Abstract
On October 27, 2017, an Mw4 earthquake occurred close to the municipality of Montesano sulla Marcellana, less than 10 km external to the concession of the largest European onshore hydrocarbon reservoir—the Val d’Agri oilfield (Southern Italy). Being a weak event located outside the extended monitoring domain of the industrial concession, the relevance of this earthquake and the possible links with the hydrocarbon exploitation were not extensively discussed. Actually, the analysis of shallow seismic events close to subsurface exploitation domains plays a significant role in the definition of key parameters in order to discriminate between natural, triggered, and induced seismicity, especially in tectonically active regions. The study of weak-to-moderate earthquakes can improve the characterization of the potentially destructive seismic hazard of this particular area, already struck by M > 6.5 episodes in the past. In this work, we analyze the source parameters of this Mw4 earthquake by applying advanced seismological techniques to estimate the uncertainties derived from the moment tensor inversion and identify plausible directivity effects. The moment tensor is dominated by a NW–SE oriented normal faulting with a centroid depth of 14 km. A single ML2.1 aftershock was recorded and used as the empirical Green’s function to calculate the apparent source time function for the mainshock. Apparent durations (in the range 0.11–0.21 s, obtained from S-waves) define an azimuthal pattern, which reveals an asymmetric bilateral rupture with 70% of the rupture propagation in the N310°W direction, suggesting a rupture plane dipping to the SW. Our results tally with the activation of a deeper fault segment associated with the Eastern Agri Fault System close to the basement as the origin of the Montesano earthquake. Finally, the Coulomb stress rate induced by depletion of the oilfield is calculated to quantify the trigger potential estimated for the Montesano earthquake yielding relatively low probabilities below 10%. Our analyses point toward the conclusion that the Mw4 event was more likely due to the local natural tectonic stress, rather than induced or triggered by the long-term hydrocarbon extraction in the Val d’Agri oilfield.
Highlights
Induced seismicity can nowadays be considered as a relevant and pressing problem of increasing interest for the general public
Taking advantage of a wide coverage of seismic stations deployed in the Val d’Agri (VA) region, we describe a detailed and robust analysis of the source parameters for the Montesano earthquake in order to decipher its genesis and apply a probabilistic approach to address the question of discriminating between natural, triggered, and induced origin
The dense network of permanent and temporal seismic stations in the VA region provides us a great opportunity to decipher the genesis of weak events, such as the 2017, Mw 4 Montesano earthquake located outside the outer border of the DE
Summary
Induced seismicity can nowadays be considered as a relevant and pressing problem of increasing interest for the general public. The discrimination problem between a natural, triggered, or induced event is addressed through a probabilistic scheme to quantify the likelihood of a correlation with the depletioninduced stress perturbations (Dahm et al, 2015) This methodology based on physical-statistical seismicity models considers an earthquake occurring close to an oil- or gasfield, that has continuously produced over a period of several years and is depleted; this is the case of the Montesano earthquake. The result of this analysis is the probability that the target event has been triggered by the stressing rate induced by the depletion of the oilfield. Note a slight increase in the trigger probability is observed if we consider a shallower earthquake in the same epicentral location of our target event; still, low probabilities are reached, for instance, values around 15% at a depth of 4 km (Supplementary Figure S6)
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