Spatial variation of coda wave attenuation in northwestern Colombia

Abstract

S U M M A R YOne thousand seven hundred and eighty-six vertical-component, short-period observations of microearthquake codas from regional earthquakes recorded by 17 stations belonging to the National Seismological Network of Colombia were used to estimate seismic wave attenuation in Colombia. Local magnitudes range from 2.9 to 6.0 and only events occurring at hypocentral distances up to 255 km were considered for the analysis. The frequencies of interest lay between 1 and 19 Hz and the analysis was performed for each seismic station separately. Coda-wave attenuation (Q −1 c ) was estimated by means of a single-scattering method whereas the separation of intrinsic absorption (Q −1 i ) and scattering attenuation (Q −1 s ) from total attenuation (Q −1 t ) was performed using a multiple lapse time-window analysis based on the hypothesis of multiple isotropic scattering and uniform distribution of scatterers. A regionalization of the estimated Q 0 (Q c at 1 Hz) values was performed and a contour map of seismic coda attenuation in Colombia is presented, where four zones with significant variations of attenuation related to different geological and tectonic characteristics can be observed. The highest attenuation is linked to the central and western regions (Q 0 around 50 and 56) whereas a lower attenuation (Q 0 around 69 and 67) is assigned to the northern and eastern regions. Results show that the Q −1 values are frequency dependent in the considered frequency range, and are approximated by a least-square fit to the power lawThe exponents of the frequency dependence law ranged from η = 0.65 to 1.01 for Q −1 c , η = 0.62 to 1.78 for Q −1 i , η = 0.28 to 1.49 for Q −1 s , and η = 0.53 to 1.67 for Q −1 t . On the other hand, intrinsic absorption is found to dominate over scattering in the attenuation process for most of the stations and frequency bands analysed. Some discrepancies have been observed between the theoretical model and the observations for some frequency bands which indicate that it would be necessary to consider models for depth-dependent velocity structure and/or non-isotropic scattering patterns.

Characteristics of coda wave attenuation in Yunnan area

“…Comparing with these results, the results of Yunnan area lie within the value ranges and its average value is close to that of Canary Islands and Southern California. Northwestern Colombia (Vargas et al, 2004); PAS station, Southern California (Jin et al, 1994); Kanto Tokai, Japan (Fehler et al, 1992); Northern Chile (Hoshiba et al, 2001); Canary Islands (Canas et al, 1998); Colombian Andes (Ugalde et al, 2002); Southern Apennine zone, Italy (Bianco et al, 2002); South-Central Alaska (Dutta et al, 2004); Wakayama, southwest Japan (Matsunami and Nakamura, 2004) …”

“…Later, Fehler et al (1992) and Hoshiba (1993) developed a multi-lapse time window (MLTW) method, which separates scattering dispersion and intrinsic attenuation by fitting the function of hypocentral distance and time integral of seismic coda energy. The MLTW method has been applied to different seismic tectonic regions (Fehler et al, 1992;Hoshiba, 1993;Jin et al, 1994;Canas et al, 1998;Hoshiba et al, 2001;Bianco et al, 2002;Ugalde et al, 2002;Dutta et al, 2004;Vargas et al, 2004;Matsunami and Nakamura, 2004), and the results show that not only 1 c − Q has great distinctness in these regions but also Yunnan area locates in the southeastern margin of the Indo-Asian collision zone. There are abundant active faults.…”

Deep Earth Structure: Seismic Scattering in the Deep Earth

“…Single-scattering studies include Sato (1978), Aki (1980), Pulli (1984), Dainty et al (1987), Kosuga (1992), and Baskoutas (1996). Multiple lapse-time studies are summarized in Figure 5 and include Fehler et al (1992), Mayeda et al (1992), Hoshiba (1993), Jin et al (1994), Akinci et al (1995), Canas et al (1998), Ugalde et al (1998Ugalde et al ( , 2002, Hoshiba et al (2001), Bianco et al (2002Bianco et al ( , 2005, Vargas et al (2004), Goutbeek et al (2004), Mukhopadhyay and Tyagi (2008), Badi et al (2009), Chung et al (2010, Carcolé and Sato (2010), Lee et al (2010), and Del Pezzo et al (2011). Values of the coefficient go for S-to-S scattering range from about 0.002 to 0.05 km À1 (mean free paths of 20-500 km) for frequencies between 1 and 30 Hz.…”

Deep Earth Structure – Seismic Scattering in the Deep Earth

“…Single-scattering studies include Sato (1978), Aki (1980), Pulli (1984), Dainty et al (1987), Kosuga (1992), and Baskoutas (1996). Multiple lapse-time studies are summarized in Figure 5 and include Fehler et al (1992), Mayeda et al (1992), Hoshiba (1993), Jin et al (1994), Akinci et al (1995), Canas et al (1998), Ugalde et al (1998Ugalde et al ( , 2002, Hoshiba et al (2001), Bianco et al (2002Bianco et al ( , 2005, Vargas et al (2004), and Goutbeek et al (2004). Values of the coefficient g 0 for S-to-S scattering range from about 0.002 to 0.05 km À1 (mean free paths of 20-500 km) for frequencies between 1 and 30 Hz.…”