Transient Weakening of a Natural Fault Zone

In 2011 and 2016 two near-identical earthquakes near Mochiyama, Japan ruptured the same fault, in the same place, with a similar magnitude. The unusually short repeat time between the two earthquakes provides a rare opportunity to estimate the evolution of stress on a fault through an earthquake cycle, as the stress drop in the first earthquake provides a reference value from which we can infer variations through time in the stresses required to cause earthquake rupture. I will argue that the fault experienced a decrease of 1–5 MPa in the shear stresses needed to generate earthquake rupture (20-50% of the first earthquake’s stress drop). I will describe geodetic and seismological evidence of the inter-event period that might have hinted at the mechanisms that led to this fault weakening.