Apparent Stress and Corner Frequency Variations in the 1999 Taiwan (Chi-Chi) Sequence: Evidence For a Step-Wise Increase at Mw ~5.5

Abstract

Apparent stress and corner frequencies are measured for the Chi-Chi, Taiwan sequence beginning with the Mw 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 ~Mw 5.5 exhibit lower apparent stress with larger scatter, ranging between ~0.08 and 0.8 MPa and are spatially variable; 3) The Brune [1970, 1971] omega-square source model fits the spectral shape for events 4.7<Mw<7.6, however a step-wise break in self-similarity exists at ~Mw 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.