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The Pelona–Orocopia–Rand (POR) schists were emplaced during the Farallon flat subduction in the early Cenozoic and now occupy the root of major strike-slip faults of the San Andreas Fault system. The POR schists are considered frictionally stable at lower temperatures than other basement rocks, limiting the maximum depth of seismicity in Southern California. However, experimental constraints on the composition and frictional properties of POR schists are still missing. Here, we study the frictional behavior of synthetic gouge derived from Pelona, Portal, and Rand Mountain schist wall rocks under hydrothermal, triaxial conditions. We conduct velocity-step experiments from 0.04 to 1 μm/s from room temperature to 500ºC under 200 MPa effective normal stress, including a 30 MPa porefluid pressure. The frictional stability of POR schists in the lower crust is caused by a thermally activated transition from slip-rate- and state-dependent friction to inherently stable, rate-dependent creep between 300ºC and 500ºC, depending on sample composition and slip-rate. The mineralogy of POR schists shows much variability caused by different protoliths and metamorphic grades, featuring various amounts of phyllosilicates, quartz, feldspar, and amphibole. Pelona and Portal schists exhibit a velocity-weakening regime enabling the nucleation and propagation of earthquakes when exhumed in the middle crust, as in the Mojave section of the San Andreas Fault. The contrasted frictional properties of POR schists exemplify the lithological control of seismic processes and associated hazards. 

Abstract We investigate the mechanism underlying lower hybrid waves associated with high altitude echoes recently detected in the post‐sunset equatorial topside ionosphere and inner plasmasphere by the Jicamarca VHF radar. These waves are visible as prominent sidebands in the echo Doppler spectra. New experimental results and newly processed incoherent scatter radar (ISR) datasets are presented that provide clues as to the conditions in which the echoes and associated waves occur. Numerical simulations are presented which demonstrate the feasibility of an inverse energy cascade coupled with a short wavelength instability, that is, the lower hybrid drift instability, in explaining the waves. An inverse cascade is required for short wavelength lower hybrid waves to extend to the 3 m wavelengths measured by the Jicamarca radar. The simulations were able to reproduce some features of the measurements including the lower hybrid sidebands at 3 m wavelengths, asymmetry in the sidebands, and the damping effect of higher densities and lower altitudes.