Seismo Blog

There are plenty of volcanoes in the Cascade Mountains of the Pacific Northwest, produced by the subduction of the Juan de Fuca Plate beneath the North American plate along the Cascadia Subduction Zone (CSZ). The nearby Olympic Mountains of Washington State, however, do not have any volcanoes. This raises the question: why are the Cascade Mountains volcanic, but the Olympic Mountains are not? They both lie along the same section of the CSZ, and they’re only a few hundred kilometers apart, separated by the Puget Sound lowlands. What makes the two mountain ranges so different?

(Figure 3. Source)

Subduction zones like the CSZ tend to produce the same basic pair of geologic features: volcanoes and accretionary wedges. Volcanoes form above the zone where the subducting slab reaches about 100 km depth and begins to release water into the overlying mantle rock. Just like salt lowers the melting temperature of ice, water and other contaminants lower the melting temperature of solid rock, allowing magma to form without an increase in temperature. The magma eventually migrates toward the surface and erupts as lava from a volcano. Until the slab reaches that specific depth, the temperature and pressure conditions aren’t quite right for melting to happen, which is why our Cascade volcanoes are so far inland from the Cascadia Fault.

The Olympic Mountain Range is non-volcanic because it is actually a huge accretionary wedge. Unlike volcanoes, accretionary wedges form near the shallow end of the subducting slab as pieces of oceanic crust and marine sediment get plowed into a “wedge” by the overlying plate (imagine using your hand to sweep sand or dirt across a table). Often, accretionary wedges are hidden below sea level or produce gentle coastal ranges, but the Olympics are a rare exception - the slab beneath the Olympic Peninsula is colder and dips at a shallower angle than other sections of the CSZ, which may explain the unusually high elevation of the Olympic Range. Because it is an accretionary wedge, the western side of the Olympic Peninsula is largely made of up ocean sediments scraped off the Juan de Fuca Plate. If you were to hike to the tip of Hurricane Ridge, you might even find shelly limestones and sandstones with marine fossils in them!
 

Another debris flow avalanche at Mount Baker

June 15, 2016

by Steve Malone

Seismic signals on May 25 are evidence for the size and timing of yet another on of these avalanches.

All the mountains, oceans, and islands on Earth exist because of plate tectonics. Different plate boundaries produce different geologic features: divergent boundaries spread apart to form mid-ocean ridges and rift valleys, transform boundaries slide past one another to form strike-slip faults like the San Andreas, and convergent boundaries collide to form tall mountains, deep trenches, and volcanoes. This type of plate boundary is responsible for the numerous volcanic arcs around the Pacific Rim (often called the “Ring of Fire”), and formed our iconic Cascade Volcanoes. Here in the Pacific Northwest, the Juan de Fuca plate is subducting beneath the North American plate along a convergent plate boundary called the Cascadia Subduction Zone (CSZ). Subduction zones like this are the only fault systems capable of producing very large megathrust earthquakes, but they only do so occasionally - over the last 100 years, there have been 84 earthquakes of magnitude 8.0 or greater worldwide, and only 4 of them were greater than an M9.

 


The simplest answer to the question “Will there be another large earthquake on the CSZ?” is yes. However, the question of “when” is much more difficult to answer. Seismologists don’t know exactly when the next large earthquake will occur on the CSZ, but we do have a good picture of when they have happened over the past 10,000 years. If we divide 10,000 years by the number of ~M9 earthquakes found in that time period, the average recurrence rate for M9 earthquakes along the CSZ is roughly 550 years. We are 316 years past the last great CSZ earthquake in 1700, and we estimate that there is about a 15 % chance that an M9 will occur on this fault within the next 50 years. However, research on submarine landslide deposits shaken loose by big earthquakes indicate that M8+ earthquakes occasionally strike off the coast of Oregon in between “full rip” M9 events. This research suggests that there is a greater probability of reoccurence of a great earthquake in Southern Oregon than off the Washington coast, but there is not a consensus within the geophysical community as to specifically how much greater the hazard is.

One is a guess, and the other is an educated guess.
Earthquakes happen on faults, but where are the faults in Oregon and Washington? The new "Display Faults" tool on the PNSN Recent Events map can help you explore the locations of faults in the Pacific Northwest.

Exotic Events (not erotic events)

April 1, 2016

by Steve Malone

Seismically recorded non-earthquakes now have their own page at the PNSN.

Negative depth earthquakes?

March 30, 2016

by Steve Malone

Why do some earthquakes in our list have negative depths now?

Explosion "Earthquakes"

March 10, 2016

by Steve Malone

Two recent large explosions generated acoustic waves recorded on seismographs.

West Coast Earthquake Early Warning System on the Horizon

February 9, 2016

by Shelley Chestler

Last week the White House hosted the first ever Earthquake Resilience Summit. One of the main goals of the meeting was to discuss the potential for fully funding a west coast earthquake early warning system.

Back-to-back ETS events, maybe

February 6, 2016

by Steve Malone

Following the recent "standard" northern Washington ETS another has apparently started heading south toward Oregon.

Slow Earthquake Trembles beneath Vancouver Island

January 7, 2016

by Shelley Chestler

Did you know that there is a type of earthquake that happens so slowly that we can’t feel it? One of these slow earthquakes is happening under Vancouver Island and northern Washington right now!

A Perspective on Tremor Activity

January 7, 2016

by Aaron Wech

With tremor activity occurring in the Pacific Northwest, it's important to provide perspective.

Don't Sweat the Little Ones

December 14, 2015

by Shelley Chestler

The recent earthquakes around the Puget Sound are probably not indicative of the “big one.”

A new look for pnsn.org

December 4, 2015

by Jon Connolly

The new pnsn.org website aims to provide a better user experience for all devices, prioritize features, and provide robust availability during a seismic event.
The July 2015 New Yorker article “The Really Big One,” by Kathryn Schulz, shook up the Pacific Northwest (PNW) more than any earthquake has since the Magnitude-6.8 Nisqually earthquake in 2001. In the article’s most dooming statement, the head of the Cascadia FEMA division was quoted saying, “everything west of I-5 will be toast.” This assertion scared the living daylights out of PNW residents, creating a sense of terror and hopelessness that was the antithesis of what the article meant to do: to spur the region into preparing for this potentially devastating event.

Unusual earthquake swarm south of Bend, OR

October 23, 2015

by Steve Malone

A somewhat unusual earthquake swarm started early on Oct 22, 2015 in an area about 65 km (40 miles) southwest of Bend, OR (25 km WSW of La Pine, OR). 36 events have been detected and located by the PNSN as of noon on Oct. 23, the largest only Magnitude 2.5. This ongoing swarm is in the same area that had similar swarms in 2001 and 2012 and is likely just the same sort of thing taking place again. For more details and updates....

How big was that earthquake?

September 24, 2015

by Steve Malone

Determining an earthquake's size seems to often result in different and inconsistent estimates. The "Magnitude" of an earthquake can be determined by several different methods, all of which should have some relationship to one another and, at least be consistent one earthquake to another. Unfortunately that's often not the case. PNSN seismologists spend lots of time estimating (measuring and calculating) earthquake magnitudes and end up discussing (arguing over) different techniques and complaining (whining) about inconsistencies and criticizing (belittling) certain results. Recently some effort is being made to try and refine (improve) our standard, routine ways of determining magnitude. We are starting to upgrade our published catalog with these "improved" magnitude estimates so you may see these estimates change from what was in the catalog before. Don't worry. The earthquakes have not changed, just our estimate of how big they are. For the gory details of how this is being done......

Summer rockfall time, yet again

August 21, 2015

by Steve Malone

With hot dry weather it is not surprising that the seismic records for volcano stations show lots of signals consistent with rockfall/avalanches and other exotic seismic events. In fact it is a bit of a surprise that we have not seen more and bigger such events this summer........ so far. Recent activity at Mount Rainier has included a debris flow (probably related to a jökulhlaup) and a moderate sized rockfall from high on the southwest side of the volcano. For some of the seismic details.....

Why earthquakes disappear

May 31, 2015

by Renate Hartog

Earthquakes have been appearing and disappearing from the U.S.G.S. webpages, this blog explains why.

PNW Earthquake Early Warning prototype goes live

February 18, 2015

by Steve Malone

Earthquake Early Warning (EEW) is now officially working for the Pacific Northwest (Washington and Oregon) in the same way it has in California for the past two years. It even got tested the first night in operations and worked...sort of. A workshop hosted by the PNSN at the University of Washington on Feb 17 introduced the topic and featured software to a group of about forty invited participants. For a few details on the workshop and the system's first live test......
The Seattle Seahawks' win over the Green Bay Packers in over time ended up so exciting that many of us serious scientists forgot to be serious and analyze the seismograms. Yikes! What a crazy ending. In fact, most of the game was seismically quiet (and disheartening for Seahawk fans), but the final half hour produced several seismic events that challenges the original "Beast Quake" for seismic supremacy. For our semi-scientific analysis......

Panther versus Seahawk Game Analysis

January 11, 2015

by Steve Malone

Both the PNSN experiment and the Seahawks were successful Saturday evening. Both got off to a slow start. The PNSN QuickShake display had several bad dropouts during the first half and at half-time the Seahawks were only ahead by four points. When working properly QuickShake provided us with "early Warning" of a successful play that would show up on TV a few seconds later. None of the signals compared to the size of those during the original "Beast Quake" of 2011 but some interesting patterns were seen. For more detailed analysis......
Last year the PNSN used the vibrations generated by enthusiastic Seahawk fans at CenturyLink Field to test instruments, data acquisition and web based displays. Some might say the seismic monitoring inspired fans to greater cheering resulting in the Seahawk's successful Super Bowl run. With new instruments recently acquired and improved data processing and display techniques developed we are again looking for somewhere to test them. With the Seahawks again in the playoffs with home field advantage why not watch/help them again? For the details.....

Canadian ETS morphing to Washington one?

November 18, 2014

by Steve Malone

Over two weeks of tremor in central Vancouver Island has been progressing southward. Though we don't know about the geodetic component we suspect that this represents a slip event that is propagating southward. Since a southern Vancouver Island-Northern Puget Sound ETS is due about now the question is will this current activity continue all the way to southern Puget Sound. For some details.....

Great ShakeOut, Great success!

October 16, 2014

by Angel Ling

Congratulations and thank you for your participation in the Great ShakeOut Earthquake Drill! Hopefully it gave you a great opportunity to practice "Drop, Cover and Hold on", and review and update emergency preparedness plans and kits/supplies. Thanks again to make the Pacific Northwest safer. Read more to see what the Great Washington ShakeOut looks like at the PNSN!