Why are Earthquakes Felt So Widely over the Eastern US?

3.9 Earthquake in Newhall

Several weeks ago, I woke up to the sound of my "Quakealarm" ringing, feeling a very small rolling motion a few seconds later. After that I immediately went back to sleep. I later checked USGS and found out that I awoke to a magnitude 3.9 earthquake near Newhall, CA. The epicenter was about 40km away from my house.

Several weeks earlier, on the East Coast, there was a magnitude 4.0 earthquake in Southern Maine. My friend who was in Connecticut,  felt the shaking (she is a native Californian, so she has felt many earthquakes). The shaking amount was a little more (About III-IV) than the amount I had felt in the earthquake (About II) in Newhall. Interestingly, my friend was over 200km away from the epicenter. How is an earthquake on the East Coast felt farther than a similar size earthquake with a similar depth, in California?

4.0 Earthquake in Maine

Well, the answer is simple.

In California the geology of the earth is very different than the geology in the East Coast. In California, the ground is less dense than on the East Coast, where the ground is more solid. Lucy Jones in USGS has said many times to TV reporters: "A solid bell rings more than a cracked bell."