The rupture process of the great 1979 Colombia earthquake: evidence for the asperity model.

We have determined the asperity distribution in the segment of the Ecuador-Colombia subduction zone that was ruptured by the 1979 (M_w = 8.2) and 1958 (M_w = 7.7) earthquakes. For the 1979 earthquake, we are able to document directivity in the P-wave source time functions and located the moment release spatially. By combining results from both P-wave and surface wave directivity, we have defined three regions indicating that the rupture zone is composed of an asperity 60 km in length, bounded by two weak (small displacement) regions. The displacement at the asperity is equal to the cumulative tectonic displacement between 1979 and 1906 (the year of the previous great earthquake in this segment). The 1958 earthquake is characterized by the rupture of a small asperity (approx 25 km) in the epicentral region. This detailed study of the spatial variation in moment release offers strong support for the asperity model of large earthquake occurrence. -after Authors