An earthquake can cause others: aftershocks, but also earthquakes at a greater distance
In the early hours of Monday, February 6, a magnitude 7.8 earthquake shook Turkey and Syria. Earthquakes in this part of the world are common, but the magnitude of this one was extraordinary: to find an earthquake this strong on this fault line, you’d have to go back to 1114.
Ten minutes after the strongest quake, a magnitude 6.7 aftershock struck near the epicenter and further aftershocks continue today in an area stretching more than 350 kilometers from eastern Turkey to the Syrian border. These “aftershocks”, the earthquakes that occur after a major earthquake, are to be expected and their statistical behavior is well known.
More surprising and dramatic, a second magnitude 7.5 quake hit further north around noon on Monday. This shock was not an aftershock: According to the first live data processed by the main international seismological agencies, it occurred on an east-west fault that intersected the main fault.
We still don’t have all the information from satellite images and GPS measurements, but it is possible that the second earthquake was caused by the first, a hypothesis that will have to be verified in the coming days with the data that will come in a trickle.
This significant seismic activity on two nearby faults reflects the gradual reorganization of earthquake-causing stresses. The region’s other major fault (the North Anatolian Fault) has seen a succession of earthquakes throughout the 20th century, like a series of dominoes, toward the Sea of Marmara and Istanbul.
The entire scientific community, as well as the Turkish authorities, expect an earthquake near this city of 8 million. We don’t know when this earthquake will happen or what it will be like. No one, with current knowledge, can suggest a date and magnitude for this earthquake, and Monday’s shock is a sad reminder that Turkey could be hit hard elsewhere as well.
The behavior of the aftershocks following Monday’s earthquake is no surprise. In 1894 Omori already observed a logarithmic decrease in the number of aftershocks with time (according to a 1/t law, where t is the time elapsed since the main shock).
The same empirical laws, known as scaling laws, predict that the largest aftershock will be one order of magnitude smaller than the main shock: in this case, the largest aftershock of the first earthquake had a magnitude of 6.7, close to the expected 6.8. Remember that this scale is logarithmic and a magnitude 6 earthquake releases 30 times less energy than a magnitude 7 earthquake.
Aftershocks stop when the forces generated by the main shock settle, kind of like when the grains keep rolling one after the other after kicking against a pile of sand and then settle.
But Turkey’s latest magnitude-7.5 earthquake completely breaks the pattern that has been statistically verified for thousands of earthquakes around the world since 1894: It’s not an aftershock, but a second quake. It should also be noted that it happened on a fault that appears to be oriented 45° to the East Anatolian Fault, as evidenced by the shape of the aftershock swarm that followed.
Therefore, here we will speak more of a “caused earthquake”, or at least we will try to investigate the mechanisms that would explain the temporal coincidence between these two large earthquakes.
Some earthquakes are actually related: By absorbing stresses that build up on fault lines, they release energy and rearrange these stresses, which can trigger new earthquakes.
The highly active North Anatolian Fault, which supports a relative displacement of about 2 centimeters per year between the Anatolian and Eurasian plates, has witnessed a series of earthquakes of magnitude greater than 7 from east to west over a length of 20th century. of about 800 kilometers. .
Most notably, the entire length of the North Anatolian Fault ruptured between 1939 and 1999. The last continuous segment is in the Sea of Marmara near Istanbul between the 1999 Izmit and 1912 Ganos earthquakes.
This sequence of earthquakes is explained by the transfer of tectonic forces from one segment of the fault to another. An earthquake locally releases the stresses accumulated by the relative motion of the plates, but at the same time increases those of the adjacent fault segments, which are therefore approaching future rupture.
If this segment is already well loaded (close to cracking), one earthquake can trigger another. Otherwise, we will have to wait for the movement of the tectonic plates to provide the rest of the stress needed to cause the shaking. This is known as “static uptake” because the state of the crust after the earthquake is what causes the next earthquake.
There is also a type of activation called “dynamic”. In some cases, the voltage variation due to a large earthquake is not large enough to explain the occurrence of certain earthquakes, especially if they are several hundred kilometers from the epicenter of the main shock.
For example, after the Landers (1992) and Hector Mine (1999) earthquakes in California, seismic swarms were observed several hundred kilometers from the epicenter. It has been shown that these earthquakes occurred precisely during the passage of the strongest seismic waves emitted by these two earthquakes.
Similar observations have been made in the lab to show that as these seismic waves pass, the material that forms the core of the fault weakens, causing stress from sliding, i.e. an earthquake.
This kind of behavior stems from the physics of granular media, which can behave like fluids when moved. By shaking a pile of sand quickly, it spreads under its own weight, while holding together very well without stirring.
So if you shock a fault quickly, it can slip and cause earthquakes. It has also been observed that these seismic waves can cause slow landslides over long distances. The seismic waves emitted by the 8.9-magnitude Maule earthquake, which struck Chile in 2010, caused a slow slide along the subduction of Mexico, about 7,000 kilometers from the epicenter.
Source: La Verdad
I am an experienced and passionate journalist with a strong track record in news website reporting. I specialize in technology coverage, breaking stories on the latest developments and trends from around the world. Working for Today Times Live has given me the opportunity to write thought-provoking pieces that have caught the attention of many readers.
