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The formation of frictional melt likely impacts the coseismic and, when solidified (pseudotachylyte), the interseismic strength of faults. Here we investigate these effects through experiments using a new energy‐controlled rotary shear machine (ECoR) on simulated faults made of a transparent rock analog material (polymethyl‐methacrylate). As in nature, ECoR allows (a) elastic strain energy to accumulate at different loading rates and (b) the spontaneous nucleation of slip events. ECoR is equipped with a high‐speed camera, thermocouples, and transducers to monitor the surface, temperature, and acoustic emissions (AEs), respectively. We perform experiments at normal stresses of ∼3.5 MPa across loading rates from 0.15 MPa/s, phase A, to 2.5 MPa/s, phases B‐C‐D. In phase A, the temperature remains constant, and slip events occur without visible melting every 3.3–6.4 s with 0.5–0.7 MPa stress drops and 3–7 mm displacements. In phases B‐C, slip events occur in the presence of melts every 0.5–0.9 s, and the bulk temperature increases progressively. Melt solidification increases static friction yielding slip events with stress drops up to 5 MPa and displacements up to 3 cm. Samples produce high‐frequency AEs during slip acceleration and deceleration. Once the bulk temperature reaches ∼110°C, a "final" and silent long displacement event occurs in the presence of melts (phase D). Experimental observations suggest that melt formation modulates the coseismic (flash melting, melt lubrication, and viscous braking) and interseismic (welding) stages. Furthermore, AEs associated with coseismic fault weakening and strengthening may have their natural equivalent and could be observed in seismograms through near‐fault instrumentation. Plain Language Summary: During earthquakes, rocks slide against each other result in frictional heat. In some cases, enough heat is generated that rocks melt, influencing earthquake behavior. We investigate this behavior with experiments using a newly devised laboratory apparatus on a rock‐analog material, polymethyl‐methacrylate glass (commercial name, Plexiglas®). The apparatus reproduces the natural earthquake cycle by compressing two cylinders (top and bottom sample) in the vertical direction, then gradually loads a spring, applying a horizontal shear force where the two samples meet. Slip between the samples occurs when the force from the spring overcomes the ability of the material to resist shearing. This process mimics earthquakes along faults at depth, generating heat and melts. We find that during slip, such melts initially lubricate the fault, then cool, thicken, and increase viscosity to resist slip. Moreover, the solidification of melts increases the fault strength leading to larger successive earthquakes. We used acoustic sensors to monitor the sound and vibrational waves emitted from slip events. These recordings reveal the occurrence and timing of the processes happening during faulting. As monitoring technology improves, the waves' characteristic of these processes may also become evident in seismograms of natural earthquakes and help us understand how earthquakes work. Key Points: We use a newly conceived experimental machine on polymethyl methacrylate to investigate the seismic cycle in the presence of meltsEnergy flux pulses from the machine to the fault yield fault weakening (flash melting + lubrication) and strengthening (viscous braking)Distinct acoustic emission signals correlate with the activation of coseismic fault weakening/strengthening mechanisms [ABSTRACT FROM AUTHOR]

The Hikurangi subduction zone hosts shallow slow‐slip events, possibly extending to the seafloor. The mechanisms allowing for this behavior are poorly understood but are likely a function of the frictional properties of the downgoing seafloor sediments. We conducted friction experiments at a large range of effective stresses, temperatures, and velocities on incoming sediment to the Hikurangi subduction zone to explore the possible connection of frictional properties to slow‐slip events. These experiments were conducted on multiple apparatuses, allowing us to access a wider range of deformation conditions than is available on any one machine. We find that the material frictionally weakens and becomes less velocity strengthening with increasing effective stress, whereas temperature has only a small effect on both friction and frictional stability. When driven at the plate convergence rate, the sediment exhibits velocity‐weakening behavior. These results imply that the frictional properties of the sediment package subducting at Hikurangi could promote slow‐slip events at the pressures, temperatures, and strain rates expected along the plate boundary thrust up to 10‐km depth without requiring elevated pore fluid pressures. The transition to velocity‐strengthening behavior at faster slip rates could provide a mechanism for limiting unstable slip to slow‐sliding velocities, rather than accommodating deformation through ordinary earthquakes. Key Points: Sediments subducting at the Hikurangi Trench become weaker and less velocity strengthening with increasing depthSediments exhibit a cutoff velocity of 1 μm/s, possibly explaining the presence of shallow slow slip at the Hikurangi TrenchSlow slip at the Hikurangi Trench could result from frictional instability, in addition to factors such as elevated pore fluid pressures [ABSTRACT FROM AUTHOR]

The presence of calcite in and near faults, as the dominant material, cement, or vein fill, indicates that the mechanical behaviour of carbonate-dominated material likely plays an important role in shallow- and mid-crustal faulting. To better understand the behaviour of calcite, under loading conditions relevant to earthquake nucleation, we sheared powdered gouge of Carrara Marble, >98 per cent CaCO3, at constant normal stresses between 1 and 100 MPa under water-saturated conditions at room temperature. We performed slide-hold-slide tests, 1-3000 s, to measure the amount of static frictional strengthening and creep relaxation, and velocity-stepping tests, 0.1-1000 μm s-1, to evaluate frictional stability. We observe that the rates of frictional strengthening and creep relaxation decrease with increasing normal stress and diverge as shear velocity is increased from 1 to 3000 μm s-1 during slide-hold-slide experiments. We also observe complex frictional stability behaviour that depends on both normal stress and shearing velocity. At normal stresses less than 20 MPa, we observe predominantly velocity-neutral friction behaviour. Above 20 MPa, we observe strong velocity-strengthening frictional behaviour at low velocities, which then evolves towards velocity-weakening friction behaviour at high velocities. Microstructural analyses of recovered samples highlight a variety of deformation mechanisms including grain size reduction and localization, folding of calcite grains and fluid-assisted diffusion mass transfer processes promoting the development of calcite nanograins in the highly deformed portions of the experimental fault. Our combined analyses indicate that calcite fault gouge transitions from brittle to semi-brittle behaviour at high normal stress and slow sliding velocities. This transition has important implications for earthquake nucleation and propagation on faults in carbonate-dominated lithologies. [ABSTRACT FROM AUTHOR]

Severe coronary artery disease may occur early in the course of mucopolysaccharidosis. The authors present radiologic, scintigraphic, and pathologic findings in five patients with Hurler syndrome. Thallium-201 myocardial scintigraphy and selective coronary angiography obtained within 2 days to 6 months were correlated in five patients. Postmortem examination of the coronary arteries was obtained in one patient within hours of angiography. Interobserver agreement on grading of scintigraphic abnormalities was poor. Nonspecific findings on Tl-201 studies included septal abnormalities in all five patients. Scintigraphic findings were corroborated by angiography and postmortem results in only 3 of 5 patients. Our findings suggest that resting Tl-201 scintigraphy has limited value in the detection of coronary artery disease in patients with Hurler syndrome.

Endomyocardial biopsy (EMB) is the standard method of monitoring heart transplant recipients for the development of allograft rejection. To date, noninvasive methods to detect cardiac allograft rejection have lacked adequate sensitivity and specificity for wide clinical application. In this study, limiting dilution analysis (LDA) was used to quantitate the number of donor alloantigen-reactive helper T lymphocytes (HTLs) in the peripheral blood of cardiac transplant recipients. Cadaveric donor splenocytes were cryopreserved, providing a source of donor alloantigenic stimulation for these assays. Peripheral blood mononuclear cells were harvested from cardiac transplant recipients before transplantation and at the time of EMB. LDA of donor-reactive HTLs was conducted simultaneously on all time points to minimize experimental variation, and these data were related to EMB scores. Frequencies of donor-reactive HTLs in pretransplant samples were highly variable, ranging from 1/1381 to < 1/200,000, and correlated poorly with the degree of HLA disparity. During episodes of moderate rejection, donor-specific HTL frequencies increased an average of 6 times their post-transplant baseline frequency. Additionally, 10-fold increases in HTL frequencies were seen preceding EMB-diagnosed rejection in several individuals. These data indicate that episodes of allograft rejection are associated with increases in the number of circulating donor-reactive HTL which are frequently detected before the development of histologically defined rejection. Thus, monitoring HTL frequencies may serve as a non-invasive method for detecting and predicting cardiac allograft rejection. Furthermore, this assay may provide a valuable means of assessing the in vivo efficacy of various immunosuppressive therapies.