I analyze the stiffness characteristics of SSEs produced in the laboratory at cm/yr driving rates and 10 MPa effective normal stress. The samples are mostly from major plate-boundary fault zones obtained via scientific drilling projects (Japan Trench, southern Cascadia, Costa Rica, Barbados, Hikurangi subduction zone, Alpine Fault (New Zealand), San Andreas Fault, Woodlark Basin), as well as one synthetic gouge (crushed pyrite). The testing apparatus is equipped with two displacement sensors in the shearing direction, which provides four direct stiffness measurements: the loadup and unloading stiffness of the sample itself, and the loadup and unloading stiffness of the surroundings (apparatus).