by P. Voosen, Sep 23, 2020 in AAAS Science
Many of the world’s most dangerous earthquake faults are a silent menace: They have not ruptured in more than a century. To gauge the hazard they pose to buildings and people, geologists cannot rely on the record of recent strikes, captured by seismometers. Instead, they must figure out how the faults behaved in the past by looking for clues in the rocks themselves, including slickenlines, scour marks along the exposed rock face of a fault that can indicate how much it slipped in past earthquakes.
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Earthquakes don’t happen all at once. Rather, the slip between rocks begins at one spot on the face of the fault—the hypocenter—and travels along it, like a zipper being unzipped. As the rupture advances, the earthquake waves it generates pile up and intensify, like the siren of an approaching ambulance. Los Angeles lies at the northern terminus of the southern San Andreas fault, Ampuero notes. “If it breaks north, toward LA, that would be pretty bad.”
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by C. Rotter, Sep 20, 2020 in WUWT
Despite climate change being most obvious to people as unseasonably warm winter days or melting glaciers, as much as 95 percent of the extra heat trapped on Earth by greenhouse gases is held in the world’s oceans. For that reason, monitoring the temperature of ocean waters has been a priority for climate scientists, and now Caltech researchers have discovered that seismic rumblings on the seafloor can provide them with another tool for doing that.
In a new paper publishing in Science, the researchers show how they are able to make use of existing seismic monitoring equipment, as well as historic seismic data, to determine how much the temperature of the earth’s oceans has changed and continues changing, even at depths that are normally out of the reach of conventional tools.
They do this by listening for the sounds from the many earthquakes that regularly occur under the ocean, says Jörn Callies, assistant professor of environmental science and engineering at Caltech and study co-author. Callies says these earthquake sounds are powerful and travel long distances through the ocean without significantly weakening, which makes them easy to monitor.
Wenbo Wu, postdoctoral scholar in geophysics and lead author of the paper, explains that when an earthquake happens under the ocean, most of its energy travels through the earth, but a portion of that energy is transmitted into the water as sound. These sound waves propagate outward from the quake’s epicenter just like seismic waves that travel through the ground, but the sound waves move at a much slower speed. As a result, ground waves will arrive at a seismic monitoring station first, followed by the sound waves, which will appear as a secondary signal of the same event. The effect is roughly similar to how you can often see the flash from lightning seconds before you hear its thunder.
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IMAGE: AN ARTIST’S RENDERING OF UNDERSEA EARTHQUAKE WAVES. view more CREDIT: CALTECH
by Duke University, January 17, 2019 in ScienceDaily/Nature
Engineers at Duke University have devised a model that can predict the early mechanical behaviors and origins of an earthquake in multiple types of rock. The model provides new insights into unobservable phenomena that take place miles beneath the Earth’s surface under incredible pressures and temperatures, and could help researchers better predict earthquakes — or even, at least theoretically, attempt to stop them.
The results appear online on January 17 in the journal Nature Communications.
“Earthquakes originate along fault lines deep underground where extreme conditions can cause chemical reactions and phase transitions that affect the friction between rocks as they move against one another,” said Hadrien Rattez, a research scientist in civil and environmental engineering at Duke. “Our model is the first that can accurately reproduce how the amount of friction decreases as the speed of the rock slippage increases and all of these mechanical phenomena are unleashed.”
For three decades, researchers have built machines to simulate the conditions of a fault by pushing and twisting two discs of rock against one another. These experiments can reach pressures of up to 1450 pounds per square inch and speeds of one meter per second, which is the fastest underground rocks can travel. For a geological reference point, the Pacific tectonic plate moves at about 0.00000000073 meters per second.
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Hadrien Rattez, Manolis Veveakis. Weak phases production and heat generation control fault friction during seismic slip. Nature Communications, 2020; 11 (1) DOI: 10.1038/s41467-019-14252-
c/o Luc Trullemans, août 2019 in PublicMétéo
Introduction
Une forte relation à été observée ces dernières années entre de l’activité sismique dans les océans et le récent réchauffement climatique (CSARGW ,Correlation of Seismic Activity and Recent Global Warming) .
Cette corrélation entre de l’activité sismique océanique et le réchauffement climatique avait déjà été remarquée de 1979 à 2016 (CSARGW16) et vient d’être confirmée jusqu’en 2018.
Dans cette note, on démontre que l’activité sismique dans les océans ( =>tremblements de terre de magnitude 4-6) provoque des flux géothermiques sous-marins et ont une relation importante avec les fluctuations de la température globale des océans (SST) et de la température globale de l’air (GT).
Ceci avance une nouvelle l’hypothèse selon laquelle l’activité sismique océanique pourrait être un des paramètres les plus importants dans la variation de la température globale.
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by Durham University, Augsut23, 2018 in ScienceDaily
The timing and size of three deadly earthquakes that struck Italy in 2016 may have been pre-determined, according to new research that could improve future earthquake forecasts.
A joint British-Italian team of geologists and seismologists have shown that the clustering of the three quakes might have been caused by the arrangement of a cross-cutting network of underground faults.
The findings show that although all three earthquakes occurred on the same major fault, several smaller faults prevented a single massive earthquake from occurring instead and also acted as pathways for naturally occurring fluids that triggered later earthquakes.
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by Utah State University, June 26, 2018 in ScienceDaily
The discovery of the Durmid Ladder reveals the southern tip of the San Andreas Fault changes fairly gradually into the ladder-like Brawley Seismic zone. The structure trends northwest, extending from the well-known main trace of the San Andreas Fault along the Salton Sea’s northeastern shore, to the newly identified East Shoreline Fault Zone on the San Andreas’ opposite edge.
“We now have critical evidence about the possible nucleation site of the next major earthquake on the San Andreas Fault,” says Jänecke, professor in USU’s Department of Geology. “That possible nucleation site was thought to be a small area near Bombay Beach, California, but our work suggests there may be an additional, longer ‘fuse’ south of the Durmid Ladder within the 37-mile-long Brawley Seismic zone.” …
by Seismological Society of America, May 17, 2018 in ScienceDaily
The experiments conducted by Lawrence Livermore National Laboratory researcher Kayla Kroll and her colleagues were prompted by a recent spike in induced earthquake activity related to oil and gas production in the U.S. and Canada. The rise in induced earthquakes has some scientists proposing changes in injection or production processes to reduce the fluid pressures that destabilize faults in these regions.
In their simulations, Kroll and colleagues “found that active management was most advantageous for wells that were closest to a fault. This scenario is most successful at reducing the total number of seismic events and also the maximum magnitude of those events,” Kroll said. In their simulations, a “close well” was one to four meters away from a fault (…)
by GFZ GeoForschungsZentrum Potsdam, Helmholtz Centre, May 1, 2018 in ScienceDaily
The metropolitan area of Istanbul with around 15 million inhabitants is considered to be particularly earthquake-prone. In order to be able to assess the risk correctly, researchers must decipher the processes underground. Below the Marmara Sea, an international research team detected earthquakes that were not directly caused by tectonic stresses but by rising natural gas.
by Tevor Nace, November 20, 2017 in WhoaScience
Scientists have found strong evidence that 2018 will see a big uptick in the number of large earthquakes globally. Earth’s rotation, as with many things, is cyclical, slowing down by a few milliseconds per day then speeding up again.
You and I will never notice this very slight variation in the rotational speed of Earth. However, we will certainly notice the result, an increase in the number of severe earthquakes.
Geophysicists are able to measure the rotational speed of Earth extremely precisely, calculating slight variations on the order of milliseconds. Now, scientists believe a slowdown of the Earth’s rotation is the link to an observed cyclical increase in earthquakes.
La géologie, une science plus que passionnante … et diverse
