The Faults and Earthquakes of the Pacific Northwest

As I was writing my previous blog on the 7/12/2019 Three Lakes earthquake, I kept getting sidetracked and found myself writing about the mechanisms behind earthquakes in general (perhaps that’s the reason why this blog is so late to get out!) And while there’s so much more I’d like to cover, like the different types of earthquake waves or the idiosyncrasies of the major crustal faults in Western Washington, I thought it would be best to focus on faults, as faults – and the tectonic forces that create them – are the source of almost all the sizable earthquakes we experience in modern life.

I’ll do here what countless before me have already done with far more eloquence and precision – summarize the different types of faults in an internet blog post. I’ll also touch upon the three general “earthquake types” we experience here in the Pacific Northwest, and from which faults they arise. And I’ll try and resist any “fault” puns along the way.

The Different Types of Faults: 

A “fault” is simply a fracture in the Earth’s crust where the crust on one side of the fracture is moving relative to the other, resulting in force buildup and displacement along this fracture.

Faults can be divided into three main subtypes: dip-slip (of which there are “normal” and “reverse” faults), strike-slip, and oblique-slip, which is a combination of dip-slip and strike-slip.

Dip-Slip Faults:

Dip-slip faults occur along an incline and have significant vertical displacement. The fault plane separates the “hanging wall,” which is above the fault plane, and the “footwall,” which is below it. To find out which block is the hanging wall and which is the footwall, suppose you are an adventurous miner and are digging a tunnel along this fault zone. The hanging wall would be above you (i.e., the wall you’d hang your lamp on), while you’d be walking on the footwall.

Normal Fault:

In a normal fault, the two sections of crust diverge, and as a result, the hanging wall drops below the footwall. Due to this divergence, tensional force (i.e. the force that occurs when stretching a rubber band) builds, and when the tensional force becomes so great that the rock splits apart (the rubber band snaps), you get an earthquake.

Reverse Fault:

As the name suggests, a reverse fault is simply the reverse of the normal fault, with convergence along the fault line and the hanging wall above the footwall.

Due to this convergence, compressional force builds. A common example of compressional force is stepping on an empty soda can. It can take a certain amount of compressional force, but once the force exceeds a certain threshold, the can crumples. The video below tests how much compressional force various objects can take before they crumple… the full coke can (begins at 4 minutes) is my favorite!

Strike-Slip Faults

Strike-strip faults are nearly vertical faults where masses of rock slide laterally past each other instead of on an inclined plane like they do in a dip-slip fault. There are two types – left-lateral and right-lateral, and they are based on the direction the rocks are sliding. To determine if a strike-strip fault is left-lateral or right-lateral, imagine you are standing directly on top of the fault. If the rock on your left is moving towards you, you are on a left-lateral fault (and vise versa for a right-lateral fault).

The sliding motion along strike-strip faults creates shearing force. Scissors utilize shearing force, as each blade applies a force in the opposite direction, shearing the object apart. There’s a reason scissors are also called shears!

The San Andreas Fault is an example of a fast-moving, dangerous right-lateral strike-slip fault between the Pacific and North American Plates, and the Cascadia Subduction Zone is a giant, 1,000 km long reverse fault capable of producing 9.0+ earthquakes. The Wasatch Fault is one of the largest normal faults in the world and is statistically overdue for a 7.0+ earthquake that could cause significant damage to Salt Lake City, Provo, and many other cities in Utah and the Intermountain West.

Oblique-Slip Faults:

Oblique-slip faults are the lovechild of dip-slip and strike-slip faults, as they have both significant vertical displacement (as you see in a dip-slip fault) and horizontal displacement (like a strike-slip fault). All faults are a dip-slip/strike-slip compromise to a degree, but to be classified as an oblique-slip, the fault needs to have a pretty even proportion of both. The San Andreas Fault, though primarily a strike-strip fault, also has significant oblique-slip sections.

Earthquakes of the Pacific Northwest:

The earthquakes of the Pacific Northwest arise from the interaction of three tectonic plates: The Pacific Plate, the North American Plate, and the Juan de Fuca Plate. The Pacific Plate covers most of the Pacific Ocean, while the North American Plate spreads northward from Mexico to the North Pole and extends all the way from the Central Atlantic to Eastern Siberia and Japan! The Juan de Fuca Plate is comparatively minuscule in size and lies just of the coast of the Pacific Northwest.

There are three different “types” of tectonic Cascadia earthquakes: Cascadia Megathrust, Deep Intraplate, and Crustal Faulting earthquakes. Volcanes and even Seahawks fans also produce earthquakes, but these are generally too small to cause damage directly (though the latter may make you deaf!)

Cascadia Megathrust

Cascadia Megathrust earthquakes are the “Big Ones” often mentioned in the media. Megathrust earthquakes are the most powerful earthquakes by far in the world, and because they occur in subduction zones (really long reverse faults where one tectonic plate moves underneath another) off the coast, they often produce massive tsunamis that can destroy everything in their path. The two most infamous earthquakes of the 21st century – the Indian Ocean Earthquake of 2004 and Tōhoku Earthquake of 2011 – were both megathrust quakes with magnitudes >9.0. A similar quake occurred along the Cascadia subduction zone in 1700, and while Cascadia megathrust quakes generally have a recurrence interval of 500-600 years, some have occurred only 300 years apart. A 2010 study found a 37% chance of a >=8.2 magnitude quake in the next 50 years and a 10-15% chance of a >=9.0 event.

Deep Intraplate

Deep Intraplate quakes are the most common major earthquakes over the Pacific Northwest. While Cascadia Megathrust quakes occur at the convergence of the North American and Juan de Fuca plates, Deep Intraplate quakes occur solely on the Juan de Fuca plate. As the Juan de Fuca plate is forced under the North American plate and pulled towards the mantle by gravity, strain builds up along fault lines in the plate, and when these faults rupture, an earthquake occurs. The 2001 Nisqually Earthquake is the most recent example of a Deep Intraplate earthquake, but similarly strong earthquakes occurred in 1949 and 1965, and chances are that us millennials will witness another Deep Intraplate quake within our lifetimes.

Crustal Faulting

Crustal earthquakes don’t get as much attention as the Cascadia Earthquakes, and major ones are much rarer than the Deep Intraplate earthquakes that typically occur once 2-3 times a century. But even though the damage from a Crustal earthquake would be far more localized than a megathrust quake along the entire Cascadia Subduction Zone, it would likely be far more severe in the immediate vicinity of the quake. Crustal earthquakes occur along a complex network of faults in the North American plate and are primarily originate from the shearing stress between the Pacific and North American plates along the San Andreas Fault, with lesser contributions from the Cascadia Subduction Zone off our coast. The July 12, 2019 Three Lakes earthquake was an example of a weak and relatively deep Crustal earthquake, but far stronger, shallower, and more destructive crustal earthquakes have occurred in the past. Two particularly dangerous faults – the South Whidbey Fault and Seattle Fault – have spawned extremely intense earthquakes in the past, and a significant quake from either fault could cause extreme damage to parts of Puget Sound.

USGS modeled a hypothetical 7.2 earthquake on the Seattle fault (link) and 7.4 earthquake on the South Whidbey Fault (link). If you find yourself sleeping a bit too easily at night, give these a read… they’ll keep you up at night!

Speaking of which, I better get to bed. Thanks for taking the time to read this series of posts, and I promise I’ll try to be a bit more punctual with my posts in the future!

Sources: 

Czajkowski, J. L., & Bowman, J. D. (2014, May). Faults and Earthquakes in Washington State. Retrieved July 14, 2019, from https://www.dnr.wa.gov/publications/ger_ofr2014-05_fault_earthquake_map.pdf

Jones, C. E. (n.d.). Normal Faults. Retrieved July 14, 2019, from http://www.pitt.edu/~cejones/GeoImages/7Structures/NormalFaults.html

Oregon State University. (2010, May 25). Odds are about 1-in-3 that mega-earthquake will hit Pacific Northwest in next 50 years, scientists say. Retrieved July 25, 2019, from https://www.sciencedaily.com/releases/2010/05/100524121250.htm

Pacific Northwest Seismic Network. (n.d.). PNW Earthquake Sources Overview. Retrieved July 25, 2019, from https://pnsn.org/outreach/earthquakesources

Rey, P. (n.d.). Lecture 3: Joints, Fractures, and Faults. Retrieved July 18, 2019, from http://www.geosci.usyd.edu.au/users/prey/Teaching/Geol-1002/HTML.Lect3/sld007.htm

Schuske, K. (2013, August 1). Explore Utah Science – Earthquake Risk in the Salt Lake Valley. Retrieved July 15, 2019, from http://www.exploreutahscience.org/science-topics/science-and-society/item/126-earthquake-risk-in-the-salt-lake-valley

Washington DNR. (n.d.). Geologic Provinces – Puget Lowland. Retrieved July 25, 2019, from https://www.dnr.wa.gov/programs-and-services/geology/explore-popular-geology/geologic-provinces-washington/puget-lowland#faults.1