Seismo Blog

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.

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.