The Los Humeros Volcanic Complex (LHVC) is a large silicic caldera complex in the Trans-Mexican Volcanic Belt (TMVB), hosting a geothermal field currently in exploitation by the Comisión Federal de Electricidad (CFE) of Mexico, with an installed capacity of ca. 95 MW of electric power. Understanding the structural architecture of LHVC is important to get insights into the interplay between the volcano-tectonic setting and the characteristics of the geothermal resources in the area. The analysis of volcanotectonic interplay in LHVC benefits from the availability of subsurface data obtained during the exploration of the geothermal reservoir that allows the achievement of a 3D structural view of the volcano system. The LHVC thus represents an important natural laboratory for the development of general models of volcano-tectonic interaction in calderas. In this study, we discuss a structural model of LHVC based on morphostructural and field analysis, integrated with well logs, focal mechanism solutions and magnetotelluric imaging. The structural analysis suggests that inherited regional tectonic structures recognized in the basement played an important role in the evolution of the magma feeding system, caldera collapses and post-caldera deformations. These inherited weak planes have been reactivated by resurgence faults and post-caldera magma-driven hydrofractures under a local radial stress field generated by the shallow LHVC magmatic/hydrothermal system. The local stress field induced caldera resurgence and volcanotectonic faulting. The results of this study are important to better constrain the structural architecture of large caldera complexes. Also, our study is useful to understand the structure of the Los Humeros geothermal field and support the exploration of deeper Super-Hot Geothermal Systems (SHGSs) and engineering of Enhanced Geothermal Systems (EGSs) for electric power production in the LHVC and other active resurgent calderas.

Since the installation of an extensive digital strong motion array by Fundación Javier Barros Sierra in 1987 three moderate earthquakes have been recorded by the array and by the accelerographs operated by Instituto de Ingeniería, UNAM. Using this new data and results from the analysis of previous accelerograms we present spectral ratios at 40 sites in the valley of Mexico with respect to a hill zone site in Ciudad Universitaria (CU). Clear evidence for nonlinear behaviour of the clay is found at Central de Abastos Oficina (CDAO) site during the great Michoacán earthquake (Ms=8. 1). At four other lake bed sites this behaviour is not seen either because none occurred or because of poorer quality of data. The spectral ratio at a given site appears to be roughly independent of magnitude (except, perhaps, during great earthquakes when lake bed sites may behave nonlinearily), azimuth, and depth of earthquakes with epicenters ≥ 200 km from the city. On the lake bed sites of the valley the relative amplification (RA) varies between 8 and 56 and the natural period lies between 1.4 to 4.8 sec. Relative amplification maps at periods centered at 3, 2.5, 2, 1.5, and 1 seconds are presented. The area where severe damage and collapse of buildings in the city was concentrated during the Michoacán earthquake correlates well with the area with RA≥14 in the period range of 1.75 to 2.75 sec.

The spectral ratio technique is a common useful way to estimate empirical transfer function to evaluates site effects in regions of moderate to high seismicity. The purpose of this paper is to show that it is possible to estimate empirical transfer function using spectral ratios between horizontal and vertical components of motion without a reference station. The technique, originally proposed by Nakamura to analyze Rayleigh waves in the microtremor records, is presented briefly and it is discussed why it may be applicable to study the intense S-wave part in earthquake records. Results are presented for three different cities in Mexico: Oaxaca, Oax., Acapulco, Gro., and Mexico City. These cities are very different by their geological and tectonic contexts and also by the very different epicentral distances to the main seismogenic zones affecting each city. Each time we compare the results of Nakamura's technique with standard spectral ratios. In all three cases the results are very encouraging. We conclude that, if site effects are caused by simple geology, a first estimate of dominant period and local amplification level can be obtained using records of only one station.

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The period at which peak in the microtremor Fourier velocity spectra occurs in the transition and lake bed zones of the valley of Mexico is found to be the natural period of the site. These periods in the valley are compiled from the microtremor measurements carried out by Instituto de Ingeniería, UNAM and scientists from Japan (for a total of 181 sites). Using this data and the natural periods estimated from strong motion recordings (36 sites), an isoperiod contour map of the valley of Mexico is presented. This map may be useful in future design of important structures.