Keywords

1 Introduction

A significant part of Mexican territory is shaken by earthquakes of different types engendered by the tectonic activity of the region. The Cocos and the North American plates at the Mexican subduction zone have a convergence rate of about 5 cm/year. This motion induces intraplate earthquakes (M ≤ 7) on the North American plate, intermediate normal fault events on Cocos Plate (M ~ 6) and large subduction earthquakes (M > 8). However, the distribution of intensities and effects of these earthquakes differ. Among the most important subduction events occurred in twentieth century we count:

  • The M7.6 earthquake of June 7, 1911 with a rupture area of ca. 60% the one of the 1985 earthquake (UNAM Seismology Group GRL 1986)

  • The M7.8 earthquake of July 28, 1957

  • The M7.4 Petatlan event of March 14, 1979 which ruptured essentially at the zone of the 1985 major aftershock

  • The M7.3 Playa Azul shock that occurred in October 25, 1981 with a rupture area of about 40 × 20 km2 (Havskov et al. 1983) (Fig. 1).

    Fig. 1
    figure 1

    Elevation map of central Mexico. Mexico City Valley is delineated with a yellow line in the middle of Trans Mexican Volcanic Belt. The estimated ruptures of five subduction events are shaded with red (from left to right: 09.19.85 M8.1; 10.25.81 M7.3; 09.21.85 M7.5; 03.14.79 M7.4; 07.28.57 M7.8). The dashed red line indicates the trench of Cocos and North American plates and the accelerometric stations of Guerrero Array are plotted with black dots

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These earthquakes caused minor damage in Mexico City, which is roughly 300 km away from the subduction zone, in comparison with the damage caused by the 1985 earthquake.

Nowadays, although the 1957 and 1979 earthquakes could be seen as a kind of warnings because of conspicuous collapse of some buildings in Mexico City, the lack of reliable historical seismicity data led to minimize the seismic hazard, and therefore effects and intensity of the 1985 earthquakes could not be anticipated. Moreover, the occurrence of such events is considered rare and the systematic recording of earthquakes is relatively recent (Sánchez-Sesma and Crouse 2014). Regarding the 1985 Michoacan Earthquake, it exists a vast bibliography. Among them are the three special issues in Earthquake Spectra (1988, 1989). In this paper, a brief account of some relevant seismological facts and important actions taken as a consequence of the 1985 earthquake and its effects on Mexico City are highlighted. This may have some bearing on the reconstruction issues at the aftermath of the April 25, 2014 Nepal earthquake.

2 Geology Frame

The Mexico City Valley geological characteristics are unique. Mexico City is in a plateau at 2250 m.a.s.l., and covers a region of about 110 km N–S and 80 km E–W. Quaternary and Cenozoic volcanoes surround the Valley, and shallow lakes (Xaltocan, Zumpango, Texcoco, Xochimilco and Chalco) formed after the Pleistocene-Holocene Chichinautzin mountain range closed the Basin resulting in the formation of the lacustrine system on which the Mexico City is extended (Fig. 2) (Mooser et al. 1974; Padilla y Sánchez 1989). The City was founded in the fourteenth century and since then floodings were not rare. In the Prehispanic Aztec towns hydraulic control structures like channels and dikes proliferate. The Chinampas (kind of floating terrains that eventually became cultivated areas) were also common constructive systems. Since the sixteenth century, with the Spanish Colonization, the lakes started to be drained to avoid the recurrent floodings. Thereby, the City grew within a basin on loose layers of compound soil of clay, sand and volcanics, over the Mesozoic bedrock of marine sedimentary sequence.

Fig. 2
figure 2

Elevation map of Mexico City Valley and the mountain ranges that closed the basin: “Sierra de Las Cruces”, “Sierra Chichinautzin” and “Sierra Nevada”. Inside the basin is the Lake Zone characterized by soft lacustrine soils, shaded in dark gray. The Transition Zone is plotted in light gray and the Hill Zone (above 2300 masl) in green. The red dots indicate stations of the Accelerometric Network that operates since 1987 to the present. The yellow triangles denote accelerometer stations that recorded the 1985 earthquakes

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The geology conditions of the region constrain the soil characteristics and determine the seismic response of Valley. Mexican engineers knew the peculiar soil conditions of the City and the special requirements were generated for the design of foundations and structures since the early 50s. Marsal and Mazari (1959) classified the soils of Mexico City in the three geotechnical zones depicted in Fig. 2. The Hill Zone, known also as the hard soil, extends on the higher elevations in the Valley, it is composed of lava flows and volcanic tuffs. The transition Zone presents alluvial sandy and silty layers with occasional intervals of clay, and surrounds with some volcanic craters. The Lake Zone, occupies the central area of the Basin, characterized by lacustrine soft-bed soil composed by highly compressible deposits with 20–80 m thickness over resistant sands.

This geotechnical classification has been useful to establish the building code regulations generated after the careful study of the recorded ground motion before and after 1985. However, this layered classification cannot explain alone the large amplitudes and long durations recorded at sites of the Valley during the Michoacan Earthquake. It is now generally accepted that to forecast the response of the Valley, a complete layered model should take into account the concatenation of source, path and site effects (e.g. Singh et al. 1988; Sánchez-Sesma et al. 1988; Campillo et al. 1989; Castro et al. 1990).

On the other hand, the water supply to the city has been an issue of major concern. Aqueducts provided water but since the mid-nineteenth century, wells were installed to extract the liquid from the aquifers (between the clay formations or below them). With time, the extreme exploitation of the aquifers produces subsidence and the ensuing changes of dominant period of the ground at some sites (Ovando-Shelley et al. 2007). All these effects combined make the Valley of Mexico highly vulnerable to the seismic threat despite of its long distance to the Pacific trench.

3 The 1985 Earthquake Characteristics

Since early 1985, seismologists were waiting for a big quake at the so called Guerrero Gap and a series of accelerometric recording stations have been installed along the Mexican Pacific coast in the Guerrero Array. In fact, the 19 September earthquake occurred in the Michoacán seismic gap, which had been identified as a zone with high seismic potential and was well recorded by the array. This mega-thrust earthquake was caused by subduction of the Cocos Plate beneath Mexico (Fig. 1). The station at Caleta de Campos was just above the epicenter (Anderson et al. 1986). On September 19, 1985, at 07:18 h (local time), a M8 earthquake of occurred with epicenter close to Pacific coast of Mexico, with some damage in the epicentral area and Ciudad Guzman, Jalisco. However, the damages in Mexico City were unprecedented.

The fault rupture area was of about 200 km length along the boundary of the Mexican subduction zone in the Pacific coast of Michoacán and a width of about 50 km. The thrust source mechanism had a 288° fault plane strike and a dip of 9° (Eissler et al. 1986). Strong ground motion investigations indicate that this major earthquake consisted of two main sub-events at 16 km depth, separated 27 s in time and about 95 km SE in distance (Fig. 1). The larger M7.5 aftershock occurred thirty-six hours later, in the SE edge of the same area. The total seismic moment was of about 11 × 1028 dyne-cm (Astiz et al. 1987) and the rupture area was of about 104 km2 (see Fig. 1).

4 Effects of Soil Amplification and Structural Damage

The 1985 earthquake produced large intensities in Mexico City, in the Mercalli scale they reached level IX. Maximum accelerations reached 0.17 g, thus exceeding building codes limits (Anderson et al. 1986) and the spectral amplifications and durations were unprecedented (Singh et al. 1988). These features are quite unusual for an event which epicentral distance to the site is of about 400 km. The earthquake destroyed or severely damaged about six thousand buildings. The causalities are estimated to be between 10,000 and 35,000 people with tens of thousands injured or homeless. The cost of damages has been estimated to be larger than the 2% of the gross domestic product in 1985.

The influence of soft soil on the seismic response of alluvial valleys may produce significant amplifications of the ground motion. This problem has been the subject of numerous studies (e.g. Bard and Bouchon 1980a, b; Sánchez-Sesma 1987; Aki 1988; Kawase and Aki 1989; Campillo et al. 1989). The large amplitude ground motion and long duration records in the lacustrine plain sites in the Valley of Mexico result from the combination of significant source, path and site effects (e.g. Singh et al. 1988; Sánchez-Sesma et al. 1988; Campillo et al. 1989; Castro et al. 1990). Ordaz and Singh (1992) discovered a large regional amplification. They demonstrated, using a data set of seismograms from stations located both inland and along the cost, that seismic waves at hill zone sites in Mexico City are amplified. Such amplifications at the so called “hard zone” are remarkable as they reach at least 10 times at critical frequencies for Mexico City soil conditions (1–5s). Considering that the uppermost soft layers trap the seismic wave energy causing significant ground motion amplifications, reaching 10–50 times, in lake-bed sites with respect to the hill sites at periods between 1 and 5 s. This results in an amplification of 100–500 with respect to the hard sites with similar hypocentral distances, located inland or along the coast. Moreover, their study shows that the amplification effects and damages seem to be independent from the earthquake magnitude. These observations strongly suggest that amplification effects in Mexico City are not only due to the lacustrine soil characteristics. The mechanisms are still to be elucidated as amplifications that are beyond the levels predicted by simple 1D models (Sánchez-Sesma et al. 1988; Singh et al. 1998). All this brings on the validity of the term ‘site amplification’ to adequately explain the observed devastation.

In addition, the number of Pre-Hispanic structures (e.g. channels, tunnels, temples), under which have been built modern structures is unknown. Their condition may cause important effects in the seismic response during earthquakes. Padilla y Sánchez (1989) showed a link between more than 2000 collapsed and damaged buildings and Pre-Hispanic and Colonial structures, during the 1985 earthquake.

Many of the structural collapses were buildings that fulfilled the requirements of the Complementary Technical Standards (NTC) 1976 of the Building Regulations of the Federal District (effective in 1985). This was due to the large values of peak ground acceleration in the Lake Zone (Fig. 2) as compared to the expected values. Ductility factors (Q) was allowed to reach 6, which further decreased the design acceleration.

On the other hand, the duration of ground motion imposes critical conditions to structural response. Based on recorded data Reinoso and Ordaz (2001) proposed expressions to estimate duration in Mexico City in terms of magnitude distance and the dominant period of site. In some sites the duration of strong ground motion in 1985 exceeded 5 min., particularly in the lakebed zone (Fig. 2). This subjected the structures to lasting oscillations, which in some cases may had caused resonance and/or structural failure due to the loss of strength after many cycles of load (Ordaz and Singh 1992). The slab punching by columns revealed lack of shear reinforcement and many systems, particularly reinforced concrete structures, but also those of steel, showed lack of strength and ductility when their periods were close to the site period.

5 New Building Code Regulations

Besides source, paths of seismic waves and the geological and subsoil conditions, irregular settlements in difficult soil conditions and poor enforcement of building regulations contributed to the collapse or failure of hundreds of structures. After the 1985 earthquake, the building codes are often revised and regular monitoring of structures is done after earthquakes to keep safer standards.

Several studies helped to characterize in detail the Mexico City subsoil; results from geotechnical and geophysical prospecting campaigns updated the shallow structure model in some areas inside the Mexico City Valley (Auvinet and Juárez 2011). Imaging investigations for subsidence monitoring and for ground water extraction are taken into account for improving the design of foundations and underground structures (Cabral et al. 2008).

Relying upon the acceleration records at many points in the City an empirical model was developed to evaluate site effects and the response of the Valley (Ordaz et al. 1994). First attempts regarding on spectral ratios were calculated with respect to a reference station at the Hill Zone (CU at UNAM’s main campus). Spectral ratios showed remarkable stability for most events, regardless their origin, their source mechanism and their magnitude (Rosenblueth and Arciniega 1992; Arciniega-Ceballos 1990). Applying some averaging techniques and interpolation scheme (Pérez-Rocha 1998) the representative spectral ratio of at each point was obtained. Then, assuming smooth spatial variation of the dominant period and applying random vibration theory (Boore 1983) the response spectra was estimated at any point within the accelerometric array.

Regarding the dominant periods of soil sites, it has recently been discovered that the frequency characteristics of the sites change with time. For instance, the site SCT had 2.0s in 1985 and now is 1.6s (Avilés and Pérez-Rocha 2010). These might be attributed to several factors that alter the soils conditions and their elastic properties (Ovando-Shelley et al. 2007). For those construction systems that are vulnerable to long lasting resonant ground motion, the emergency regulations (released just after the 1985 earthquake and the subsequent versions), included increases in steel reinforcement, like stirrups and joints. Significant strength requirements, regarding sections of columns, walls and beams, were introduced. That leads to increases in stiffness as well. Systems with rigid flat-slabs and frames without walls have now stronger requirements in both strength and ductility. Moreover, slenderness and irregular geometries were restricted. They are however not free of controversy as they leave to designers some freedom to choose, as some simplifications require higher coefficients, while more costly and detailed procedures allow weaker constrains. These specifications and other design parameters have been modified in the 1993 and 2004 versions that are currently used where the seismic coefficient of maximum design acceleration is 0.45 g.

6 Accelerometric Network, Alert System and Institutional Outreach

In 1985 the Mexico City Accelerometric Network (MCAN) had only 9 stations (marked with stars in Fig. 2). Few years after the earthquake, the monitoring systems improved increasing the MCAN around 80 in the free field (Fig. 2). Nowadays the MCAN resulted from joint efforts of collaboration between the Institutes of Engineering and Geophysics, UNAM and Centro de Instrumentación y Registro Sísmico (CIRES—a private non-profit organization). In addition, around the Valley there are 31 short-period seismometers, operated by the National Seismic Survey, which nowadays counts with a network of 56 broadband STS-2 and 9 short-period seismometers distributed around the country (see http://www.ssn.unam.mx/acerca-de/estaciones/).

A Seismic Alarm System for Mexico City, developed by CIRES, was implemented for earthquakes with magnitudes larger than M6 occurring along the Guerrero and Michoacán coast. This system gives about 50-s warning before the shear waves arrive to the Valley.

In 1988, the National Disaster Prevention Center was established; first, sponsored by the Japanese International Cooperation Agency and later by the Mexican Ministry of Interior. In the last few years an independent organism was created: The Institute for Building Safety which aim is to promote criteria to enforce the fulfillment of code requirements. This requires close collaboration with professional engineering organizations. The widespread damage after the 1985 quake, was enormous and the nature of the causes, initially was a mystery for most of the population, therefore a number of educational actions were developed to promote the culture of prevention: regular simulacrums, insurance information, formation of well-trained emergency search and rescue brigades, are all among the preventive measures adopted.

7 Conclusions

The Valley of Mexico grew on loose layers of compound soil of clay, sand and volcanics. The unconsolidated thick layers of lake sediments (circa 100 m), together with the excess draining of the aquifer, subject the City to severe processes of subsidence. These local characteristics in addition with tectonic and regional settings favor the amplification of seismic waves and long lasting codas.

It is well known that earthquakes cannot be predicted and it is as well difficult to anticipate their intensities. Specially, in the case of complex earthquakes like Michoacán 1985 and Tohoku 2011. These types of events are considered rare and our knowledge about such earthquakes is still limited regarding that the necessary dense seismic networks are relatively recent.

The study of the 1985 Michoacán earthquake and its effects in Mexico City brought on new impetus and several studies have served as an example to the world contributing to our understanding on the seismic response in basins with lacustrine and sand conditions (e.g. San Francisco Bay, CA in 1989). More recent investigations include analyses and modeling of site effects, mechanical and geometrical characteristics of the strata and subsoil and the damage distribution caused by a strong-motion event (Baena et al. 2016 and references here in).

In the same vein, the recently created Institute for Building Safety by the City government may contribute to guarantee structural safety by rigorous code enforcement. The maximum design acceleration were increased, the values of the ductility factor (Q) for different types of structures and building materials were restricted, and increases the steel reinforcement. All these actions are established to implement practical criteria for the adequate assessment of future ground motion levels and seismic risk mitigation. Nevertheless, more efforts have to be done to create awareness among public, architects, engineers planners and policy makers about the implications on living in a region with high rates of seismicity.

8 Addendum

The effectiveness of the developed programs was tested exactly 32 years after the M8.1 1985 Michoacán event by the occurrence of two intraplate earthquakes: the first, M8.2, on September 7th, 2017, in Tehuantepec, at a distance of about 600 km to Mexico City, and the second, M7.1, ironically again on September 19th, between Puebla and Morelos States, only 120 km away from Mexico City. Both events had an extensional mechanism and about the same depth of 57 km, causing considerable damage within a radius of approximately 200 km. During the M8.2 earthquake, maximum accelerations (Amax) did not exceed the values recorded in 1985, causing almost no damage in Mexico City, while during the M7.1 earthquake, Amax in hard zone (57 cm/s2), doubled the response recorded in 1985 (30 cm/s2). Damage not only happened in the soft-lake zone, also many buildings (3 to 8 floors) collapsed in the transition zone. This might be attributed to the high frequency content of the almost vertically incoming waves motion.

It is clear that this recent seismic activity put to the test the programs implemented after 1985. Building codes for different earthquakes scenarios have to be revised as well as the strict supervision of regulations by qualified and uncorrupted authorities.

The Seismic Alert System (SAS) worked during the M8.2 quake as expected but during the M7.1 event the SAS triggered late. This happened because SAS is designed for earthquakes coming from the Pacific coast. Fortunately, the M7.1 earthquake took place 2:15 hours after the annual simulacrum exercise, mitigating considerably the casualties (about 500 nationwide) compared with about 10,000 casualties of the 1985 Michoacán earthquake. This experience states the urgent revision of civil protection programs, in order to strengthen education and routinely practice simulacrum exercises. In addition, a nationwide coverage of the warning system should be achieved.