April 22, 2013 3:00 pm
On August 23, 2011 a rare magnitude 5.8 earthquake hit Virginia. The shaking cracked the Washington Monument, toppled part of the National Cathedral and shook around a third of the U.S. population. Later that week, Hurricane Irene moved into the region, wiping out power, downing trees and, according to new research presented at the meeting of Seismological Society of America, says Nature, triggering more small earthquakes in the recently ruptured fault.
The rate of aftershocks usually decreases with time, says study leader Zhigang Peng, a seismologist at the Georgia Institute of Technology in Atlanta. But instead of declining in a normal pattern, the rate of aftershocks following the 23 August, 2012 [sic], earthquake near Mineral, Virginia, increased sharply as Irene passed by.
The waves of the Virginia earthquake were felt far and wide.
Hurricanes are known to produce strong seismic waves all by themselves. Indeed, says Smithsonian‘s Surprising Science blog, Hurricane Sandy “generated seismic shaking as far away as Seattle.” But hurricane-triggered seismic waves these were not. These were real aftershocks. “Scientists did not initially notice the unusual pattern, Peng said, because the aftershocks were small (many below magnitude 2) and the hurricane itself produced a lot of seismic noise.” A careful analysis of the data, however, revealed that the aftershock activity actually rose around the time of the hurricane’s passing.
The scientists, says Nature, argue that “a decrease in pressure caused by the storm’s travel up the East Coast might have reduced forces on the fault enough to allow it to slip.” More research will be needed to definitively pin down the proposed tie between the hurricane and the earthquake. But the suggestion that the Virginia fault system would have been susceptible to the stresses caused by the hurricane aligns well with the idea that big natural systems, sometimes treated as if they act independently of the world around them, might actually all be connected.
The Irene-triggered aftershocks could have happened because the fault system that had ruptured in Virginia has memory—that is, the fact that it slipped so recently makes it easier for it to do so again. The idea of a natural system having memory is one that is becoming increasingly important for scientists trying to understand natural disasters. The idea is important to the field of complexity science. In a previous interview by this author with Surjalal Sharma, the University of Maryland astronomer explains this idea of memory:
“Memory is, essentially, a correlation in time or space. My memory of past events affects what I do now; that’s long range or long-term correlation. The bunching or clustering of events is, as we understand it, due to the memory of the events in a system. That is, a sequence of natural disasters may not be just a coincidence. [I]f we look at the data for floods, earthquakes, or solar storms, we see that their distributions are [not shaped like a bell curve.] This indicates that these are not random events. Rather, these systems have long-term memory.
So in the case of space weather, let’s imagine that a coronal mass ejection reached the Earth and disturbed the magnetosphere. There are two things about this disturbance that we need to characterize: one, how long does the visible or measurable effect of the disturbance last? The other is, how long would this system remember that the disturbance happened? If a second coronal mass ejection were then to come along within the memory time scale, the disturbance is likely to be much bigger and more prominent in some ways than the first, even if the two ejections are of similar intensity. It is in this context that we have to worry about long-term memory. As one might imagine, this is very important for extreme events.”
A fault that has slipped as an earthquake loads more stress. More research is needed, but if it turns out to be the case that hurricanes really can cause earthquakes, then Gaea just got a whole lot more dangerous.
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