Schmidt hammer and terrestrial laser scanning (TLS) used to detect single‐event displacements on the Pleasant Valley fault (Nevada, USA)

Changes in surface roughness on carbonate fault scarps often reflect varying durations of subaerial weathering. On the Pleasant Valley fault in central Nevada, the documentation of a surface rupture in 1915, a long recurrence interval of faulting, slow weathering rate, and a relatively high (2–3 m) single‐event displacement make the discrimination of the historical and penultimate slip patches unambiguous. Following from a 2018 study, we used a Schmidt hammer and terrestrial laser scanning (TLS) to further test whether these weathering patterns delineate exposed slip patches on a fault scarp. Results show that Schmidt hammer rebound value ranges (termed ΔR – the difference between minimum and maximum R‐values in repeat impacts at a point), increase by ~8–10 points across the historical–penultimate event transition zone in two separate scarp transects. TLS‐derived surface roughness also indicates a clear difference between the most recent and penultimate events. The average single‐event displacement (SED) estimated using the Schmidt hammer and TLS is 2.85 m at two transect sites and is roughly equivalent to the visually estimated 3 m. While this fault is an ideal case where we know some of the slip history, the results demonstrate that these techniques show promise for discriminating slip patches on larger carbonate fault scarps with longer paleoearthquake histories, and could be used alongside 36Cl cosmogenic exposure‐age dating to improve paleoseismic records on normal faults. © 2019 John Wiley & Sons, Ltd.The Schmidt hammer and terrestrial laser scanning (TLS) can detect hardness and mm‐ to cm‐scale surface roughness changes on bedrock fault scarps. Here, we demonstrate that these changes occur between the ‘slip patches’ of a 1915 and pre‐1915 event on the Pleasant Valley fault, yielding single event displacements of ~2‐3 m. These techniques can now be combined to estimate SEDs on larger fault scarps with longer records of paleoearthquakes.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/154454/1/esp4748.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154454/2/esp4748_am.pd

Spatiotemporal specificity of GABAA receptor-mediated regulation of adult hippocampal neurogenesis

GABAergic transmission regulates adult neurogenesis by exerting negative feedback on cell proliferation and enabling dendrite formation and outgrowth. Further, GABAergic synapses target differentiating dentate gyrus granule cells prior to formation of glutamatergic connections. GABA(A) receptors (GABA(A) Rs) mediating tonic (extrasynaptic) and phasic (synaptic) transmission are molecularly and functionally distinct, but their specific role in regulating adult neurogenesis is unknown. Using global and single-cell targeted gene deletion of subunits contributing to the assembly of GABA(A) Rs mediating tonic (α4, δ) or phasic (α2) GABAergic transmission, we demonstrate here in the dentate gyrus of adult mice that GABA(A) Rs containing α4, but not δ, subunits mediate GABAergic effects on cell proliferation, initial migration and early dendritic development. In contrast, α2-GABA(A) Rs cell-autonomously signal to control positioning of newborn neurons and regulate late maturation of their dendritic tree. In particular, we observed pruning of distal dendrites in immature granule cells lacking the α2 subunit. This alteration could be prevented by pharmacological inhibition of thrombospondin signaling with chronic gabapentin treatment, shown previously to reduce glutamatergic synaptogenesis. These observations point to homeostatic regulation of inhibitory and excitatory inputs onto newborn granule cells under the control of α2-GABA(A) Rs. Taken together, the availability of distinct GABA(A) R subtypes provides a molecular mechanism endowing spatiotemporal specificity to GABAergic control of neuronal maturation in adult brain