The occurrence of earthquakes in the continental upper mantle is highly debated, and bears directly on lithospheric rheology (e.g. “jelly-sandwich” vs. “crème-brulée” models). Because Sn waves travel only below the Moho, a detection of high Sn amplitudes indicates that the source earthquake occurred below the Moho. In contrast, because Lg waves propagate in the crust, strong Lg amplitudes signify a crustal earthquake above the Moho. In this project, we use Sn/Lg amplitude ratios as evidence to support prior identifications of mantle earthquakes. We develop a novel workflow to analyze data from the IRIS database. Using the expected velocities of Sn and Lg waves, we calculate the RMS amplitude of the Sn and Lg windows, to determine the Sn/Lg amplitude for each recording. To validate our approach, we apply our methods to the well-recorded 2013 Mw 4.8 Wyoming event, reported to be 76 km deep, and the 2016 Mw 4.8 Wyoming event, just 12 km deep. Contrary to simple expectation, the deep Wyoming earthquake does not show a strong Sn/Lg ratio. However, the Sn/Lg amplitude ratio for the deep (upper-mantle) event is significantly larger than the equivalent ratio at the equivalent station for the shallow (upper-crustal) event (Figure 1). We apply the same algorithm to Californian events reported to be in the crust, near the Moho, and in the mantle, to test our methodology and theory. We present our results in the form of raypath maps and record sections for each earthquake that we studied. Our results show that the 1D assumptions of the Sn/Lg theory are successful in Wyoming and corroborate the existence of rare deep earthquakes, which indicates that some parts of the mantle possesses brittle properties like the upper crust. However, in other areas our 1D assumptions are insufficient, as shown by contrasting results from the East African Rift (see adjacent poster by Espinal et al.).

We studied seven earthquakes in the southern East African Rift System (EARS) with catalog depths of 10 to 33km, in locations where the Moho is thought to be at ~32 km depth (CRUST 1.0). Our earthquakes include three relocated by Yang and Chen (JGR, 2010) to be significantly deeper and to be below the Moho. We independently assessed whether the events occurred above or below the Moho using the Sn/Lg method (Wang et al., AGU Fall Meeting 2019; see also adjacent poster by Chen et al.). In a 1D earth, sub-Moho earthquakes produce strong Sn and weak Lg signals, and intra-crustal earthquakes produce weak Sn and strong Lg arrivals. All seven events we studied were characterized by low Sn/Lg, including the three earthquakes interpreted as upper-mantle events by Yang and Chen (2010) (their events M3 and M5 in Malawi and T12 in Zambia). Although low Sn/Lg is elsewhere associated with crustal events we suspect that, in the East African Rift, events in the shallow upper mantle that produce strong Sn at the source may be recorded at regional distances with low Sn/Lg due to Sn-to-Lg conversion at the deepening Moho at the rift margins. CRUST 1.0 suggests crustal thicknesses reach 45 km beneath the cratons adjacent to the East African Rift, with average Moho dips of 5-10°. Hence even the deepest earthquake reported by Yang and Chen (JGR, 2010), at 44±4 km, could undergo significant Sn-to-Lg conversion. Our findings highlight the importance of careful interpretation of Sn/Lg ratios and motivates our ongoing work to model 2D propagation effects.