(1) Department of Earth and Planetary Sciences, Northwestern University Evanston, IL 60208
(2) Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, PA 18015
(3) Weston Observatory, Department of Earth and Environmental Sciences, Boston College, Weston, MA 02493
The August 2011 magnitude 5.8 Virginia earthquake is part of an intriguing and not-yet-understood seismic zone that extends along the east coast of North America. As a result, it gives us an exciting opportunity to learn more about what’s going on.
Although the August earthquake is the biggest along the coast in recent years, others have been much larger. In 1933 a magnitude 7.3 earthquake happened in Baffin Bay, and in 1929 a magnitude 7.2 earthquake occurred off the Grand Banks of Newfoundland. Shaking from Grand Banks earthquake was felt as far south as New York but did only minor damage on land. However, the earthquake generated a huge underwater landslide that moved about 50 cubic miles of sediment, causing a tsunami that destroyed homes, ships, and businesses. A smaller earthquake, with magnitude about 6, struck near Cape Ann, north of Boston in 1755. Many buildings, especially on filled land near the harbor, suffered minor damage, mostly to chimneys. One with magnitude about 7.2 struck near Charleston, South Carolina in 1886. It took the city about five years to recover from the earthquake, and some of the damaged buildings can be seen today. These include the College of Charleston’s beautiful Randolph Hall.
These earthquakes are interesting because of where they happen. Most of the world’s earthquakes occur along the boundaries between plates, like the San Andreas Fault. That’s because earthquakes happen when forces in the earth make faults move. Plate tectonics explains the faults that are part of the boundary between plates and that the force to move them comes from the motion between the plates on either side. Earthquakes inside plates are rarer because the movements between plates are much faster than motion within plates. If plates behaved perfectly there’d be no motion within them, and earthquakes wouldn’t happen there. That’s almost, but not perfectly, true. Earthquakes do happen within plates, including along the east coast.
The east coast isn’t a plate boundary, because there’s no plate boundary there. Both the continent and ocean are part of the North American plate. Coasts like these are called “passive” continental margins, in contrast to “active” continental margins like that off Oregon and Washington, where there’s a plate boundary so the two sides are on different plates. How, then, can there be earthquakes here?
The answer seems to lie in the history of the continental margin. One of the great ideas of geology, called the Wilson cycle, explains why continents and ocean basins have different histories. The continents are made up of granite rocks that are less dense than the basalt rocks under the oceans. That’s why the continents stick up above sea level. This also means that continents and oceans have different life histories.
The Wilson cycle begins when part of a continent starts to be pulled apart. The granite crust stretches like taffy and starts to break along newly formed faults, causing earthquakes and forming a rift valley. Eventually the rift is filled by enough basalt that it becomes an oceanic spreading center. Spreading at the new ridge forms a new ocean. With time, the ocean widens, looking like the Atlantic today. The older seafloor, away from the ridge, cools off. The cooler rock contracts and gets denser, so the ocean gets deeper the further it is from the ridge. Sediments produced when the continents on either side erode accumulate in thick piles along the coasts, which are passive margins like the east coast.
However, the ocean can’t keep getting wider forever. Eventually, the plate breaks along one of the passive margins, and a new subduction zone forms. The plate being subducted gets smaller because it subducts at one side faster than than it’s produced at the ridge on the other side. Eventually the entire ocean basin is closed, so the continents on either side collide. This is happening today – the ocean basin between India and Eurasia has been subducted, and the collision between the two continents is pushing up the Himalayas, the highest mountains on earth. They go up, because continents don’t subduct. Eventually the mountain building stops. This leaves two continents plastered together with a mountain range between them.
Eventually, rifting starts within a continent, and the cycle starts again. Often the new rifting starts near the site of the earlier rifting, because continents don’t heal well. The eastern U.S. is a nice example. The Appalachian Mountains formed in a continental collision that closed an earlier Atlantic Ocean about 300 million years ago. Since then, the present Atlantic Ocean opened during the past 200 million years. This process left lots of fossil faults, old weak zones where earthquakes can – and do – happen. The Virginia earthquake probably happened on one of these.
This idea makes sense, because earthquakes happen on passive margins around the world.
A tougher problem is what forces cause motion – including earthquakes – on the fossil faults. At this point, we don’t know. Lots of different forces may be acting within the plate. One comes from the great mile-thick ice sheets that covered much of North America during the Ice Age. Although the ice sheets started melting about 18,000 years ago, the land is still moving as a result. Small motions, called “post-glacial rebound,” happen because the mantle below the earth’s crust flows like a super-gooey fluid – much stickier than road tar or maple syrup. The weight of the ice sheets pushed material in the mantle beneath them away, and it’s now flowing back. GPS data show Canada rising and the U.S. sinking. Post-glacial rebound is a likely cause of the earthquakes along the northern part of the east coast, where motions are geologically faster. However, although we only have a short history of earthquakes to work with, it looks like the energy released in coastal earthquakes decreases to the south, where the glacial rebound motions are less.
Another possibility is that forces due to plate motions cause the earthquakes. The North American continent is part of the North American plate, which extends all the way out to the Mid-Atlantic ridge. At the ridge, new rock is added to the plate. Over time, this slow addition has made the wide Atlantic ocean. As this hot rock cools, it gets denser and so moves away from the ridge. This cooling causes a force called “ridge push” within the plate, which is part of what makes the plate move. Computer models predict that this push should be transmitted into the continent. There are also forces acting on the bottom of the plate, which are caused by the mantle flow. Another possible effect comes from erosion of the Appalachian mountains.
Whatever causes the earthquakes has been going on for a long time. Geological studies show that the Piedmont and Appalachians have been going up relative to the Coastal Plain for at least the past 10 Ma. Thus the earthquakes seem to be part of a long-term process that we’d like to understand.
Another interesting question is why the earthquakes happen in “patches.” The August earthquake occurred on the northern edge of a patch called the central Virginia seismic zone. There are others like it, and also places in which we haven’t seen earthquakes. We don’t know whether these patches are more active over time, or whether the earthquake activity moves around. Perhaps the patches reflect aftershocks of large prehistoric earthquakes. We also don’t know why the patches have the orientations they do. In particular, why is the central Virginia seismic zone perpendicular to the coast and mountains?
These intriguing questions will provide lots of opportunity for studies in the next few years.
Our paper, Wolin, E., S. Stein, F. Pazzaglia, A. Meltzer, A. Kafka, and C. Berti,
Mineral, Virginia, earthquake illustrates seismicity of a passive-aggressive margin, Geophys. Res. Lett., 39, L02305, doi:10.1029/2011GL050310, 2012.
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