THE PECULIAR CASE OF DEEP SIERRAN EARTHQUAKES

Seismological research in California focuses on regions like the San Andreas system and the western Basin and Range that are subject to large, damaging quakes. In contrast, the Sierra-Great Valley block is generally viewed as a nearly rigid microplate. However, small earthquakes (M<~3) have been recorded in the western Sierra over the past 50 years, mainly in and west of Yosemite National Park (e.g., Ryan et al., Geosphere, 2020). Unlike virtually all seismicity in California or the world at large, quakes are limited to the middle-lower crust with no spatially associated events reliably located in the upper crust. Yet we expect from rock physics that the lower crust is more likely to be aseismic in comparison to the upper crust or upper mantle. Other areas with earthquakes in these depth ranges have seismicity extending to the near-surface. Explanations for this unusual pattern range from the banal to the exotic. In such a low-strain-rate region, perhaps strain has not accumulated to the degree necessary to cause shallower faults to slip, or faults are slipping aseismically due to widespread serpentinite. In these cases, a stress regime like that to the east and west (associated with NW dextral shear) would be expected but is not observed. It is possible that the deep Yosemite seismicity reflects some magmatic process, akin to the Long Valley caldera to the east and associated harmonic-style tremor. Unlike putative injection events in the Lake Tahoe region, however, seismicity in Yosemite has not migrated into the upper crust and lacks focal mechanisms and alignments like those seen at Tahoe. If this seismicity is occurring below an aseismic decollement, then what shallower deformation is it connected to? The most exotic explanation (so far) is that this deformation is local and entirely in the lower crust. Ryan et al. speculated that this could reflect the initiation of foundering of the lower crust, which is consistent with indications that this deep seismicity accommodates shortening in both horizontal principal axes, and vertical thickening. Better testing of this and other ideas would incorporate more of the seismological data that is available (including a denser deployment in 2007), relocation of the events in a 3-D velocity model and fuller examination of the uncertainties of focal mechanisms in the presence of 3-D structure.