Change in paleo-stress state before and after large earthquake, in the Chelung-pu fault, Taiwan

Stress state close to seismogenic fault is a key parameter to understand earthquake mechanics. Changes in stress state after large earthquakes were documented recently in the 1999 Chi-Chi earthquake, Taiwan, and 2011 Tohoku-Oki earthquake, Northeast Japan. If the temporal changes are common in the past and in the future, the change in paleostress related to large earthquakes are expected to be obtained from micro-faults preserved in outcrops or drilled cores. In this study, we show a change in paleostress from micro-fault slip data observed around the Chelung-pu fault in the Taiwan Chelung-pu fault Drilling Project (TCDP), which is possibly associated with the stress drop by large earthquakes along the Chelung-pu fault. Combining obtained stress orientations, stress ratio and stress polygons, stress magnitude for each stress state and difference in stress magnitude between obtained stresses are estimated.

For stress inversion analysis, multiple inversion method (MIM, Yamaji et al., 2000) was carried out. To estimate the centers of clusters automatically, K-means clustering (Otsubo et al., 2006) was conducted on the result of MIM. In the result, four stress states were estimated. The stress states are named C1, C2, C3 and C4 in ascending order of stress ratio (Φ). Stress ratio is defined as (σ₁-σ₂) / (σ₁-σ₃). To constraint the stress magnitude, stress polygons are employed combining with the inverted stress states. The principal stress vectors for four stress states (C1-C4) was projected to the SHmax or the Shmin and vertical stress directions. SHmax is larger than Shmin as definition. Stress ratio was estimated by inversion method. Combining those conditions, a linear function in SHmax and Shmin space respected to Sv is obtained from inverted stress states.

We obtained two groups of stress state from the slip data in the TCDP core. One stress state has WNW-ESE horizontal sigma1 and larger stress magnitude including reverse fault regime. Another stress state has WNW-ESE horizontal sigma 2 or sigma 3 and smaller stress magnitude within normal fault or strike slip fault regimes. The switch of horizontal principal stress in the direction of WNW-ESE from sigma 1 to sigma 2 or sigma 3 may be caused by the decrease of shear stress after earthquake as observed in Tohoku-Oki earthquake (Hasegawa et al., 2012, Lin et al., 2012).