LATERAL VARIATIONS OF LG CODA-Q IN SOUTHERN MEXICO

Broad band digital three-component data recorded at UNM, a GEOSCOPE st ation, were used to estimate Lg coda Q for 34 medium size (3.9 less th an or equal to m(b) less than or equal to 6.3) 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 Q(0) (Q at 1 Hz) and the frequency dep endence parameter eta estimates averaged on the frequency interval of 0.5 to 2 Hz for the regions and the three components considered are: i ) Guerrero region 173 less than or equal to (Q) over bar(0) less than or equal to 182 and 0.6 less than or equal to <(eta)over bar>(0) less than or equal to 0.7, ii) Oaxaca region 183 less than or equal to (Q) over bar(0) less than or equal to 198 and 0.6 less than or equal to <( eta)over bar> less than or equal to 0.8, iii) Michoacan-Jalisco region 187 less than or equal to Q(0) <less than or 204 and 0.7 less than or equal to <(eta)over bar> less than or equal to 0.8 and iv) eastern po rtion of the Transmexican Volcanic Belt (TMVB) 313 less than or equal to Q(0) less than or equal to 335 and eta = 0.9. The results show a ve ry high coda Q for the TMVB as compared to other regions of southern M exico. This unexpected result is difficult to reconcile with the geoph ysical characteristics of the TMVB, e.g., low seismicity, high volcani c activity and high heat flow typical of a highly attenuating (low Q) region. Visual inspection of seismograms indicates that for earthquake s with seismic waves traveling along the TMVB, the amplitude decay of Lg coda is anomalously slow as compared to other earthquakes in southe rn Mexico. Thus, it seems that the high (2 value found does not entire ly reflect the attenuation characteristics of the TMVB hut it is proba bly contaminated by a wave-guide effect. This phenomenon produces an e nhancement 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 eart hquake. The overall spatial distribution of coda Q values indicates th at 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 e. A slight dependenc e of average coda Q(-1) on earthquake focal depth is observed in the f requency range of 0.2 to 1.0 Hz approximately on the horizontal compon ent. Deeper (h > 50 km) events yield lower values of Q(-1) than shallo wer events. For frequencies higher than 1.0 Hz no clear dependence of Q(-1) on focal depth is observed. However, due to the estimates uncert ainties this result is not clearly established.