Synthetic Seismogram Study of the Eastern Central Andes

Along the South American coast, there have been several earthquakes of great magnitude--between 7.5 and 9.5 Mw, with ruptures 50 km to 950 km long, producing maximum intensities between VII and X (MM)-particularly in two seismic gaps located in southern Peru and northern Chile. In these places, two large earthquakes occurred in the 19th century, in 1868 and in 1877, establishing the possibility of the seismic gap hypothesis: earthquake hazard increases with the time since the last large earthquake of long rupture length occurred, forming a seismic gap at whose ends will occur seismic activity of small magnitude and minor rupture length (Kelleher, 1972). In 1995, 2001, and 2007, earthquakes occurred within the limits of both gaps. They were felt and had light effects in cities in western Bolivia, especially in La Paz. Of major magnitude, they occurred within the limits of gaps located in the coupling zone of the Nazca and the South American plates. The above-mentioned earthquakes were here used to generate the characteristics of the 19th-century earthquakes. The effects of such an earthquake today would be catastrophic for the city of La Paz, situated in a basin with rugged topography, abrupt slopes, a geology of slightly compacted and highly eroded soils, and a complex hydrology of surface and underground rivers. Completing the picture are expanding, disorganized demographics. The city could be highly vulnerable to an earthquake with characteristics of the 19th-century events. The EGF method (Empirical Green Function, Irikura, 1986) was applied to an earthquake in Peru on 15/08/2007 and an aftershock on 16/08/2007, registered at the LPAZ station. Numerical analyses of the synthetic and principal seismograms (short period) show a correlation for frequencies lower than Hz and predominance between 0.05 and 2.5 Hz. Presented at the Conference on Ground-Based Nuclear Explosion Monitoring Technologies (30th), held in Portsmouth, VA, on 23-25 Sep 2008. Published in the proceedings of the conference in Monitoring Research Review, v1 p151-158, 2008. The original document contains color images. All DTIC reproductions will be in black and white.