A model of the subsurface structure of the eastern part of the Española Basin in the northern Rio Grande rift of New Mexico was constructed from geophysical data obtained since 1983 by the Summer of Applied Geophysical Experience (SAGE) field course. Approximately 742 new gravity observations, 1276 ground magnetic stations, 30 km of seismic refraction lines, 19 km of seismic reflection lines, 22 magnetotelluric stations, and several Schlumberger and dipole‐dipole resistivity lines were established. Our studies provide new information on one boundary of a major continental rift and on the depositional and structural style of an extensional basin within the rift. Integration of these data sets into a single transect indicates that the Española Basin is asymmetrical with approximately 2 to 3 km of sediments and sedimentary rocks near the center, thinning eastward to the Precambrian outcrop of the flanking Sangre de Cristo uplift. Several minor faults with throws of less than 200 m were found, but no major eastern bounding fault was observed. Thus, the Española Basin could be an asymmetrical, west‐dipping half‐graben. However, major fault offset, down toward the basin axis, may occur within Precambrian rocks of the Sangre de Cristo uplift. In either case, the geometry of the basin does not agree well with current models for the structural evolution of continental rifts, which emphasize low‐angle detachment faults which create asymmetrical, hinged half‐grabens. These models predict that major shoulder uplift should occur adjacent to the side of the graben bounded by a listric master fault rather than adjacent to the hinged side. In contrast, for the Española Basin major uplift occurred adjacent to the eastern side, which could be the hinged side of the basin. A thick wedge of older sedimentary rocks with high P‐wave velocity (4.4 km/s) and low electrical resistivity (5 Ω⋅m) was discovered under the younger Tertiary sediments and sedimentary rocks near the center of the basin. This wedge has maximum thickness of 1.2 km at the western end of the profile and thins eastward. The physical properties suggest this layer could be older Tertiary, or possibly a Mesozoic‐Paleozoic, section of rocks. If the latter, it has potential economic importance because of the possible presence of a Cretaceous section which is known to produce oil and gas in the Albuquerque Basin to the south and the San Luis Basin to the north. However, based on data from the Yates La Mesa no. 2 well, 10 km south of the transect, this wedge is likely middle Tertiary lacustrine deposits (NMOCD, 1986). The great thickness of lake deposits may represent a major lacustrine facies of the Eocene Galisteo and El Rito formations, exposed around the southern, southwestern, and northwestern margins of the basin. Magnetotelluric data suggest the crystalline basement underlying the central Española Basin may be more conductive than near the eastern margin. The entire Española Basin is also underlain by a highly conductive layer of about 1 Ω⋅m at a depth of 15 km. Both the shallow and deep low‐resistivity zones may result from hot, saline fluids. Such fluids deep within the crust may reduce the shear strength of the crust significantly and concentrate crustal extension on the west side of the rift.

No precursory variations of either resistivity or self‐potential were seen to precede an earthquake of magnitude 4.0 which occurred in December 1977 on the San Andreas Fault in central California. This result is in contrast with an earlier result, in which a variation of about 10 to 15% was seen to precede an earthquake of similar magnitude and location which occurred in June 1973. As the present accuracy of the resistivity measurement is about ± 2%, this result suggests either that the relationship between earth strain and resistivity was different for the two events, or that the earlier result was erroneous and that the amplitude of any resistivity variation preceding an earthquake of magnitude 4 in this area is less than 2%. This result implies that much better measurement accuracy will be required to detect any variations preceding earthquakes of magnitude 4 or less in this area.

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Given the degree of complexity of modern magnetotelluric (MT) instrumentation, comparison of the total performance for two or more systems is an important verification test. This paper compares the processed data from five MT systems which were designed and constructed separately, and which employ different electrode types, electrode separations, magnetometers, and methods of signal processing. The comparison shows that there is a high degree of agreement among the data from the different systems. The study also demonstrates the compatibility and reliability of the MT systems employed as part of EMSLAB Juan de Fuca (Electromagnetic Sounding of the Lithosphere and Asthenosphere Beneath the Juan de Fuca Plate). This project, proposed by a consortium of institutions, involves not only magnetotellurics studies but also studies of magnetic variation, on land and on the sea bottom. The project calls for the real‐time MT systems to occupy stations along segments of a profile in Oregon. A composite profile will be created from the segments. Prior to commencing the main MT profiling phase, one week was set aside in August, 1984, for all groups to record and process MT data sequentially at six sites in diverse geologic terrains; this experiment was called mini‐EMSLAB.

Se ha propuesto que la separación del Bloque de Jalisco es la consecuencia principal de la subducción oblicua diferencial de las placas de Rivera y de Cocos debajo de la Norte América y de la interacción entre las cuatro placas. El análisis vectorial de la subducción de la Rivera y Cocos con respecto a Norte América arroja una frontera de desplazamiento lateral entre las placas oceánicas, cuya velocidad es de 4.2 cm/año, que coincide en su orientación con lineamientos estructurales interpretados en el Bloque Jalisco y con el Graben Colima. Muchas estructuras en el rango de N30°-40°E y N60°-70°E en el Bloque Jalisco son similares a las observadas en el área de Tecpan, entre Zihuatanejo y Acapulco. Estos resultados concuerdan con la interpretación hecha con información geofísica marina de detalle obtenida a bordo de B/O "Akamedik Nikolai Strakhov" durante Febrero-Marzo de 1989 en el área frente a las costas de Manzanillo y Tecpan, incluyendo perfilado sísmico de reflexión, batimetría y 122 mediciones directas de flujo de calor. Además, se efectuaron 39 estimaciones de flujo de calor con base en la identificación a partir de los perfiles sísmicos de capas de gas en la cubierta sedimentaria del talud continental de Manzanillo. En la región limitada por cañones submarinos de la Cuenca de Manzanillo, el perfilado sísmico y la batimetría indican levantamiento recién asociado con fallamiento de desplazamiento lateral. El flujo de calor en la porción occidental de la Cuenca de Manzanillo varía entre 20 y 35mW/M2 (promedio de 28±6 mW/m2), mientras que en la porción oriental el promedio es de 51±7 mQ/m2. En ambas áreas se observa fallamiento lateral con componente vertical. El área con flujo de desplazamiento izquierdo es el Bloque de Jalisco. El flujo de calor promedio en la parte oriental de la Cuenca de Manzanillo concuerda con valores típicos medidos frente a las costas de Tecpan, Guerrero, del oren de 55±8 mW/m2. Un flujo de calor mayor de 100 mW/m2 fue medido en la pendiente oceánica de la trinchera. Se le atribuye a la edad joven de la corteza con respecto a la corteza continental o bien a la presencia de fuentes locales de actividad magmática o hidrotermal en el Graben El Gordo.