early morning research

i'm trying to find out:

what does it mean when 3 earthquakes hit the same area in 4 days? this morning i woke to another shaker (david pretty much slept through it), and the first thing i thought of was the helpful info i read last night: if you're in bed, place a pillow over your head, and hold on. the quake was over before i had a chance to use the pillow, but i'm glad my brain recalled that tip.