Wednesday, February 18, 2015

Forgotten Earthquake of October 1922

An interesting item showed up when looking through some old Anchorage climate forms. Back in October 1922, a notation was made by the Anchorage climate observer that read, "Violent earthquake 7:29 p.m. One & 1/2 minute! 2 shocks." Figure 1 shows the scanned form for October 1922.


Figure 1. October 1922 climate form for Anchorage, Alaska.

Since I had never heard of this earthquake before, I decided to look around a little to see what turned up. A search of the USGS earthquake database showed no events around that time. A search of the Alaska Earthquake Information Center database also came up empty. A cursory search of the Alaska Division of Geological and Geophisical Surveys did not find and reports or listings for an earthquake in 1922 (note: I did not read through any reports).

Having struck out with the go-to earthquake centers, I went back to NCDC and pulled more reports. It turns out that this event was noted at several other stations. Figure 2 shows the Local Climate Data report for October 1922 in Alaska.



Figure 2. Portion of October 1922 climatological summary for Alaska (p. 75).

Using the list from Figure 2 as a starting point, here are the stations within 350 miles of Anchorage and their notations for October 5, 1922:

   Seward --> 7:30 p.m. Severe earthquake on 5th
   Matanuska Exp. Farm --> Distant Earthquake at 7:27 p.m. on 5th
   Chitina --> Earthquake 8:10 p.m.
   Paxon --> Slight quake 7:30 p.m.
   Iliamna AP --> Earthquake 9:00 p.m. Intensity (illegible)
   Kodiak --> (nothing)
   Talkeetna --> (nothing)
   Dillingham AP --> (nothing)
   Valdez --> (nothing)
   Cordova --> (nothing)
   Yakutat --> (nothing)
   Chickaloon --> (nothing)
   Kennecott --> (nothing)
   Copper Center --> (nothing)
   Healy --> (nothing)
   Nenana --> (nothing)
   Fairbanks --> (nothing)

Of particular interest was the notation for Seward: "Severe earthquake on 5th." Figure 3 shows the Seward form for October 1922 and Figure 4 shows a map of the stations that "felt" the earthquake.


Figure 3. October 1922 climate form for Seward, Alaska.


Figure 4. Map showing all stations that noted the October 5, 1922, earthquake on Cooperative Observer (COOP) form.


While the terms severe and violent are quite subjective, they certainly indicated a level of shaking beyond the typical Alaska earthquake. On my way home I stopped by the UAA Consortium Library to look at old newspapers and see if there were stories about the earthquake. The Anchorage Daily Times from that time period was quite disappointing. Not only was there no story about the earthquake but there were no local stories at all – only national stories. The Fairbanks News Miner was a much better newspaper then the Anchorage Daily Times but it too did not contain any stories about the earthquake However, the Seward Gateway did have a news story about the earthquake (see Figure 5). 



Figure 5. October 6, 1922, edition of the Seward Gateway.

Where was it centered?

The newspaper noted the length of shaking at 0:28. The Anchorage climate form noted the shaking was 1:30. What does that tell us? Some references note that shaking is more intense and of shorter duration close to the epicenter. Other references say that a stronger quake will be felt longer near the epicenter. Who is right? I am not a seismologist and cannot speculate. However, given the statement that it felt like it moved from northwest to southeast, and given the fact that the Matanuska Experiment Farm station noted a "distant earthquake," it is not unreasonable to assume that the earthquake was centered somewhere very near to Anchorage. There are several active faults near Anchorage – most notably the Castle Mountain Fault.

Whose fault was it?

Since very little information is readily obtainable about the earthquake, plus the fact that several thousand people were living in the area, it is reasonable to assume that it was not a magnitude 8+ event associated with the Aleutians Megathrust Fault. However, less intense, deep earthquakes associated with this fault can occur almost anywhere in southern Alaska.

A USGS report from 1999 suggests that magnitude 7.0 is the approximate upper limit of locally generated earthquakes in the Anchorage area. These faults are located close to the surface and would generate considerable shaking. Earthquakes close to the surface generally have more aftershocks. No mention of aftershocks are noted on any of the forms in regard to the 1922 earthquake  Since the Castle Mountain fault, the main active fault near Anchorage, has been studies extensively and no record of a 1922 quake exists along that fault line, it appears doubtful that this is the source of the earthquake. Figure 6 shows the locations of major fault lines in southern Alaska,


Figure 6. Major faults in southern Alaska. Modified from original image found at:  http://www.sciencemag.org/content/300/5622/1113/F1.large.jpg .


Given the aforementioned constraints, the descriptor "violent" in Anchorage, the newspaper account of the "Severe" effects in Seward, and the fact that it was felt in cities 400 miles apart, a strong argument can be made that this was a deep earthquake somewhere in the magnitude 7 range centered near Anchorage.

Some readers are far more knowledgeable about earthquakes than I am. Any assistance you can provide in finding more information about this event would be appreciated.

October 5, 2015 Postscript

I communicated with a researcher with several published articles on historic earthquakes in southern Alaska. They read the blog post and said it was entirely possible the earthquake magnitude might be as high as 7 on the Richter Scale. That assessment is based solely on the information presented in this blog post and is not a formal opinion.

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