In August of 2011, seismologists recorded an earthquake mainshock in Virginia and this author felt it in Windsor, Ontario. A lot happened before and since, including probable foreshocks, damaged structures, and aftershocks. The foreshocks are past, the damage is repaired but the aftershocks continue to this day, and there is a rush to research them.
But the picture is not clear. There is a significant difference between studying foreshocks and aftershocks, and predicting earthquake mainshocks. Not the least difficulty is distinguishing between them. Sometimes what appears to be a mainshock is actually a large foreshock of an even larger earthquake.
What we do know, however, is that the mainshock is ultimately the quake of greatest magnitude.
In the case of Virginia, the U.S.G.S. has used known earthquake mainshock patterns to map hazard zones. One such zone is in central Virginia in the area between Charlottesville and Richmond. Four earthquakes with significant magnitudes have occurred in this zone in the last 140 years: 1875- M4.8; 1984- M4.2; 2003- M4.5; 2011- M5.8. And since the 2011 quake, scientists have recorded more than 450 aftershocks.
In response, researchers buried an array of twenty seismometers and electronic data loggers in the Central Virginia Seismic Zone. Specific locations range from Charlottesville in the west to east of Richmond, VA. The area covered is about 40 miles north and south of Interstate 64.
What is a Seismic Array?
While there is no definitive definition of a seismic array, in general it is a set of seismographs distributed over an area of the earth’s surface at spacing narrow enough that seismologists can correlate signal waveform between adjacent seismometers. They allow scientists to measure, for instance, the spatial variability of earthquake intensities near a seismic wave source.
One large scale example of an array is the just-completed $90-million Transportable Array, an ambitious effort to blanket the contiguous United States with a mobile grid of seismic monitors. As Alexandra Witze explains in Nature, the array’s 400 stations are arranged at 70-kilometre intervals in a regular grid.
Each monitor stays in the ground for roughly two years before being dug up and redeployed to the array’s leading edge. The stations capture seismic waves traveling through the ground from earthquakes near and far (and) allow researchers to construct images of the deep Earth, rather like a geological computerized-tomography (CT) scan.
Virginia quakes occur on faults that lie from 3-15 miles below the surface – and their waves, traveling through relatively solid rock, reach much farther than waves in the west traveling through chopped up rock masses. Damage also extends much further. As Jennifer Young reported in Decoded Science, research into the extent of damage of the August 2011 quake revealed that it caused damage over an area many times larger than predicted or seen before.
To fully appreciate and understand this characteristic of mid-plate earthquakes, we need to revisit continental drift. Virginia, for instance, is in the middle of a tectonic plate moving away from the mid-Atlantic Ridge.
In other words, the east coast of the United States marks the boundary between continental and oceanic crust. So, while Virginia is on the edge of the continent, it is not the edge of the continental plate. California, in contrast, is at the western edge of the North American plate and marked by more distinct fault lines.
Aftershocks for Research
Aftershocks present opportunities for two areas of research, namely:
- A study of the sequence of mainshocks in order to compare and contrast large earthquakes; and
- A study of aftershocks in order to obtain an after-image of fault ruptures.
Because array seismometers are closely-spaced in a grid pattern with unprecedented density, the recorded wave amplitudes at each seismometer can clearly show through time how wave after wave progresses along the great circle path from an earthquake’s epicenter.
Central Virginia Seismic Zone
The Central Virginia seismic zone is laced with known faults but numerous smaller or deeply buried faults remain undetected. The goal in Virginia is, therefore, to “record tiny ground vibrations caused by earthquakes, and (have) the science team… use the data to better understand earthquakes in the Central Virginia Seismic Zone. Network sensors will also help determine if the earthquakes align with specific faults (such as Pacific fault) by increasing the number of earthquakes detected and improving the accuracy of the locations.”