Has there been an increase in the frequency of seismic activity? First it was New Zealand, now Japan -- two major earthquakes in a span of less than 2 months. Cults could use this opportunity to sell their end-day stories. Joking aside, is there any scientific evidence showing that seismic activity has become more and more frequent these days?

Actually not. It happened that those two earthquakes get media attention because they happen in developed countries.

Actually there are earthquakes daily around the Earth. A vast majority of them is not harmful because the magnitude is low, or also because the epicentre lies from inhabited places.

Read about Clustering illusion to know what can produce the feeling you describe.