Data from portable seismographs and a permanent local network (called RESCO) are used to locate the aftershocks of the October 9, 1995 Colima‐Jalisco earthquake (Mw 8.0). The maximum dimension of the aftershock area, which is rectangular in shape, is 170 km × 70 km. Our study shows that the mainshock nucleated ∼24 km south of Manzanillo, near the foreshock of October 6, 1995 (Mw 5.8), and propagated ∼130 km to the NW and ∼40 km to SE. The aftershock area lies offshore and is oriented parallel to the coast. The observed subsidence of the coast is a consequence of this offshore rupture area. The aftershocks reach unusually close to the trench (within 20 km). This may be due to lack of sediments with high pore pressure at shallow depth. There are some similarities between this earthquake and the two great earthquakes of 1932 (3 June, Ms 8.1; 18 June, Ms 7.8) which occurred in this region. In both cases the aftershocks were located offshore and the coastline subsided. The sum of seismic moments and the rupture lengths of the 1932 events (1.8×1021 N‐m and 280 km, respectively), however, were greater than the 1995 earthquake. Also a comparison of seismograms of 1932 and 1995 earthquakes show great differences. It seems that the 1995 event is not a repeat of either June 3 or June 18, 1932 earthquakes.

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Broad band digital three-component data recorded at UNM, a GEOSCOPE station, were used to estimate Lg coda Q for 34 medium size (3.95 m b 56.3) 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 dependence parameter p 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 5Q ( 0 5182 and 0.6 5p) 50.7, ii) Oaxaca region 1835 Q ( 0 5 198 and 0.6 5p) 50.8, iii) Michoacan-Jalisco region 187 5 Q ( 0 5204 and 0.7 5p) 5 0.8 and iv) eastern portion of the Transmexican Volcanic Belt (TMVB) 3135Q 0 5335 and p =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 MichoacanJalisco 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.

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A 150 km segment of subduction plate boundary along the Pacific coast of Mexico between the aftershock areas of the Colima earthquake (Jan. 10, 1973; Ms = 7.5) and the recent Petatlán earthquake (March 14, 1979; Ms = 7.6) has not experienced a major earthquake since 1911 and, thus, has been designated as a seismic gap. There has been considerable discussion in the scientific community about instrumenting this gap for intensive observation. An examination of the 1911 earthquake (M = 7 3/4), however, provides strong evidence that its location was about 280 km NNW of the epicenter reported by Gutenberg and Richter. Study of seismicity of Mexico in the past century gives some additional evidence that no major earthquake (M ≳ 7.5) occurred in the area. Thus, presently available evidence suggests that no large earthquake has occurred in this gap for at least the past 78 years and perhaps for as long as 178 years.