The network of strong motion accelerographs in Mexico includes instruments that were installed, under an international cooperative research program, in sites selected for the high potenial of a large earthquake. The 19 September 1985 earthquake (magnitude 8.1) occurred in a seismic gap where an earthquake was expected. As a result, there is an excellent descripton of the ground motions that caused the disaster.

https://science.sciencemag.org/content/sci/233/4768/1043.full.pdf

Strong Ground Motion from the Michoacan, Mexico, Earthquake

THE western coast of Guerrero in southern Mexico has been identified as a seismic gap on the Middle American Trench in which no large earthquakes have occurred at least since 1908. It has been suggested that the seismic energy accumulated since 1908 will eventually be released by a large earthquake. A permanent seismic network was installed to monitor the seismicity of this mature seismic gap and to understand the geometry of the subducted slab beneath this region. The seismicity defines an unusual distribution along two bands of seismic activity parallel to the coast. The resulting geometry of the subduction zone shows that the Cocos plate dips at a shallow angle beneath the North American plate to a depth of ∼40 km; from there the subducted slab is bent upward, following a subhorizontal trajectory extending inland at a depth of ∼50 km. This plate geometry is reminiscent of that found in Peru and central Argentina, two other regions where a young oceanic plate is being subducted. In Mexico, however, the slab underplates an overriding plate which is only half as thick as that observed in South America. A possible oceanic origin of the allochtonous terranes comprising southern Mexico may explain the presence of the anomalously thin lithosphere in this region.

https://horizon.documentation.ird.fr/exl-doc/pleins_textes/pleins_textes_5/b_fdi_30-30/31928.pdf

Geometry of subduction and depth of the seismogenic zone in the Guerrero gap, Mexico

The validity of the Gutenberg-Richter (G-R) frequency-magnitude relation over a wide range of magnitudes is usually accepted, and the statistics of small earthquakes is often extrapolated to predict the frequency of occurrence of large earthquakes. We take NOAA and PDE data from 1963 to 1981 related to the Mexican subduction zone ( H ≦ 65 km) and convert m b to M s (in the range of 3.3 ≦ m b ≦ 5.7). For this conversion, we use regression of M s on m b for Mexican and Central American events as well as an M s - m b regression based on worldwide data (Wyss and Habermann, 1982). We also take into account the standard error in M s ; this may be important since the number of events decreases as m b increases. The M s data of 18.5 yr is normalized to 75.5 yr and is compared to the statistics of observed large earthquakes ( M s ≧ 6.4) for the period 1906 to 1981. It is found that the G-R relation in the range 4.5 ≲ M s ≲ 6.0, when extrapolated, grossly underestimates the observed frequency of large earthquakes ( M s ≧ 6.5) for the Oaxaca and Jalisco regions. There is strong evidence that the plate interface in the Oaxaca region consists of a few (3 or 4) large fault zones each of which cyclically ruptures over its entire dimension without giving rise to smaller events in the manner implied by the G-R relation. The result is of profound importance in seismic risk analysis and in the understanding of fault mechanics. This phenomenon has been reported for only a few regions (e.g., Turkey: Bath, 1981; Japan: Wesnousky et al. , 1983); it may have escaped detection in other regions due to a variety of causes, the most important being the long recurrence periods of large earthquakes.

Statistics of small earthquakes and frequency of occurrence of large earthquakes along the Mexican subduction zone

We have analyzed seismograms of the 3 June 1932 ( Ms = 8.2) and the 18 June 1932 ( Ms = 7.8) Jalisco earthquakes and their aftershocks at Manzanillo (MNZ), Guadalajara (GUM), and Tacubaya (TAC). The aftershocks locations, the first motions at MNZ, and the isoseismic maps of the two main shocks strongly suggest that: (a) the 3 June 1932 earthquake initiated NW of but close to MNZ and propagated NW for an estimated length of rupture of 220 km; (b) the 18 June 1932 earthquake nucleated SW of MNZ (offshore) and perhaps ruptured a length of about 60 km; and (c) the width of rupture was approximately 80 km. There is no evidence of events occurring SE of MNZ even up to 1 1/2 yr after the first main shock. A gap of about 60 km remains between the aftershock areas of the 1932 Jalisco and the 1973 Colima earthquakes whose seismic potential is unknown. Although the boundary between the Rivera and Cocos plates is uncertain, there is little doubt that the 1932 earthquakes broke the shallow part of the Rivera subduction zone. Our results may help in quantifying seismic potential of tectonically similar areas such as the Juan de Fuca subduction zone in the NW United States.

The great Jalisco, Mexico, earthquakes of 1932: Subduction of the Rivera plate

An earthquake doublet in Ometepec, Guerrero, Mexico

The Playa Azul earthquake broke the central part of the Michoacan gap along the Mexican subduction zone. The aftershock area estimated from field data for a 6-day period beginning 19 hr after the main shock is 40 km × 20 km. Preliminary focal mechanism of the main shock is consistent with a shallow-dipping thrust fault. Although there is some evidence of seismic quiescence for 5 1 2 yr followed by a foreshock about 3 months before the main shock and an above average microseismic activity in the western part of the epicentral region during the last 4 1 2 months or more, the data is not sufficient quality to assert this conclusion. The Michoacan gap was previously assigned category 3 (Singh et al. , 1981) based on the classification of McCann et al. (1979). The seismic potential of the unbroken segments of this gap (∼45 km to NW and ∼35 km to the SE of the Playa Azul aftershock area) remains uncertain. Corresponding to a rupture length of 40 km, each segment could generate an earthquake of similar magnitude to the Playa Azul earthquake. Unfortunately, the recurrence period of large earthquakes in the region is not known and great/major earthquakes cannot be ruled out.

E. Nava

Papers

Playa Azul, Michoacan, Mexico, earthquake of 25 October 1981 (Ms = 7.3)