After starting this blog with the intent of describing primarily the Nisqually earthquake, it’s come to have the broader goal of chronicling a wide variety of sizable quakes in the Northwest through the decades. With that in mind, I thought some statistical descriptions of those earthquakes were in order. So, here is a chart showing the frequency of 4+ magnitude earthquakes with an epicenter in Washington or Oregon, from 1970 through 2010:
This is an alternate presentation of the above data, in the form of scattered dots. It’s interesting to see that the year-to-year variation seems to diminish when you look at dots rather than bars of color to represent the number of quakes:
You’ve probably noticed that both of the above charts omit 1980, the year of the St. Helens eruption. The reason is that the number of 4.0 plus quakes connected to the eruption dwarfs the cumulative number of quakes in all other years of these four decades. Here’s a chart that includes 1980:
By the way, according to the PNSN, the total number of Northwest quakes since 1872 with at least 4 magnitude is 519. Of those, the number associated with the St. Helens eruption in 1980 was 315, from March 24 to May 21. Also, the PNSN notes that “this list is incomplete prior to 1970. Since 1970, the list should be complete, and include all WA and OR earthquakes magnitude 4.0 and greater.” As of mid-2011, the number since May 21, 1980 was 79. There were 24 on St. Helens on May 18, 1980. 13 of the ones since 1872 had at least a 6 magnitude, and 3 had at least a 7 magnitude.
By way of comparison, here is a chart showing the number of 7 plus magnitude earthquakes happening around the world in each year from 1973 through 2010:
And, again, the same data in the form of scattered dots:
And a final comment: although the 22 7 plus quakes in 2010 is the greatest in any year since 1973, it is not extraordinarily greater than the 15 to 18 quakes that happened in many years of the past four decades. In the U.S., we tend to think of years that include devastating quakes in our country as being especially bad: and yet 1989, the year of the Loma Prieta quake, had just 7 large quakes globally, and 1994, the year of the Northridge quake, had just 13, about an average year.