Lateral Variations of Lg Coda Q in Southern Mexico

Broad band digital three-component data recorded at UNM, a GEOSCOPE station, were used to estimate Lg coda Q for 34 medium size (3.95mb56.3) earthquakes with travel paths laying in different geological provinces of southern Mexico in an effort to establish the possible existence of geological structures acting as wave guides and:or travel paths of low attenuation between the Pacific coast and the Valley of Mexico. The stacked spectral ratio method proposed by XIE and NUTTLI (1988) was chosen for computing the coda Q. The variation range of Q0 (Q at 1 Hz) and the frequency dependence parameter h estimates averaged on the frequency interval of 0.5 to 2 Hz for the regions and the three components considered are: i) Guerrero region 1735Q( 05182 and 0.65h)50.7, ii) Oaxaca region 1835Q( 05198 and 0.65h)50.8, iii) Michoacan-Jalisco region 1875Q( 05204 and 0.75h)50.8 and iv) eastern portion of the Transmexican Volcanic Belt (TMVB) 3135Q05335 and h 0.9. The results show a very high coda Q for the TMVB as compared to other regions of southern Mexico. This unexpected result is difficult to reconcile with the geophysical characteristics of the TMVB, e.g., low seismicity, high volcanic activity and high heat flow typical of a highly attenuating (low Q) region. Visual inspection of seismograms indicates that for earthquakes with seismic waves traveling along the TMVB, the amplitude decay of Lg coda is anomalously slow as compared to other earthquakes in southern Mexico. Thus, it seems that the high Q value found does not entirely reflect the attenuation characteristics of the TMVB but it is probably contaminated by a wave-guide effect. This phenomenon produces an enhancement in the time duration of the Lg wave trains travelling along this geological structure. This result is important to establish the role played by the transmission medium in the extremely long duration of ground motion observed during the September 19, 1985 Michoacan earthquake. The overall spatial distribution of coda Q values indicates that events with focus in the Michoacan- Jalisco and Oaxaca regions yield slightly higher values than those from Guerrero. This feature is more pronounced for the horizontal component of coda Q. A slight dependence of average coda Q 1 on earthquake focal depth is observed in the frequency range of 0.2 to 1.0 Hz approximately on the horizontal component. Deeper (h\50 km) events yield lower values of Q 1 than shallower events. For frequencies higher than 1.0 Hz no clear dependence of Q 1 on focal depth is observed. However, due to the estimates uncertainties this result is not clearly established.