Apr 5, 2024 02:47 AM
https://www.sciencefocus.com/planet-eart...arthquakes
EXCERPTS: When it comes to earthquakes, always expect the unexpected. That’s the message coming from seismologists Prof Éric Calais, of the École Normale Supérieure (ENS) in Paris, and Jean François Ritz, Director of Montpellier’s CNRS Laboratoire Géosciences.
Underpinning their advice is the reality that Earth sometimes shakes in places it shouldn’t. These mysterious events, called intraplate earthquakes, happen far from the active margins of tectonic plates and in places that are otherwise geologically quiet.
[...] Intraplate earthquakes can happen anywhere that a geological fault is present in the crust. In the last few centuries, they’ve been recorded as far apart as Basel in Switzerland, New York and Boston in the US, and the St Lawrence River in Canada.
More recently, they’ve caused substantial damage in the Australian city of Newcastle and, in 2017, in the southern African country of Botswana and Puebla, Mexico, the latter resulting in close to 400 deaths.
[...] One thing that both intraplate earthquakes and those at the plate margins have in common is their operating mechanism. Essentially, strain accumulates over time on a geological fault – a weakness in the Earth’s crust – until it reaches a critical level that causes the fault to break or rupture.
[...] In relation to faults that host intraplate quakes, however, there’s a problem. Such structures are often old and deeply buried, and on occasion their existence isn’t suspected at all until after they’ve hosted an earthquake.
The absence of any record of past events makes it impossible to estimate when the next one might happen. Indeed, as Calais points out, some intraplate quakes might even be so-called ‘orphan’ events that aren’t part of a series and which involve the one-off reactivation of an ancient fault that hasn’t ruptured for millions of years.
While the mechanism of strain accumulation and release is the same for both intraplate quakes and those that happen at plate boundaries, the triggers that cause rupture may well be different.
[...] At this point, it’s worth noting that a fault that’s primed and ready to rupture can be induced to do so by pressure equivalent to that of a handshake. So, while it may have taken millions of years for strain to accumulate on an ancient intraplate fault, setting it off can happen over a relatively short period of time.
[...] unloading and deformation of the crust due to the rapid melting of the great ice sheets between about 20,000 and 10,000 years ago has been implicated in the promotion of many intraplate quakes...
[...] So what does this all mean for the future? Calais highlights the difficulty of predicting intraplate quakes. “With these special earthquakes, it’s very complicated to calculate future risks, especially as they can sometimes only occur once in a given location,” he says. “We lack objective indicators to assess future intraplate seismicity”.
Nonetheless, studies have been undertaken that focus on the threat posed by intraplate quakes in areas that have been affected by them in the past. The results make for unnerving reading.
Today, more than half the world’s population is urbanised, and cities in areas affected by intraplate quakes have grown considerably in size...
[...] When it comes to future intraplate earthquakes, there’s one other major factor to consider: global heating. As glaciers and ice sheets melt, the reduced load on the crust beneath is leading to faults releasing long-accumulated strain.
[...] The crust beneath is uplifting as a consequence and an eventual hike in intraplate quake activity has been predicted.. (MORE - missing details)
EXCERPTS: When it comes to earthquakes, always expect the unexpected. That’s the message coming from seismologists Prof Éric Calais, of the École Normale Supérieure (ENS) in Paris, and Jean François Ritz, Director of Montpellier’s CNRS Laboratoire Géosciences.
Underpinning their advice is the reality that Earth sometimes shakes in places it shouldn’t. These mysterious events, called intraplate earthquakes, happen far from the active margins of tectonic plates and in places that are otherwise geologically quiet.
[...] Intraplate earthquakes can happen anywhere that a geological fault is present in the crust. In the last few centuries, they’ve been recorded as far apart as Basel in Switzerland, New York and Boston in the US, and the St Lawrence River in Canada.
More recently, they’ve caused substantial damage in the Australian city of Newcastle and, in 2017, in the southern African country of Botswana and Puebla, Mexico, the latter resulting in close to 400 deaths.
[...] One thing that both intraplate earthquakes and those at the plate margins have in common is their operating mechanism. Essentially, strain accumulates over time on a geological fault – a weakness in the Earth’s crust – until it reaches a critical level that causes the fault to break or rupture.
[...] In relation to faults that host intraplate quakes, however, there’s a problem. Such structures are often old and deeply buried, and on occasion their existence isn’t suspected at all until after they’ve hosted an earthquake.
The absence of any record of past events makes it impossible to estimate when the next one might happen. Indeed, as Calais points out, some intraplate quakes might even be so-called ‘orphan’ events that aren’t part of a series and which involve the one-off reactivation of an ancient fault that hasn’t ruptured for millions of years.
While the mechanism of strain accumulation and release is the same for both intraplate quakes and those that happen at plate boundaries, the triggers that cause rupture may well be different.
[...] At this point, it’s worth noting that a fault that’s primed and ready to rupture can be induced to do so by pressure equivalent to that of a handshake. So, while it may have taken millions of years for strain to accumulate on an ancient intraplate fault, setting it off can happen over a relatively short period of time.
[...] unloading and deformation of the crust due to the rapid melting of the great ice sheets between about 20,000 and 10,000 years ago has been implicated in the promotion of many intraplate quakes...
[...] So what does this all mean for the future? Calais highlights the difficulty of predicting intraplate quakes. “With these special earthquakes, it’s very complicated to calculate future risks, especially as they can sometimes only occur once in a given location,” he says. “We lack objective indicators to assess future intraplate seismicity”.
Nonetheless, studies have been undertaken that focus on the threat posed by intraplate quakes in areas that have been affected by them in the past. The results make for unnerving reading.
Today, more than half the world’s population is urbanised, and cities in areas affected by intraplate quakes have grown considerably in size...
[...] When it comes to future intraplate earthquakes, there’s one other major factor to consider: global heating. As glaciers and ice sheets melt, the reduced load on the crust beneath is leading to faults releasing long-accumulated strain.
[...] The crust beneath is uplifting as a consequence and an eventual hike in intraplate quake activity has been predicted.. (MORE - missing details)