When super-earthquakes trigger other earthquakes... from a distance

 On Monday 6 February, at 4.17 a.m., a 7.8 magnitude earthquake struck Turkey and Syria. Earthquakes in this part of the world are common, but the magnitude of this one is clearly impressive: to find an earthquake this strong on this fault, you have to go back to 1114.

Ten minutes after the strongest quake, a 6.7 magnitude aftershock occurred near the epicentre and other aftershocks continue to occur in an area stretching over 350 kilometres from eastern Turkey to the Syrian border. These 'aftershocks', the earthquakes that occur after a major earthquake, are expected and their statistical behaviour is well known.
More surprisingly and dramatically, a second earthquake of magnitude 7.5 occurred at 13:24 local time, further north. This earthquake is not an aftershock: according to the first data processed live by the major international seismological agencies, it occurred on an east-west fault cutting the main rupture.

We do not yet have all the information provided by 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 is arriving in dribs and drabs.

This major seismic activity on two nearby faults reflects the fact that the stresses that cause earthquakes are gradually shifting. The other major fault in the region (the "North Anatolian" fault) has seen a sequence of earthquakes spread throughout the 20th century, like a series of dominoes, to the Sea of Marmara and the megalopolis of Istanbul.

The entire scientific community, as well as the Turkish authorities, are expecting an earthquake near this city of 8 million inhabitants. We do not know when this earthquake will occur or how big it will be. No one can, at this stage of knowledge, propose a date and magnitude for this coming earthquake, and this Monday's earthquake is an unfortunate reminder that Turkey can also be hit hard elsewhere.

When giant earthquakes trigger other earthquakes... at a distance

There is also a type of triggering known as 'dynamic'. In some cases, the stress variation resulting from a large earthquake is not large enough to explain the occurrence of other earthquakes, especially if they are located several hundred kilometres from the epicentre of the main shock.

For example, following the Californian Landers earthquake in 1992 and the Hector Mine earthquake in 1999, earthquake swarms were observed several hundred kilometres from the epicentre. It has been shown that these earthquakes occurred exactly during the passage of the strongest seismic waves emitted by these two earthquakes.

Similar observations have been made in the laboratory to show that when these seismic waves pass, the material that makes up the core of the fault weakens, causing a sliding stress release, i.e. an earthquake.

This kind of behaviour comes from the physics of granular media, which when shaken can behave like fluids. Rapid shaking of a pile of sand will cause it to flatten under its own weight, whereas without the shaking it will hold together very well.

Shaking a fault quickly can therefore cause it to slip, producing earthquakes. It has also been observed that these seismic waves can trigger slow slippage at colossal distances. The seismic waves emitted by the Maule earthquake, a magnitude 8.9 earthquake in 2010 in Chile, caused a slow slip along the Mexico subduction, about 7000 kilometres from the epicentre.