On the afternoon of August 23rd, 2011, an earthquake rocked the Mid-Atlantic region. Seismometers recorded its signature at 1:51 PM as it measured a magnitude of 5.8 on the Richter scale. The epicenter of the seismic discharge was located in northern Virginia, 38 miles northwest of Richmond. The shock waves from the subterranean shift were felt in areas of New Jersey, as far north as Montreal, Quebec, east to Rhode Island and as far south as Georgia.
It was a few minutes before 2 o’clock when I experienced the effects of the tremor (I was in my New Jersey apartment at the time). At first, I looked out the window to see if there was a strong gust or a nearby truck that could have generated the vibration. As the shaking gained momentum, I suspected it was an earthquake: how long would it last, how intense would it be, what danger was I in…there was no way to tell.
My cat was in the bedroom seemingly unable to move: her arms and legs were spread out as if to prevent herself from sliding. I picked her up, moved under the door frame of the bathroom and waited for the seismic impulse to pass. As I braced my arm against the door frame, I noticed the initial vibration gave way to a gentle sway: the resonance continued to build as if the apartment was slowly, rhythmically swinging on a giant pendulum. The standing picture frames were shaking in synchronization; all the while, I heard a low rumbling coming from outside. In the bedroom, the remote control hooked to the fan tower fell…at that moment the rocking stopped. It was silent for a few moments following the tremor then a commotion of startled residents descended outside. The entire event lasted about 30 seconds, but it seemed to unfold in slow motion.
I checked online and turned on the television to the local channel—an earthquake was confirmed. Many people I spoke with in the Tri-State area experienced the tremor. My brother was in his house a few minutes north of me, yet he did not feel the effects from the propagating vibrations. A friend living in Kentucky, west of the epicenter, did not notice the event.
The seismic waves of the Virginia quake traveled further distances and its effects were more intense compared with California tremors of equal size. Unlike the West Coast, which sits between two sliding tectonic plates, the Eastern U.S. is situated in the middle of the North American plate. The earth’s crust in the Eastern U.S. is more solid and dense, aiding in the propagation and amplification of the vibrations (the shaking). Seismic waves travel faster through solid rock like granite compared to gravel and soil. When subjected to strong shaking, moist sediment like silt (fine sand) becomes susceptible to liquefaction (process by which a solid behaves like a liquid).
Predicting the magnitude (size), intensity (effects), when and where an earthquake will hit is different from forecasting severe weather events such as tornadoes. U.S. meteorologists use Doppler radar to identify Tornadic Vortex Signatures (TVS), bow echoes and hook echoes from mesocyclones—as the event takes shape. Recent improvements in technology and training have led to a lower average lead time of 11 minutes for tornado warnings.
Japan has the most advanced early warning system for earthquakes. On March 11th, 2011, Japan broadcast a nationwide alert within seconds after a powerful 8.9-magnitude quake was detected, yet Tokyo residents had a lead time of just 80 seconds before the devastating tremor reached the city. Why? The answer lies in the phase velocity of seismic waves. Although tsunamis can reach speeds up to 500 miles per hour, seismic waves move significantly faster.
More about the velocity in a moment; let’s go over the four main types of seismic waves. Primary or P-Waves compress and dilate the medium (i.e. rock; soil) in which it passes through as it propagates in the direction of the underground force. Secondary or S-Waves oscillate perpendicular to the horizontal momentum of energy—up and down and side-to-side. P-Waves and S-Waves are referred to as body waves because it radiates through the Earth’s body (from the hypocenter below the surface). The other two distinct seismic vibrations are Love Waves and Rayleigh Waves, which are referred to as surface waves because it travels along the Earth’s surface (from the epicenter). Love Waves shift the ground side-to-side as it moves forward. Rayleigh Waves roll along the vertical axis in an undulating motion.
The shorter P-Waves cause little to no damage, and are often too faint to be felt by people (some animals such as dogs and elephants can sense vibrations from P-Waves). The longer S-Waves are typically the first vibrations we experience. S-Waves are capable of causing ground shifts and structural damage to buildings. Love Waves and Rayleigh Waves are more intense, causing the most damage, because the vibrations radiate along the ground instead of below the surface.
With respect to phase velocity, P-Waves are the fastest, followed by S-Waves then Love Waves and Rayleigh Waves. Put another way: body waves radiate more quickly than surface waves. The average velocity of an S-Wave is 2 – 3 miles per second; its speed varies depending on the composition of the Earth’s crust. Located 230 miles southwest of the epicenter, Tokyo residents would have experienced the S-Wave from the March 11th quake in about 90 seconds.
The typical speed of a tornado is 30 miles per hour; the fastest twisters move 60 plus miles per hour. To put it into perspective, I felt the effects of the Mid-Atlantic earthquake approximately three minutes after the seismic discharge, which was 330 miles southwest of my location. Hence, the subterranean speed of the tremors traveled near 7,000 miles per hour! In retrospect, the vibrations and effects I experienced that day were a series of distinct seismic waves arriving one after another.
[Side note: News and social media coverage of the Virginia earthquake was non-stop until the attention shifted to the approach of Hurricane Irene. 12 hours earlier at 1:46 AM EST on August 23rd, a quake, magnitude of 5.3, rattled Colorado (centered 180 south of Denver). It was the second tremor originating from the same location within a 7-hour period (first temblor, magnitude 4.6, struck at 7:30 PM EST on August 22nd). There was not as much coverage in the Denver news media or on twitter regarding the back-to-back events. Earthquakes of magnitude 5+ are uncommon east of the Rocky Mountains. The highest risk for seismic activity east of the Rockies is in the Ozark region.]
Another aspect of an earthquake’s power is the observed impact on built structures, above and below the ground, and its effect on the surrounding environment. This is referred to as an earthquake’s intensity. The modified Mercalli Intensity scale, the description of its values, and an approximate comparison to a quake’s seismographic measurement is provided in the following table.
The energy release from an earthquake is also determined by seismometers, and its magnitude is measured logarithmically. U.S. Geological Survey has an online calculator for comparing the size and strength of varying magnitude. Included is a table with differential references based on change in ground motion and energy release.
References: Sites visited / links viewed in researching information and creating tables for this piece.
Date (mm/dd/yy) article published in parenthesis. [Hyperinks are checked periodically. Previously referenced URLs no longer available, and added links are noted in brackets.]
http://www.diffen.com/difference/Mercalli_Scale_vs_Richter_Scale [added 8/23/17]
https://en.wikipedia.org/wiki/Mercalli_intensity_scale [added 8/23/17]
https://en.wikipedia.org/wiki/Moment_magnitude_scale [added 8/23/17]
https://pnsn.org/outreach/about-earthquakes/magnitude-intensity [added 8/23/17]
http://earthquake.usgs.gov/earthquakes/eqarchives/year/eqstats.php [webpage no longer available]
http://earthquake.usgs.gov/earthquakes/eqinthenews/2011/usc0001xgp/ (03/11/11) [webpage no longer available]
http://earthquake.usgs.gov/earthquakes/eqinthenews/2011/usc0005ic9/ (08/22/11) [webpage no longer available]
http://earthquake.usgs.gov/earthquakes/eqinthenews/2011/usc0005idz/ (08/23/11) [webpage available but no longer maintained]
http://earthquake.usgs.gov/earthquakes/eqinthenews/2011/usc0005ild/ (08/23/11) [webpage no longer available]
http://churmura.com/lifestyle/environment/earthquake/20286/ (10/21/09) [webpage no longer available]
http://earthquake.usgs.gov/earthquakes/recenteqsww/Quakes/usb0001igm.php (02/21/11) [webpage no longer available]
http://wcatwc.arh.noaa.gov/physics.htm [webpage no longer available]
http://www.geo.mtu.edu/UPSeis/magnitude.html [webpage no longer available]
http://www.geology.siu.edu/people/pinter/pdf/EQMagIntensity.pdf [webpage no longer available]
http://www.theblaze.com/stories/why-was-virginias-earthquake-felt-all-the-way-in-canada/ (08/23/11) [webpage no longer available]