Seismic coda attenuation after theMw= 6.2 Armenia (Colombia) earthquake of 25 January 1999

Abstract

[1] Seismic wave attenuation in the central region of the Colombian Andes is studied using coda waves. Most of the events used occurred in the region on the occasion of the Armenia 25 January 1999 earthquake. The estimation of the decay rate of coda amplitudes (called coda Q−1 or Qc−1) is performed by means of the single isotropic scattering method of Sato [1977]. The attenuation parameters Qi−1 (intrinsic absorption), Qs−1 (scattering loss), and Qt−1 (total attenuation) are also estimated using the multiple lapse time window method of Hoshiba et al. [1991]. The frequencies of interest lie between 1 and 15 Hz. Results show that scattering attenuation is predominant for the frequency bands 1.5 ± 0.5 and 13.5 ± 1.5 Hz, whereas the intrinsic absorption and scattering contribute in equal shares to total attenuation for the 3 ± 1, 5 ± 1, 7.5 ± 1.5, and 10.5 ± 1.5 Hz frequencies. A comparison among the estimated attenuation parameters indicates that Qc−1 is close to total attenuation for all the studied frequency bands. On the other hand, no evidence in support of temporal changes of Qc−1 before and after the main shock has been found. Finally, the regionalization of Qc−1 values seems to correlate well with the geotectonic characteristics of the region.