The fluids that cross the earth's crust are involved in the processes that generate earthquakes and from their analysis it is possible to obtain information on the dynamics within our planet. Thanks to its characteristics, helium (the lightest noble gas) is a useful tracer for studying the processes that occur in the crust, including those preceding earthquakes.
These are the results of the study "Earthquakes control the impulsive nature of crustal helium degassing to the atmosphere" recently published in Nature's 'Communications Earth & Environment'.
Researchers from the Istituto Nazionale di Geofisica e Vulcanologia (Ingv), the University of Naples Federico II, the University of Basilicata and the Institute of Methodologies for Environmental Analysis of the National Research Council of Italy (Cnr-Imaa) have calculated the volumes of the rocks involved in the recent seismicity of Irpinia, in Italy, by analyzing the seismic catalog of the last 12 years. This analysis made it possible to calculate the fluxes of radiogenic helium (4He) produced in the fault zones which the researchers then compared with the helium fluxes through the entire crust.
In particular, helium is present in natural fluids with two isotopes: primordial helium (3He), whose origin on our planet is mainly attributable to the origin of the planet itself and the radiogenic helium, which is continuously produced by the decay of elements such as uranium (U) and thorium (Th), contained in terrestrial rocks. In the continental areas of our planet where plate tectonics is active, the overall analysis of helium has proved to be a powerful tool for reconstructing the processes involving fluids in their transfer or stationing in the earth's crust.
The researchers found that low-magnitude earthquakes (i.e. magnitudes less than 4) cause variations in the crustal helium flux in the atmosphere, confirming the increase in terrestrial degassing in tectonically active continental regions. Scientists therefore concluded that there is a quantitative relationship between crustal helium fluxes and volume in fault zones. This suggests that changes in helium flux may represent an indicator of changes in fault stress thus correlating it to earthquakes.
“Our idea was to observe the processes that generate earthquakes using a multidisciplinary, geophysical and geochemical approach. In this way, it will be possible to
understand the volumes of rock involved in the processes of breaking the crust, using the earthquake catalogs and, consequently, to demonstrate that the genesis of an earthquake can modify the degassing of helium in the atmosphere. In this way we will also have new elements for the study of the evolution of the earth's atmosphere", says Antonio Caracausi, researcher at INGV and coordinator of the study.
The research was conducted in Irpinia in the area of the 1980 earthquake which recorded a magnitude of 6.9. Using the data of the seismic catalog of the last 12 years and the information concerning the source parameters of thousands of small earthquakes of magnitude less than 4 occurred in the same area, it was possible to calculate the degassing of radiogenic helium released into the atmosphere.
The researchers, therefore, were able to estimate that large quantities of helium are released into the atmosphere in coincidence with low-magnitude seismic activity (M <4). Previous scientific evidence, on the other hand, was related only to high-magnitude earthquakes, such as that of Kobe in 1995 and of Kumamoto in 2016, both in Japan with a magnitude of no less than 6.
"We used the data collected by the seismic network in Irpinia (ISNet) because it represents a natural laboratory for the study of the evolution and breakdown processes of faults in an Apennine sector characterized in the past by strong earthquakes and the emission of gas of deep origin, mainly carbon dioxide”, continues Antonio Caracausi.
"The important consequence of this study is that high-frequency monitoring of helium is essential for examining and calibrating regional models capable of describing the relationship between helium degassing and the seismogenic processes responsible for large earthquakes. Our study emphasizes therefore the need to implement new solutions for the field measurement of helium fluxes that allow data to be acquired on a daily basis. The creation of innovative helium monitoring systems, applied in different contexts, could also help to reconstruct the temporal evolution of natural processes such as volcanic eruptions and earthquakes”, concludes the researcher.
Cnr-Imaa: Tony Stabile, email: firstname.lastname@example.org