Seismo Blog

Earthquake swarm NE of Bremerton

May 11, 2017

by Renate Hartog

The Seattle Fault Zone runs West to East right through downtown Seattle and has the potential of being the source of a large (M6+) earthquake. It was therefore quite exciting when a small earthquake (Ml=3.3) happened right below the Seattle Fault Zone near Bremerton last week and was followed by aftershocks, and then again by a Ml=3.4 earthquake and another Ml=3.6 earthquake last night (as I am writing this our prototype Early Warning App is telling me another earthquake just happened, it is 10:57PDT on the 11th, this one turns out to be near Whidbey Island instead).


Rather than call it a mainshock-aftershock pattern, we can now call it a swarm, which basically means a cluster of earthquakes close in space and time that doesn't have an obvious mainshock. Swarms of small earthquakes are not unusual in our region. In fact, I found at least 6 more PNSN blog posts about swarms in WA or OR (links at the bottom of this post). However, this swarm is interesting in that it might be related to the Seattle Fault Zone. 


See the mapview below for locations of the earthquakes in this swarm. Important to know is that the smallest dots on the map are probably not in the correct location. These earthquakes are so small that they are not well-recorded on all the nearby seismometers, which makes it difficult to locate them well. We are quite convinced that these ocurred in the same area as the bigger ones (except for one, but more about that later) because their waveforms look similar to the bigger ones on those seismometers that did see all of them.


Locations of earthquakes in Bremerton swarm as of 5/11/2017 9:30 PDT


When we draw a North-South (i.e. perpendicular to the Fault Zone) cross-section right through the well-located part of the swarm, we see that these earthquakes are very deep, around 25km depth.


Location of cross-section profile


Cross-section through well-located part of swarm


The next maps shows where the various strands of the Seattle Fault Zone may come (close) to the surface. As you can see, the Seattle Fault runs right through the area of the current swarm. But, remember, these earthquakes are very far below the surface and can therefore not be on any of the Seattle Fault strands.


Approximate surface traces of Seattle Fault Zone fault strands as retrieved from USGS quartenary map server


To understand why, I am about to put in a very complicated and not very well re-produced figure from a scientific paper published in 2004 that summarizes some ideas about what the Seattle Fault Zone might look like at depth. I put red circles on the plot to show where exactly the current swarm is located.

(a) Schematic map and (b, c, d, e, and f) cross sections along lines A-A′, B-B′, and C-C′ showing different structural interpretations for the central Puget Lowland and seismicity. Hypocenters are from the Pacific Northwest seismic network for the years 1970–2001 and have magnitude ≥2 and depth uncertainty of up to ∼4 km. Swaths of hypocenters projected onto cross-section lines are 25 km wide. BI, Bainbridge Island; Br, Bremerton; EPZ, East Passage zone; PS, Puget Sound; RM, Rosedale monocline; S, Seattle; SFZ, Seattle fault zone; T, Tacoma, TF, Tacoma fault; VI, Vashon Island. (From Johnson et al., Tectonophysics, 2004,  DOI: 10.1029/2003TC001507)


Almost twenty years ago, in June of 1997, a magnitude 4.9 earthquake occurred very close to this swarm (see legacy page to read about it: That earthquake and its aftershocks were much shallower and therefore more likely actually located on a strand of the Seattle Fault. In fact, you can see those earthquakes at 10km and shallower above the red circle in the previous plot.



So, to answer, what is going on? We are having a nice vigorous swarm deep below the Seattle Fault Zone. The focal mechanism of the bigger events are consistent with North-South contraction.  But we don't really understand what causes these swarms. It is possible that more, and bigger, earthquakes happen. It is also possible that this area quiets down again. The swarm is providing us with some nice local waveforms that might be able to tell us more about the local crustal structure. For now, all we can do is what we do everyday, keep monitoring the seismicity in our region! What you can do is to make sure to be prepared for emergencies, check out the CREW website for ideas:

Volcano Preparedness May 2017

May 1, 2017

by Nancy Sackman

May is Volcano Preparedness Month for Washington State
On Monday morning (April 10) the Pacific Northwest Seismic Network (PNSN) was buzzing with activity, but not seismic activity. The Network hosted a press conference to announce the rollout of a new version of the earthquake early warning (EEW) system, ShakeAlert, which is now fully integrated across the entire West Coast of the United States.

Next ETS Expected any time now

January 24, 2017

by Steve Malone

Already over and then going again. Back-to-back ETS and finally over as of Apr 6.

Another Seahawks game experiment - Jan 7

January 6, 2017

by Steve Malone

Seahawks fans shake up the PNSN instruments.... again.
This Thursday, 50 million people around the world will drop, cover, and hold on for the 8th Annual Great ShakeOut, the largest earthquake drill in the world. This year at the PNSN, our motto is “drop, cover, hold on, and do something else too". We are thinking about other ways that we can enhance our preparedness for a major earthquake. This week on the SeismoBlog, we are outlining a few other strategies to supplement this year’s drill.

Cascade Volcano Seismology - a Tutorial

October 5, 2016

by Steve Malone

A newly modified tab on each volcano page gives a nice overview of each's earthquake history. Here is an introduction and some hints for interpreting these plots.

iMUSH: Adventures in the Field

September 12, 2016

by Lauren Burch

Seismology graduate student Mika Thompson shares a thrilling tale of wasps, fallen trees, and other impediments to science.

The Long Trek to MH09

September 8, 2016

by Shelley Chestler

Sometimes uninstalling a seismic monitoring station doesn't go quite as planned. Here is tale of my team's first attempt to take out a particularly stubborn station for the iMUSH (Imaging Magma Beneath St. Helens) project:
Both the Cascade Mountains and the Olympic Mountains are products of subduction, but not all mountain ranges are created in the same way.

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

December 4, 2015

by Jon Connolly

The new 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....