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Why New Jersey's earthquake was so strong in Philadelphia

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PHILADELPHIA (CBS) -- Friday's 4.8 magnitude earthquake in northern New Jersey is the strongest centered in the Garden State in centuries. Not since what is believed to have been a 5.3 magnitude quake shook the New Jersey Highlands in 1783.

The quake was centered 60 miles north of Center City in Whitehouse Station, New Jersey [Hunterdon County], and felt more than 100 miles away. In Philadelphia, we felt what was a steady shaking for up to 30 seconds. It was also a loud quake with rumbling ground that sounded almost like a freight train.

Of the nearly 200 earthquakes recorded in the United States on April 5, the 4.8 quake in New Jersey was the strongest.

Much of the northeast is considered to be on the interior of Earth's nearby plates in the Atlantic. There are old fault lines in the region and when enough stress builds up between the plates there is a release of energy. Sometimes that can trigger old faults beneath us. In the western U.S., the plates have created active faults like the infamous San Andreas Fault in California.

So, why would we feel a quake not directly linked to an active fault?

First, it's the type of soil and rocks beneath our feet. We have a loose, soft type of soil in the northeast which allows the energy to travel farther. In areas with dense rock, the energy does not travel as far.

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Second is the depth of the quake. In this case, the depth was just under three miles which is considered shallow. The more shallow the quake, the easier for the energy to travel along the surface. The energy from deeper quakes 43 miles [70km] or more beneath the surface takes longer to make its way to the surface and doesn't travel as far from the epicenter.

You will also feel more shaking on the upper levels of a building as opposed to ground level where simple road traffic could mask a quake.

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