Strong over weak shock tracers as accounted by SiO J = 2 − 1/HNCO (4_0, 4–3_0, 3) (left) and SiO J = 5 − 4/HNCO (10_0, 10–9_0, 9) (right) line ratios obtained from ALCHEMI data (Martín et al. 2021). A sigma clip of 3.0 was applied for both SiO and HNCO before obtaining the line ratios. Regions where we detect Class I maser emission in the J₋₁ → (J − 1)₀ − E series (see Sect. 4.1.2) are labeled in red and with slightly larger numbers. The center and edges of the inner Lindblad resonance from Iodice et al. (2014) are denoted with a black ellipse and dash-dotted gray ellipses, respectively. A square root stretch has been applied for an easy visualization.