family and multi family; cause landslides, and a possibility of sympathy earthquakes from local, smaller faults.  Not to mention, interruption to electricity, water, gas, and other utilities we rely on heavily day-to-day.  While California has done an excellent job of educating the populace about earthquake preparation, there is still a long way to go to be completely prepared.  Japan is one of the, if not the most, seismically prepared country on earth; the 2011 Tohoku, Japan earthquake exemplified that even the most rigorous preparation may not be enough.




Since the Cascadia Subduction Zone is relatively obscure, and there hasn’t been any major blockbuster movies made about the fault – in fact, there haven’t been any movies (that I’m aware of) made about it, so we don’t have any mega Hollywood myths to dispel.

The Cascadia Subduction Zone is a fault line stretching over a 600 mile area from southern British Columbia to Northern California, ranging from 75 miles to approximately 100 miles off the western coastline of the US and Canada.  The fault itself is where the Juan de Fuca Plate is subducting under the North  American Plate, at a rate of about 60 MM per year or just about two-and-a-half inches.  Over the last 10,000 years, the zone has produced over 40 magnitude 8.5 + earthquakes, the last one being the January 26th 1700 event mentioned above.  The fault has averaged one major (8.5+) quake about every 250 – 300 years.

This relatively unknown has produced 42 massive quakes over the last 10,000 years over magnitude 8.5.  The last seismic event created by the Cascadia Subduction Zone was an estimated magnitude 9.1 that occurred little more than three hundred years ago.  The massive earthquake caused widespread fear and panic throughout the regions many Native American tribes.  The massive quake produced a tsunami that left much of the low lying coastal plains of Oregon and Washington washed over in salt water, causing ghostly evidence still visible today.  Stands of dead cedar and sitka spruce trees in swampy low lands, are one of the key reminders of the destruction brought about by the last seismic event.  Known as the “Ghost Forests” of Oregon and Washington.  Written accounts of the tsunami inundating small villages along the coast of Japan, causing widespread death and destruction correlate with lore of the local Native Americans of Oregon and Washington.

Since the last event was before the advent of pretty much so everything that we take for granted on a daily basis, what would another mega quake do to our region and what would its affect be on us today?

*Tsunami:  with each passing day, pressure between the Juan de Fuca plate and the North American plate continues to increase.  When the building stress is finally at the breaking point, the North American plate will release, causing a seismic lift at the point of the release, which could be localized or along the entire length of the fault line.  Whatever the height of the lift is, will roughly match the height of the tsunami.  If the lift is 30 meters, the tsunami height will be roughly the same.  I use that number as an example – it could be one meter or thirty – chances are, a mega quake’s release will be closer to the thirty than to the one.

A tsunami has the capability to travel at extremely high speeds, with the mass and energy of the ocean itself speeding toward the coastline.  There would be extremely limited time between a major seismic event at the fault, and the arrival of the resulting tsunami.  Scientists and experts agree that the tsunami would most likely reach several miles inland, causing mass destruction in its path.  The exact distance it would travel inland is unknown as factors such as height, location of epicenter, and magnitude will play a hand in the size and power of the ensuing tsunami.   A study from Cascadia Region Earthquake Workforce (CREW) suggests “Once it hits shore, a single tsunami wave can take as much as an hour to finish flowing in. The height of the wave and how far inland it travels will vary with location: In places along Cascadia’s coast, the tsunami may be as high as 30–40 feet”.  A tsunami from a magnitude 8.5+ may have the same destructive power as the 2004 Indonesia tsunami that killed 250,000+ people.  The depth into the interior of the coast line may not be as severe as the 2004 Indonesia or 2011 Tohoku quakes due to the mountainous topography of the region.  Still, damage will be severe and widespread.

*Collapsing of high rise buildings.  The epicenter location and the magnitude of the next Cascadia mega quake will dictate the degree of destruction.  An epicenter towards the southern end of the zone will result in less damage to highly populated regions like Portland or Seattle than one centered off the northern coast of Oregon or off the coast of Washington.  In the case of the latter, the majority of the bridges in the Portland area would sustain a high degree of damage or total destruction.  Most of the high rise buildings – especially those completed in the nineteen eighties and before, will most certainly sustain a high degree of damage.  Windows will shatter, spraying the streets below with glass, curtain wall (exterior part of more modern structures encompassing both window and wall) sections may break loose and fall, structures may collapse completely, others partially, and unreinforced masonry buildings will most likely experience complete destruction  – except for those that have been seismically retrofitted, which will most likely suffer some damage, but not to the degree of those not seismically retrofitted.  Landslides are highly possible in Portland’s West Hills, Mt Tabor, Mt Scott and other areas withing the metropolitan area.

Seattle will fair about the same, but on a greater scale given its larger footprint, more high rise buildings, bridges, and miscellaneous structures.

*Access to Columbia River Commerce.   International shipping is a commerce mainstay in the Pacific Northwest, with the Columbia River serving the main port of Portland, OR along with several other ports along it’s lower length on the Washington side of the river.  A tsunami would not be able to make its way very far inland against the strong current of the Columbia River, but it could cause havoc on the rip-rap break waters protecting the mouth of the Columbia, as well as cause major damage immediately inland to the Astoria Bridge, which spans the Columbia just shy of the mouth.

An intense verbal description of a potential chain of events in Discover paints a graphic mental picture of the scale of destruction:

The sheer potential magnitude of such a quake, located so close to dense population zones is truly beyond the comprehension of most of us.  it is imperative to be prepared.  However, being prepared is only part of the equation for survival.  Knowing what to do before, during and after is paramount.  American Red Cross has a publication (outlined in detail in previous blogs) called  “Prepare!   A Resource Guide” that outlines methods of preparation, working with neighbors and other resources for being prepared.  Contact your local and regional authorities to learn where more about rally points; radio stations that broadcast current updates and pertinent information.  Information about volunteer opportunities before, during and after a major seismic event can be obtained both through the American Red Cross and your local authorities.

I prefer to not be all doom and gloom when writing a blog, but at the same time, I think it is important for us to know the true potential of a mega quake and what we need to do to be prepared.  We are so far behind Japan in preparation – I truly hope we are able to better prepare before the next mega quake makes its introduction.





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