Geochemical variations in soil gas concentrations have been identified, favored by crustal expansion associated with the 2012 seismic activity in Emilia Romagna. The research, conducted by INGV and the University of Ferrara in the "hot lands of Medolla", an agricultural area near Modena - historically known for the presence of high temperatures, methane fumes and absence of vegetation - was published in Scientific Reports of the Nature group
The analysis of soil gas geochemistry in seismically active areas represents an effective tool for identifying buried faults (not visible on the surface) and provides an important contribution in discriminating gas migration linked to seismic activity. These results have been achieved by research, which began in 2012 and lasted about 3 years, conducted by the National Institute of Geophysics and Volcanology (INGV), in collaboration with the Department of Physics and Earth Sciences of the University of Ferrara (UNIFE ). The study Learning from soil gas change and isotopic signatures during 2012 Emilia seismic sequence, published in Scientific Reports of the Nature group (www.nature.com/articles/s41598-017-14500-y) has identified and measured, for the first time, a marked increase (up to three orders of magnitude) in the concentrations of methane (CH4 ), hydrogen (H2) and carbon dioxide (CO2) in soils, during the 2012 seismic sequence, compared to pre-earthquake values.
“The increase in concentrations during the earthquake and in the following months”, explains Alessandra Sciarra, INGV researcher and first author of the work, “was favored by crustal expansion linked to seismic activity which allowed the geogas to rise towards the surface. The anomalous concentrations of gas have highlighted a buried fault in an east-west direction that connects 3 zones with no vegetation and gaseous emanations, known as macroseeps.
In fact, it is known in the literature that earthquakes and the consequent crustal deformation can alter the hydraulic properties of soils, such as porosity and permeability, favoring the migration of fluids along preferential routes due to variations in pressure and temperature.
“The study of the geochemistry linked to earthquakes is complex”, continues the INGV researcher. “Generally geochemical measurements are made only after a seismic event has occurred, thus precluding the possibility of comparing pre- and post-event data. In this work, however, the co-seismic and post-seismic concentrations were exceptionally compared with data from 2008".
The study area, known as “Terre Calde di Medolla” is an agricultural area where the presence of high soil temperatures (up to 48°C) and natural methane emissions has been known since 1893. In 2008 it was A geochemical study of soil gases has already been conducted to characterize the distribution and origin of methane. After the seismic sequence of 2012, the monitoring was repeated to understand what had changed.
"The analysis of the isotopes of methane and CO2 on the macroseeps, to trace the origin of the gases, highlighted two distinct sources, one deep and thermogenic, the other superficial (microbial) due to the alteration of the organic matter, whose different contribution has varied over time”, continues Sciarra. "In particular, with the earthquake there was a shaking of the more superficial layers (Plio-Pleitocene), which increased the natural phenomenon of methane emanation already existing in the Medolla area, increasing the migration from the more superficial layers".
Long-term geochemical monitoring allowed to identify a trend in the temporal distribution of gaseous species. In fact, after an initial variation in the distribution of gases in the soil, linked to seismic activity, gas concentrations in 2015 are slowly returning to pre-earthquake values, most likely due to a partial closure of the migration routes after the overpressure generated by the earthquake has subsided.
“Hence the idea” concludes Sciarra “of starting further investigations to correlate the geochemical measurements with geoelectric investigations, to define the geometry of the buried fault, and techniques aimed at measuring the deformations of the earth's surface. The results obtained could encourage research on the geochemistry of soil gases in seismically active areas, also highlighting the importance of having a data set before, during and after earthquakes.
ABSTRACT
Learning from soil gas change and isotopic signatures during 2012 Emilia seismic sequence
Authors: Alessandra Sciarra, Barbara Cantucci, Massimo Coltorti
Soil surveys were performed in Medolla (Italy), a peculiar area characterized by spotty high soil temperature, gas vent, and lack of vegetation, to determine the migration mechanisms and spatial behavior of gas species. Hereby we present soil gas measurements and their isotopic ratios measured between 2008 and 2015, including the 2012 Emilia-Romagna seismic sequence. We found that soil gas concentrations markedly changed during the main shocks of May 20 and 29, 2012 (Mw 6.1 and 6.0, respectively), highlighting the presence of a buried fault intersecting the gas vents. We suggest that crustal dilation associated with seismic activity favored the uprising of geogas towards the surface.
Changes in the isotopic signature highlight the contribution of two distinct sources, one deeper, thermogenic and another superficial related to organic-rich layer, whose relative contribution varied before, during and after the earthquake. We suppose an increase of microbial component likely due to the ground shaking of shallower layers linked to seismic sequence, which masks the thermogenic contribution. Although the changes we detect are specific for an alluvial plain, we deduce that analogous processes may be active elsewhere, and that soil gas geochemistry represents a useful tool to discriminate the gas migration related to seismic activity.
cs 05122017 earthquake geochemistry
Figure 1. (a) Seismotectonic framework of the study area. Solid black lines represent the major active thrust faults of the area18,36. The orange circles are the relocated aftershocks of the first year after the two main shocks37, while the green circles are 3-D relocated aftershocks of the first month of the sequence18 [this figure has been constructed using Esri ArcGIS ArcMap 10.2.1 http://www.esri.com/software/arcgis/arcgisfor-desktopfor Desktops]. (b) Satellite imagery of the study area with evidence of M20, M3, and M14 macroseeps [basemap was obtained from Esri ArcGIS ArcMap 10.2.1]. (c) Landscape picture showing extension of a macroseep, delineated by the absence of vegetation.