Numerical and Observational Study of Sn-To-Lg Conversion Due To Crustal-Thickening: Implications for Identification of Continental Mantle Earthquakes

We study Sn-to-Lg conversion due to significant crustal-thickening, using Sn and Lg amplitude ratios (Sn/Lg) to identify continental mantle earthquakes. We perform 2.5D simulations up to 5 Hz and 2,000 km using an enhanced AxiSEM3D. Our simulations compare propagation in a constant-thickness crust from a source at three depths straddling the Moho, to 48 models of the same three sources propagating through Moho ramps of four different widths (dips) at four different distances from the source, so perturbations of Sn/Lg are only due to crustal-thickening. We compare our synthetics to data from 12 earthquakes recorded on the HiCLIMB array across Tibet, of which six previously studied events from northwestern Tibet traverse no major crustal-thickness variation, and six new events are located south of the Himalaya across a major Moho ramp. Our simulations show that Sn/Lg for mid-crustal earthquakes is consistently lower than for mantle earthquakes, especially for areas beyond the end of the ramp. Our observations on real data, with similar travel paths for each group of events (ramp-crossing and non-ramp-crossing), display bi-modal separation of Sn/Lg for both groups, identifying new mantle earthquakes in northern India. Sn-to-Lg converted waves may be readily detected by enhanced high-frequency Lg content, as suggested by previous mode-coupling studies, verified here with synthetics, and observed in data. Lastly, we observe higher frequency content in Lg from crustal earthquakes than from mantle earthquakes in simulations and in data, offering a new discriminant for continental mantle earthquakes based on frequency content of Lg waves alone.