The variations of seismic waves and the identification of the movement of fluids in the subsurface could help monitor the evolution of the hazard of a seismic sequence.
A new study conducted by a team of researchers from theIstituto Nazionale di Geofisica e Vulcanologia ofUniversity of California Berkeley and United States Geological Survey (USGS), developed a method for monitoring fluid migration in the Earth's crust, showing that these processes are closely correlated with seismicity, especially during the most intense sequences.
The research, entitled “Temporal and spatial changes in Seismic Attenuation Associated with Inferred Fluid Migration in the 2016 Central Apennines Earthquake Sequence”, and published in the scientific journal Bulletin of the Seismological Society of America, aims to understand the complex evolution of the phenomena that characterized the seismic sequence of the Central Apennines, which occurred between Amatrice, Visso, Norcia and Capitignano in the period between 24 August 2016 and the end of February 2017.
The study analyzes the temporal variations in the attenuation of seismic waves observed during the 2016-2017 sequence, identifying signals compatible with the migration of pressurized fluids along the fault system responsible for the events. “Before a major earthquake - explains Luca Malagnini, Research Director of INGV and first author of the study - Faults can act as impermeable surfaces, capable of impeding the movement of fluids in the Earth's crust. However, when a fault, or a system of faults, is activated during a strong earthquake, it can change behavior and become a highly permeable channel, allowing crustal fluids to migrate both vertically and laterally.
In the specific case analyzed, high-pressure CO₂ migration, triggered by the Amatrice earthquake of August 24, 2016, of magnitude 6.0, could having contributed to weakening nearby faults and favoring other seismic events like the Norcia earthquake of October 30, 2016, of magnitude 6.5.
The authors of the study noted that the behavior just described, which can produce a series of high-magnitude events, is typical of large seismic sequences in the Central Apennines (and perhaps the entire Apennines), and highlighted how analyzing seismic waves observed during the 2016-2017 crisis could help detect episodes of high-pressure fluid migration occurring in the Earth's crust. In the future, monitoring temporal variations in the parameters that regulate the dissipation of elastic energy transported by seismic waves could help improve the assessment of the evolution of seismic hazard, in space and time, during a seismic sequence.
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Fig. 1 - These three graphs illustrate the cumulative dilation induced by the entire Amatrice-Visso-Norcia-Capitignano sequence (red: dilation; blue: contraction) and the resulting diffusion of crustal fluids (most likely CO2) expected during the Central Apennines seismic sequence. The axes indicate the distance from the epicenter of the Amatrice earthquake. The cumulative dilation was calculated using the dislocations produced by M>3.0 events. The arrows show the diffusion of crustal fluids expected at a depth of 5 km in a confined aquifer (direction and relative magnitude). After the Amatrice event, and until the day before the Visso earthquake (panel a), crustal fluids were expelled in a northwesterly direction, toward the epicenter of the Visso earthquake. From the day of the Visso earthquake until the day before the Norcia earthquake, crustal fluids were expelled from the epicentral area of the Visso event and directed south-southeastward toward the epicenter of the Norcia event (panel b). From the day of the Norcia event until the day of the Capitignano subsequence (18/01/2017), crustal fluids were violently expelled from the area surrounding the Norcia epicenter and conveyed southeastward toward the Capitignano epicenters.
Fig. 2 - Graphs (a) and (b) show how a parameter that measures the attenuation of seismic waves (essentially, how much earthquake vibrations are "damped" as they travel along crustal paths) changed over time before and during the series of strong shocks in central Italy in 2016. Measurements taken in the part of the rock above the fault plane (the so-called "roof" of the fault, or "hanging wall") are shown in black. Measurements taken in the part of the rock below the fault plane (the so-called "wall" of the fault, or "footwall") are shown in red. In panel (a), the geometry of the Amatrice fault is used as a reference to define the "roof" and "wall" areas, since this geometry is very similar to that of the faults that generated the main Visso and Norcia earthquakes. In panel (b), the geometry of the Amatrice fault is used until shortly before the main Visso shock. Next, the geometry of the Capitignano fault is adopted. Two marked increases in attenuation are noted in the area "above" the fault (black line) just before the Capitignano sequence, labeled "1" and "2." These increases indicate significant changes in the properties of crustal rocks. Graph (c) shows regional seismic activity during the same period. The seismic events used to calculate the attenuation parameter are a subset of this general catalog.
Fig. 3 - This graph illustrates the epicentral distance, measured from the epicenter of the main Amatrice earthquake of August 24, 2016, as a function of the "reduced time" for all events whose epicenters are within 5 km of a line defining the orientation of the Amatrice fault. The "reduced time" is defined as the square root of the elapsed time (in days) since the main Amatrice earthquake of August 24, 2016. The graph highlights: the three main earthquakes of the sequence (Amatrice - AM, Visso - VI and Norcia - NO); the first event of the Capitignano (CA) sub-sequence; two significant episodes labeled "1" and "2" in the previous figure; Some seismicity alignments: those going from bottom to top represent seismicity migrations (induced by the diffusion of high-pressure crustal fluids) occurring along the fault system in a northwest (NW) direction, while those going downward represent migrations going in the opposite direction. This graph helps scientists visualize how the Central Apennines seismic sequence has evolved in space and time: a crucial element for understanding the progression of seismic phenomena in the area. In particular, it is important to note how the migrations triggered by the Amatrice (AM) earthquake are moving towards the epicenters of Visso (26/10/2016) and Norcia (30/10/2016).