A magnitude 5.8 earthquake occurred in Mineral, Virginia today. The shaking was felt all over East coast, even by yours truly. My gut reaction to the slight swaying of our office building was not fear, but excitement!
Last year, I blogged about the trials and tribulations of my graduate earthquake engineering class (and I aced it!) Who knew I cared so much about earthquakes?
I felt validated today for all that hard work, and distinguished knowing a bit more about earthquake design than the average civil engineer. So, I thought I would share my enthusiasm and answer some common questions regarding today’s quake:
Since when do we have earthquakes on the East Coast?
Since 1755! Believe it or not, the one of the earliest reported U.S. earthquakes occurred off the coast of Cape Ann, Massachusetts in 1755! Eventually, the West was won and California took the seismic crown, recording its first earthquake in 1769. Earthquakes happen all over the U.S. and the USGS regularly maps seismic hazard on maps like the one shown below.
As you can see, Virginia is a hot spot in terms of seismicity, comparable with the West Coast. Some of the southern states (Missouri, Arkansas, Tennessee, Kentucky) are also quite active.
Usually, earthquakes occur along the tectonic plate boundaries, shown below.
This explains the heavy seismic activity all along the West coast. We also see that Japan and Indonesia are located along a plate boundary known as the Ring of Fire. Less commonly, quakes pop up in the middle of plates as individual hot spots, Hawaii being the hottest of all. There is still research being done on these regions to explain why the ground is susceptible to quaking.
Why did we feel the earthquake so far away from the epicenter?
Attenuation is the word of the day! Earthquake waves on the East coast travel farther than on the West coast. That’s why a quake originating in VA was felt all the up here in Boston, and as far west as Michigan. Waves attenuate (lose intensity) more slowly on the East coast because the grouns is easier to travel through.
In California, there are more fault lines to deflect and absorb the wave energy. A magnitude 6 quake in L.A. could go unnoticed in San Francisco!
How come I didn’t feel anything?
Even though shaking was recorded all along the East coast, many people missed it. I happened to be in a 10-story structure in Back Bay, and we felt a fair amount of sway lasting for almost a minute. My boyfriend, just 10 miles away, felt nothing. The difference is that I was in a tall building, founded on soft soils. He was on the 2nd floor of a building built on rock.
Earthquake waves travel through the ground at a certain frequency. Depending on what type of building you are in, or what kind of ground you are standing on, the waves may shake you a little or a lot.
Tall, long structures resonant with low frequency shaking. By the time the waves from Virginia reached us, they were low and slow enough to shake my office building.
Could a quake in Virginia result in a tsunami?
Most likely, no! Tsunamis occur in subduction zones where two plates are colliding and one is being ‘subducted’ or pushed beneath the other. We saw this with the Japanese earthquake a few months ago.
There is potential for this to happen in the Pacific Northwest, it is pretty much the exact scenario depicted above.
Is this the end of the world?
I hope not, but it is kind of crazy!
FYI: A magnitude 10 earthquake would blow up half the earth.