Wednesday, August 24, 2011

Quake Zone: The Passive Coastal Margin of North America

"Did you feel it?"

"Did it used to touch?" is the big question everyone in this office building was asking after rushing out into the street when it began to quiver and shake. Many workers in the Financial District of Boston are certain that the building on the right never used to touch the building on the left,
that is until 1:15 PM yesterday.

Yesterday, August 23, 2011 at 1:15 PM (EST), a 5.8 magnitude earthquake rocked the Mid-Atlantic East Coast. On the Richter Scale, 5.8 connotes a quake that is in the high intermediate range; 6.0 is considered to be noteworthy. The USGS reported that the quake occurred at a shallow depth of 3.7 miles with its epicenter at Mineral, Virginia, 40 miles northwest of Richmond. The movement apparently lasted for no more than 30 seconds. The quake sent shock waves up and down the East Coast to a third of the U.S. and was reported to have been felt in at least 22 states and northward into CanadaThe coordinates of the epicenter (the position of the quake on the surface of the Earth) in units of latitude and longitude were 37.936°N, 77.933°W. The source of this information is the Southeast U.S. Seismic Network.  

Mineral, Virginia, the epicenter of the quake, is located about 75 miles southwest of Washington, D.C. The town is situated within the Piedmont physiographic zone of the Appalachian Mountain chain.

About 12 million people living close to the epicenter reported intense shaking as powerful as that of a freight train. An overall feeling of jitters and rattled nerves prevailed for many of those that had never experienced an earthquake before, including myself. Reports of people running into the streets abound. Early reports of damage, which appears to have been minimal, include collapsed walls, snapped church steeples, damaged roofs and dismantled chimneys. Communication system phones were briefly overwhelmed. Numerous airports on the East Coast were temporarily closed, while flights were diverted. But overall, the damage appears to have been minimal with few reports of injuries and without any loss of life; however, the quake was disruptive in that buildings up and down the East Coast were immediately evacuated, including National Monuments in D.C., the Pentagon, and even the White House. Chivers ran high with the approach of 911's ten-year anniversary.

The epicenter was not far from Dominian Virginia Power’s North Anna nuclear plant. Its two reactors were protectively shut down without incident and power was lost, but its diesel generators were up and running. As an interesting twist of events, people on the west coast were calling friends and family on the east coast asking if they were all right.

The USGS 2008 hazards map
The seismic zones in central Virginia are in the low mid-range for hazard potential.

According to the USGS, the earthquake occurred as reverse faulting on a north or northeast-striking plane within a previously recognized seismic zone, the Central Virginia Seismic Zone. The CVSZ has produced small and moderate earthquakes since at least the 18th century. The previous largest historical shock from the Zone occurred in 1875. The 1875 shock occurred before the invention of effective seismographs, but the felt area of the shock suggests that it had a magnitude of about 4.8. A magnitude 4.5 earthquake on 2003, December 9, also produced minor damage.

Previous seismicity in the CVSZ has not been causally associated with mapped geologic faults. Analyses of past seismicity includes diverse focal mechanisms over a wide swath of land. This means that the seismicity is likely caused by several smaller faults instead of one general, causative fault such as the San Andreas Fault System.  

The locale of the Central Virginia Seismic Zone: The underlying bedrock is composed of a variety of igneous and metamorphic rocks that range in age from Proterozoic (~1,100 Ma) to Paleozoic. These rocks form the internal core of the Appalachian Mountain belt.

Previous, smaller, instrumentally-recorded earthquakes from the CVSZ have had shallow focal depths (average depth about 8 km). They have had diverse focal mechanisms and have occurred over an area with length and width of about 120 km, rather than being aligned in a pattern that might suggest that they occurred on a single causative fault.

Individual earthquakes within the CVSZ occur as the result of slip on faults that are much smaller than the overall dimensions of the zone. The dimensions of the individual fault that produced the earthquake will not be known until longer-term studies are done, but other earthquakes of similar magnitude typically involve slippage along fault segments that are 5-15 km long.

Most earthquakes that occur on the Eastern Seaboard of North America are the result of movement on ancient faults due to the relief of stress. Unlike West Coast quakes where one plate (the North American Plate) is pushing under or sliding past the other (the Pacific Plate or remnants of the extinct Farallon Plate), slippage in a "hidden" fault occurred near the surface at a relatively shallow depth.

Fractured bedrock on the West Coast dampens a quake's energy, while East Coast quakes are often felt widely because the bedrock is more solid. This dissipates the quake's energy over a longer distance.

Earthquakes in the central and eastern U.S., although less frequent than in the western U.S., are typically felt over a much broader region. East of the Rockies, an earthquake can be felt over an area as much as ten times larger than a similar magnitude earthquake on the west coast. A magnitude 4.0 eastern U.S. earthquake typically can be felt at many places as far as 100 km (60 mi) from where it occurred, and it infrequently causes damage near its source. A magnitude 5.5 eastern U.S. earthquake usually can be felt as far as 300 miles from where it occurred (Boston, my hometown, is 470 miles from the epicenter), and sometimes causes damage as far away as 25 miles.

The continental margin present on the east coast of North America, in fact on both sides of the Atlantic Ocean, is an “Atlantic-style” margin. It is considered to be "passive" in that regionally there are no plate collisions, no subduction zones, no volcanic activity, no mountain-building and no earthquakes (well, sort of). In contrast, “active” margins, such as those that surround the Ring of Fire around the Pacific Ocean, are zones of plate collision, and therefore possess all those things that a passive margin typically doesn't have. That isn't to say that the East Coast's passive, Atlantic margin possesses no activity. Active processes certainly are present in the form of active subsidence, sedimentation, growth faulting and migration.

Passive margins develop as continents rift apart to form new ocean basins, which is exactly what happened when the supercontinent of Pangaea began to break apart during the Late Triassic. The splitting initially occurs at a divergent plate boundary, but as spreading proceeds with the ocean basin widening, the active boundary remians in a mid-plate position at the center of the new ocean basin. That leaves the Atlantic margins passive in the sense that they are no longer active.

The tectonic framework in the vicinity of the seismic zone is extremely complex. The precise location of the pre-Rodinian (pre-Grenville) continental margin is unknown (although much of it can be assumed) and is thought to be west (using contemporary perspectives) of the seismic zone. Subsequent accretionary events such as the Grenville Orogeny, the closing of the Iapetus Ocean, the formation and subsequent rifting of Pangaea, etc. have cumulatively imprinted the Eastern Seaboard. Therefore, assigning the Central Virginia Seismic Zone to a particular tectonic event is difficult at this time, and will surely be a subject of discussion at an upcoming meeting of the Geological Society of America. 

Map showing the complex tectonic framework in eastern North America. Color-coded are the assembly of Rodinia, opening of the Iapetus Ocean, assembly of Pangaea and the opening of the Atlantic Ocean.
(From Tectonic Inheritance at a Continental Margin, William A. Thomas, 2005 GSA Presidential Address)

The USGS cautioned that a concern is that the quake represents a foreshock, implying that the worst is to come. If not, then perhaps aftershocks will follow. At least one is reported to have occurred. The following days will provide additional information, this report being posted the morning after the quake.

It all reminds me of two quotations. Will Durant, a U.S. historian, writer and philospher, once said "Civilization lives by geological consent, subject to change without notice." And, the poet Ralph Waldo Emerson who said "We learn geology the day after the earthquake." How apropos!

This post is a only preliminary report. More information is available at the USGS website located at

On the lighter side:

By Shelly 1990

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