April 8, 2013
by John Vidale
The event on Saturday night (well, early Sunday morning), and its half dozen aftershocks big enough to locate, were typical of Seattle region deep seismicity. The events broke the ground 20-25 km deep, in a region that has had similar small earthquakes before during the 40 years that the PNSN has been running.
To see more detail, I selected all earthquakes in our catalog from 47.25 to 47.4° N and 122.6 to 122.9° W, plotted them on the map, and then plotted a cross section.
The earthquakes last night are the blue ones in a cluster in the middle of the map. More details about last night's activity may be seen on our usual seismicity page. A few have been in the area previously, and many have been nearby.
The A-B cross section through the seismicity (all the earthquake hypocenters projected at right angles onto the red A-B line, then their depth plotted vs their A-B position) is shown below, and the depth of the earthquakes is typical. Last night's events are among the cluster of about 10 events around the one marked by the mainshock with a green circle, in the middle of plot. The events have several km uncertainty in their location, so we cannot interpret the pattern observed beyond that the events all locate quite close together.
In 1972, another M3.4 event was observed in the area (the nearby green circle shallower in the cross-section, at 5km depth, and the yellow symbol on the map). You can make similar maps, too, with the Custom Search function on our web page.
The events last night and in 1972 are in the neighborhood of the Tacoma fault, but whether the fault extends that deep and locates precisely there are not clear.
The potential for activity on the Tacoma Fault is serious, as pointed out by this review, but an M3.5 does not indicate much danger. As usual, we seismologists just watch a little more carefully following such earthquakes, but consider that nothing too alarming is happening.
- Why earthquakes disappear
- PNW Earthquake Early Warning prototype goes live
- Packers versus Seahawks game analysis -- too exciting
- Panther versus Seahawk Game Analysis
- Seismology will again watch/help the Seahawk's playoff run
- Canadian ETS morphing to Washington one?
- Great ShakeOut, Great success!
- The Great ShakeOut 2014 is Tomorrow!
- Three Cascadia ETS events in past month??
- Is Mount St. Helens seismicity increasing?
- 2015 (5)
- November (1)
- October (2)
- September (1)
- July (1)
- May (1)
- April (1)
- March (2)
- February (3)
- January (4)
- October (1)
- September (1)
- August (1)
- June (1)
- April (3)
- March (4)
- February (1)
- January (2)
- December (1)
- November (2)
- October (3)
- September (1)
- August (3)
- July (2)
- June (4)
- May (4)
- April (2)
- The wech-o-meter takes over all of Cascadia
- Keystone Cops: Italy prosecutes seismologists for failure to predict deadly quake
- UFOs in eastern Washington? No, rather UTEs (Unidentified Terrestrial Events)
- New Sodo Seattle Liquefaction Array Installed
- Why we should constantly watch the deformation of the seafloor
- Mystery chirp near Newberry Volcano
- Planting seismographs causes earthquakes? or maybe ice-quakes?
- Tunneling rumbles south under Capitol Hill
- 15 years of mostly silent magma inflation near Three Sisters, Oregon
- Mount Hood earthquake swarm of Feb 23, 2012
- Web glitches: duplicate (and even triplicate!) earthquakes
- How earthquake magnitude scales work
- Mine blast masquerades as volcanic tremor
- The Spokane Swarm about 10 years ago
- Another hum around Mount St. Helens
- Slow slip: A new kind of earthquake under our feet
- PNSN and social media
- 3am M3.4 earthquake in St. Helens Seismic Zone
- The wrong kind of volcano noise
- Fast chatter on Rainier an hour ago
- Can slush-mageddon trigger earthquakes?
- Rainier Repeating Earthquakes Update and Comparison with Weather Patterns
- 22-minutes drumbeat icequakes(?)
- Mount Rainier popping away
- Repeating Earthquakes on Mount Rainier - are glaciers the culprit?
- Debunking another SEC football myth by the PAC-12
- One year ago, Seattle Seahawks 12th Man Earthquake
- The odds this year of a megaquake on the Pacific Northwest coast
- Is the plague of great earthquakes this decade a sign of increased danger?
- Nile Valley landslide talks to PNSN seismologists
- Good vs evil in central US earthquake hazard analysis
- Why does a volcano scream?
- Predicting big quakes from patterns of little ones
- 1-hour warning for Japanese M9 earthquake?
- Sound Transit train under Interlaken keeps a rollin'
- Invisible changes under the hood at the PNSN
- Sound Transit Tunneling Noise
- "Visionary" toads
- Earthquake early warning in the PNW
- November (1)
- December (13)