geotechnical aspects of the 2017 mw 7.1 puebla-morelos ... · plate and the northamerican plate,...

Geotechnical Aspects of the 2017 Mw 7.1 Puebla-Morelos Earthquake

Felipe Ochoa-Cornejo(1), Cesar Pasten(1), Francisco Hernandez(1), Rodrigo Astroza(2)

(1) Profesor Asistente, Departamento de Ingeniería Civil, Universidad de Chile (2) Profesor Asistente, Facultad de Ingeniería y Ciencias Aplicadas, Universidad de los Andes

Abstract

This work presents the geotechnical observations related to the Mw 7.1 Earthquake that hit the region of Puebla-Morelos, in Mexico, on September 19th of 2017, which caused more than 350 deaths and losses equivalent to 0,1% of the GDP of the north-american country. The most severe geotechnical damage observed after the Puebla-Morelos Earthquake was the origination of cracks in the southern part of Mexico City, moderate rock falls, and lateral displacements in the area of a bridge in a road to Jojutla, close to the epicentral area. In addition, geophysical measurements were made to set a case of site effects in Jojutla, where a concentrated area of damage was observed. A repository of photographs documenting the geotechnical, and some structural, damages is also presented.

Palabras-Clave: Puebla-Morelos Earthquake, México, Ground motion, Cracks, CDMX, Jojutla

1 Introduction

The 2017 Mw 7.1 Puebla-Morelos Earthquake occurred on September 19th of 2017 at 13:14:40 local time, 32 years after the 1985 Mw 8.1 Great Michoacan Earthquake. This seismic event had its epicenter 12 kms southeast of Axichiapan, in the border of the states of Morelos and Puebla, 120 kms from the capital Mexico City (CDMX). In particular, the earthquake affected the states of Puebla, Morelos, Guerrero, Hidalgo, Tlaxcala and CDMX [1,2]. As most of the significant earthquakes occurring in Mexico the 2017 Puebla-Morelos Earthquake was of subductive nature; particularly, it was an intraplate earthquake with a normal fault mechanism at an average depth of 57 kms inside the Cocos Plate [3]. In this context, this earthquake confirms the high subductive activity of the area due to the strong interaction between the Pacific, Cocos, Rivera, Caribean, and North American tectonic plates, depicted in Figure 1 [4]. Particularly the Cocos Plate has a significant influence in the occurrence of earthquakes in Mexico due to its convergence rate, which is 6 cm/year, less than the rate existing in between the Nazca and Southamerican Tectonic Plates in Chile [5], with most of the earthquakes occurring along the coast, as depicted in Figure 2.

The earthquake-induced catastrophic damage was especially significant in structures and foundations through several locations around the epicentral area, particularly in Mexico City, Puebla City, and Jojutla. More than 8400 buildings were affected, with more than 2000 declared as “severely affected”; 360 of these buildings were demolished. 44 buildings and some pedestrian bridges fully collapsed. The consequences of the structural damage caused that more than 30000 people were displaced from their homes. In this context, the Puebla-

XII Congreso Chileno de Sismología e Ingeniería Sísmica ACHISINA 2019 Valdivia, 03 al 05 de Abril 2019

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Morelos Earthquake caused more than 360 deaths, most of them in Mexico City and Morelos, and other close counties. In economic terms, the estimated financial losses due to this earthquake were over USD 4 Billions, close to 0,1% of the Mexican GDP [6,7].

Few days after the earthquake, the authors completed an earthquake reconnaissance campaign mainly in Mexico City and Jojutla, one of the most affected locations due to the strong ground motion, very close to the epicentral area. During the reconnaissance, the earthquake induced damage was addressed, focusing on both structural and geotechnical aspects.

The reconnaissance was completed covering the inspection of facilities such as buildings, houses and residential buildings (masonry and concrete), bridges, roads, etc. Regarding the geotechnical damage, the most significant damage was the development of large shallow cracks, which varied in extension and depth, following clear directional patterns. Illustrative pictures are presented to document the geotechnical damage from the earthquake. In this context, this paper focuses on the damages observed after the earthquake, providing an engineering perspective.

2 Seismic context of Mexico, and the Pueblo-Morelos earthquake

The subductive environment of Mexico is particularly active due to the strong interaction among the Pacific, Cocos, Rivera, Caribean, and North American tectonic plates. In this context, particularly, the Cocos Plate has a significant influence in the occurrence of earthquakes in Mexico.

A significant portion of Mexico is along the frictional tectonic border formed by both the Cocos Plate and the NorthAmerican Plate, which belong to the Pacific Ring of Fire. These tectonic plates mainly interact in the south of Mexico, along the coastal trench that is known as the Middle American Trench. This type of subduction is similar to the one observed along the Nazca- Southamerican Trench.

Figure 1. Interaction between five tectonics plates: Southamerican, North American, Cocos, Rivera, and Caribbean [4]


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The Cocos Plate is the one inducing most of the significant earthquakes in Mexico due to its convergence rate, with them depicted in Figure 2 [4]. The oceanic Cocos Plate subducts under the continental North American plate, triggering earthquakes that range from small to very large. These earthquakes are due to the stress accumulation from the interlocking and deformational processes happening in the frictional contact.

The Middle America Trench (MAT) of Mexico is a highly active subduction zone, presenting a significant range of steep dip angles in the west and east part, with a flat portion -running 150 kms beneath Mexico- in between these sections, as shown in Figure 4. Particularly, the seismicity is concentrated between the trench and the coast of Mexico.

Figure 2. Rupture area of the most important earthquakes occurred in México [4]

Figure 3. Cross section of the subduction scenario of México formed by the Cocos and NorthAmerican plates [8]


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Figure 4. Cross section of the subduction scenario of México formed by the Cocos and NorthAmerican plates

3 Geotechnical features of Mexico City

Mexico City is in the interior of Mexico, approximately 400 kms from the nearest coast, at an altitude of 2250 meters above the sea level. Figure 5 depicts the geographic description of the city of México with respect to its coastal area, where most of earthquakes affecting Mexico are triggered. Although Mexico City high in the mountains, mainly in rocky environments, its shallow geomorphological features are conditioned and constrained by a large basin dominated by old lakes: The Texcoco Lake and the Xochimilco-Chalco Lakes. The Texcoco Lake is to the north of Mexico City, and close to the northern edge of the Xochimilco-Chalco Lake. These lakes were the product of the endorreic system that controls the flow of water in the basin formed by volcanic rocks and lacustrine sediments. Nowadays, these lakes have disappeared due to the historical development of the city. On one hand, the water consumption drained the hydrological levels of the lake, phenomenon that has significantly increased its rate in the last century due to the population growth rate of CDMX. On the other hand, the overpopulation reached, demanded an increase of buildings construction and urban development, forcing land occupation and, therefore, the displacement of the forementioned lakes.

https://www.sciencedirect.com/topics/earth-and-planetary-sciences/volcanic-rock

https://www.sciencedirect.com/topics/earth-and-planetary-sciences/lacustrine-deposit


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Figure 5. Geographical location of Mexico City, the Geographical distribution of the Texcoco Lake and the Xochimilco-Chalco Lakes, and their evolution [7,9,10]

It is well recognized that lakes are low energy environments that promote the formation of soils with poor geotechnical quality. Therefore, Mexico City is settled in a geological setting in which rock and good geotechnical quality soils are found in the periphery, former shores of the lake, while low geotechnical quality, or lacustrine, soils are found in most of the city, corresponding to the inner portions of the lake. As a general description, the areas with good soils are hard soil deposits with shear waver velocities over 400 [m/s] in the outer areas of the city while the areas fill with clay deposits -of high compresibility, low stiffness, and high plasticity- are in the central part of the city [9,11,12].

Mexico Coast

CDMX


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The soils of Mexico City have several origins due to the wide variety of geologic events that have formed the basin in which the city is. Volcanic activity, tectonic movements, as well as changing geography due to the existence of rivers, as well as intense rainy and drought seasons, defined the existence of different types of soils. In general terms, the soils of Mexico City are formed by volcanic ashes, sand, fine sands, cemented sands, carbonatic sands, silt, clay, gravels, basaltic rocks, lacustrine sediments, as depicted in Figure 6. The dominant soil formation beneath Mexico City has a depth of approximately 100 m, with a shallow, middle, and deep clay formation. In between the clay layers there are intrusions of volcanic and hard soils. The clays deposited on the Xochimilco-Chalco lake, to the south of CDMX, are more rigid than those in the Texcoco Lake, in downtown of Mexico City [11–13]. In addition to the previous soils, Mexico City presents diatomaceous soil particles. This type of soil is particularly due to the lacustrine environment. Figure 7 presents pictures of some examples of many of the soils extracted, and imaged, from Mexico City.

Figure 6. Soil profile of Mexico City, in the west-east direction [13]

https://www.sciencedirect.com/topics/earth-and-planetary-sciences/soil-profile


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Figure 7. Soil particles of some soils extracted from Mexico City. Modified from Diaz-

Rodriguez [14,15]

In the geotechnical context presented, the geotechnical zoning of CDMX for seismic engineering design purposes, considers the following areas: 1) Zone I: Hills, formed by rocks or, generally, firm soils that were deposited outside the lacustrine environment, but in which there may be, superficially or interbedded, loose sandy deposits or cohesives relatively soft. 2) Zone II: Transition, in which the deep deposits are at 20 m depth, or less, and composed predominantly by sandy layers and sandy silt interbedded with lacustrine clay layers; the thickness of these is variable between tens of centimeters and few meters. 3) Zone III: Lacustrine, composed of highly compressible clay deposits, separated by sandy layers with diverse silt or clay content. These sandy layers are generally moderately compact to very compact and of varying thickness from centimeters to several meters. Lacustrine deposits are usually covered superficially by alluvial soils, dried materials and artificial fillings; The thickness of this set can be greater than 50 m. Zone III was subdivided into zones IIIa, IIIb, IIIc and IIId taking into account the depth of the clay deposits changing from the hilly areas to the center of the old lakes area. The areas are depicted in Figure 8, along with the fundamental periods of vibration assigned for site classification.

a. Silty particles

b. Ostracodes Valves

c y d. Diatomaceous particles

e. Pyrite framboides

f. Dominios de partículas con

estructura floculada

In addition, there is presence of

volcanic altered glass, and

minerals like alophans


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Figure 8. Geotechnical zoning of Mexico City. Image modified from the SSN 2017

In particular, the diatomaceous particles and soils that are present in the soft soil deposited in the areas of the ancient lakes, mainly zones Zone II and III, have shear wave velocities below 100 [m/s] with depth. Soil profiles in zone III, for instance, provisions consider a Vs30 ranging between 60-70 [m/s]. They have a low stiffness degradation (G / G0), and low damping ratio increase rate: the shear modulus decreases to the 90% of its original value for cyclic shear strains around 0.1%; the damping ratio increases from 2% to 3% for the same level of cyclic shear strains. Therefore, these soils present poor geotechnical quality, being prone to experience seismic amplification. Diatomaceous soils were deposited on the basin, with variable thickness, influencing the natural periods of the surface soil, and the seismic classification depicted in Figure 8.

In the context of the high seismicity of Mexico, with the occurrence of several large earthquakes that have affected the country, Mexico City appears as a “site amplification effect observatory”, due to the overall presence of low geotechnical quality soils. The seismic waves entering the basin in which Mexico City is, present important reverberations due to the poor geotechnical quality of its soil to resist shear wave distorsions. The best example of this effect was observed in the 1985 Mexico City earthquake, in which the attenuated seismic signals,

Area Fundamental

Period [s]

I 0.5

II 0.5 – 1.0

IIIa 1.0 – 1.5

IIIb 1.5 – 2.5

IIIc 2.5 – 3.5

IIId > 3.5


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originated in the coast of Mexico, were significantly amplified once they hit the basin of Mexico City, almost a minute after the earthquake was triggered. Figure 9 illustrates the seismic amplification registered in the large earthquake of 1985.

Figure 9. Site effect observed in Mexico City in its geological/geographical context [16]

The interplate 1985 Mw 8.1 Michoacan Earthquake that hit the northwestern coast of Mexico on September 19, 1985, seriously affected CDMX although there were 400 km of distance between the epicentrer of the event and CDMX. The damage observed in the peripheral area and short buildings in the city was almost none. However, the site effects due to the amplification of the long period seismic waves was significantly observed in tall buildings as they reached resonance. The 1985 earthquake collapsed more than 350 buildings; many of them of great relevance for the Mexican citizen. Most of the collapses occurred in areas where the soil period varied between 0, and 2,2 seconds. The 1985 earthquake cause close to 10000 deaths, and an economic cost of approximately US 8 Billion.

4 Main Geotechnical Observations Post-earthquake

4.1 Surface cracks in the southern área of Mexico City

Until some centuries ago, the basin of Mexico City was of endorreic nature, and mainly characterized, as presented before, by the large and shallow Texcoco and Xaltocan lakes. However, it became an open basin as urban development increased, therefore becoming an exorreic system, with the lakes draining, and eventually, practically disappearing. As a consequence, Mexico City developed over a poor geotechnical environment, with highly plastic soft clays. The upper, lower, and deep clay formations partially, yet significantly, control the geotechnical behavior of the city, with the contribution of anthropic fill areas. In addition, the area is affected by dry and wet cycles. In this context, Mexico City lays on a soil


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profile where the normally consolidated behavior is dominant, inducing significant settlement of the city [17].

In addition, the overpopulation of Mexico City, currently over 20 millions of people, has induced a significant overconsumption correspondingly causing a significant decrease of the water table to approximately 50 m depth, increasing the rate of settlement as pore pressures decrease, inducing a height decrease of over 8 m in the last 100 years [18]. The combination of overpopulation, water consumption and geotechnical quality of the soils in Mexico City explains the regional subsidence of the city, which usually is expressed as the formation of induced faulting, due to subsidence, which has been of the order of 10 m, being less in some other areas [17,18]. In this context, provided the nature of the basin in which Mexico City lays, surface faults and horizontal shearing planes along weak transition areas between rock and soils in the interfaces of the basin with the mountain, and controlled by the presence of clayey and silty compressible materials that yield to cracking, associated with the decrease of the water table. The latter has expressed in “ground fracturing” areas in some zones of Mexico City. One of the main triggering factors of the development of surface cracks identified in Mexico City is the mechanical behavior of the basin, which presents a strong variability of soil conditions in both the horizontal and vertical directions, in addition to the excessive groundwater consumption in short periods. These large and extended systems of shallow cracks has been well reported by Carreon-Freyre for both Mexico City and other areas of the central part of the country [19,20]. In this context, events like earthquakes can accelerate the rate of occurrence of these surface cracks. The latter was confirmed during the earthquake reconnaissance after the Puebla Morelos Earthquake of 2017, evidencing the significant impact that static and dynamic loads have on the subsoil, and therefore the surface, of some areas of México City. The red pin shown in Figure 10 presents a particular location –out of many- to the south of Mexico City that previously presented surface cracks. The cracks in this area have been long identified.


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Figure 10. Cracks in the southern part of Mexico City

As shown in the upper right picture of Figure 10, these cracks present a directionality across the streets and neighborhood as they pertain to a larger system of cracks, shown in the lower-left picture of Figure 10. In general, these cracks have presented a very low increase in their size and extension, with this phenomenon occurring at a very low rate, which has allowed the urban development of neighborhoods without presenting significant problems. However, after the Pueblo-Morelos Earthquake, they were significantly magnified by the ground motion, producing an increase and size and extension, accelerating the shearing processes, inducing deformations of up to 1 m in the vertical direction (Figure 10, lower right pictures). The cracks increased in size crossed houses, streets, buildings, as well as parks and free fields. Part of the damage induced by this accelerated process is show in Figure 11.


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Figure 11. Structural damages observed due to the cracks in the southern part of Mexico City

Geophysical measurements were performed in the areas close to the cracks observed. In particular, H/V spectral ratios and SPAC measurements were obtained to describe the dynamic properties of the ground in where the cracks were. H/V recordings were obtained from singular measurements with TROMINO geophones, while the dispersion curves to relate the profile of shear wave velocity with depth was made with spatial arrays of four TROMINOS simultaneously measuring on the ground. In Figure 12, the results of these measurements are presented. The information shows that H/V spectral ratios present one single peak located at a fundamental frequency of the ground -in the area of the cracks- of the order of 0,5 Hz, i.e. 2 s,, while the dominant shear wave velocities should be of the order of 100 m/s in the shallower parts of ground as inferred from the dispersion curves . These magnitudes confirm the results obtained by previous studies regarding fundamental periods in that area of Mexico City, accusing the existence of low quality soils.


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Figure 12. Geophysical measurements in the area of the cracks observed in the southern part of Mexico City. Measurements obtained with TROMINO geophones.

Figure 13. Bridge damage on a road between Mexico City and Jojutla

4.2 Bridge on the way to Jojutla, close to epicentral area


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On the way to the city of Jojutla, in an interior road close to the epicentral area, a bridge with significant, yet fixable, damage was found. The following observations in this bridge were made, which are shown in Figure 13: 1) significant relative displacement between road decks in vertical, lateral, and longitudinal direction of up to 30 cm, 2) significant settlement that reached up to 20 cm, 3) significant lateral displacements of the shore of the river crossed by the bridge that ranged from 0,5 m up to 3 m, 4) relative displacements of the deck with respect to the bridge abutments, 5) and liquefaction cones of ejected sand through the earthfill of the foundation system of the bridge.

4.3 Area of Jojutla, close to the epicentral area

The town of Jojutla is approximately 150 kms to the south of Mexico City. On the way to the city of Jojutla, except for the bridge previously mentioned and a particular rock fall observed in a mountain cliff, little to none-damage was observed. However, the downtown commercial area of Jojutla presented significant and severe damage. Houses, schools, buildings, commercial centers, hospitals, and banks presented medium to very high damage levels, with many collapses of one/two floors structures. This area had an extension of approximately 2 km2. Provided the damage observed was specifically located in one single and concentrated area, and considering the existence of a river close to the city, which also passed through it in the past, there are suspicions that site effects could be the reason to observe such a concentrated effect on structures.

Figure 14. Visit to the city of Jojutla, where geophysical measurements were completed as site effects could be a possible reason to explain such a concentrated amount of damage

In the context presented, geophysical measurements were performed in different places of Jojutla; mainly, H/V spectral ratios and SPAC measurements to obtain some information


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regarding the dynamic properties of the ground beneath the city. H/V results show the predominance of two peaks at 0,5 Hz, and almost 4 Hz, implying periods between 0,25 and 2 seconds. Dispersion curves suggest the soil profile could be mainly dominated by silty sand deposits, provided the Rayleigh wave velocities obtained in the dispersion curves.

5 Conclusions

A field reconnaissance was performed after the earthquake that hit Mexico on September 19th of 2017. The main observations are related to the geoenvironmental conditions of the soil of Mexico, in particular the poor quality of the soils, mainly of lacustrine origin, features that promote the site amplification effect observed in the Mexico City basin. Geophysical measurements of fundamental periods and dispersion curves performed by the Chilean reconnaissance team confirm the poor quality of the soil of Mexico, which are well known by the earthquake engineering community.

6 References

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[17] Rodríguez-rebolledo JF, Auvinet-guichard GY, Martínez-carvajal HE. Settlement analysis of friction piles in consolidating soft soils Análisis de asentamientos de pilas de fricción en suelos blandos compresibles. Dyna 2015;82:211–20.

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geotechnical aspects of the 2017 mw 7.1 puebla-morelos ... · plate and the northamerican plate,...

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