An earthquake that actually "shudder" throughout the continent

July 24, 2020

Updated on July 26, 2020 at 8h34

An earthquake that actually “shudder” throughout the continent

Jean-François Cliche

The Sun

24 to the SECOND / on Wednesday, in the wee small hours of the morning, a major earthquake of 7.8 magnitude shook Alaska. It has certainly not been felt by Mr. and Mrs. All-the-World on this side of the continent, but the seismographs are instruments so sensitive that its signal has been recorded up in Haiti !

iris.edu

The IRIS, a consortium of american research in seismology, drew, the amazing animation above, which shows how the vibrations spread from one end of the continent to the other. Include from the outset that it is a condensed : the video only lasts 36 seconds, but shows the events that have stretched over nearly three-quarters of an hour. We see the epicenter of the earthquake, marked by a star, as well as a station seismograph of reference in the western United States, marked with a triangle.

Each point corresponds to a station seismology. These points become red when the flip the raises, and blue when they come down. The black lines that appear show when the horizontal movements of the soil, indicating both their direction and their amplitude. The whole shows a kind of “ballet” doubly beautiful, both from an aesthetic point of view and because the “waves” that sweep across the continent to see could not be more clearly different types of seismic waves and how they follow each other. Let’s look at it more ready.

As explained by the researcher in geology and a specialist in earthquakes, Université Laval Richard Fortier, “there are two major categories of seismic waves : body waves and surface waves. The waves propagate in the earth in all directions and in a straight line, while the surface waves, they are guided by the surface, they follow the surface of the earth.”

Among the waves, he continues, there is a distinction between waves of compression, also referred to as “P waves”, which form a pattern of compression and dilation in the rock. “The first sound we hear in an earthquake, it is often a kind of big bang. This is the compression wave,” said Mr. Fortier. On the graph in the bottom of the video, the blue line horizontal marked with the letter “P” indicates the moment where the compression wave arrived at the station seismograph of reference (the triangle on the map). As this is the fastest, moving at speeds of 5 to 7 km per second (!) in the crust of the earth, they are detected first.

The second wave type of volume is called “shear-wave”, or “S-wave”. They do move around the earth in a manner perpendicular to their axis of propagation. They are less quick qiue the P-wave — “only” 3 to 4 km/s, and come after (that is, the line “S” on the chart).

But the P waves and the S are only the “appetizers”, so to speak. “The waves contain the most energy, the ones that do the most damage are surface waves,” explains Mr. Fortier. They also come in two different types : the Rayleigh waves, which shake the ground in the manner of a wave, and the waves of Love, that shake horizontally. The Rayleigh waves are marked on the graph by the letters “Rl”; those of Love are not indicated, but one sees very well that the strongest shaking recorded by this station-there begin with the arrival of the Rayleigh waves.

“The surface waves can also be spread less rapidly than body waves”, complete-t-he, which explains why they are recorded later.

Note that the acronym “ScS” on this graph indicates a “wave reflection”, that is, an S-wave that was going in the direction of the center of the Earth and that has somehow “bounced” on the earth’s mantle. It’s only a engage in to arrive at the same time that the strongest vibrations, ” said Mr. Fortier. The small vertical movements (blue dots and red pale), which continue to be recorded until well after the passage of these waves are other “reflections” or “echoes” of the earthquake, in any way.

It should be noted, ” says Mr. Fortier, the shaking was detected by the seismograph of the University of Laval, as shown in the chart below. Each of the lines of turquoise that you see represents 30 minutes of reading of the movements of the ground — the hours to the right indicates the end of a period of 30 minutes, in time “universal” time (Greenwich, England). “It has not detected waves are surface waves that we see,” explains the researcher. It goes to 6h22 UTC [editor’s note : 2: 22 in the morning, hour of Quebec] and it continues for several minutes. It is interesting to note here, is that the video shows that the earthquake occurred at 6: 12 in Alaska, and it detects the vibrations at the University of Laval to 6h22. Then it shows that the wave took just 10 minutes to cross the entire continent.”