| Abstrakt: |
We evaluate three identifiers of continental mantle earthquakes (CMEs) motivated by surface‐wave normal‐mode theory: the amplitude ratio of Sn to Lg, and the frequency content of Sn and of Lg, after wave propagation through continental crustal thinning. These flexible and easily applicable methods allow for potential new discoveries of CMEs. They rely on guided waves whose propagation is dependent on the uniformity of their waveguides. For a range of Moho models, we perform 2.5D axisymmetric simulations that reach the conventional distance and frequency ranges of observational studies; we compare results from four different source depths straddling the Moho. Our synthetics, and six south‐Tibet earthquakes recorded by an array in Bangladesh, show our Sn/Lg identifier is robust in the presence of crustal thinning, but the identifying frequency contents of Sn and Lg are easily obscured. These results strengthen the utility of Sn/Lg methods for global studies of CMEs. Plain Language Summary: That continental mantle earthquakes (CMEs) occur is at last widely accepted, but the tectonic settings in which they occur remain conjectural. Systematic mapping of CMEs has been impossible because of the difficulty of distinguishing the depth of small earthquakes relative to the Moho, hence whether these earthquakes occur in crust or mantle. Our new method separates CMEs from conventional crustal earthquakes using amplitude and frequency ratios of early and late arriving waves known as Sn and Lg. The basic wave physics requires uniform waveguides for Sn (the curvature of the crust‐mantle boundary, i.e., the Moho) and for Lg (the crust), so changing waveguide shapes could influence these discriminants. Here we focus on the effect of crustal thinning. Our advanced computer simulations, and real data from earthquakes in South Tibet that cross the major crustal‐thinning Himalayan Moho ramp, demonstrate that the amplitude‐ratio metric is robust but frequency‐ratio metrics are less resilient. Our synthetic results deviate from a flat‐Moho model only due to crustal‐thinning, so we can isolate this effect, enabling our results to be quickly adaptable to other regions/applications utilizing Sn and Lg waves. Our guided‐wave‐based discriminants may permit a shift from reporting individual CMEs to mapping their world‐wide distribution. Key Points: Amplitude ratio of Sn and Lg is a robust discriminant for continental mantle earthquakes (CMEs) even across extreme crustal thinningFrequency content of Sn and Lg for CME discrimination is less robust than Sn/Lg amplitudes, with or without crustal thinningBy comparing nearby earthquakes recorded on the same stations, Sn/Lg may illuminate the as‐yet‐unknown world‐wide distribution of CMEs [ABSTRACT FROM AUTHOR] |
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