They’re also studying the ongoing aftershock sequence. The Mineral event isn’t truly over, because the fault is still generating small quakes, though fewer and fewer. Herein lies a research opportunity for understanding enigmatic East Coast tremors — how long does it take for the earth to calm down?
And Mineral is also a warning. No one was shocked when a moderate quake hit the Central Virginia Seismic Zone — that’s precisely where you might expect an earthquake. But it was much bigger than any recorded tremor in the zone. The previous standard was a roughly 5-magnitude event in 1875, its strength estimated from the damage it caused.
The obvious implication is that other seismic zones in the east are capable of generating earthquakes more powerful than anything in the historical record.
“The Virginia earthquake suggests that what we’ve seen in the past is not as bad as what we could potentially see in the future,” said John Ebel, a Boston College geophysicist who spoke with reporters in a conference call Wednesday from a meeting of the Seismological Society of America in San Diego.
Based on what happened in Virginia, “we have a seismic hazard that we really have to take seriously” in other major East Coast cities, including Philadelphia, New York, Providence and Boston, Ebel said.
‘Highly stressed’ rocks
East Coast earthquakes are inherently mysterious. There are no tectonic plate boundaries nearby — no big slabs of crust grinding and sliding and heaving past one another. The Mineral quake occurred on an unmapped fault.
Scientists have studied the aftershocks and have been able to model the fault and understand how the earthquake had such strong effects in Washington. This earthquake broke in such a way that it essentially threw a haymaker at the nation’s capital.
The Mineral quake nucleated about five miles below the surface on a fault near Interstate 64. The rupture propagated to the northeast. An earthquake doesn’t generate waves like a pebble dropping into a pond, but rather can send them preferentially in one direction. In this case, the way the fault ruptured led to intense shaking in Washington and along the Chesapeake.
All of the East Coast is theoretically vulnerable to a major earthquake, but such events are assumed to be very rare. Martin Chapman, a Virginia Tech geophysicist who has been studying the Central Virginia Seismic Zone since the 1970s, said Wednesday that a quake the size of August’s occurs roughly every 400 or 500 years in that area.
“It’s something that I’ve been sort of expecting at some point, but I didn’t expect to get a magnitude 5.8 in my professional lifetime,” Chapman said.