A few possible factors to consider. In the yearly cycle, though this only changes slightly over time, the Sun moves through the 12 constellations of the zodiac. If I understand my astrology then the Sun is in conjunction with the galactic center in Nov-Dec when it is in Sagittarius. The opposite time of year May-June the Earth is between the Sun & galactic center, but in both instances the gravity and other forces are at their peak, and these alignments are close to the same dates which only slowly change over long periods due to various precession, sidereal & etc. as the signs don't exactly match the constellations over the 26k year cycle. But by all appearances the peaks occur close to the same dates twice per year. You get an idea of these tidal gravity forces by how the moon effects the tides and other 28 day cycles like female periods, etc..
The black hole in the center of our galaxy in Sagittarious holds all the stars in the Milky Way spiral arms so they orbit it, its gravity stretches huge distances, and there's lots of talk of other effects such as the galactic wave theory that some were tying to Dec. 21, 2012 and supposed Gamma rays.
My point is this could account for the stated effects which would be greatest twice yearly but accentuated perhaps in Springtime when the Earth is between the Sun & the galactic center.
Even though those dates might be ahead of the effect you are seeing it might be caused as we begin to enter that gravitational lineup sort of like surfing the wave as it is forming like an incoming tide. In any event these periods must peak & ebb on a regular basis and there might be an ebb effect when the Earth is at right angles to this line also.
These periodic cycles would also be bookended by the Solstices & Equinoxes for example the Winter solstice is about Dec. 21 as the Sun leaves Sagittarious and is between Earth and the galactic center.
Now there is a chance that the HAARP operators are aware of these energy fluxes and coincide their "attacks" the amplify certain cosmic waves. It would be interesting to compare your graphs against the HAARP readings & it's well known here something was up the week before the Japanese quake and that there were releases of energy along Japan's faults that sattellites picked up.
Perhaps they are timing it to add to the resonance effect & focus it, such as scalar weapons and Tesla effects such as his resonance device that could bring down a skyscraper with a relatively small energy input. Is that enough doom for you? I emphasize the galactic wave which you can Google, and it references a gamma ray event about 14,000 years ago that might have wiped out some species & caused geologic effects, and is due to recur.