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Effects of Realistic Topography on the Ground Motion of the Colombian Andes – A Case Study at the Aburrá Valley, Antioquia

Doriam Restrepo, Jacobo Bielak, Juan Gómez, Juan Jaramillo, & Ricardo Serrano

Published 2015, SCEC Contribution #2106

This paper presents a set of deterministic 3D ground motion numerical simulations in an earthquake-prone region exhibiting moderate-to-strong surficial irregularities known as the Aburrá Valley in the Colombian Andes. We created the first Velocity Model of the Aburrá Valley region using the geological structures as a proxy for shear-wave velocity. We introduced the highly heterogeneous surficial topography by implementing a Virtual Topography (VT) scheme into Hercules, the octree-based finite-element earthquake simulator developed by the Quake Group at Carnegie Mellon University. From the computational point of view, the VT methodology proves to be accurate, efficient, and preserves the salient features of multi-resolution cubic finite elements. We proposed a 50x50x25 km$^3 volume to perform our simulations, and four Mw=5 rupture scenarios along a segment of the Romeral fault, a significant source of seismic activity of Colombia. In order to examine the sensitivity of the ground motion to the irregular topography and to the three-dimensional effects of the valley, each simulation was conducted under three different assumptions: (i) realistic 3D velocity structure plus realistic topography, (ii) realistic 3D velocity structure without topography, and (iii) homogeneous half space with realistic topography. Our results show how surface topography affects the ground response. In particular, our findings highlight the importance of the combined interaction between source-effects, source-directivity, focusing, soft-soil conditions, and 3D topography. We provide quantitative evidence of this interaction and show that topographic amplification factors can be as high as 500 percent at some locations. In other areas the topographic effects result in relative reductions, but these always lie in the 0-150 percent range.

Restrepo, D., Bielak, J., Gómez, J., Jaramillo, J., & Serrano, R. (2015). Effects of Realistic Topography on the Ground Motion of the Colombian Andes – A Case Study at the Aburrá Valley, Antioquia. Geophysical Journal International, 204, 1801-1816.