BLACKSBURG, Va. (WDBJ7) A magnitude 6.5 earthquake struck about 100 miles off the Northern California coast on Thursday morning, according to the U.S. Geological Survey. The quake triggered tsunami watches which were later dropped with no significant waves reported.
Two thousand miles away at Virginia Tech's Seismological Observatory, a helicorder was recording signals from the seismometer.
Seven minutes after the California quake, the device in Blacksburg recorded several spikes, seen in red on the image above.
Martin Chapman, Research Associate Professor of Geophysics at Virginia Tech examined the plot Thursday evening.
"The California earthquake is near the bottom of the graph. The P-wave of the earthquake is the largest amplitude arrival which is the red spiking, 4th line from the bottom. It should arrive slightly less than seven minutes after the origin time of the earthquake," explains Chapman.
P-waves are a type of elastic wave, and are one of the two main types of elastic body waves, called seismic waves in seismology, that travel through a continuum and are the first waves from an earthquake to arrive at a seismograph.
The seismic waves travel at more than 8.5 kilometers per second, so it takes only a few minutes to cross the entire continent. Martins points out that a surface wave appears in the plot as a green colored trace at about 10:13 a.m.
READING A HELICORDER PLOT
The different colored lines are there to help determine the time. Each horizontal line is 10 minutes long. The vertical lines are 1 minute apart. Time advances from left to right. The time is listed in Eastern Standard Time in the left margin, and Universal time (London, UK) in the right margin.
Typically, the traces are fairly flat, with only a small amount of waviness. Sometimes you can see a distant quarry blast or other small event: blast. You may even see a smaller earthquake, which appear in on the device quite often, but are never felt by humans.