Fluid-induced anthropogenic and natural earthquake swarms driven by aseismic slip

Abstract

Anthropogenic fluid injections at depth induce seismicity which is generally organized as swarms, clustered in time and space, with moderate magnitudes. Earthquake swarms also occur naturally in different tectonic contexts. While some similarities between natural and injection-induced swarms have already been observed, whether they are driven by the same mechanism is still an open question. Indeed, they are commonly related to fluid pressure processes, while recent observations suggest the presence of aseismic slip driving seismicity. Based on such observations, we propose a simple model that combines fluid and aseismic processes, in which seismicity is triggered by fluid-induced aseismic slip. The model reconciles the seismicity migration observed in natural and anthropogenic swarms, and allows us to quantify the seismic-to-total moment ratio. By validating our approach using 22 earthquake swarms, both from natural and anthropogenic origins, our findings provide a generic explanation of the swarm driving process. Hosted fileessoar.10509508.1.docx available at https://authorea.com/users/559641/articles/608111-fluidinduced-anthropogenic-and-natural-earthquake-swarms-driven-by-aseismic-slip Hosted file agu_adv_danre_supp.docx available at https://authorea.com/users/559641/articles/608111-fluidinduced-anthropogenic-and-natural-earthquake-swarms-driven-by-aseismic-slip Fluid-induced anthropogenic and natural earthquake swarms driven by aseismic slip

Inferring fluid volume during earthquake swarms using seismic catalogues

“…In the Corinth case, the volume of fluids circulating during the 2015 swarm (De Barros et al 2020) would be of around 5100 m 3 . Interestingly, this swarm presents many similarities with the 2006 Basel sequence in terms of duration, propagation of seismicity and magnitude (Danré et al 2021). Finding for Corinth a volume estimate similar to the injected volume in Basel (11 500 m 3 ) highlights this similarity.…”

“…First, the effective stress drops and the estimated seismic-to-total ratio falls within the same range for natural and injection-induced seismic swarms. Effective stress drops are indeed found between ∼1 kPa and ∼1 MPa in both cases (Danré et al 2021). Moreover, the estimation of the volume allows the computation of the seismogenic indexes for natural swarms (Table 1).…”

“…A proper way to discriminate between fluid-driven or slow-slip driven swarms, if possible, is therefore required. One possibility for that lies in the migration velocities, as fluid-induced sequences show lower migration velocities than slow-slip triggered swarms, even if it depends on the swarm duration (Danré et al 2021).…”

“…Assuming that natural swarms are caused by the same mechanisms as injection-induced sequences (Danré et al 2021), we can therefore apply eqs ( 4) and ( 10) to those sequences, to infer the volume of circulating fluids. After computing the seismic observables (completeness magnitude, b-value, number of events, effective stress drop), we can reconstruct fluid volume values for the nine natural swarms studied here, with the two methods developed previously.…”

“…Migration duration for swarms is determined empirically as the time during which the distance of the seismicity front to the first few earthquakes increases (Danré et al 2021). For injection-induced sequences, migration duration is nearly identical to the injection duration (Fig.…”