PARI's Sensitive Seismometer: an Overview
Seismic activity is measured using a three axis sensor system and a program called WinSDR [http://www.webtronics.com].
Z-axis (vertical) motion is captured using a Mark Products L-4 sensor called a geophone. This is our most sensitive seismic channel and is capable of resolving quakes of moderate magnitude worldwide. Even quakes too small to be felt register in seismometers hundreds of miles away.
To read the seismic traces, called helicorders, think of it as one long sentence starting at the top left of the graph. Each line ends on the right and then continues from the left side on the line just below, exactly as one would read a paragraph. 24 lines constitute 24 hours of data. The strength of the shaking broadens the up/down width of the line.
As with most seismic stations of this sensitivity, in inhabited areas, truck traffic, or even lawn mowers at PARI, may show up in the traces from time to time. A car door slamming for instance will register as a sharp spike. Heavy rain or high winds show up as a broadening of the baseline and have a very random nature. An example of 40mph winds and extremely heavy rain is at the top of the bottom picture on the left. After the storm front passed through the recordings returned to normal straight lines.
Earthquakes generally consist of several minutes of gradually decreasing vibration after a quick increase. With practice and comparison with other online seismometers, the distinctive signature of an earthquake will become obvious. The most useful confirmation of an earthquake is by comparing helicorder traces for the same time frame from widely separated stations. Remember that the earthquake will arrive first at the station closest to the epicenter of the quake.
Two waves propagate outward from the shear zone or epicenter where a quake originates. A “P” or Primary wave and an “S” or Surface wave. The P wave arrives first because it travels a more direct route. The S wave to P wave delay gives a measure of distance to the origination point. A blast at the Etowah Quarry 20 miles away for instance has almost simultaneous arrival of P and S waves. A quake in Chile 7 thousand miles away might have a six minute delay between the P and S arrival times.
A local Magnitude 2.6 earthquake 3 miles below the surface of the Earth, East of Winston Salem, NC on 10/17/2006 at 4:56am registered on the seismometer at PARI a mere 30 seconds after the actual event 162 miles away. These events are termed microquakes and feel very much like the close low frequency rumble of thunder.
The December 26th 2004 Sumatra M9 earthquake ( 0058 UTC) took 18 minutes to travel halfway around the world arriving at PARI at 01:16 UTC. Almost two hours of intense seismic activity followed before the aftershocks quietened down to the original baseline readings. Many large presumably related quakes occurred for days afterwords.
A Magnitude 8.7 earthquake in Sumatra on March 28th 2005 showed a very similar ring down. Almost an hour after the initial event the motion became a slow rhythmic motion coming in 20 second cycles and lasting on and off for the next hour on the instruments at PARI. No tsunami accompanied this large quake, but a number of people died in Indonesia in the ensuing panic fleeing the coastlines in fear of another tsunami.
The December Sumatra quake and tsunami that followed were the results of a tectonic plate making an abrupt shift in position. Normally this plate shifts in small movements totaling about 2 inches per year. During the magnitude 9.0 quake it moved over 50 feet in just a few minutes. This undersea movement displaced cubic miles of seawater resulting in the tsunami waves that inundated low areas throughout the Indian Ocean basin.
For data and further confirmation of recent earthquakes (size, location) go to the US Geologic Survey website ( http://www.usgs.gov ).
Below is the trace from PARI's Seismometer Channel 1 showing the 5.8 magnitude quake on August 23, 2011 that was centered near Charlottesville, Virginia
Many other good Earthquake webpages and tutorials are located atand .
Contact Ben Goldsmith (firstname.lastname@example.org) for questions regarding the instrumentation at PARI.