Why do earthquakes occur in front of volcanoes
The coupling of earthquakes and volcanoes
Observations show that eruptions occur frequently at tectonic plate boundaries after mega-quakes. In the meantime, researchers can also say how this coupling works on the basis of simulations.
In 1707, an earthquake struck mainland Japan - the strongest in the country's history. The so-called Hōei earthquake, with an estimated magnitude of 8.6, caused considerable damage to land, triggered a tsunami, and claimed over five thousand lives. Only a few days after this earthquake, the Fuji volcano rumbled near Tokyo. On December 6, 1707 it broke out with devastating effects and buried large parts of Japan under a layer of ash.
Eruption of the Karymsky volcano
About three hundred years later, history seems to repeat itself. On March 11, 2011, a magnitude 9.0 quake shook the seabed off Japan. Another devastating tsunami was triggered, which also cost the lives of around 19,000 people due to the greater energy of the quake and the denser settlement of the coastal region. In addition, the tsunami led to the disaster of a nuclear power plant. The quake also changed the topography of the country - not only on the fault itself, but also on volcanoes at a distance of 100 to 150 kilometers.
New measurement data from satellite radar show that several regions with active volcanoes sagged by 5 to 15 centimeters in the course of 2011. Japanese researchers suspect that the earthquake of March 11, 2011 changed the tension under the earth's surface. That led to the sinking of the areas. It is not clear to the researchers whether this is an indication of volcanic activity; however, they consider it highly advisable to keep the volcanoes under constant surveillance.
The concerns are well founded. In the past, in regions where one tectonic plate dips beneath another - the so-called subduction zones - it has happened several times that a severe earthquake later resulted in volcanic eruptions. For example in Chile: two days after the catastrophic earthquake on May 22, 1960, the Cordón-Caulle volcano erupted. In the months after this largest earthquake ever recorded with a magnitude of 9.5, the Planchón-Peteroa, Tupungatito and Calbucu volcanoes also began to become active.
Looking for the mechanisms
Of course, it could in principle have been a coincidence in Chile or Japan. But researchers have made similar observations in other subduction zones - such as the Aleutian Islands in Alaska and Kamchatka, the Philippines and Indonesia, Central America and Italy. That is why more and more researchers are convinced that strong earthquakes can bring volcanoes out of a dormant phase. These volcanoes are mostly very close to the focus of the earthquake, but in exceptional cases they are also thousands of kilometers away. For some years now, geoscientists have been looking for the mechanisms that could take place in such couplings.
World map of earthquakes and volcanoes
On the basis of three-dimensional model calculations, Thomas Walter from the Helmholtz Center Potsdam believes that several mechanisms are responsible for the fact that earthquakes can activate volcanoes. These mechanisms can work individually or together, which intensifies the effect. On the one hand, a number of waves propagate in the earth's crust from the epicentres. In principle, they belong to the same seismic waves that cause damage to the earth's surface. The temporary deformations also shake the magma reservoirs under volcanoes. It can happen that liquids or gases, which were dissolved in the hot rock slurry up to that point in time, emerge from the solution. In addition, fractures are formed or reactivated and liquid and gaseous substances are set in motion. All of this can cause pressure changes that eventually lead to the volcano's eruption.
Second, as in Japan and Chile, megaquakes change the static conditions in the vicinity of the subduction zone. Due to the displacement during the quake, the elastically deformable earth's crust is compressed in some areas, but pulled apart in others. Walter and colleagues recognized that volcanic activity increases primarily in regions in which the volume increases, i.e. the earth's crust is stretched.
Positive feedback effect
At first glance, the diagnosis seems a bit nonsensical. Why should an eruption develop in regions in which the magma reservoirs of volcanoes suddenly have more space available? Walter admits that the thought contradicts intuition. But there are plausible approaches for an explanation.
Again, fluids, i.e. liquid and gaseous substances, play a key role. If the volume of a magma chamber expands, carbon dioxide and other fluids, for example, can escape from the solution. By relieving pressure on further ascent, a liquid can turn into gas. This can expand even more easily. In this way, a positive feedback effect can eventually trigger an eruption.
Activation of volcanoes at a subduction zone
There are other processes that could play a role in the relationship between earthquakes and volcanic activity. The vibrations and deformations that a quake triggers may mix different types of magma. This can also lead to an oversaturation of gases in the magmas. And ultimately, it cannot be ruled out that an earthquake could widen the volcanic channels between the magma reservoirs and the surface. It has not yet been clarified whether all of these mechanisms work individually or in their entirety. It is also unclear why geysers and mud volcanoes - in which there are no magmas involved - are activated by earthquakes in a similar way to volcanoes.
What does the coupling with earthquakes mean for the assessment of the danger emanating from volcanoes? In the case of mega quakes, the volume expansion extends over a distance of sometimes more than two thousand kilometers. Walter assumes that the hazard from volcanoes in such a region must be re-evaluated after a mega quake. Comparative studies show that increased volcanic activity can be expected even years after a strong earthquake. In short: the frequency and risk of volcanism fluctuate over time - and are closely linked to earthquakes.
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