Apparent stress and corner frequency variations in the 1999 Taiwan (Chi‐Chi) sequence: Evidence for a step‐wise increase at M w ∼ 5.5

Apparent stress and corner frequencies are measured for the Chi‐Chi, Taiwan sequence beginning with the M w 7.6 mainshock on 20 September 1999. Using the coda source ratio method, we obtained stable source ratio estimates using broadband stations on Taiwan. We find the following: (1) For the mainshock and 3 of the larger aftershocks, apparent stress is clustered around 0.8 MPa (±0.1 MPa); (2) Events below ∼ M w 5.5 exhibit lower apparent stress with larger scatter, ranging between ∼0.08 and 0.8 MPa and are spatially variable; 3) The Brune [1970a, 1970b] omega‐square source model fits the spectral shape for events 4.7 < M w < 7.6, however a step‐wise break in self‐similarity exists at ∼ M w 5.5. We hypothesize that larger events are subject to the regional state‐of‐stress, whereas smaller aftershocks are sensitive to the local state‐of‐stress from stress‐field redistribution following the mainshock and/or fault zone lubrication that affects only larger events.