Group B, Poster #044, Seismology

Causes for continental lower-crustal and upper-mantle earthquakes inferred from Tibet Sn/Lg analysis

Xiaohan Song, & Simon L. Klemperer
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Poster Presentation

2022 SCEC Annual Meeting, Poster #044, SCEC Contribution #12315 VIEW PDF
Almost all continental earthquakes nucleate in the crust. Claims of sub-Moho earthquakes in southern California (Inbal et al., Science, 2016) have recently been challenged with questions raised about depth uncertainties (Yang et al., Sci.Adv., 2021), so it is important to continue to search for such events and understand their potential causes.

We use the waveguide theory that earthquakes above or below the Moho will excite different energy in their Sn and Lg seismic phases to search for mantle earthquakes. With this method we do not attempt to measure earthquake depth depth or Moho depth, but instead only whether the earthquake is above or below the Moho. Our synthetics show...
a ubiquitous sharp increase of the Sn/Lg ratio for sources beneath a low-velocity crust and, if a deep-crustal high-wavespeed layer is present (e.g. eclogitized lower crust) we see a smaller decrease & rebound of the Sn/Lg ratio. We apply our method to Tibet, a continental plate-boundary zone rich in deep seismicity, and measure Sn/Lg ratios of 171 earthquakes in west Tibet and 113 earthquakes in south Tibet from 1998 to 2022 with reported hypocentral depths >30 km and magnitudes >3.2. As predicted by our synthetics, these earthquakes show Sn/Lg ratios that undergo a step change in amplitude near the independently determined Moho. Numerous earthquakes with high Sn/Lg ratios and with reported depths spanning the reported Moho are identified between the Karakoram and Altyn-Tagh (ATF) strike-slip faults in west Tibet. We attribute these earthquakes, occupying 100-km cross-strike and 250-km along-strike and spanning a 50-km depth range, to an eclogitized Indian lower crust colliding with the Tarim craton beneath the ATF, and downwelling/delaminating. Similarly in south Tibet, dense seismicity occurs in the upper mantle and deep crust beneath the High Himalaya, spanning a 60-km depth range for 100 km across- and 300 km along-strike of the Main Himalaya thrust, suggestive of an eclogitized Indian lower crust beginning to delaminate south of the Yarlung-Zangpo suture.

Our Sn/Lg analysis strongly suggests the earthquake nucleation occurs both above and below the Moho vertically beneath the continental plate-boundary. Analogous tectonics in southern California including Moho-offsetting San Andreas strike-slip faults and mantle down-welling beneath the Transverse Ranges convergence may lack sub-Moho seismicity due to thinner crust and higher temperatures than in Tibet.