Love-wave normal modes discriminate between upper-mantle and crustal earthquakes: Simulation and demonstration in Tibet

We discriminate upper-mantle earthquakes from crustal earthquakes based on the amplitude ratio of seismic waves Sn and Lg ('Sn/Lg'), a prominent feature of regional seismograms that is visible to the naked eye. Crucially, our new method uses only the waveforms of the candidate earthquake, unlike previous methods to identify upper-mantle earthquakes that introduce potentially large errors by comparing hypocentral depths with independent measures of crustal thickness. Our synthetics show that the Love-wave higher modes that form individual Sn and Lg Airy phases on the transverse component are preferentially excited when the source is respectively below or above the Moho. We use three previously recognized mantle events from southern Tibet to validate our new approach and show that focal mechanism, intrinsic attenuation, geometrical spreading etc., can be ignored to first order. We then identify two new upper-mantle earthquakes in NW Tibet where, previously, only lower-crustal events had been reliably demonstrated, thereby showing this NW-Tibet lithosphere is seismogenic at all depths i.e. that upper-mantle and lower-crustal earthquakes co-exist. Our method has potential for expanding the global catalog of continental earthquakes reliably determined to be close above or close below the Moho. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The method distinguishes upper-mantle earthquakes from crustal earthquakes based on the amplitude ratio of seismic waves Sn and Lg ('Sn/Lg'), showing potential for expanding the global catalog of continental earthquakes.