Frictional Segmentation of the Chilean Megathrust From a Multivariate Analysis of Geophysical, Geological, and Geodetic Data
2021
Molina F., Diego | Tassara, Andrés | Abarca, Rodrigo | Melnick, Daniel | Madella, Andrea
We present a formal seismic segmentation model of the Chile Margin derived from a multivariate analysis of gravity anomalies, basal friction and interplate locking from GPS velocities. These independent proxies of the frictional structure along the megathrust integrate over timescales of 10¹–10⁷ years. We analyze their covariance using Principal Component Analysis (PCA) and extracting the Empirical Orthogonal Functions (EOFs) and Principal Components (PCs). We find that (a) the first and first + second PCA modes for the triple combination of the proxies explain (respectively) 41.4% and 69.4% of the total variance and (b) the first‐order spatial pattern of each isolated proxy is largely reproduced when it is combined with the other two proxies. This suggests that the three proxies share a first order along‐trench tendency that must be connected through a common physical process. Comparing our results with rupture areas of earthquakes and using the rate‐and‐state friction theory, we infer that trench‐perpendicular PCs reflect the limits of the velocity‐weakening (VW) seismogenic zone. Polarity changes in along‐trench EOFs (mostly for gravity) mark changes between unstable and conditionally stable frictional regimes inside the VW zone and we used them to define the boundaries of 17 unitary segments. These segments correlate with first‐order tectonic features of the Andes at >10³‐km scales as well as with zones of multi‐segment ruptures at 10²‐km scale. We analyze the combined influence of subducting and upper‐plate geologic features on the nature of persistent seismic barriers. Our results have implications for understanding seismotectonic processes along the Andean margin and elsewhere.
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