Some associate it with a truck, others with a train that passes in a hurry, and few do not perceive it, but that noise that is heard seconds before a movement of earth responds to the sound that occurs after the fracture of the earth’s crust.
This was established by the geologist and former director of the Puerto Rico Seismic Network (RSPR), Christa Von Hillebrant-Andrade, explaining that everything results from the clash between the rocks that develops every time an earthquake occurs, regardless of its magnitude.
“This sound is very common in places near where an earthquake is generated. When the seismic wave reaches the surface it creates an acoustic wave that is what you hear with that noise and then the tremor occurs”, Hillebrant-Andrade explained, while pointing out that all this process and transformation between waves occurs in a few fractions of seconds.
Each seismic wave produces an acoustic wave with different frequencies. The frequency and capacity that is perceived depends directly on its interaction with the surface and the fraction of a second that occurs before the earthquake.
“Some of these frequencies are perceived and others are not, some are audible only by animals. And that has been proven. In some cases these sounds are not audible to humans, but to animals, so if you have a pet you will have noticed that sometimes they become impatient and start barking before the tremor, ”Said the expert.
A work team of United States Geological Survey (USGS) compiled a series of records of earthworks and their frequencies to transform it into sound.
The agency managed to capture the noise in some earthworks and, when compared, concluded that the acoustic wave responds similar to a kind of explosion that occurs deep in the earth.
“This moment is still studied by experts, because there are different theories and every time the earthquake occurs the sound wave can be different. What is known is that it is normal for this rumble to occur and that it is perceived seconds before the tremor, because the acoustic wave travels faster than the seismic wave, ”said Hillebrant-Andrade.
However, the expert, who directs the Tsunami program of the National Oceanic and Atmospheric Administration (NOAA, in English) on the island, said it is not crazy to relate the sound to an explosion, because “with the earthquake there is a release of energy and a fracture in the earth that is going to be heard abruptly.”
“For example, there is a group of seismologists who have studied the relationship between the gases in the earth and earthquakes. There may come a time when a gas is emitted, such as radon gas, and that is why this research sought to conclude whether the release of gas could serve as a precursor to an earthquake, ”said Hillebrant-Andrade.
They will investigate fractures in the ground
Within this process between the noise and the earthquake, a ground fracture occurs that, according to the expert, will be the reason why the USGS will bring a seismology team to investigate the seismic sequence that affects Puerto Rico since last 28 December.
Judging by Hillebrant-Andrade and the director of the Puerto Rico Seismic Network, Victor Orphan, a change in the surface of the earth and the relief near the epicenters in the southwest must already be undergoing due to the large number of earthquakes that have been recorded.
“A ship from the United States Geological Survey will come to investigate the epicenters that have developed south of Guánica because almost all of them have been deep in the sea and, at the moment, a series of fishermen have approached me to tell me that They have already noticed something strange in the area where they used to fish. I don’t know, but it will be investigated ”Orphan said.
He stressed that the USGS team will seek to develop a map that shows some kind of change in the relief after the thousands of earthquakes that have occurred in the same area for a short period of time.
“The exhortation is to contact us or send photos if they perceive something strange. There may be alterations in animal behavior or even obvious changes in the surface and all that will be investigated, ”said Hillebrant-Andrade.
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