Eastern North America has older rocks, some of which formed hundreds of millions of years before those in the West. Faults in these older rocks have also had more time to heal, which allows seismic waves to cross them more effectively when an earthquake occurs.
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Eastern North America has older rocks, some of which formed hundreds of millions of years before those in the West. Faults in these older rocks have also had more time to heal, which allows seismic waves to cross them more effectively when an earthquake occurs.