Analysis of the 2016–2018 fluid-injection induced seismicity in southern Italy

What is it about?

Our study addressed the challenge of detecting small earthquakes caused by injecting fluids into the ground. We identified 196 low-magnitude earthquakes belonging to the fluid-injection induced seismicity cluster located in the High Agri Valley, Southern Italy, using a template-matching algorithm on the continuous seismic data streams acquired by the INSIEME network. These findings provided insights into the damage zone of a SW dipping fault, characterized by a variety of fractures critically stressed in the dip range between ~45° and ~75°. Notably, the earthquakes occurred in clusters and were found to be closely linked to the volume and pressure of the injected fluids.

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Photo by Zbynek Burival on Unsplash

Photo by Zbynek Burival on Unsplash

Why is it important?

The main findings of this study are: 1. The single-station template matching algorithm allowed us to detect weak events with local magnitudes in the range − 1.2 ≤ Ml ≤ 1.2 and with a completeness magnitude Mc = − 0.5, much lower than the completeness magnitude obtained for this seismicity cluster in previous studies. 2. The seismicity is characterized by a swarm time-dependent earthquake occurrence with no identifiable mainshock and a b-value of 1.37 ± 0.05, inferred from the frequency-magnitude distribution of seismicity (Fig. 3). Furthermore, the seismicity well correlates with injection operational parameters (Fig. 5 and Table 1) with only 8 events occurred with injection rates equal or less than 100 m3/day. These observations suggest the reduction of the frictional fault strength due to pore fluids pressure. 3. The distribution of hypocenters obtained from relative locations depicts the damage zone of a SW dipping fault (Fig. 4). It may constitute a hydraulic connection path between the bottom-hole of the CM2 well and the NE dipping back-thrust which was originally reactivated by injection operations in the period 2006–2012 (Fig. 6). 4. The maximum injection pressure at the bottom-hole of 6 MPa operated by the oil company may critically stress fractures dipping in the range from ~ 45° to ~ 75° (Fig. 7), thus justifying the observed variety in the earthquake waveforms (Supplementary Figs. S3 and S4). 5. A strong increase in the number of seismic events is observed after acidification operations and for injection rates greater or equal than 1900 m3/day, but never exceeding the magnitude threshold (M = 1.5) that activates the attention level introduced in the Italian guidelines. When acid treatment is not executed, injection rates of about 2000 m3/day do not significantly affect the earthquake production.

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