Seismo Blog

Entiat area earthquakes and other seismicity

August 9, 2017

by Steve Malone

Entiat Area Seismicity

A persistent producer of small earthquakes is located near the town of Entiat, WA just south of Chelan.  This area has been known to be active since 1975 when the first set of seismic stations were installed near this area.  While the largest earthquake in this area was a magnitude 3.7 event in July, 1997 the PNSN locates about 55 earthquakes here each year, most less than magnitude 2 and thus not felt.  Below is a detailed map color coded by event depth (red- <1 km, blue- > 10 km) of all located events over this 46 year period.  The main zone of activity is centered on Entiat and is slightly elongated stretching about 24 km in the E-W direction and 16 km in the N-S direction.  Most events have a depth between 2 and 8 km.  Looking at cross-sections at many different angles there are no obvious structures indicating a single or few faults.

Unlike other places, particularly in eastern Washington the seismicity near Entiat does NOT take place in strong, isolated swarms.  While some of the larger (Mag > 3) events may have a few aftershocks most seismicity just putters along at a fairly uniform rate with minor fluctuations over weeks to months.  Here is a cumulative seismicity plot over the 42 years showing a nearly straight line over this whole period.

So the question has always been, why are these earthquakes here?  Why do they have a different time sequence than most other sequences in the Pacific Northwest?  Also, why have there been no larger earthquakes during this period. Based on a normal Gutenberg-Richter size distribution there should have been at least 2 earthquakes greater than magnitude 4 in the past 46 years.

While always suspected as being possibly related to a magnitude ~7 earthquake that took place somewhere in north central Washington in 1872, it is only in recent years that there has been increasing evidence that the Entiat zone is the location of that large earthquake.  Bakum and others (doi: 10.1785/0120010274 Bulletin of the Seismological Society of America December 2002 vol. 92 no. 8 3239-3258) using modern techniques for interpreting an extended set of felt reports suggested the area south of Chelan as the 1872 earthquake source zone rather than areas in southern British Columbia and the north Cascades as pervious studies had suggested.  More recently Brian Sherrod presented a paper that used LIDAR images to pinpoint a suspected surface fault just to the north-east of Entiat.  Close up investigations suggest this fault had a 2.5 m off-set sometime in the past few 100 years.  Just this year Brocher and others presented a paper examining the historical (felt report) catalog of possible earthquakes in this area to suggest that the on-going sequence is consistent with it being largely aftershocks from the 1872 event.  Both of these latter studies have not yet been published though the Brocher study is currently "in press".

Thus the Entiat earthquakes of the last few weeks are nothing new and there is no reason to think that these sorts of events will not continue for years if not longer.

Catalog Magnitude Changes

Those that keep a close watch on our recent earthquake page may see a very recent event (red on the map) change its magnitude from that first reported; usually getting smaller.  Our automatic detection and locating system will often get the location of an earthquake quite accurately but does a poorer job with magnitude.  Any event located where there are few seismic stations or ones that are noisy or problematic for other reasons can get an artificially inflated automatic magnitude.  A duty seismologist (human eyes) can easily spot the problems with such errors and then make appropriate corrections.  In some cases the magnitude (and even location) may change hours later.  Our master analyst reviews all events sooner or later.  There are about 6 or 8 rotating duty seismologists who are tasked with responding quickly to review the automatic events.  For consistency we have one super expert review everyone elses work some time later.

Bogus Tremor Events

Since we are on the subject of correcting errors, some may have noticed a scattering of isolated tremor events on the tremor monitoring page (Wech-o-meter) up and down Cascadia.  Most of these recent isolated events are bogus; ie false locations.  We have been having trouble with one of our wave-form storage systems that seems to generate small data gaps now and then.  Normally, this would be no problem but when many occur at nearly, but not exactly the same time it can generate an apparent set of wiggles in filtered data that fool the wech-o-meter locater.  Our policy is to just leave these locations in the catalog unless there are lots of them and are clearly all bogus.  Recently that is not the case.  In fact there has been a very typical southern Oregon - Northern California ETS going on for the past several weeks.  Here is a map of the tremor locations from Jul 22 to Aug 8 color coded by time (blue - oldest, red- newest).  Note that this ETS started just west of Weed and spread both south and north and then jumped farther north with a small gap near Grants Pass.  It may still be going on.  However, note that there are a few isolated locations north near Corvallis and others near and south of Eureka.  Looking in detail at the data during these events it seems that most (maybe all) are due to these data gap glitches and are not real tremor locations.  One of our engineers may have solved the problem yesterday and we are waiting to see if it is a long term solution.

Earthquake swarm NE of Bremerton

May 11, 2017

by Renate Hartog

Earthquake swarm near Bremerton; what is going on?

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 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.