Summary -Laboratory room temperature triaxial friction tests on sawcut granite and serpentinite specimens suggest that stick-slip at high confining pressures is preceded by dilatancy in the intact rock adjacent to the shear surface. By using a fast-reacting servo-loading system in combination with a high resolution digital computer recording system it is possible to obtain a more realistic picture of the stickslip mechanisms and the stress drop associated with unstable slip. Fracturing in granite under biaxial loading leads to a significant anisotropy in dilation and P-wave velocity of the rock. Velocity decreases remarkably in the directions of minor principal stresses with no indication for velocity recovery before macroscopic shear fracture development.The results suggest that dilatancy and velocity anomalies may precede crustal earthquakes under certain tectonic conditions. Key words: Dilatancy; Velocity changes; Friction and fracture.ZusammenJassung -Triaxiale Scherversuche an polierten Scherftachen in Granit-und Serpentinitproben (Normaltemperatur) lassen schlieBen, daB instabile Gleitvorgange auf den Scherftachen bei sehr hohen Manteldriicken mit vorhergehenden Auftockerungen (Dilatanz) des intakten Gesteins in der Umgebung der Scherftache verbunden sind. Durch die Verwendung eines reaktions-schnellen Servobelastungssystems in Verbindung mit einem hoch-auftosenden digitalen Aufzeichnungssystem konnte erstmals ein Einblick in den tatsiichlichen Ablauf des instabilen Gleitprozesses gewonnen werden. Der Bruch in Granit bei biaxialer Belastung fiihrt zu einer signifikanten Anisotropie der Auflockerung des GesteinsgefUges und damit der Geschwindigkeit seismischer Wellen. Die Geschwindigkeit der P-Wellen nimmt in Richtung der heiden kleineren Hauptspannungen mit fortschreitender Deformation kontinuierlich ohne Anzeichen eines Geschwindigkeitsanstiegs vor der Entwicklung des makroskopischen Scherbruchs abo Die Ergebnisse lassen den SchluB zu, daB Dilatanz und damit verbundene Geschwindigkeitsanomalien Erdbeben in der Kruste bei bestimmten tektonischen Bedingungen vorangehen konnen.
Actigraphy has been used for more than 60 years to objectively measure sleep–wake rhythms. Improved modern devices are increasingly employed to diagnose sleep medicine disorders in the clinical setting. Although less accurate than polysomnography, the chief advantage of actigraphs lies in the cost-effective collection of objective data over prolonged periods of time under everyday conditions. Since the cost of wrist actigraphy is not currently reimbursed, this method has not enjoyed wide acceptance to date. The present article provides an overview of the main clinical applications of actigraphy, including the recommendations of specialist societies.
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