May 28, 2012
by John Vidale
At the PNSN, we're switching software this year, which makes it a good time to improve the earthquake catalog. I spent today pulling weeds from the list of earthquakes offshore Oregon and Washington.
Here is a minor mystery, the only near-shore events between 1980 and now that are puzzling.
There are a pair of M2.4 and M2.5 events that are 90 minutes apart and nearly identical, and odd in other ways as well! They struck in the late afternoon, on September 2nd 1992, 20 km offshore (see map). All the surrounding events from all the years of the catalog looked much more normal (higher in frequency content, with clear S waves).
These are the two closest stations, OOW and OSR, the little tick marks are one second apart. The P waves are unusually long-period, and the seismograms show no S waves, and so I suspect non-earthquake origins (but 20km offshore?). To be precise, they struck at at 23:13 and 00:44 UTC, on 9/2 and 9/3.
Here's a closer look in the 3rd figure, click on it to zoom in, the seismograms are VERY similar. Even the second or two BEFORE the clear arrival looks somewhat similar, maybe the start of the signal at this station is emergent, although it starts abruptly on adjacent stations. They start to differ 30-40s back, toward the end of the plot. Maybe that could be used to estimate the distance between the two sources.
Here are all the stations for the second event (click to zoom) to illustrate the quality of the data. Not great, but enough to locate it and see that the S waves are negligible, at best.
Any ideas? Explosions at sea? Undersea sliding? They could just be two earthquakes that happen to be very similar, but then why so long period and without S waves?
It's the day of a famous M7 slow Nicaraguan earthquake that caused a tsunami, but that's probably irrelevant.
- Three new ways to view recent earthquakes in the Northwest
- M3.5 event west of Tacoma early Sunday morning
- Oregon ETS is over, but....
- Small swarm near Mount McLoughlin last night
- Earthquake early warning workshop quick report
- thePNSN Facebook discussions
- Deep Tremor over much of Cascadia
- Small earthquakes under Gold Bar
- The Last Cascadia Great Earthquake and Tsunami; 313 Years and Ticking
- A flash in the sky, a thump in the ground
- 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)