David A. Lockner Nicholas M. Beeler Diane E. Moore Tamara Nicole Jeppson 2023 <div class="article-section__content en main"><p>The evolution of a fault's frictional strength during the interseismic period is a critical component of the earthquake cycle, yet there have been relatively few studies that examine the time-dependent evolution of strength at conditions representative of seismogenic depths. Using a simulated fault in Westerly granite, we examined how frictional strength evolves under hydrothermal conditions up to 250°C during slide-hold-slide experiments. At temperatures ≤100°C, frictional strength generally increases with hold duration but, at 200 and 250°C, an initial increase in strength transitions to rapid time-dependent weakening for holds longer than 14&nbsp;hr. Forward modeling of long hold periods at 250°C using the rate and state friction constitutive equations requires a second, strongly negative, state variable with a long evolution distance. This implies that significant hydrothermal alteration is occurring at 250°C, consistent with microstructural observations of dissolution and secondary mineral precipitation.</p></div> application/pdf 10.1029/2023GL105517 en American Geophysical Union Time-dependent weakening of granite at hydrothermal conditions article