Fast MW estimation of microearthquakes recorded around the underground gas storage in the Montello-Collalto area (Southeastern Alps, Italy)

Abstract

Underground fluid injection and extraction is able to change pore fluid pressure at depth and make faults unstable, due to friction-force reduction, with an increased possibility of triggering earthquakes. Studying the local seismicity, down to microearthquakes, and stress field in areas where such activities are developed are essential steps to discriminate between natural and induced events. In this context, the moment magnitude (M W) is a key-parameter to both evaluate the energy balance and the stress involved in earthquake rupture process and assess seismic hazard accurately. Here, we focus on the fast M W estimation of microearthquakes recorded around the underground gas storage of Collalto (Northeastern Italy) by a dedicated seismic monitoring network. The area of Montello-Collalto, where this industrial activity is carried out, is densely populated and characterized by relevant seismic hazard. We compute M W from the response spectra (SA) calculated at fixed periods (i.e., 1.0 and 0.3 s); we show that log (SA) and M W scale as 2/3 and extend our method to microseismicity by using response spectra at 0.1 s. We eventually estimate M W for 1659 events (0.4 ≤ M W ≤ 3.5) and find that M L and M W scale as 2/3 too. The discrepancy between these two magnitude scales affects both the Gutenberg-Richter parameters and completeness magnitude estimations; therefore, it has consequences when those quantities are used for physical interpretation. Our procedure shows to be efficient and suitable to be implemented within standard routine analyses of real-time monitoring and feed decision-making processes about plant management, such as the traffic light protocols.

An unusually productive microearthquake sequence brings new insights to the buried active thrust system of Montello (Southeastern Alps, Northern Italy)

“…We compute M W for 324 of 407 earthquakes identified in August 2021; the low signal-to-noise ratio does not allow M W computation for the remaining 83 events. Figure 4A shows that this dataset satisfactorily fits the relationship previously reported by Lanzoni et al (2020) for 1659 events with M W 0.4-3.5, during 2012-2018. Similar to previous studies (e.g., Deichmann, 2017;Moratto et al, 2017), magnitudes scale nearly 1:1 for the strongest events (for M > 1.8), while M W is higher than M L and scales 2:3 for the weakest earthquakes.…”

“…The cluster centroid shifts about 20 m northward from its original position; the average location errors (2sigma) are a few hundred meters, with average time residuals of about 17 ms. 5. Moment magnitude data set (M W ) for 324 of 407 earthquakes computed from response spectra (Moratto et al, 2017;Lanzoni et al, 2020) and seismic energy estimates (Kanamori et al, 2020): they allow comparison with M L and help to understand the dynamic properties of the sources. 6.…”

“…In this study, M 0 is derived from M W , which in turn is estimated from vertical response spectra (SA) computed at 1.0, 0.3, and 0.1 s and properly corrected for path effects (Moratto et al, 2017). This approach assumes that the empirical relationships between M W and the values of SA have been previously calibrated for the studied area, as done by Lanzoni et al (2020) for the Montello-Collalto area.…”

Monitoring the Microseismicity through a Dense Seismic Array and a Similarity Search Detection Technique: Application to the Gas-Storage Site of Collalto, North Italy

“…The field is equipped with 17 active wells through which the gas is seasonally injected into the reservoir during the November-March period and extracted during the April-October period [6,8]. In ten years of operation, the Collalto seismic network recorded thousands of microearthquakes of natural origin in the surrounding areas, reaching a completeness magnitude Mc for the whole area of 0.6 and 1.2 for the local [8], and moment [9] magnitude, respectively, and of 0.1 for a smaller region around the reservoir [10].…”

Preface to the special issue “Induced seismicity: observations, monitoring, and risk management strategies”

“…Finally, Lanzoni et al (2020) focus on fast M w calculation of microseismic events recorded around the underground storage facility of Collalto (Italy) by a dedicated seismic monitoring network. The authors compute M w from the response spectra calculated at fixed periods (i.e., 1.0 and 0.3 s) showing that the spectra and magnitude scale as 2/3.…”