Above or Below the Moho? Depth analysis of continental U.S. earthquakes using Sn/Lg ratio

Abstract:

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.).