California earthquake more mellow than East Coast quake

California earthquake vs Virginia quake

Wondering why the 6.0-magnitude California earthquake in 2004 was felt only 200 miles away (as far away as San Francisco and Los Angeles) while the less powerful magnitude 5.8 Virginia quake last Tuesday felt as much as 500 miles away (as far north as Ottawa, Canada; as far south as Georgia, and as far west as Chicago)?

According to U.S. Geological Survey (USGS) seismologist Lucy Jones, it is because of the differing geologic conditions of these two areas.

"The crust out in the East Coast is older, colder and harder, and does a better job transmitting the energy. So it's felt over a much wider area," Jones told LA Times.

In contrast, Jones added that in California, "the bad news is we have a lot of faults. The good news is all those faults break up our rock. And just like a cracked bell doesn't resonate as well as a solid bell, our cracked-up crust [weakens] the waves as they pass through. It dies off pretty rapidly.
"But out there, the rocks are old and cold and hard, and do a lovely job of transmitting the energy," Jones said.

A USGS article had also explained the phenomenon.

"Western rock is relatively young, which means it absorbs a lot of the shaking caused by earthquakes. Thus, western earthquakes result in intense shaking close to the epicenter, but fade more quickly the farther the earthquakes travel.

"In the eastern United States, on the other hand, the rock is far older, and so earthquakes can have a much larger and more widespread impact. Earthquake energy can therefore spread farther and have a greater impact." USGS wrote.