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Fluid-induced seismicity typically refers to (minor) seismic events that (partially) involve fluid flows. Examples range from natural flows associated with rainfalls and volcanic eruptions to human-made contexts including wastewater injection wells, hydraulic fracturing, and geothermal power plants. Recently, anthropogenic sources have lead to an extraordinary surge of seismic activities in different parts of the United States. The most extreme cases are reported in Oklahoma and Southern Kansas where most seismic events are potentially linked to large-scale wastewater disposals. The USG survey indicates that no more than five (tectonic) earthquakes per year with magnitude 𝑚≥3 had been previously reported over almost three decades, in sharp contrast to the one thousand 𝑚≥3 earthquakes recorded in 2016.
Figure 1) Magnitude timeseries and daily activity rates of the earthquakes incurred in a) Oklahoma and southern Kansas.
In this context, it is essential to identify potential anthropogenic origins and relevant secondary triggering mechanisms, which has important consequences in terms of seismic hazard assessment, earthquake forecasting and effective mitigation strategies. In particular, the secondary triggering processes --- such as static and dynamic stress changes due to preceding seismic events leading to aftershocks --- might determine the spatial extent of fluid-induced seismicity. In Karimi et al. (2020), we present for the first time a direct and systematic comparison of the features of such triggering in fluid-induced situations associated with industrial settings to those of natural swarms. They are believed to be close counterparts in that both settings involve fluid flows. Using a statistical seismology framework, we show in both cases the importance of secondary triggering, which is characterized by a much more narrow spatial footprint --- compared to tectonic aftershock sequences --- and predominantly swarm-like triggering cascades.
In addition, our results indicate an elevated triggering production associated with large magnitude events in anthropogenic settings compared to natural swarms. Both observations can be directly incorporated in statistical forecasting schemes for seismic activity such as the ones based on the Epidemic-Type Aftershock Sequence (ETAS) model, which presents the current gold standard in the field.
Figure 2) Result of the clustering analysis. Density map on a scaled distance -scaled time domain. The dashdotted line separates the triggered events on the lower left from background part on the upper right.
We believe that our novel findings has potentially high impacts on a broad range of geophysics research topics in seismology, nonlinear geophysics, tectonophysics, hydrology as well as natural and anthropogenic hazards.
Figure 3) Triggering cascade in Oklahoma. The yellow circle indicates the trigger. The red circles denote the triggered events. The radius indicates the events' magnitude.
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