I was trying to think of a reason -if these were, in fact, planned explosions - what else they might be doing besides inflicting harm on California, as Cher has suggested. That reasoning didn't seem plausible. However it did seem plausible to me that explosions might be used for releasing pressure along the fault at key points (thus causing these acute, localized 'swarms' of quakes in Oregon, CA and maybe even the one in Ill.). So I googled some of those key words and found this article.
May seem far fetched but this article at least puts it within the realm of possibility:
Israeli startup develops method to predict and prevent earthquakes 2 Jan 2006
Terramoto, an Israeli company, has developed a three-stage method that its founder believes will provide the answers that scientists need to prevent major earthquakes from occurring along known fault lines....snip...
To give an accurate prediction of when this quake might occur, two lasers are placed on the different tectonic plates, and the interference pattern they create is measured to evaluate the relative velocity at which the two plates are moving, and in turn the energy accumulation inside the rock.
When a high-magnitude earthquake has been predicted, Terramoto's method calls for explosives to be drilled into the rock to create a series of controlled explosions that will weaken the rock and let the energy accumulated over the years be released gradually in a number of small quakes, rather than one devastating one.
The method can be adapted for fault lines along the ocean, with geophones sunk down to sea level with large cement blocks. Nachman admits that drilling to relieve pressure in these locations is more difficult than on land, but adds that it is not impossible.
The technological development of Terramoto's earthquake detection and prevention system is supported by scientists in the field from both Tel Aviv University and the Hebrew University in Jerusalem. Nachman believes that each system, which includes 6-10 geophones, will cost customers in the region of $1 million....>
http://www.mfa.gov.il/MFA/Israel+beyond+politics/Israeli%20startup%20develops%20method%20to%20predict%20and%20prevent%20earthquakes%202-Jan-2006