By Fabian Schmidt
Researchers have discovered an interesting similarity in two of the largest recent earthquakes in Japan and Chile: a strange large-scale ground movement back and forth in the months leading up to the quake.
Whole Continental Plate 'Wobbles'<p>Bedford and his colleagues analyzed how the ground stations in Japan and Chile had moved in the five years before the two quakes. They noticed that the motion of the continental plate on which the stations are located had been reversed several times in the last five months before the quake (in the case of Japan) and seven months (in the case of Chile). The researchers published <a href="https://doi.org/10.1038/s41586-020-2212-1" target="_blank">their results</a> in the scientific journal <a href="https://www.nature.com/articles/s41586-020-2212-1" target="_blank">Nature</a>.</p><p>Both plate boundaries are subduction zones. Subduction is the process of one plate diving under another. In both study regions, the oceanic plates are subducting underneath the continental plates and meet at a place called the trench, which for both cases is submerged under the ocean. Normally, the continental plate is pressed by the oceanic plate and thus pushed away from the trench. However, the geophysicists have now discovered that this movement was first reversed in the direction of the trench, then away from the trench, then back towards the trench again.</p>
Same Movement Over Thousands of Kilometers<p>They call this phenomenon "wobbling," due to its appearance in the GNSS time series. The amplitude of this movement is not particularly great — it was only between 4 and 8 millimeters — but Bedford points out that this is significant compared to the relative plate motion that can be a few centimeters per year. Furthermore, the spatial extent of the signal extended thousands of kilometers along the plate boundaries.</p><p>"It is a common assumption that deeper subduction proceeds at a fairly constant speed in between large earthquakes," says Bedford. "Our study shows that this assumption is an oversimplification. In fact, its variability might be a key factor in understanding how the largest earthquakes nucleate." </p><p>With global satellite tracking now becoming better and better, and with accurate data available for the first time in decades, earthquake researchers have an ever-increasing ability to make such observations.</p><p>"We can now trace movements back decades," says Bedford. "In the next stage, we'd like to monitor the changes in near real time."</p>
Not Suitable as an Early Warning System<p>Until now, seismologists have been more able to say <em>where</em> large earthquakes are likely to occur — less able to say <em>when</em>. By simply calculating the magnitude of the last large earthquake in a region and knowing the average relative plate velocity, one can estimate when that fault will be mature enough to sustain a repeat event, although there are great uncertainties in this approach. This is because sometimes a fault will rupture only over a smaller area (e.g. with a "large" magnitude 8 event) and other times it will rupture over many magnitude 8 regions all at once (e.g. in a "mega" magnitude 9 event).</p><p>Could an observed untypical plate movement therefore give us a better warning of an imminent earthquake? Not really. </p><p>"It would not be wise for a geophysicist to issue such a warning," is Bedford's sobering answer. "The observed signals of this study are not necessarily precursory movements of a major quake."</p><p>More research is necessary and as a matter of principle, people in known earthquake areas should not let their guard down. </p><p>"The general public should always be prepared," Bedford warns.</p>
Idaho residents were rattled Tuesday evening by the biggest earthquake to shake the state in almost 40 years.
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Croatia's capital of Zagreb was rocked Sunday by its strongest earthquake in 140 years, which pushed people out into the streets at exactly the moment they were supposed to stay indoors to fight the spread of the coronavirus.
A 7.7 magnitude earthquake shook the Caribbean Tuesday, rattling people from Miami to Mexico.
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By Richard Aster
Multiple strong and damaging earthquakes in southern Puerto Rico starting around Dec. 28, 2019 have killed at least one person, caused many serious injuries and collapsed numerous buildings, including a multistory school in the town of Guánica that luckily was empty at the time. These quakes are the most damaging to strike Puerto Rico since 1918, and the island has been under a state of emergency since Jan. 6, 2020.
Multiple faults crisscross the eastern Caribbean. Those outlined in red have a potential to generate a large earthquake. The arrow at top right shows the direction of the North American plate's motion relative to the Caribbean plate. Red stars denote intensity centers for past earthquakes. USGS
The second-most active volcano in the Philippines belched to life on Sunday when it sent a cloud of ash miles into the air that forced thousands to evacuate and shuttered the airport in the capital of Manila.
Mysterious hums that were heard around the world in 2018 have now been identified as the rumblings of a magma-filled reservoir deep under the Indian Ocean, announcing the birth of an underwater volcano, according to a new study, as CNN reported.
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A powerful 6.4-magnitude earthquake struck Puerto Rico just before dawn on Tuesday after a week of heavy seismic activity. The early morning quake caused power plants to shut down to protect themselves, leaving the island in a blackout until power is restored later in the day. It also killed at least one person, injured at least eight and collapsed buildings, as the AP reported.
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By Matthew Ross
Modern society relies on metals like copper, gold and nickel for uses ranging from medicine to electronics. Most of these elements are rare in Earth's crust, so mining them requires displacing vast volumes of dirt and rock. Hard rock mining – so called because it refers to excavating hard minerals, not softer materials like coal or tar sands – generated $600 billion in revenues worldwide in 2017.
Along with metals such as gold, silver and iron, mines also produce materials including sand and gravel, crushed stone and Portland cement. USGS