Tectonic setting and fault roughness vs earthquake double couple

The occurrence of earthquakes is the mirror of the stress patterns inside the brittle lithosphere. The statistical properties of seismicity are known to be affected by several factors such as the presence of fluids and the rheological features of rocks (Amitrano, 2012), with cross-scale effects on faulting and geologic structures (e.g., Schorlemmer et al., 2005; Diederichs et al., 2019). We show that the tectonic setting directly impacts on the moment tensors of earthquakes. By analysing global and regional focal mechanisms of shallow seismicity (Dziewonski et al., 1981), we find that thrust events are characterized by higher double-couple contribution with respect to normal and strike-slip fault earthquakes. Different trends are also observed as a function of magnitude, i.e., reverse fault seismicity is featured by linearly increasing double-couple percentages as magnitude raises, while this phenomenon is not observed in normal and mixed events; on the contrary, the double-couple percentage of transcurrent fault sliding appears to be negatively correlated with their own sizes at large magnitudes. Our results are also coherent with the broadly studied stress dependence of the scaling exponent b-value of the Gutenberg-Richter law, which turns out to be anticorrelated to the double-couple contribution. Our work suggests that the structural and tectonic control of seismicity may seep out of the complexity of the seismogenic source marking the roughness of faults and the width of the dislocation zones, coherently with previous research (e.g., Frohlich, 1994; Julian et al., 1998; Zaccagnino et al., 2022); the sharper and concentrated the slip along a single fault as along thrust faults, the higher the double couple (Fig. 1). In this regard, we warn its potential effect in introducing a significant source of bias in magnitude estimation with dramatic impact for seismic forecasting.